comparative levels and types of microbial contamination

APPLIED MICROBIOLOGY, July, 1966Copyright © 1966 American Society for Microbiology

Vol. 14, No. 4Printed in U.S.A.

Comparative Levels and Types of MicrobialContamination Detected in Industrial Clean Rooms

MARTIN S. FAVERO, JOHN R. PULEO, JAMES H. MARSHALL,AND GORDON S. OXBORROW

Technology Branch, Communicable Disease Center, U.S. Public Health Service, Phoenix, Arizona

Received for publication 19 January 1966

ABSTRACTFAVERO, MARTIN S. (Communicable Disease Center, Phoenix, Ariz.), JoHN R.

PULEO, JAMEs H. MARSHALL, AmD GoRDoN S. OxBoRRow. Comparative levels andtypes of microbial contamination detected in industrial clean rooms. Appl. Micro-biol. 14:539-551. 1966.-The primary objective of this study was to determinequantitatively and qualitatively the predominant types of microbial contamina-tion occurring in conventional and laminar flow clean rooms. One horizontallaminar flow, three conventional industrial clean rooms, and three open factory areas

were selected for microbiological tests. The results showed that as the environmentand personnel of a clean room were controlled in a more positive manner withrespect to the reduction of particulate contamination, the levels of airborne andsurface microbial contaminants were reduced accordingly. The chief sources ofmicrobial contamination were associated with the density and activity of cleanroom personnel. In addition, the majority of microorganisms isolated from theintramural air by air samplers were those indigenous to humans. Studies on thefallout and accumulation of airborne microorganisms on stainless-steel surfacesshowed that, although there were no significant differences in the levels of microbialcontamination among the conventional clean rooms, the type of microorganismdetected on stainless-steel surfaces was consistently and significantly different. Inaddition, the "plateau phenomenon" occurred in all environments studied. It wasconcluded that the stainless-steel strip method for detecting microbial accumula-tion on surfaces is efficient and sensitive in ultra-clean environments and is the mostreliable and practical method for monitoring microbial contamination in futureclass 100 clean rooms to be used for the assembly of spacecraft which will be steril-ized.

The National Aeronautics and Space Adminis-tration (NASA) requires that spacecraft destinedto impact Mars must not introduce viable ter-restrial organisms. There are two basic reasonsfor this decision: (i) introduction of a terrestrialorganism might result in devastating growth thatwould profoundly alter indigenous extrater-restrial environments, and (ii) contaminationwith a terrestrial organism could preclude demon-stration of the existence of extraterrestrial life.Since dry heat is to be employed for sterilization,the probability of obtaining a sterile spacecraft isenhanced significantly if the level of microbialcontamination is relatively low prior to the heattreatment. In accordance with this basic premise,it is necessary to assemble and test spacecraft, re-iquired to be sterile, in areas where microbial con-tamination can be maintained at an extremely lowlevel (2). Both conventional and laminar flow

industrial clean rooms have been suggested forthis purpose.Such areas are used primarily to control partic-

ulate contamination such as dust or lint whichcould affect the reliability of precision instru-ments. In laminar flow clean rooms, the entireceiling or one wall consists of a series of ultra-high-efficiency filters. The air stream, whichtravels at approximately 100 ft/sec, is exhaustedthrough a perforated floor (vertical laminarflow) or wall (horizontal laminar flow), depend-ing on which system is used. The air moves, es-sentially, in parallel patterns and is recirculatedback to the filters. Laminar flow systems, conse-quently, provide for the rapid change of ultra-clean air. Approximately 150 to 800 changes perhour can be achieved, depending on the size ofthe area. The cleanliness of the air is virtually

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CHANGE ROOM

FiG. 1. General floor plan ofclean room A. SS = stainless-steel strips; A, B. and C = air-sampling sites.

independent of activity occurring down- orcross-stream.Clean rooms are graded according to levels and

size of particulate contamination controlled intoclasses 1, II, I1I, and IV, or, 100,000, 10,000, and100, with classes IV and 100 being the mostclean. The theory, design, and operation of cleanrooms were discussed extensively by Austin andTimmerman (1).The primary objective of this study was to

determine quantitatively and qualitatively thepredominant types of microbial contaminationfound in conventional industrial clean rooms andin clean rooms which employ laminar air flowto control particulate contamination.

MATERIALS AND METHODS

Four industrial clean rooms and three generalmanufacturing areas involved in aerospace activitiesin the Phoenix area were selected for microbiologicaltests. Since the processes conducted in these rooms re-quired various levels of environmental control, theprovision for excluding contaminants differed in thethree areas. Table I contains a description of thephysical and operational characteristics of the fourclean rooms. The three general manufacturing areasA, C, and D were open factory environments. Manu-facturing area C was a machine shop. In manufactur-ing areas A and D conventional electronic components

were assembled, labeled, and packed for shipment. Noenvironmental control was required for these activi-ties.

Figures 1 and 2 illustrate the typical layout of aconventional clean room (A) and a horizontal laminarflow room (D). Typical sampling sites also are indi-cated. Clean rooms B anid C were similar to A. Inaddition, clean room C contained three laminar flowwork benches which were located at one end of theroom. The personnel and activities associated withclean room B (Table 1) were transferred to laminarflow clean room D after its construction.The detailed methods of air and surface sampling

and the technique used to enumerate the number ofmicroorganisms which accumulate on stainless-steelsurfaces have been described previously (4). In eacharea studied, air samples were obtained during fullworking days with slit samplers (Reyniers and Sons,Chicago, 111.). Surface contamination on bench topswas measured by the Rodac plate technique (3). Theaccumulation rates of microorganisms on surfaceswere determined by exposing 50 sterile stainless-steelstrips (1 by 2 inches; 2.5 by 5 cm) to the intramural airof the test area for a period of 21 weeks. At intervalsof 3 weeks, six strips were returned to the laboratory,and the numbers of mesophilic aerobic and anaerobicvegetative cells and spores were determined. Theculture medium for all sampling was Trypticase SoyAgar. Cultures were incubated at 32 C for 72 hr.Brewer jars were employed for anaerobic incubation.Each jar was flushed with nitrogen gas three times,

VOL. 14, 1966 541

FAVERO ET AL.

FiG. 2. Generalfloor plan ofclean room D. A, B, and C = stainless-steel strips; 1, 2, 3, and 4 = air-samplingsites.

filled with hydrogen, and connected to an electricalsource for 45 min.

Stainless-steel strips which were exposed to theintramural environment of the horizontal laminarflow clean room were assayed in a laminar flow cleanbench. Also, the strips were assayed at 1-week ratherthan 3-week intervals.

Culture plates from the air, surface, and stainless-steel strip samplings were selected at random, andrepresentative colonies were picked and subcultured.The cultures were Gram-stained, subjected to perti-nent biochemical tests, and identified. In the case ofmicroorganisms accumulating on stainless-steel sur-faces, 30 to 40 colonies were picked randomly aftereach 3-week assay period. In all, approximately 1,600cultures were examined.

RESULTS

Typical air sampling results in clean rooms A,B, C, and D are presented in Tables 2 and 3. It isevident that in areas not requiring strict environ-mental control, e.g., clean room A, the level ofviable particles per cubic foot was essentially thesame as that of the adjacent corridor. However,in clean rooms B, C, and D the numbers of viableparticles per cubic foot were consistently lowerthan in the noncontrolled areas. The number ofpersonnel and their activity influenced signifi-cantly the levels of contamination. Wheneverpersonnel left the area, the levels of viable par-ticles decreased accordingly. A definite level ofbackground contamination existed in clean room

TABLE 2. Levels ofairborne microbial contaminationper cubic foot in conventional clean rooms

and manufacturing-factory areas

No. of viable particles percubic foot of aira

Area

Avg Range

Clean room ASite A................... 5.29 2.93-8.56Site B.................. 6.04 3.91-7.58Change room............ 7.82 6.86-8.96Corridor outside cleanroom A................ 6.74 6.48-7.00

Factory area adjacent toclean room A.......... 16.01 12.96-24.13

Clean room BSite A................... 1.01 0.66-1.36Site B................... 1.38 1.01-1.63Corridor outside cleanroom B ................ 8.42 6.98-11.31

Clean room CSite Ab .................. 0.21 0.07-0.35Site B ................... 0.80 0.28-1.16Site E................... 1.12 0.70-2.07

Manufacturing area CSite A................... 12.19 7.42-18.42Site B ................... 3.85 1.93-5.39

a Average of seven consecutive 1-hr samples.b Site A was nearest and site E farthest from

three laminar flow benches located at one end ofthe clean room.

542 APPL. Mic:RoBIOL.

VOL. 14, 1966 MICROBIAL CONTAMINATION IN CLEAN ROOMS 543

TABLE 3. Comparative levels of airborne viable particles in horizontal laminar flow cleanroom D

No. of viable particles per cubic foota

Location of air samplers Day 1 Day 2 Day 3 Day 4

Avg Range Avg Range Avg Range Avg Range

Site 16 inches down-stream fromfilter wall. 0....-.0. 0 -

Site 23 ft downstreamfrom filter wall;1 ft upstreamfrom frequentlyused intercom. 0.0023 0-0.016 0.057 0-0.017 0 - 0

Site 332 ft downstreamfrom filter wall. 0.32 0.07-0.58 0.224 0.017-0.47 1.03 0.55-1.53 0.44 0.10-0.50

Site 46 inches upstreamfrom exhaustwall.0.64 0.22-0.93 1.15 0.58-1.95 0.62 0.23-1.40 0.63 0.28-1.40

a Average of seven consecutive 1-hr samples.

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FIG. 4. Airborne microbial contamination within clean room D.

A, even in the absence of peisonnel. This wasusually not the case in clean rooms B and C.Table 2 also shows that in clean room C thelevel of viable particles increased progressivelythe farther the sampling site was from threelaminar flow work benches located at one end ofthe clean room (sites A, B, and E, respectively).The effect of shoe-cleaning operations on the

level of airborne contaminants is illustrated inFig. 3. Significant aerosols of viable particles weregenerated whenever the shoe cleaner was inoperation. This particular shoe cleaner wassituated in the change room of clean room C.Figure 4 shows typical air sampling results ob-tained in the horizontal laminar flow clean room.No viable particles were detected at the filterwall. As the air moved past personnel towardthe exhaust wall, the level of microbial contamina-tion increased progressively. It was also foundthat, when samplers were placed at floor level,higher levels of contamination were detected thanwhen placed at the 6-ft level. At both samplingsites, however, the number of viable particles de-creased to zero when personnel left the area.

Figure 5 shows the comparative levels of viableparticles per cubic foot in manufacturing areas

A and C, and clean rooms A and D. The valuesfor clean rooms B and C, which were not plotted,fall between those for clean rooms A and D.Table 4 contains the results of the surface

sampling on bench tops in several clean roomsand manufacturing areas by the Rodac platemethod. In general, the level of surface con-tamination appeared to increase after personnelhad been in the area for 6 hr. In addition, benchtops in open factory areas contained higher levelsof contamination. The types of microorganismsdetected in clean rooms and on bench tops inclean rooms were mainly those indigenous tohumans. Table 5 shows typical patterns. Similarresults were obtained in clean rooms C and D.Few sporeformers (Bacillus spp.), molds, andactinomycetes were detected. The vast majoritywere Staphylococcus spp. and Micrococcus spp.,which are associated with human skin, hair, andrespiratory tract.The results of tests designed to detect the levels

of microbial contamination accumulating onstainless-steel surfaces exposed to the intramuralenvironments of clean rooms A, B, and C, andmanufacturing area C, are presented in Table 6.The comparative levels of aerobic mesophilic

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FIG. 5. Comparative levels of airborne microbial contamination in two manufacturing areas, a conventionalclean room, and the exhaust wall ofa horizontal laminarflow clean room.

microorganisms are shown in Fig. 6. In cleanroom A, the level of microbial contamination in-creased to a maximum in the 9th to 12th weeksand then remained constant. Similar results wereobtained in clean rooms B and C. In manufactur-ing areas C and D, the levels remained relativelyconstant throughout the 21-week exposure period.

Trays containing stainless-steel strips wereplaced at three sites in the horizontal laminarflow clean room. One site was located at the filterwall and the other two at the exhaust waU (Fig.2). Figure 7 shows the comparative levels ofcontamination at the three sites. Site B showedthe highest level of microbial contamination.This was probably due to the airstream passingby more personnel who were located directlyupstream. Both exhaust sites showed no signifi-

cant increase in the level of microbial contamina-tion throughout the 7-week exposure period. Forthe first 5 weeks of the study, no viable microor-ganisms were recovered from stainless-steelsurfaces placed immediately downstream from thefilter wall (site A). Between the 5th and 6th weeks,the air-handling system failed for several hours,owing to a loss of electrical power. Followingthis failure, a three-log increase in the level ofmicrobial contamination occurred. These latterresults point out the sensitivity of this samplingmethod in an ultraclean environment and alsothe efficiency of laminar air flow systems.The types of microorganisms which accumu-

lated on stainless-steel surfaces in the four cleanrooms and in manufacturing areas C and D arepresented in Table 7. Each value is the average

VOL. 14, 1966 545

6FAVERO ET AL.

TABLE 4. Comparative levels of surface contamina-tion on bench tops in clean room A and the ad-jacent assembly-factory area, clean room B, andclean room C

Avg no. of No.Site and time of sampling colonies Range of

per Rodac sam-plate ples

Clean Room AIst test series

Before personnel entered.. 12.5 2-31 206 hr after personnel en-tered.................. 24.9 1-100 20

2nd test seriesBefore personnel entered.. 18.7 0-63 1006 hr after personnel en-tered.................. 27.3 6-62 100

Assembly-Factory Area;site C

Middle of work day...... 28.1 3-51 50

Assembly-Factory Area;site D

Middle of work day...... 65.2 0-130 50

Clean Room BBefore personnel entered.. 3.65 0-25 906 hr after personnel en-tered.................. 9.89 1-32 90

Clean Room CBefore personnel entered.. 9.16 0-52 1006 hr after personnel en-tered.................. 13.80 0-102 100

of seven assay periods. The pattern in each areadid not change significantly throughout the entireexposure period. The vast majority of microor-ganisms which were found in clean room B wereStaphylococcus spp., Micrococcus spp., and theCorynebacterium-Brevibacterium group. Theseare indigenous to human skin, hair, and respira-tory tract. Few sporeformers (Bacillus spp.),molds, and actinomycetes, which are associatedwith soil and dust, were detected. The personnelof clean room B were most rigidly controlledwith respect to apparel and occupancy (Table 1).This environment also was the most rigidly con-trolledamongthe three conventional cleanrooms.Clean room A, which required the least

amount of environmental and personnel control,showed a significantly different pattern with re-spect to types of microorganisms. Most of themicroorganisms accumulating on stainless-steelsurfaces were sporeformers and molds. Only asmall portion of the microbial population con-sisted of microorganisms of human origin. This

TABLE 5. Types of aerobic mesophilic microor-ganisms in the intramural air and on surfaces intwo clean rooms

Clean room A Clean room BType of microorganism

Air Surface Air Surface

% % % %Staphylococcus

epidermidis ......... 41.7 20.5 70.2 54.2S. aureus ............. 8.3 15.4 5.9 0Micrococcus spp....... 5.5 25.6 3.6 12Streptococcus spp..... 0 0 1.2 0Sarcina spp....... 8.3 0 0 1.2Gaffkya spp........... 2.8 0 1.2 0Bacillus spp........... 0 7.7 0 3.6Corynebacterium spp... 2.8 0 9.5 16.9Flavobacterium spp.... 2.8 15.4 0 0Pseudomonas-Achromobacter spp.. 13.9 15.4 3.6 1.2

Neisseria catarrhalis... 0 0 0 1.2Yeasts........... 2.8 0 0 0Molds ............... 8.3 0 1.2 2.4Actinomycetes ........ 0 0 1.2 1.2Unidentified .......... 2.8 0 2.4 6

Total number ofcolonies examined... 36 78 84 83

same type of pattern also appeared in manu-facturing areas C and D.Clean room C exhibited a pattern which re-

flects its environmental control. The air-cleaningsystem was good in that ultrahigh-efficiencyfilters were employed and three laminar flowbenches were located at one end of the cleanroom. Personnel control, however, was similarto that in clean room A (Table 1). Although themajority of microorganisms detected wereStaphylococcus spp., Micrococcus spp., and theCorynebacterium-Brevibacterium group, spore-formers, as well as molds, also were present in ahigher ratio than in clean room B.

In horizontal laminar flow clean room D, thequalitative pattern at both exhaust sites wassimilar to that of clean room C, except thathigher percentages of sporeformers were de-tected. These latter data point out the importanceof personnel apparel. As mentioned in Materialsand Methods, the personnel and activity in cleanrooms B and D were identical. However, whenthe laminar flow room was used, some personnelconstraints were relaxed (Table 1). For example,no air showers or plastic booties were required.These factors, especially the latter, could accountfor the higher number of sporeformers. In addi-tion, tests were performed to determine the pat-tern of air flow in the immediate area upstream



TABLE 6. Avercage levels of microbial contamination which accumulated on stainless-steelstrips exposed within three clean rooms and one manufacturing areaa

Vegetative microorganisms Spores

Location Weeks ofLocation i exposure Anaerobes AneosAerobes (no./ft)2 (no./ft2) Aerobes (no./ft2) (no./f t2)

Clean room A 3 1,728 720 936 4176 3,168 1,224 1,944 7929 11,664 1,296 2,520 1,65612 6,480 3,312 2,808 2,34015 9,072 900 3,197 41818 12,312 3,060 2,808 1,20221 13,968 1,138 3,060 720

Clean room B 3 4,736 1,656 180 1156 2,880 655 180 1809 4,082 302 482 012 9,000 1,857 684 11515 24,718 3,362 936 54018 6,422 1,440 238 12221 9,238 1,260 720 238

Clean room C 3 4,918 1,318 720 1226 8,698 2,398 1,562 6489 7,848 1,202 1,620 23812 16,862 1,318 2,218 72015 26,280 1,678 3,693 30218 18,274 1,872 5,940 1,36821 14,818 2,398 4,248 1,678

Manufacturing area C 3 31,500 1,922 6,358 1,6786 22,464 6,178 7,654 3,7229 41,278 5,998 5,638 1,72812 34,704 7,020 11,642 3,60015 31,680 3,118 11,282 2,39818 17,640 5,040 7,200 3,09621

a Each value is the mean of 6 samples.

from the exhaust wall and sampling sites. Airvelocities were measured with a thermoane-mometer. The results indicated that the air pat-tern ceased to be laminar as it approached theexhaust wall and tended to rise faster toward thetop portion of the exhaust wall. This factor, too,could have influenced the types of microorgan-isms found.

Table 8 shows that very few obligate anaerobicmicroorganisms were detected on stainless-steelstrips exposed to the environments studied. Themajority were facultative microorganisms whichwere able to grow both aerobically and anaero-

bically.

DIscussIoN

It is evident from the results obtained in thisstudy that as the environment and personnel of a

clean room were controlled in a more positive

manner with respect to the reduction of par-ticulate contamination, the levels of airborne andsurface microbial contamination were reducedaccordingly. This was especially true in the hori-zontal laminar flow clean room. The chiefsources of microbial contamination were asso-ciated with people working in the clean room, asshown by the increase or decrease of airborneviable particles, depending on the number ofpersonnel in the room and their activity. In addi-tion, the majority of microorganisms isolatedfrom the air by air samplers and on bench topsurfaces by the Rodac plate method were Staphy-lococcus spp., Micrococcus spp., and the Coryne-bacterium-Brevibacterium group. These bacteriaare indigenous to human skin, hair, and respira-tory tract.

Studies on the fallout and accumulation ofairborne microorganisms on stainless-steel sur-

547VOL. 14, 1966

FAVERO ET AL.

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faces showed several interesting phenomena.First, there was no significant difference in thelevels of microbial contamination among thethree conventional clean rooms A, B, and C.(Table 6 and Fig. 6) and the exhaust wall sites ofthe horizontal laminar flow clean room D (Fig.7). Contamination levels were in the range of103 to 104 microorganisms per square foot. Inthe two open factory areas, the level of microbialcontamination was approximately 1 log higherthan in the clean rooms.

Second, the types of microorganisms whichaccumulated on stainless-steel surfaces were dif-ferent among the clean rooms and the two manu-facturing areas (Table 7). Vegetative microor-ganisms of human origin such as Staphylococcusspp., Micrococcus spp., and the Corynebacterium-Brevibacterium group accounted for the majorityof microbial contamination detected on stainless-steel strips in clean room B. Few microorganismsassociated with soil and dust, such as spore-formers, molds, and actinomycetes, were de-tected. In clean room A and manufacturing areas

C and D, the reverse was found. More soil typesand fewer microorganisms indigenous to humanswere detected. Clean rooms C and D were moreor less midway between these two extremes.These data definitely show that the degree ofenvironmental control is reflected more ade-quately by types than by the number of microor-ganisms present. This was especially true in theconventional clean rooms. Personnel apparel, inaddition to strict environmental control, werethe main variables which influenced these qualita-tive results. In clean room B, the only parts of thebody of personnel exposed were the eyes, nose,mouth, and cheeks. In addition, ultrahigh-efficiency filters were employed, and bench topsand the floor were routinely wiped down andvacuum cleaned. The personnel in clean rooms Aand C wore lint-free gowns over street clothes.Clean room C, however, employed ultrahigh-efficiency air filters and also contained threelaminar flow clean benches.

Third, the "plateau phenomenon" (4, 8, 9, 10;Favero et al., unpublished data; G. S. Michaelson,



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unpublished data) occurred in all areas studied.The levels of microbial contamination resultingfrom the faUout of airborne microorganisms ontostainless-steel -surfaces did not increase signifi-cantly during the relatively long exposure periodof 21 weeks. In some cases, there was a slightincrease up to 8 to 12 weeks, with levels subse-quently stabilizing. These results confirm thoseof other investigators in different geographicalareas of the United States. One study showedthat stainless-steel surfaces exposed to the intra-mural air of an industrial clean room for 1 weekcontained the same level of microorganisms as

those exposed for 52 weeks (10).It must be emphasized that the plateau phe-

nomenon is the result of a dynamic rather than a

static system, and one which is influenced bymultiple factors. The most plausible explanationfor the plateau is that the number of microor-ganisms deposited on or surviving on surfaces isbalanced by the number of microorganisms dying

on the same surface. It has been shown that themicrobial population, especially in the vegetativestage, is constantly decreasing on the stainless-steel surfaces (Favero et al., unpublished data).Such factors as the absence of nutrients, relativehumidity, temperature, and type of microor-ganism influence the survival rate on surfaces(5, 6, 7). Mechanical or physical dislodgment ofviable particles from the surfaces also may playa role, but there has been no proof of it up to thepresent time.The results of this study also show that the use

of stainless-steel collecting surfaces is a muchmore sensitive and reliable method for assessingairborne microbial contamination in clean rooms

than is the use of air samplers. This was apparentespecially in the laminar flow clean room. Volu-metric air samplers detect airborne viable par-ticles that have been freshly generated by per-sonnel in the immediate area of the samplers.Since the majority of these are vegetative micro-

VOL. 14, 1966 549

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TABLE 7. Types of aerobic mesophilic microorganisms which accumulated on stainless-steelsurfaces exposed to the intramural air offour clean rooms and two manufacturing areas

Clean room DClean room Clean room Clean room

D

Manufacturing IManufactur-Type'of microorganism A B C area C ing area DSite B Site C

Staphylococcus aureus 0 0 0.5 0 0.5 0 0 0S. epidermidis .............. 1.9 47.2 22.1 33.2 23.5 12.7 14.6Micrococcus spp.4.3 6.4 8.6 5.2 7.5 1.6 6.1Streptococcus spp.0 0.9 2.9 0.9 0 0 2.3Bacillus spp. (spore-

formers) ................. .55.5 10.1 17.9 35.1 32.5 30.9 21.5Brevibacterium-Corynebac-

terium group.14.3 28.4 35.7 8.5 20.5 10.3 23.8Gram-negative micro-organisms ................ 0 0 0 0.5a 0 15.1a 0

Yeasts ..................... 1.9 0.9 0.7 2.4 1.0 1.6 0.8Molds ..................... 12.9 2.7 6.4 11.8 11.0 25.4 23.8Actinomycetes and strepto-mycetes .................. 5.7 1.4 2.1 1.9 2.0 2.4 6.9

Unidentified or lost uponsubculture ............... .3 1.4 0 0 0 0 0

a Pseudomonas spp.

TABLE 8. Occurrence offacultative microorganisms and obligately anaerobic microorganismswhich accumulated on stainless-steel surfaces exposed to the intramural environment of

four clean rooms and two general factory areasa

Facultative microorganisms Obligately anaerobicmicroorganismsArea No. examined

From nonheat- From heat- From non- From heat-shocked samples shocked samples heat-shocked shocpkled

Clean room A 52 39 13 0 0Clean room B 104 97 7 0 0Clean room C 106 76 24 6 0Manufacturing area C 103 62 39 1 1Manufacturing area D 214 167 44 3 0

Total 579 441 127 10 1

a All cultures were isolated from plates incubated in Brewer jars at 32 C for 72 hr.

organisms (Table 5), they would not survive on asurface for an appreciable length of time. Thestainless-steel strip method, on the other hand,assesses the fallout of airborne microorganismson surfaces and their subsequent survival andaccumulation.

Obviously, the sensitivity and reliability of thestainless-steel strip method is best in an ultra-clean environment. This is illustrated in Fig. 7.No microorganisms were detected until after theair-handling system had failed for several hours.In aU probability, air samplers would not haveadequately detected this failure. Consequently

this technique appears to be the more reliablefor monitoring laminar flow clean rooms wherespacecraft required to have an extremely lowmicrobial load prior to heat sterilization are as-sembled.The most efficient system for controlling micro-

bial contamination to which spacecraft would beexposed during assembly was found to be thelaminar flow clean room.

ACKNOWLEDGMENTThis investigation was supported by contract R-137

with the National Aeronautics and Space Administra-tion.


VOL. 14, 1966 MICROBIAL CONTAMINAI

LITERATURE CITED1. AUSTIN, P. R., AND S. W. TIMMERMAN. 1965. De-

sign and operation of clean rooms. BusinessNews Publishing Co., Detroit.

2. HALL, L. B., AND C. W. BRUCH. 1964. Proceduresnecessary for the prevention of planetary con-tamination. Life Sci. Space Res. 3:48-62.

3. HALL, L. B., AND M. J. HARTNET-r. 1964. Measure-ment of bacterial contamination on surfaces inhospitals. Public Health Rept. U.S. 79:1021-1024.

4. MCDADE, J. J., M. S. FAVERO, AND G. S.MICHAELSEN. 1966. Environmental micro-biology and the control of microbial con-tamination. Proc. Natl. Conf. SpacecraftSterilization Technol. National Aeronauticsand Space Administration, Washington, D.C.

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comparative levels and types of microbial contamination

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