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Probing young drinking water biofilms with hard andsoft particles

Tony Parisa,b,1, Salaheddine Skali-Lamia, Jean-Claude Blockb,*aLaboratoire d’Energetique et de Mecanique Theorique et Appliquee (LEMTA), UMR 7563, Nancy-University, CNRS, 2 avenue de la Foret de

Haye, BP 160, 54504 Vandoeuvre-les-Nancy, FrancebLaboratoire de Chimie Physique et Microbiologie pour l’Environnement (LCPME), UMR 7564, Nancy-University, CNRS, 405 rue de

Vandoeuvre, 54600 Villers-les-Nancy, France

a r t i c l e i n f o

Article history:

Received 31 July 2008

Received in revised form

20 September 2008

Accepted 7 October 2008

Published online 18 October 2008

Keywords:

Biofilm

Drinking water

Particles

Accumulation

Wall shear rate

* Corresponding author. Tel.: þ33 (0) 383 68 5E-mail address: jean-claude.block@pharm

1 Present address: IPL sante environnemenFrance. Tel.: +33 (0) 383 50 36 90; fax +33 (0)0043-1354/$ – see front matter ª 2008 Elsevidoi:10.1016/j.watres.2008.10.009

a b s t r a c t

The aim of our study was to investigate, through the use of soft (Escherichia coli) and hard

(polystyrene microspheres) particles, the distribution and persistence of allochthonous

particles inoculated in drinking water flow chambers. Biofilms were allowed to grow for 7–

10 months in tap water from Nancy’s drinking water network and were composed of

bacterial aggregates and filamentous fungi. Both model particles adhered almost exclu-

sively on the biofilms (i.e. on the bacterial aggregates and on the filamentous structures)

and not directly on the uncolonized walls (glass or Plexiglas). Biofilm age (i.e. bacterial

density and biofilm properties) and convective-diffusion were found to govern particle

accumulation: older biofilms and higher wall shear rates both increased the velocity and

the amount of particle deposition on the biofilm. Persistence of the polystyrene particles

was measured over a two-month period after inoculation. Accumulation amounts were

found to be very different between hard and soft particles as only 0.03& of the soft

particles inoculated accumulated in the biofilm against 0.3–0.8% for hard particles.

ª 2008 Elsevier Ltd. All rights reserved.

1. Introduction Such microorganisms can enter the systems through breaks,

Drinking water networks can be regarded as biological reactors

which host a wide variety of microorganisms (bacteria,

protozoa, fungi, etc.), both in the bulk water and on the pipe

surfaces (Amblard et al., 1996; Block et al., 1997; Berry et al.,

2006). The biomass attached to the pipe surface, known as

biofilm, is a particularly sensitive concern. Indeed, most of the

biomass present in the drinking water network is located at the

pipe walls (Flemming et al., 2002). Moreover, the accumulation

of waterborne pathogens in drinking water biofilms, which

may promote their survival or growth, is a main issue for public

health and drinking water distribution system management.

2 43; fax: þ33 (0) 383 27 5a.uhp-nancy.fr (J.-C. Blot durables Est, rue Lucie

350 36 99.er Ltd. All rights reserved

leaks and insufficient treatment (Westrell et al., 2003). Partition

between bulk and surface, and pipe wall adhesion of the

microorganisms of interest (Escherichia coli, Bacillus atrophaeus,

Legionella, Mycobacterium, Cryptosporidium, viruses, etc.) have

been relatively well documented (Dailloux et al., 2003; Fass

et al., 2003; Westrell et al., 2003; Langmark et al., 2005; Lehtola

et al., 2007; Szabo et al., 2007; Helmi et al., 2008). However, the

biofilm never completely covers the wall (Martiny et al., 2003;

Paris et al., 2007) and the relative contribution of the drinking

water biofilm itself versus the uncolonized wall in trapping

particles has not been documented. Furthermore, the distri-

bution of the particles adhering to the substratum has never

4 44.ck).n Cuenot, site Saint-Jacques II, BP 51005, 54521 Maxeville cedex,

.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6118

been assessed either. Such knowledge should be helpful for

future strategies of surface disinfection and cleaning.

Fluorescent microspheres have been used as a surrogate in

a wide variety of applications to examine the accumulation and

fate of particulate materials on pipewalls (i.e. without distinction

between colonized and uncolonized walls) (Drury et al., 1993;

Okabe et al., 1997; Reichert and Wanner, 1997). Recently, a study

of Langmark et al. (2005) introduced the use of polystyrene

particles in drinking water systems as a tool to discriminate the

respective roles of biological (e.g., grazing, mortality, etc.) and

physical (e.g., erosion) phenomena in the loss of particles from

thebiofilm. Hypothesizing that these particles were not sensitive

to grazing or disinfection, they concluded that erosion played

a key role in the loss of biofilm mass. In addition, these authors

observed that the accumulation of particles in their biofilms was

less important when the biofilm bacterial density increased.

Nevertheless, in their study, hydrodynamic conditions were not

taken into account.

Hydrodynamic conditions are known to affect the organi-

zation of drinking water biofilms (Percival et al., 1999; Manuel

et al., 2007; Paris et al., 2007). Moreover, hydrodynamic

conditions can also modify the accumulation of particles into

biofilms through transport phenomena. In order to control the

hydraulic conditions, flow chambers have been used to

explore the adhesion of particles or microorganisms to

surfaces (McClaine and Ford, 2002; Purevdorj et al., 2002).

From these pioneering works, it appears that the accumula-

tion of model particles in a biofilm should be investigated

under controlled hydrodynamic conditions and in systems

allowing the localization of the accumulated particles.

Thus, the aim of this study was to probe young biofilms (i.e.

covering less than 30% of a glass or Plexiglas wall), fed with tap

water from Nancy’s drinking water network, with two model

particles recognized as hard (polystyrene) and soft (E. coli)

particles (Oshima, 2002; Gaboriaud and Dufrene, 2007). Such

particles, with significant differences in their surface proper-

ties (in particular their hydrodynamic permeability), should

behave differently in terms of sorption/desorption kinetics. As

hydrodynamic conditions and bacterial density are known to

influence particle accumulation, these two parameters were

investigated through the inoculation of particles at various

wall shear rates (70 and 460 s�1) in biofilms of different ages (6–

10-month-old). In order to interpret the accumulation curves,

a model describing bacterial transport and accumulation was

used and allowed us to get information about the adhesion

affinity of the bacterial cells for the uncolonized wall and for

the biofilm. The relative importance of the uncolonized wall

versus the colonized surface was also explored by localizing the

adhered particles. Eventually, the dynamic of the biofilm was

studied through the persistence of the particles over time.

2. Materials and methods

2.1. Flow chamber experimental setup

The experiments were carried out with the experimental

setup described in Paris et al. (2007). More details on the flow

chamber can be find in the Supporting data S1, S2 and S3. The

experimental setup consisted of 4 flow chambers fed in

parallel with tap water from Nancy’s drinking water network

and coupled to an inverted microscope (Axiovert 200, Zeiss,

Germany) equipped with a 100� objective (LD EC ‘‘Epiplan-

Neofluar’’ 100�/0.75 DIC M27, w¼ 4 mm). In order to prevent

massive contamination of the flow chambers by amoebas,

a sediment filter (porosity: 1–3 mm, Bioblock, France) was

added at the feeding tank inlet. The flow chambers were

constantly fed with water from Nancy’s drinking water

network. No chlorine residual was measured in the feeding

tank. pH (7.8� 0.1), O2 concentration (5.8� 0.6 mg O2 L�1) and

conductivity (259� 56 mS cm�1) did not show particular

trends over the course of the experiment. However, temper-

ature increased from 15 �C to 22 �C during the experiment

which was performed from January to October. The TOC of

the water was 1.5� 0.3 mg C/L and its saturation index was

�0.78� 0.26.

The flow chambers consisted of a Plexiglas� block, a silicon

gasket (e¼ 0.512 mm) and a glass slide (70� 44� 0.19–

0.21 mm, PolyLabo, France) fastened by means of vacuum

suction and were positioned at approximately 45� from the

horizontal, glass slide above to avoid gas bubbles accumula-

tion. The dimensions of the test channel were

45� 15� 0.512 mm. The long working distance objective

allowed the observation of both sides of the channel (i.e. the

glass slide and the Plexiglas block).

A laminar parabolic Poiseuille flow was maintained in the

channel and the wall shear rate (g) was calculated using the

following equation (McClaine and Ford, 2002):

g ¼ 6QH2w

(1)

with Q being the flow rate, H the height of the channel and w

its width.

The wall shear stress (s) can be expressed as follows:

s ¼ mg (2)

with m the viscosity of the water (Pa s).

A wall shear rate of 460 s�1 was applied in all the flow

chambers and, when necessary, modified during the inocu-

lation assays. The experiments were started simultaneously

in flow chambers 1–3. Flow chamber 4 was started simul-

taneously with the particle inoculation assay in flow

chamber 3.

This experimental setup was completed by a syringe pump

(Harvard Apparatus 22) which was used (Paris, 2008) to

perform the inoculation assays. It was connected to the

system 5 cm upstream of the flow chambers by the same

tubing used in the other sections of the setup.

2.2. Particle characteristics and inoculation conditions

Two kinds of particles were used in the inoculation assays:

polystyrene particles and E. coli.

Fluoresbrite polychromatic red microspheres (Polysciences

Inc., PA, USA) were used as model inert particles. These

spheres were 0.513� 0.04 mm in diameter, fluoresced bright

red when excited between 475 and 490 nm, and had a diffu-

sion coefficient of 8.7� 10�13 m2 s�1. Their zeta potential and

electrophoretic mobility were obtained with the Zetasizer

Nano ZS (Malvern Instruments, UK) and were respectively

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6 119

�35.5� 0.07 mV and �2.78� 0.06 mm cm/V s. The inoculum

was prepared by diluting the commercial solution

(3.7� 1011 particles mL�1) with tap water to a concentration of

5� 107 particles mL�1. This solution was injected into the

system by the syringe pump at a flow rate of 1.8� 10�3 mL s�1

(to obtain a concentration of 3.7� 108 particles L�1 in the flow

chambers for a wall shear rate of 460 s�1). All the injections

were performed over a 6-h period. The particle density being

close from the water density, the characteristic settling length

is too long compared the length of the flow chamber channel

to allow sedimentation.

E. coli strain CM161 (rod-shaped, around 1 mm in length)

was obtained by transforming strain PHL818 (Prigent-Com-

baret et al., 2001) with plasmid pPROBE-GT (Miller et al., 2000)

using the same electrotransformation procedure as described

by Yu et al. (2000). This strain was chosen for its mutation on

the ompR gene that stimulates the production of curli and

increases its ability to form a biofilm. The plasmid was

chosen for its reporter gene ( gfp), its stability and the pres-

ence of a gentamicin resistance gene. A 3.9� 1010-cell L�1

suspension of CM161 was prepared in a 0.1 MgSO4 buffer and

injected into the system by the syringe pump. Two flow rates

were used during the inoculation period (1.7� 10�4 and

0.5� 10�2 mL s�1) to get two different E. coli concentrations in

the flow chambers (2.1� 107 and 6.3� 108 cells L�1 respec-

tively). The inoculation process was divided into three pha-

ses: from t¼ 0 to 6 h, a final concentration of 2.1� 107 E.

coli L�1 was maintained in the flow chamber; from t¼ 6 to 8 h,

this concentration was increased to 6.3� 108 E. coli L�1; and

from t¼ 8 to 24 h, the concentration was brought back to

2.1� 107 E. coli L�1.

Quantitative assessment of the surface-accumulated

polystyrene particles and E. coli CM161 was performed by

epifluorescence microscopy. Two filter sets were used for

epifluorescence microscopy: a gfp filter set (38HE eGFP, ex: 470/

40 nm, em: 525/50 nm, Zeiss, Germany) and a propidium

iodide filter set (no. 14, ex: 510–560 nm, em: >590 nm, Zeiss,

Germany). A 8/10-bit digital camera (XCD-SW910, Sony) with

a spatial resolution of 1280� 960 pixel and the Matlab Image

Acquisition Toolbox 1.9 (Mathworks Inc., USA) were used for

image acquisition. For each counting, 35 images (60� 90 mm)

Table 1 – Overview of the inoculation experiments carried outapplied during inoculation. All the biofilms were grown under

Biofilm age (months) 0a 6

Flow chamber 1 E. coli CM161

v¼ 460 s�1

Flow chamber 2

Flow chamber 3

Flow chamber 4 Polystyrene

particles

v¼ 70 s�1

a This experiment without biofilm was run in parallel, with the flow cha

were acquired at random positions throughout the test

channel surface.

2.3. Inoculation experiments

Six inoculation experiments were performed in the flow

chambers (Table 1) and allowed us to explore how various

factors affect the accumulation of particles on the wall, the

presence of a biofilm on the wall, the wall shear rate (70 and

460 s�1), the material type (glass and Plexiglas) and the nature

of the particles (hard or soft particles).

2.4. Determination of the position of the attachedparticles

The position of the polystyrene particles on the surface (i.e.

on the bacterial aggregates, on the filamentous structures

covering some areas of the biofilms or directly on the glass

slide) was determined for a 10-month-old biofilm inoculated

at a wall shear rate of 460 s�1. In order to perform this task,

areas of the flow chamber were chosen randomly: images

obtained by optical microscopy allowed the localization of

the biofilm and images obtained by epifluorescence

microscopy allowed the localization of the polystyrene

particles. When the particle could be clearly localized, it

was attributed to one of the categories cited above. When it

was not the case, the particle was attributed to the

‘‘undefined’’ categories. This localization was performed on

100 particles.

2.5. Bacterial transport and accumulation model

A model based on convective-diffusion (Son and Hanratty,

1969) describing transport and accumulation of bacterial cells

to the surface was developed elsewhere (Paris, 2008) to char-

acterize the effect of biofilm ageing on the kinetics of accu-

mulation and on the saturation level of the surface by their

deposition. Briefly, this model describes the number of

bacterial cells deposited (nf, bacterial cells cm�2) at a given

time (t, s) as a function of the bacterial concentration in the

bulk water (C0, % of volume), of the average surface occupied

in the 4 flow chambers. g represents the wall shear ratea wall shear rate of 460 sL1

7 8 10

Polystyrene

particles

v¼ 460 s�1

Polystyrene

particles

v¼ 460 s�1

Polystyrene

particles

v¼ 70 s�1

Polystyrene

particles

v¼ 70 s�1

mber 3 experiment.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6120

by the bacteria (s, cm2), of their average volume (v, cm3), of

their diffusion coefficient (D, cm2 s�1), of their diameter (d0,

cm) and of the wall shear rate (g, s�1):

nf ¼1

a2sa1þ zexp½ � 1:21a2C0t�� (3)

with t* being an adimensional time expressed as:

t� ¼

gD2

d40

!1=3

t (4)

Fitting this model against experimental data leads to

the identification of three coefficients: a1, a2 and z (cm2).

The coefficient a1 characterizes the affinity of the particles

for the wall, a2 characterizes the kinetics of accumulation

of the particles on the wall and their affinity for the bio-

film, the ratio a2/a1 characterizes the saturation level of

the wall with the accumulated particles and z is the

number of particles accumulated on the surface at the

initial time.

3. Results

3.1. Biofilm characteristics

The inoculation experiments were performed on 6–10-month-

old biofilms grown in flow chambers under a laminar Pois-

euille flow at a wall shear rate of 460 s�1. The evolution of

a number of biofilm characteristics (surface area covered,

biomass distribution, presence of filamentous structures) with

time was used to investigate the influence of biofilm ageing on

particle accumulation (Table 2). The surface area covered

increased with biofilm age (15% for the 6-month-old biofilm

and up to 30% for the 10-month-old one) and the maximal

height of the biofilm aggregates was around 10 mm. Moreover,

the distribution of the bacterial biomass (i.e. isolated cells

versus aggregates) was also modified by biofilm ageing. Indeed,

55% of the biofilm surface was organized as aggregates (cell

clusters with a major axis length> 6.6 mm) in a 6-month-old

biofilm whereas this percentage reached 63% in a 10-month-

old one. After 5 months of biofilm colonization, some fila-

mentous structures, presumably of fungal origin (as demon-

strated by Calcofluor white M2R staining using the method

described by Goncalves et al. (2006)), were forming a web

covering some areas of the wall (Fig. 1) and located a few

micrometers above the wall and the bacterial aggregates.

Table 2 – Evolution of the biofilm characteristics withageing (Wall shear rate [ 460 sL1).

Biofilm age (months) 6 7 8 10

Surface area covered with the

biofilm (i.e. single cells and aggregates) (%)

15 18 22 30

% of surface area occupied by the

aggregates within the biofilm

55 58 59 63

% of micrographs showing

filamentous structures

31 46 43 46

3.2. Effect of wall shear rate on particle accumulation

The effect of wall shear rate on the accumulation of the 0.5-

mm-diameter hard polystyrene particles was studied through

a set of two experiments conducted on 7-month-old biofilms

grown under identical conditions at a wall shear rate of

460 s�1. These two biofilms were inoculated with a 3.7� 108-

particle L�1 suspension for 6 h at wall shear rates of 70 and

460 s�1, respectively.

By the end of the inoculation period, polystyrene particles

had been injected in equal amounts in the flow chambers, but

three times more particles had accumulated in the biofilm

exposed to a wall shear rate of 460 s�1 compared to that

of 70 s�1 (1.2� 0.3� 106 particles cm�2 against 0.4� 0.2�106 particles cm�2, respectively) (Fig. 2). Respectively, 0.3 and

0.8% of the injected polystyrene particles accumulated in the

biofilm during these experiments.

3.3. Effect of biofilm ageing on particle accumulation

In order to investigate the influence of biofilm ageing (i.e.

increase in the surface area covered and modification of the

surface properties of ‘‘young’’ biofilm) on particle accumula-

tion into the biofilm, two kinds of experiments were per-

formed. On one hand, two distinct biofilms (a 7-month and

a 10-month-old one) were inoculated at the same wall shear

rate (70 s�1) (Fig. 3). On the other hand, a biofilm grown in the

same flow chamber was inoculated twice, with a 1-month

interval, at months 7 and 8 (Fig. 4). Each time the older biofilm

accumulated more particles than the younger one: twice as

much for the 10-month-old one compared to the 7-month-old

one, and 10% more for the 8-month-old one compared to the

7-month-old one (to make this calculation, residual particles

from the first inoculation were subtracted to the amount of

particles accumulated after 8 months).

In order to characterize the effect of biofilm ageing on

particle accumulation, the experimental data presented in

Fig. 4 were confronted to a model describing particle transport

and accumulation to the surface. The fitting of this model

to the experimental data was performed separately on the

data obtained for the two inoculation assays and therefore,

the coefficients identified for the two assays were different

(Table 3).

The coefficient a2, which characterizes the kinetics of

accumulation of the particles and their affinity for the biofilm,

was 1.5 times higher in the second inoculation experiment.

Therefore, the accumulation of the polystyrene particles was

favored on the wall exhibiting the greatest biofilm-covered

surface area.

The ratio a2/a1, which characterizes the saturation level of

the surface with particles, was 1.95 times higher in the second

inoculation experiment. Thus, as the increase in surface area

covered with the biofilm was only 1.37, the ability of the bio-

film to accumulate particles increased faster than the surface

area it covered. The saturation level of the biofilm with

particles was calculated for inoculations 1 and 2, and found to

be 1.16 and 2.25� 106 particles cm�2, respectively.

The coefficient z, which characterizes the number of

particles accumulated on the surface at the initial time, was

higher in the second inoculation assay due to the presence of

Fig. 1 – Optical microscopy micrographs of 6-month-old biofilms. A. Area covered with filamentous structures. B. Bacterial

aggregates. Localization of the E. coli cells is shown by the white circles.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6 121

previously deposited particles. At the early beginning of the

inoculation experiments, the numbers of particles accumu-

lated were respectively of 7� 104 and 8.3� 106 particles cm�2

for experiments 1 and 2.

3.4. Accumulation of particles in the biofilm: effect of thesubstratum and localization

Using both sides of the test channel of one flow chamber, the

accumulation of particles at a wall shear rate of 460 s�1 in a 7-

month-old biofilm appeared to be identical whether the bio-

film had formed on the glass slide or on the Plexiglas wall

(Fig. 5).

In addition, the accumulation of particles on a clean glass

slide and in a 10-month-old biofilm was investigated in

parallel at a wall shear rate of 70 s�1. The amount of particles

accumulated on the clean glass slide represented only 1.5% of

the particles accumulated in the 10-month-old biofilm. Thus,

the presence of a biofilm covering 30% of the wall was favor-

able to the accumulation of particles.

Optic microscopy combined with epifluorescence micros-

copy allowed us to determine the position of the particles on

the wall (i.e. on bacterial aggregates, on filamentous structures

or on the glass slide). Polystyrene particles could never be

localized with certainty on the uncolonized glass slide and

only 0–4% of them may have been attached directly on the

glass. A large majority of the particles were attached to the

bacterial aggregates (63%) and 17% appeared associated with

Fig. 2 – Effect of wall shear rate on particle accumulation in

a 7-month-old biofilm grown on glass substratum.

the filamentous structures forming a web over the aggregates

on some areas of the biofilm. Twenty percent of the particles

could not be attributed with certainty to any of the above-

mentioned categories due to the overlapping of bacterial

aggregates and filamentous structures.

3.5. Persistence of the particles accumulatedin the biofilm

The persistence of the particles within a biofilm was investi-

gated by performing two inoculation assays in the same bio-

film (after a 7- and 8-month growth periods) at a wall shear

rate of 460 s�1 and by counting the remaining particles two

months after the second inoculation (Fig. 6).

One month after the first inoculation assay (at month 8),

approximately 60% of the polystyrene particles accumulated

during this inoculation were still present in the biofilm. Two

months after the second experiment (at month 10), only

2� 105 particles (z8%) remained.

3.6. Accumulation of gfp targeted E. coli into the biofilm

In order to evaluate the ability of the biofilm to accumulate

another kind of particles (soft particles), the accumulation of

gfp targeted E. coli at a wall shear rate of 460 s�1 into a 6-

month-old biofilm was investigated through a three-step

inoculation process (Fig. 7).

Fig. 3 – Effect of biofilm age on the accumulation of

polystyrene particles at a wall shear rate of 70 sL1.

Particles were inoculated for 6 h.

Fig. 4 – Effect of biofilm age on polystyrene particle accumulation at a wall shear rate of 460 sL1 in 7 and 8-month-old

biofilms grown in the same flow chamber. Full and dashed lines show the curves obtained through the application of the

bacterial transport and accumulation model.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6122

During the first phase of inoculation (t¼ 0–6 h), the biofilm

was inoculated with a 2.1� 108-bacterial cell cm�2 suspen-

sion. The bacterial accumulation was quite slow and reached

7� 7� 102 bacterial cells cm�2 after 6 h. Therefore, E. coli

concentration in the flow chamber was increased for 2 h

(6.3� 108 bacterial cells cm�2), resulting in a significant rise in

bacterial accumulation (3.7� 2.1� 103 bacterial cells cm�2 at

t¼ 8 h). Then, the E. coli concentration in the flow chamber

was brought back to the first phase concentration (t¼ 8–24 h).

A rapid decrease in E. coli surface density was observed

between t¼ 8 and 9 h. Afterward, the E. coli surface density

remained fairly constant (z2� 103 bacterial cells cm�2).

Comparison of optical microscopy and epifluorescence

microscopy images indicated that E. coli cells were almost

exclusively attached on the aggregates or on the filamentous

structures (Fig. 1), as observed for the polystyrene particles.

Between t¼ 6 and 8 h, only 0.03& of the E. coli injected

attached to the biofilm.

Persistence of E. coli within the biofilm was not assessed

beyond 28 h because of the low stability of the gfp fluoro-

chrome over several days.

4. Discussion

The biofilms grown in this study were relatively young, non-

confluent, and morphologically diversified (from single cells to

Table 3 – Values of the coefficients identified from thebacterial accumulation model.

Coefficient Inoculation assay 1 Inoculation assay 2

a1 340 260

a2 2� 109 3� 109

a2/a1 5.9� 106 1.1� 107

z (cm2) 1.4� 10�5 1.2� 10�6

filamentous structures). They were fed with Nancy’s drinking

water in order to be as close as possible from actual drinking

water biofilms. However, as the substrata used in this study

did not match that of the drinking water network, care must

be taken when extrapolating the results obtained to actual

drinking water biofilms.

In order to probe these young biofilms grown in flow

chambers under controlled wall shear rates, two models of

particles, one with a hard surface, the other with a soft

interface, were used. Their behavior was a priori suspected to

be very different not because of their electrophoretic mobility

(in the range of �1 to �2 mm cm V�1) but as a result of their

different hydrodynamic permeabilities which should limit

soft particle adhesion. Indeed, based on the recent electroki-

netics theory for soft biocolloidal particles, published data

indicates that peripheral layers of bacteria exhibit more or less

high hydrodynamic flow permeation as compared to hard

polystyrene particles which are quite impermeable (Duval,

2007; Gaboriaud and Dufrene, 2007; Gaboriaud et al., 2008).

Such physico-chemical features have been reported to govern

adhesion to different supports (Gaboriaud et al., 2006;

Tatchou-Nyamsi-Konig et al., 2008). Hard particles have been

previously used to measure transport in thick biofilm chan-

nels (de Beer et al., 1994) and to assess how they accumulate

on materials colonized by thin and patchy drinking water

biofilms (Langmark et al., 2005). We clearly observed that hard

polystyrene particles did stick very efficiently compared to

soft E. coli. Indeed, only 0.03& of the E. coli injected attached to

the biofilm whereas for polystyrene particles this percentage

was z1%. This latter percentage is quite similar to the one

reported by Langmark et al. (2005) (ranging from 0.3 to 0.8%).

The relatively long persistence of biofilm-associated poly-

styrene particles observed in our work (up to two months)

showed both a high sticking efficiency of the particles to the

biofilm and a low biofilm turnover which was reported in

biofilms with very low growth rates (Block et al., 1993; Boe-

Hansen et al., 2002). However, such persistence can vary a lot

depending on the microorganism, the particle or the method

Fig. 5 – Comparison of the accumulation of polystyrene particles in biofilms grown on two substrata (glass and Plexiglas) at

wall shear rates of 460 sL1 and 70 sL1 and on a clean glass slide. For more clarity, standard deviations are not presented.

However, they are identical to those presented in Figs. 1 and 3.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6 123

used to trace accumulation within the biofilm. Indeed, Lang-

mark et al. reported that, 38 days after inoculation, the FISH-

positive Legionella pneumophila cells represented 75% of the

original amount whereas the cultivable cells had undergone

a 5-log decrease. Helmi et al. (2008) reported the same trend

for three model viruses (vaccinal Poliovirus-1, PhiX174 and

MS2): infectious viruses were detected in experimentally

contaminated drinking water biofilms up to 6 days after

inoculation whereas viral genomes were observed up to the

end of the experiment (34 days). Our study highlights the

inaccuracy of using hard particles to model the accumulation

of microorganisms into biofilms. Indeed, even with a rela-

tively similar size and electrophoretic mobility, hard and soft

particles appeared to accumulate very differently (0.3–0.8%

and 0.03& of the particles injected for hard and soft particles,

respectively). Thus, polystyrene particles may not be an

appropriate indicator to monitor the fate of a pathogen in

drinking water biofilms.

Fig. 6 – Persistence over time of the polystyrene particles accum

inoculation experiment was performed for 6 h.

In the system used in this study, two kinds of substrata

were investigated for growing biofilms: glass and Plexiglas.

These two materials are not representative of drinking water

distribution pipes and may be considered as ‘‘low adhering’’

materials. However, the experiments performed with these

materials showed that they were not of a critical importance

as far as accumulation was concerned. Indeed, when a biofilm

was present, polystyrene particles and E. coli cells were rarely

seen attached directly on the uncolonized surface. Moreover,

when comparing accumulations of particles on a clean glass

slide and on a glass slide colonized by a 10-month-old biofilm,

it appears that the particles accumulated on the clean glass

slide only account for 1.5% of the particles accumulated on the

colonized one. This clearly demonstrates the higher ability of

the biofilm to be colonized by (the polystyrene) particles.

Furthermore, accumulation experiments conducted on both

materials (glass and Plexiglas) lead to the same results.

Nevertheless, care must be taken when extrapolating these

findings to other materials, like corroded iron or copper

ulated in the biofilm at a wall shear rate of 460 sL1. Each

Fig. 7 – Accumulation of gfp targeted E. coli in a 6-month-old biofilm at a wall shear rate of 460 sL1.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6124

surfaces, as many bacteria are known to adhere very effi-

ciently to such materials (Appenzeller et al., 2002; Noyce et al.,

2007).

The wall shear rates applied to the biofilms grown in our

study are representative of those found in drinking water

distribution networks (generally lower than 500 s�1). For

instance, in a PVC pipe (roughness¼ 0.0015 mm) of 10 cm of

diameter, a wall shear rate of 150 s�1 is equivalent to a flow

velocity of 0.1 m s�1 (around 0.8 L s�1). Using a high wall shear

rate (460 s�1 against 70 s�1) during the inoculation experi-

ments appears to favore the accumulation of polystyrene

particles into the biofilms. This result is quite in accordance

with the colloid transportation models (Bowen et al., 1976;

McClaine and Ford, 2002). However, these models were

established considering a clean substratum but it is known

that the presence of the biofilm, through its roughness and

surface properties, modifies the transportation of the particles

to the wall. Thus, a new model describing the transport and

accumulation of bacterial cells to the wall was applied in order

to characterize the influence of biofilms on particle accumu-

lation (Paris, 2008). This model which takes into account the

affinity of the particles for the biofilm and for the uncolonized

wall, as well as the saturation level of the wall with biofilms,

was able to describe the particle accumulation on the wall.

The ability of this model to fit the experimental data under-

lines the critical importance of convective-diffusion on the

accumulation of particles on the wall. Moreover, the identifi-

cation of a parameter characterizing the adhesion affinity

of the particles for the biofilm (a2) made it possible to describe

the evolution of the biofilm properties with ageing: the older

the biofilm, the higher the affinity of the particles for the

biofilm. Thus, hydrodynamic conditions are not the only

parameter controlling particle accumulation into the biofilm.

Indeed, particles accumulated more as biofilms grew older

due (1) to a rise in the biofilm-covered surface area (from 18%

for the 7-month-old biofilm to 30% for the 10-month-old one),

(2) to modifications in the biofilm organization (58% of aggre-

gates in a 7-month-old biofilm against 63% in a 10-month-old

one), and (3) possibly to its adhesion properties. Helmi et al.

(2008) observed that the accumulation of their model viruses

was higher in thick waste water biofilms as compared to thin

drinking water biofilms, which corroborates our results. Thus,

biofilms appear to play a key role for particle accumulation on

the wall and may therefore behave as a reservoir for

allochthonous bacteria independently of the material consti-

tuting the wall.

5. Conclusions

The primary objective of this study was to acknowledge the

respective contribution of the biofilm and of the uncolonized

wall to particle accumulation. The polystyrene particles as

well as the E. coli injected accumulated preferentially on the

aggregates and on the filamentous structures making up the

biofilm and only slightly on the uncolonized glass and

Plexiglas walls. Moreover, particle accumulation in a 10-

month-old biofilm was twice as high as in a 7-month-old

one, indicating that expansion of the biofilm-covered area,

structural changes and surface properties favored particle

accumulation.

The fairly long persistence of the polystyrene particles

within the biofilm suggested both an efficient sticking of the

hard particles to the biofilm and a low turnover of the biofilm

biomass.

The use of hard and soft particles to trace particle accu-

mulation in biofilms indicated that their behavior was

completely different despite their relatively similar size and

electrophoretic mobility. Indeed, hard particles accumulated

approximately 100 times more than soft particles.

Increasing the wall shear rate from 70 s�1 to 460 s�1 led to

a significant rise (�3) in the number of particles accumulating

in the biofilm. Moreover, a new model describing bacterial

transport and accumulation enabled us to characterize the

kinetics of accumulation of the particles on the wall and the

adhesion affinity of these particles for the biofilm: it appeared

that convective-diffusion played a key role in particle accu-

mulation on the wall. However, hydrodynamic parameters

were not the only ones to control accumulation. Indeed,

accumulation was also enhanced by increased biofilm

coverage and by changes in the biofilm properties through

ageing.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6 125

Acknowledgments

We would like to thank Christophe Merlin who helped us

constructing E. coli CM161. We also acknowledge Xiong Wang

who introduced us to the use of flow chambers. The results of

this work were obtained within the scope of a study (Biofim VII)

coordinated by the Centre International de l’Eau de Nancy

(NANCIE), through a CIFRE fellowship, and supported by the

following partners: Anjou-Recherche and VEOLIA EAU, the

Syndicat des Eaux d’Ile de France, the Agence de l’Eau Seine-

Normandie, the Communaute Urbaine du Grand Nancy

(CUGN, France) and the Centre International de l’Eau de Nancy.

Supplementary data

Supplementary data associated with this article can be found

in the online version, at doi:10.1016/j.watres.2008.10.009.

r e f e r e n c e s

Amblard, C., Bourdier, G., Carrias, J.F., Maurin, N., Quiblier, C.,1996. Saisonary evoution of the structure of microbialcommunity in a drinking water tank (Evolution saisonnierede la structure des communautes microbiennes dans unreservoir d’eau potable). Water Res. 30 (3), 613–624.

Appenzeller, B.M.R., Duval, Y.B., Thomas, F., Block, J.C., 2002.Influence of phosphate on bacterial adhesion onto ironoxyhydroxide in drinking water. Environ. Sci. Technol. 36 (4),646–652.

Berry, D., Chuanwu, X., Raskin, L., 2006. Microbial ecology ofdrinking water distribution systems. Curr. Opin. Biotechnol.17 (3), 297–302.

Block, J.C., Haudidier, K., Paquin, J.L., Miazga, J., Levi, Y., 1993.Biofilm accumulation in drinking water distribution systems.Biofouling 6, 333–343.

Block, J.C., Sibille, I., Gatel, D., Reasoner, D.J., Lykins, B.,Clark, R.M., 1997. Biodiversity in drinking water distributionsystems: a brief review. In: Sutcliffe, D. (Ed.), TheMicrobiological Quality of Water. Royal Soc. Public HealthHyg, London, pp. 63–71.

Boe-Hansen, R., Albrechtsen, H.J., Arvin, E., Jorgensen, C., 2002.Bulk water phase and biofilm growth in drinking water at lownutrient conditions. Water Res. 36 (18), 4477–4486.

Bowen, B.D., Levine, S., Epstein, N., 1976. Fine particle depositionin laminar flow through parallel plate and cylindricalchannels. J. Colloid Interface Sci. 54 (3), 375–390.

Dailloux, M., Albert, M., Laurain, C., Andolfatto, S., Lozniewski, A.,Hartemann, P., Mathieu, L., 2003. Mycobacterium xenopi anddrinking water biofilms. Appl. Environ. Microbiol. 69 (11),6946–6948.

de Beer, D., Stoodley, P., Lewandowski, Z., 1994. Liquid flow inheterogenous biofilms. Biotechnol. Bioeng. 44 (5), 636–641.

Duval, J.F.L., 2007. Electrophoresis of soft colloids: basic principlesand applications. In: Wilkinson, K.J., Lead, J.R. (Eds.),Analytical and Physical Chemistry of Environmental Systems,vol. 10, pp. 315–344.

Drury, W.J., Stewart, P.S., Characklis, W.G., 1993. Transport of1-mm latex particles in Pseudomonas aeruginosa biofilms.Biotechnol. Bioeng. 42, 111–117.

Fass, S., Block, J.C., Boualam, M., Gauthier, V., Gatel, D., Cavar, J.,Benabdallah, S., Lahoussine, V., 2003. Release of organicmatter in a discontinuously chlorinated drinking waternetwork. Water Res. 37, 493–500.

Flemming, H.C., Percival, S.L., Walker, J.T., 2002. Contaminationpotential of biofilms in water distribution systems. Water Sci.Technol. Water Supply 2 (1), 271–280.

Gaboriaud, F., Dague, E., Bailet, S., Jorand, F., Duval, J., Thomas, F.,2006. Multiscale dynamics of the cell envelope of Shewanellaputrefaciens as a response to pH change. Colloids Surf.B: Biointerfaces 52, 108–116.

Gaboriaud, F., Dufrene, Y.F., 2007. Atomic force microscopy ofmicrobial cells: application to nanomechanical properties,surface forces and surface recognition forces. Colloids Surf.B: Biointerfaces 54, 10–19.

Gaboriaud, F., Gee, M.L., Strugnell, R., Duva, J.F.L., 2008. Coupledelectrostatic, hydrodynamic and mechanical properties ofbacterial interfaces in aqueous media. Langmuir 24 (19),10988–10995.

Goncalves, A.B., Santos, I.M., Russel, R., Paterson, M., Lima, N., 2006.FISH and Calcofluor staining technique to detect in situ filamentousfungal biofilms in water. Rev. Iberoam. Micol. 23, 194–198.

Helmi, K., Skraber, S., Gantzer, C., Willame, R., Hoffmann, L.,Cauchie, H., 2008. Interactions of Cryptosporidium parvum,Giardia lamblia, vaccinal Poliovirus-1, bacteriophages PhiX174and MS2 with a drinking water biofilm and a wastewaterbiofilm. Appl. Environ. Microbiol. doi:10.1128/AEM.02495-07published ahead of print on 15 February 2008.

Langmark, J., Storey, M.V., Ashbolt, N.J., Stenstrom, T.A., 2005.Accumulation and fate of microorganisms and microspheresin biofilms formed in a pilot-scale water distribution system.Appl. Environ. Microbiol. 71, 706–712.

Lehtola, M.J., Torvinen, E., Kusnetsov, J., Pitkanen, T., Maunula, L.,von Bonsdorff, C., Martikainen, P.J., Wilks, S.A., Keevil, C.W.,Miettinen, I.T., 2007. Survival of Mycobacterium avium,Legionella pneumophila, Escherichia coli, and Caliciviruses indrinking water-associated biofilms grown under high-shearturbulent flow. Appl. Environ. Microbiol. 73 (9), 2854–2859.

Manuel, C.M., Nunes, O.C., Melo, L.F., 2007. Dynamics of drinkingwater biofilms in flow/non-flow conditions. Water Res. 41 (3),551–562.

Martiny, A.C., Jorgensen, T.M., Albrechtsen, H., Arvin, E., Molin, S.,2003. Long-term succession of structure and diversity ofa biofilm formed in a model drinking water distribution system.Appl. Environ. Microbiol. 69 (11), 6899–6907.

McClaine, J.W., Ford, R.M., 2002. Characterizing the adhesion ofmotile and nonmotile Escherichia coli to a glass surface usingparallel-plate flow chamber. Biotechnol. Bioeng. 78, 179–189.

Miller, W.G., Leveau, J.H.J., Lindow, S.E., 2000. Improved gfp andinaZ broad-host-range promoter-probe vectors. Mol. Plant.Microbe Interact. 13 (11), 1243–1250.

Noyce, J.O., Michels, H., Keevil, C.W., 2007. Inactivation ofinfluenza A virus copper versus stainless steel surfaces. Appl.Environ. Microbiol. 73 (8), 2748–2750.

Okabe, S., Kuroda, H., Watanabe, Y., 1997. Uptake and release ofinert fluorescent particles by mixed population biofilms.Biotechnol. Bioeng. 53, 459–469.

Oshima, H., 2002. Electrophoretic mobility of a charged sphericalcolloidal particle covered with an uncharged polymer layer.Electrophoresis 23, 1995–2000.

Paris, T., 2008. Accumulation and organization of drinking waterbiofilms. Effect of wall shear rate (in french). PhD Dissertation.Ecole doctorale EMMA. Nancy, Nancy-University, pp. 182.

Paris, T., Skali-Lami, S., Block, J.C., 2007. Effect of wall shear rateon biofilm deposition and grazing in drinking water flowchambers. Biotechnol. Bioeng. 97 (6), 1550–1560.

Percival, S.L., Knapp, J.S., Wales, D.S., Edyvean, R.G.J., 1999. Theeffect of turbulent flow and surface roughness on biofilm

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 1 7 – 1 2 6126

formation in drinking water. J. Ind. Microbiol. Biotechnol. 22,125–159.

Prigent-Combaret, C., Brombacher, E., Vidal, O., Ambert, A.,Lejeune, P., Landini, P., Dorel, C., 2001. Complex regulatorynetwork controls initial adhesion and biofilm formation inEscherichia coli via regulation of csgD gene. J. Bacteriol. 183 (24),7213–7223.

Purevdorj, B., Costerton, J.W., Stoodley, P., 2002. Influence ofhydrodynamics and cell signalling on the structure andbehaviour of Pseudomonas fluorescens biofilms. Appl. Environ.Microbiol. 68, 6899–6907.

Reichert, P., Wanner, O., 1997. Movement of solids in biofilm-significance of liquid phase transport. Water Sci. Technol. 36,321–328.

Son, J.S., Hanratty, T., 1969. Velocity gradients at the wall for flowaround a cylinder at Reynolds numbers from 5*103 to 105.J. Fluid Mech. 35, 353–368.

Szabo, J.G., Rcie, E.W., Bishop, P.L., 2007. Persistence anddecontamination of Bacillus atrophaeus subsp. globigii spores oncorroded iron in a model drinking water system. Appl.Environ. Microbiol. 73 (8), 2451–2457.

Tatchou-Nyamsi-Konig, J.-A., Dague, E., Mullet, M., Duval, J.F.L.,Gaboriaud, F., Block, J.-C., 2008. Adhesion of Campylobacterjejun and Mycobacterium avium onto polyethyleneterephthalate (PET) used for bottled waters. Water Res. 42 (19),4751–4760.

Westrell, T., Bergstedt, O., Stenstrom, T.A., Ashbolt, N.J., 2003.A theoretical approach to assess microbial risks due tofailure in drinking water systems. Int. J. Health Res. 13,181–197.

Yu, D., Ellis, H.M., Lee, E.C., Jenkins, N.A., Copeland, N.G.,Court, D.L., 2000. An efficient recombination system forchromosome engineering in Escherichia coli. Proc. Natl. Acad.Sci. 97, 5978–5983.