chemotaxis in vibrio cholerae

www.fems-microbiology.org

FEMS Microbiology Letters 239 (2004) 1–8

MiniReview

Chemotaxis in Vibrio cholerae

Markus A. Boin, Melissa J. Austin, Claudia C. Hase *

Department of Microbiology, Oregon State University, 220 Nash Hall, Corvallis, OR 97331, USA

Received 23 June 2004; received in revised form 19 August 2004; accepted 27 August 2004

First published online 11 September 2004

Edited by G.M. Ihler

Abstract

The ability of motile bacteria to swim toward or away from specific environmental stimuli, such as nutrients, oxygen, or light

provides cells with a survival advantage, especially under nutrient-limiting conditions. This behavior, called chemotaxis, is mediated

by the bacteria changing direction by briefly reversing the direction of rotation of the flagellar motors. A sophisticated signal trans-

duction system, consisting of signal transducer proteins, a histidine kinase, a response regulator, a coupling protein, and enzymes

that mediate sensory adaptation, relates the input signal to the flagellar motor. Chemotaxis has been extensively studied in bacteria

such as Escherichia coli and Salmonella enterica serovar Typhimurium, and depends on the activity of single copies of proteins in a

linear pathway. However, growing evidence suggests that chemotaxis in other bacteria is more complex with many bacterial species

having multiple paralogues of the various chemotaxis genes found in E. coli and, in most cases, the detailed functions of these poten-

tially redundant genes have not been elucidated. Although the completed genome of Vibrio cholerae, the causative agent of cholera,

predicted a multitude of genes with homology to known chemotaxis-related genes, little is known about their relative contribution to

chemotaxis or other cellular functions. Furthermore, the role of chemotaxis during the environmental or infectious phases of this

organism is not yet fully understood. This review will focus on the complex relationship between chemotaxis and virulence in

V. cholerae.

� 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.

Keywords: Cholera; Signal transduction; Motility; Virulence

1. Introduction

Extensive structural and genetic analyses of the chem-

otaxis behavior of Escherichia coli and Salmonella ent-

erica serovar Typhimurium have deciphered the

complexity of the coordination of movement in response

to environmental stimuli (reviewed in [1–3]). In E. coli,

the signal for a chemical attractant or repellent is re-

ceived by four membrane-spanning methyl-accepting

chemotaxis proteins (MCPs) that respond to a change

0378-1097/$22.00 � 2004 Federation of European Microbiological Societies

doi:10.1016/j.femsle.2004.08.039

* Corresponding author. Tel.: +1 541 737 7793; fax: +1 541 737

0496.

E-mail address: [email protected] (C.C. Hase).

in concentration of a limited number of periplasmic

chemoeffectors. When an attractant binds or a repellentleaves the periplasmic domain of the MCP, a conforma-

tional change occurs [4]. This signal is transmitted

through the cytoplasmic linker protein CheW to the sol-

uble protein kinase CheA [5,6]. CheA autophosphory-

lates and this phosphate is transferred to a response

regulator, CheY. CheY-P binds to the flagellar motor

causing a switch from counterclockwise to clockwise

rotation, resulting in reorientation of the cell [7]. Adapta-tion to a stable background level of attractant is accom-

plished by varying the degree of methylation of specific

residues in the cytoplasmic signaling domain of the

MCP. Methyl groups are transferred to the MCP by a

. Published by Elsevier B.V. All rights reserved.

mailto:[email protected]

2 M.A. Boin et al. / FEMS Microbiology Letters 239 (2004) 1–8

constitutively active methyltransferase, CheR, and re-

moved by a methylesterase, CheB, whose enzymatic

activity is increased on phosphorylation by CheA-P

[8,9]. The chemotaxis machinery in E. coli and Salmo-

nella depends on the activity of single copies of proteins

in a linear pathway. Fig. 1 depicts a general schematic ofthe interactions of the key components related to this sig-

nal transduction cascade. However, chemotaxis in sev-

eral other bacteria is more complex. The genomes of a

large number of bacterial species, including Vibrio chol-

erae. Pseudomonas aeruginosa, Rhodobacter spaeroides,

Myxococcus xanthus, Borrelia burgdorferi, and Yersinia

pestis, encode for multiple gene paralogues of the various

chemotaxis genes found in E. coli (recently reviewed in[10]). In most cases, the detailed functions of these redun-

dant gene paralogues have not been elucidated.

In the present review, we analyze the V. cholerae

chromosome related to chemotaxis-related genes and

summarize the current understanding of the chemotaxis

behavior of V. cholerae with an emphasis on the intrigu-

ing interplay between the chemotaxis and virulence

traits in this organism.

2. V. cholerae and human disease

Cholera is the most severe of many diarrheal diseases

that affect humans and is responsible for significant mor-

bidity and mortality especially among children in devel-

oping countries. The causative agent of cholera, Vibriocholerae, is a Gram-negative highly motile bacterium

with a single polar flagellum that inhabits rivers, estuaries

or other aquatic environments. Cholera is a water-borne

disease and the bacteria are usually transmitted via con-

taminated food or water. Upon ingestion, the organisms

colonize the small intestine where they elaborate the po-

tent cholera toxin (CT) that is directly responsible for the

profuse diarrhea characteristic of the disease. The V.

Fig. 1. General diagram of the chemotaxis signal transduction

cascade. For simplicity, some proteins and interactions are not

included. Not to scale.

cholerae bacteria are shed in large numbers in the typical

‘‘rice water’’ stool into the environment, where they can

associate with other members of the ecosystem until they

are ingested again, thus completing the life cycle of this

organism. Motility is an important virulence factor in

many pathogenic species and in some cases is inverselyregulated with the expression of virulence traits [11].

Although the role of motility of V. cholerae in its ability

to cause cholera has not been clearly established, there

is growing evidence that chemotaxis plays an important,

but somewhat unusual, role in pathogenesis [12–14].

Chemotaxis probably plays a role in both the environ-

mental and pathogenic phases of V. cholerae and there-

fore is an important aspect of its life cycle.

2.1. V. cholerae chemotaxis genes

The genome sequence of V. cholerae revealed a rela-

tively large number of chemotaxis-related gene homo-

logues [15]. In total, there are 68 V. cholerae open

reading frames that have been annotated as putative

chemotaxis-related genes (www.tigr.org), with 22ORFs coding for homologues of che genes and 46 ORFs

encoding possible MCPs (Table 1). Most of the ORFs

encoding the different Che homologues are clustered in

three different regions distributed on both chromosomes

(Fig. 2). Cluster I contains cheY-1 (VC1395), cheA-1

(VC1397), cheY-2 (VC1398), cheR-1 (VC1399), cheB-1

(VC1401), and a putative cheW gene (VC1402). Cluster

II contains cheW-1 (VC2059), cheB-2 (VC2062), cheA-2(VC2063), cheZ (VC2064), and cheY-3 (VC2065),

whereas cluster III contains cheB-3 (VCA1089), cheD

(VCA1090), cheR-3 (VCA1091), cheW-2 (VCA1093),

cheW-3 (VCA1094), cheA-3 (VCA1095), and cheY-4

(VCA1096). Clusters I and II are located on chromo-

some I, and cluster III on chromosome II. In addition,

there are several che gene homologues that are not lo-

cated within any of the three clusters. For instance,the cheR-2 and cheV-3 genes are found next to each

other in a separate area of chromosome I and are lo-

cated adjacent to several genes involved in flagellar pro-

duction (Fig. 2). In contrast, a putative cheY gene, the

cheV-1, cheV-2 genes as well as a putative cheV gene

are found in separate locations throughout the two

chromosomes. Interestingly, in between the chemo-

taxis-related genes within the che cluster I, three ORFsencoding various hypothetical proteins are found

(Fig. 2). It is tempting to speculate that these genes en-

code for novel proteins potentially involved in chemo-

taxis or whatever other cellular functions will be

assigned to this gene cluster. An ORF located within

cluster II shows strong homology to the parA gene of

E. coli, encoding an ATPase protein involved in chro-

mosome partitioning, suggesting a linkage between celldivision and chemotaxis.

http://www.tigr.org

Table 1

Chemotaxis genes of V. cholerae

Gene # Protein homology Putative function

Chromosome I

Cluster I

VC1394 MLP Receptor

VC1395 CheY-1 Response regulator

VC1396 Hypothetical protein

VC1397 CheA-1 Histidine kinase


VC1399 CheR-1 Methyltransferase


VC1401 CheB-1 Methylesterase

VC1402 putative CheW Coupling protein

VC1403 MLP Receptor

VC1404 Putative protein

VC1405 MLP Receptor

VC1406 MLP Receptor

Cluster II

VC2059 CheW-1 Coupling protein


VC2061 ParA Cell division

VC2062 CheB-2 Methylesterase

VC2063 CheA-2 Histidine kinase

VC2064 CheZ Phosphatase


Others

VC1316 putative CheY Response regulator

VC1602 CheV-1 Coupling and adaptational


VC2201 CheR-2 Methyltransferase


Chromosome II

Cluster III

VCA1088 MLP Receptor

VCA1089 CheB-3 Methyltesterase

VCA1090 putative CheD Deaminase

VCA1091 CheR-3 Methyltransferase

VCA1092 MLP Receptor

VCA1093 CheW-2 Coupling protein

VCA1094 CheW-3 Coupling protein

VCA1095 CheA-3 Histidine kinase

VCA1096 CheY-4 Response regulator

Others

VCA0895 Chemotaxis transducer-related

protein

VCA0954 Putative CheV Coupling and adaptational

*Escherichia coli K12-MG1655.

Additional methyl-accepting chemotaxis-like proteins (MLPs):

VC0098, VC0216, VC0282, VC0449, VC0512, VC0514, VC0825 (tcpI),

VC0840 (acfB), VC1248, VC1289, VC1298, VC1313, VC1413,

VC1535, VC1643, VC1859, VC1868, VC1898, VC1967, VC2161,

VC2439, VCA0008, VCA0031, VCA0923, VCA0068, VCA0176,

VCA0220 (hlyB), VCA0268, VCA0658, VCA0663, VCA0773,

VCA0864, VCA0906, VCA0974, VCA0979, VCA0988,

VCA1031(putative), VCA1034, VCA1056, VCA1069.

V. cholerae gene summary: 45 MLPs + 1 putative; 3 CheAs; 3 CheBs; 1

CheD, putative; 3CheRs; 3CheVs + 1putative; 3CheWs + 1putative; 4

CheYs + 1 putative; 1 CheZ; 1 chemotaxis transducer related protein.

M.A. Boin et al. / FEMS Microbiology Letters 239 (2004) 1–8 3

The V. cholerae genome predicts several che genes

that are not present in E. coli, but are found in various

other bacterial and archaeal species, including several

cheV genes as well as a putative cheD gene (Table 1).

Although the roles of these proteins have not been fully

elucidated, the CheD protein is involved in receptor acti-

vation, whereas the CheV protein functions as an adap-

tation and coupling protein [10]. All together, there are

three cheA, three cheB, three cheR, three cheV, threecheW, four cheY, and one cheZ, genes. In addition to

these full-length ORFs, several putative che genes were

identified, including one putative cheD, one putative

cheV, one putative cheW, and one putative cheY gene

(Table 1). Among the ‘‘standard’’ set of Che proteins

found in E. coli, the V. cholerae CheA-3, CheB-3, and

CheR-3 paralogues, encoded in the che cluster III, are

the most similar to those of E. coli. Among the fiveCheY paralogues, the most similar to the E. coli CheY

protein is CheY-3, encoded in the che cluster II. Thus,

it is hard to predict from sequence analyses alone which

of these gene clusters is involved in chemotaxis or other

cellular functions, although one might assume that the

genes within a cluster function together.

Most chemoeffectors are detected by a small family of

closely related chemoreceptors that are localized in thecytoplasmic membrane (reviewed in [16]). The effect of

ligand binding causes a conformational change in the

MCP, which is sensed by the CheA–CheW complex

and alters the activity of the CheA protein kinase, and

the susceptibility of the MCP to methylation and deme-

thylation. In addition to the various che genes men-

tioned above, the V. cholerae genome encodes for at

least 46 MCP-like proteins (called MLPs) as summa-rized in Table 1. In contrast to the che genes, most of

the genes encoding possible chemoreceptors are found

throughout the genome, with roughly half being located

on either of the two chromosomes.

An increasing number of annotated bacterial gen-

omes predict multiple sets of che genes, frequently found

arranged in distinct clusters. However, in most cases the

exact roles of the multiple sets of genes have not beenelucidated. Interestingly, some chemotaxis-like proteins

were found to be involved in cellular functions not re-

lated to chemotaxis. Several chemosensory systems have

been implicated in the regulation of gene expression,

including swarmer cell differentiation in E. coli [17], pili

expression in Synechocystis [18,19], and fibril biogenesis

in M. xanthus [20]. Moreover, in several cases Che para-

logs are involved in regulating the expression of surfacestructures, such as pili of fibrils and although the mech-

anism for control of pilus-based motility has not been

determined for any organism, chemotaxis-like proteins

have been shown to affect expression of pil genes rather

than regulation of pilus function [21]. Moreover, the

M. xanthus frz genes, showing homology to chemotaxis

genes, are required for activating a signaling cascade

resulting in cell development [22,23]. Based on theseexamples from other organisms we can perhaps expect

similar gene expression changes in the V. cholerae che

Fig. 2. A schematic diagram of the three che gene clusters on the two chromosomes in V. cholerae. The numbers below the arrows indicate the VC or

VCA numbers assigned by TIGR. MLP, MCP-like protein; H.P. Hypothetical protein.


mutants. The V. cholerae genome is tantalizing with re-

spect to chemotaxis, as it suggests an additional com-

plexity over that of E. coli due to the multiplicity of

chemotaxis-related genes. The presence of potentially46 MCP-like proteins suggests a tremendous capacity

for sensory responses in V. cholerae.

2.2. Functional analyses of the V. cholerae chemotaxis

genes.

To date, there have been a limited number of studies

involving genetically defined mutants ofV. cholerae lack-ing the various chemotaxis genes. In-frame deletions in

each of the three cheA genes as well as a triple mutant

strain were generated [24]. Interestingly, only the cheA-

2, but not cheA-1 or cheA-3, gene was found to be essen-

tial for chemotaxis under standard conditions. It is

possible that the multiple cheA genes serve as secondary

chemotactic genes, and we have yet to discover the con-

ditions under which these genes are needed, or theymight function in other cellular processes. Complemen-

tation studies suggested that only the V. cholerae cheA-

2 gene encodes a functional homologue of the E. coli

CheA protein. Insertional disruption of the cheY-4 gene

in the chromosome resulted in slightly decreased motil-

ity, whereas insertional duplication of the cheY-4 gene

resulted in slightly increased motility [25]. Unfortu-

nately, these genetic constructs are expected to have po-lar effects on some of the other chemotaxis-related genes

with the region encoding cheY-4 (Fig. 2) and it is there-

fore impossible to assign these phenotypes exclusively

to the CheY-4 protein. Similarly, an insertion mutant

in a V. cholerae cheR gene was generated and failed to

display chemotactic behavior [26]. Although the authors

did not specify which of the three V. cholerae cheR gene

was disrupted, by analyzing the deposited GenBanksequence (Accession No. AF139167) it is clear that the

affected gene was cheR-2. Although, polar effects on

downstream genes cannot be ruled out, it is interesting

to point out that this gene is not located in the cluster

II with the functional cheA-2 gene. In fact, cluster II

seems to be missing a cheR homologue. A recent abstract

reported that of the three cheR paralogues, only thecheR-2 gene is required for chemotaxis and that deletion

of cheA-2, cheW-1, and cheR-2 resulted in non-chemo-

tactic, counterclockwise-biased phenotypes [27]. During

an elegant selection scheme to identify regulatory pro-

teins involved in transcriptional activation of the main

V. cholerae virulence genes, several transposon-derived

mutants in genes related to chemotaxis were identified

[28]. Among those, the cheA-2, cheY-3, and cheZ trans-poson insertion mutants were non-chemotactic, whereas

the MLP encoded by VC2161 did not seem to contribute

to chemotaxis. Although these transposon insertions are

expected to have polar effects, strains engineered to con-

tain specific point mutations in cheA-2 or cheY-3 were

also found to be non-chemotactic [28], confirming earlier

results implicating the CheA-2 protein in chemotaxis.

Unexpectedly, the single cheZ gene, when deleted, wasnot required for chemotaxis of V. cholerae [13]. Another

abstract recently reported that only overexpression of

CheY-3, but not any of the other V. cholerae cheY para-

logues, affected flagellar rotation [29]. We recently gener-

ated in-frame mutants in the V. cholerae cheY-1, -2, -3,

and -4 genes and found that only the cheY-3 deletion

strain showed a chemotaxis defect (unpublished data).

Thus far, all of these reports indicate that only thegenes located in che cluster II, as well as the cheR-2 gene,

are important in V. cholerae chemotaxis and that the

other systems might control other physiological func-

tions. Clearly, a comprehensive genetic and biochemical

analysis of the different chemotaxis-related proteins en-

coded in the V. cholerae genome is sorely needed.

2.3. Chemotactic behavior of V. cholerae

E. coli uses two signal transduction strategies to

migrate to preferred microenvironments, including


metabolism-dependent and metabolism–independent

chemotaxis. In metabolism-independent chemotaxis a

change in the concentration of molecules binding to a

transmembrane chemoreceptor is sensed. Metabolism-

dependent behavior, termed �energy taxis�, is far less wellcharacterized and includes aerotaxis, electron acceptortaxis, redox taxis and phototaxis. Although the V. chol-

erae genome predicts 46 MLPs, indicating an enormous

sensory transducing potential, very little is known about

the tactic behavior of V. cholerae. An early study by

Freter and O�Brien [30] reported that V. cholerae was at-

tracted to all 20 L-amino acids tested and that the chem-

otaxis response to amino acids was stronger than that to

carbohydrates. LL-Fucose had no attraction for thisstrain, whereas a pepsin digest of rabbit mucosal scra-

pings or tryptone strongly attracted vibrios. Chemotaxis

of V. cholerae toward chitin oligosaccharides, such as

GlcNAc, a substance abundantly found in marine envi-

ronments has been reported [31]. Furthermore, V. chol-

erae cells displayed a chemotactic response toward bile

and intestinal mucin from humans and several other ani-

mal sources [26]. Deciphering the roles of the variousV. choleraeMLPs will be a challenging task, particularly

because there might be redundancy or overlap of signa-

ling receptors. Furthermore, a comprehensive analysis

of the chemotatic and energy taxis behavior of V. chol-

erae is lacking thus far.

2.4. Chemotaxis and virulence in V. cholerae

Although the role that motility plays in virulence of

V. cholerae is not fully understood, motility has been

identified as an important virulence factor in some ani-

mal models. However, differences in biotypes as well as

animal models greatly affected these findings [32].

Unfortunately, many of the early studies in the litera-

ture use non-motile mutants that have not been genet-

ically characterized or use non-isogenic strains for thecomparison of motile and non-motile strains making

it hard to critically interpret the different results. In

more recent studies, defined non-motile strains of V.

cholerae were found to be attenuated in the infant

mouse model [12,28]. Similarly, the role of chemotaxis

during infection by V. cholerae has not been fully elu-

cidated. It was observed that motile V. cholerae organ-

isms rapidly enter the mucus gel and can be found inintervillous spaces within a short time period and that

motile V. cholerae direct themselves to the mucosal sur-

face in response to chemoattractants [30]. In the rabbit

ileal loop, the growth rate of chemotactic vibrios was

significantly higher than that of non-chemotactic mu-

tants and this was correlated with a higher degree of

association of the chemotactic organisms with the

intestinal mucosa [33]. These results were supportedby in vitro studies with rabbit mucosal slices [34]. In

contrast, in the infant mouse model of cholera, non-

chemotactic mutants survived better than the wild-type

strain and produced a more rapid and severe disease

[35]. Again, the conflicting results concerning the

growth rates and adherence abilities of non-chemotac-

tic strains compared to wild-type cells in the different

animal models are hard to interpret, as the mutantsused were not genetically characterized. In a more re-

cent study, defined non-polar mutants in the cheA-2

and cheY-3 genes were used and both Che� strains dis-

played greatly increased colonization fitness compared

to the wild-type parent [28], supporting a similar find-

ing reported earlier [35].

These observations are quite counterintuitive, as

chemotaxis is expected to enhance the ability of the bac-teria to swim towards the preferred colonization sites,

rather than being a deleterious trait. Several models

were proposed to explain the out-competition of non-

chemotactic bacteria, including increased growth rates

and preferred killing of the Che+ bacteria. In fact, the

non-chemotactic vibrios, unlike their parent strain, were

found to colonize the stomach as well as the entire

length of the intestine. In addition to the aberrant distri-bution, a greater number of Che� cells were recovered

from distal half of the small intestine, where most of

the wild-type cells were found [28]. A similar aberrant

distribution and increased ‘‘fitness’’ of the non-chemo-

tactic bacteria had previously been observed by Freter

and O�Brien [35]. Nonchemotactic mutants display a

lower infectious dose and an accelerated time to death

in the infant mouse model [28,35]. All of these observa-tions are consistent with the hypotheses that the process

of chemotaxis is responsible for guiding wild-type

V. cholerae to the distal half of the infant mouse intes-

tine and that there might be an antibacterial mechanism

in the deeper layers of the intestinal epithelium of infant

mice to which chemotactic V. cholerae are attracted to,

but where non-chemotactic bacteria fail to go. Interest-

ingly, the out-competition phenotype appears to bedependent on the direction of flagellar rotation and

independent of the main adhesion factor of V. cholerae,

TCP [13]. Specifically, only non-chemotactic strains with

counterclockwise (CCW) flagellar rotation displayed in-

creased colonization, whereas a non-chemotactic strain

with clockwise (CW) flagellar rotation was attenuated

in the mouse model. A CCW-biased mutant displays re-

duced tumbling and thus increased smooth swimming,whereas a CW-biased mutant excessively tumbles. As a

result, a CW-biased strain is confined to the lumen of

the small intestine. It appears that the out-competition

phenotype of the non-chemotactic strains is not due to

the failure to chemotact per se, but instead is a complex

multifactorial phenomenon requiring motility, expanded

colonization distribution, and smooth swimming. How-

ever, it is important to note that all of these observationswere obtained in animal models and human studies

might provide all together different results.


Several lines of evidence suggest that there is an inti-

mate relationship between motility and/or chemotaxis

and virulence gene regulation. Expression of the main

virulence factors, CT and TCP, is coordinately regulated

via a cascade of regulatory proteins, with the ToxR/S

and TcpP/H proteins activating expression of the toxT

gene, encoding ToxT, a key regulatory protein in the

regulon. Interestingly, at least two genes within the

ToxR regulon, tcpI and acfB, encode MLPs and loss

of either of these genes resulted in increased swarm cir-

cles in semisolid medium as well as reduced colonization

abilities of the bacteria [36,37]. Furthermore, toxR mu-

tant strains displayed a hypermotile phenotype, whereas

some spontaneous hypermotile strains lack expressionof CT and TCP under normally inducing conditions

[12]. However, in most cases, it is not yet clear if these

effects are due to hypermotility per se or to an increase

in chemotaxis-directed motility. Although, some non-

motile mutants show constitutive expression of CT

and TCP and increased toxT transcription, deletion of

the cheA genes did not alter virulence gene expression

[24]. An in vivo screening and further characterizationrevealed that several chemotaxis genes [mcpX

(VC2161), cheZ, cheA-2, and cheY-3] appear to be re-

quired for the induction of the cholera toxin (ctx) and

toxT promoters upon infection of mice, although the

induction of the ctx promoter in vitro does not require

any of these genes except cheZ [28]. These findings

suggest a complex interplay between the chemotaxis-

signaling system and virulence gene regulation in vivo.Using several global transcription profile approaches,

many V. cholerae chemotaxis genes were identified as

differentially regulated genes when comparing gene

expression patterns between in vivo vs. in vitro growth

conditions [38]. Human shed V. cholerae show high

expression of many motility genes but low expression

of chemotaxis genes, including the cheA-1 gene, cheY-2

gene, all three cheW genes, all three cheR genes, threeout of four cheV genes, as well as 17 genes encoding

MLPs [14]. Interestingly, expression levels of the cheA-

2, cheY-3 and cheZ genes were not altered. As the

cheA-2, cheY-3, cheW-1, and cheR-2 genes encoded the

paralogues that appear to be involved in chemotaxis, it

is hard to predict the chemotactic state of the V. cholerae

bacteria exiting the human host. At the same time, it im-

plies a role for the che genes that have not been found tobe involved in chemotaxis in the dissemination of V.

cholerae. However, stool V. cholerae are believed to have

CCW-biased flagellar rotation [13], which supports a

model whereby human colonization creates a hyperin-

fectious bacterial state that is maintained after dissemi-

nation and that may contribute to epidemic spread of

cholera [14]. In a separate study analyzing human

stool-derived V. cholerae, several chemotaxis genes wereidentified as highly expressed, including the cheA-1 gene

that was particularly strongly upregulated [39]. Consist-

ent with this, the CheA-1 protein was identified as

uniquely expressed during human infection with

V. cholerae [40]. At this point it is hard to explain the

different expression level findings; however, a possible

explanation could be the use of different microarray

reference sample. In summary, further analyses of thecomplex regulatory patterns involving chemotaxis genes

are required to gain further insights into the intricate

link between chemotaxis behavior and pathogenesis of

V. cholerae.

3. Summary

With the increased sequence data availability for

many prokaryotic genomes, it is becoming clear that

we have a very limited understanding of the complexity

governing the chemotactic behavior of microbes and

that E. coli represents a rather simple example of bac-

terial chemotaxis. The V. cholerae chemotaxis system is

particularly complex and its importance during patho-

genesis of cholera is not well understood. There areseveral outstanding questions in this field. One of the

future challenges will be to identify which cellular proc-

esses are controlled by the various CheA/CheY two

component regulatory systems encoded in the che gene

clusters that are not required for traditional chemo-

taxis. In various organisms some chemotaxis-related

genes are involved in swarmer cell differentiation, type

IV pilli mediated surface migration such as twitchingmotility and social gliding. Although V. cholerae has

not yet been shown to display twitching motility it does

encode several different type IV pili [15] and has been

shown to display an unusual kind of surface migration

[41]. It is tempting to speculate that some of these

chemotaxis genes play a role in non-flagella based

motility as well as other cellular systems that have

yet to be discovered.Several lines of evidence suggest that there is an inti-

mate relationship between chemotaxis and virulence

gene regulation. Interestingly, Che� strains displayed

greatly increased colonization fitness compared to the

wild-type parent and this out-competition phenotype

appears to be dependent on the direction of the flagellar

rotation and independent of the main adhesion factor of

V. cholerae, TCP [16,30]. Moreover, many V. cholerae

chemotaxis gene were identified as differentially regu-

lated genes when comparing the transciptome of in vivo

vs. in vitro grown V. cholerae. Human shed V. cholerae

show high expression of many motility genes but low

expression of some chemotaxis genes. The complex

interplay between chemotaxis and virulence should be

an important aspect of future research. Chemotaxis

probably plays a role in both the environmental andpathogenic phases of the V. cholerae life cycle and stud-

ying the mechanisms of chemotaxis is of vital impor-


tance to understand the survival strategies of this

bacterium.

Acknowledgement

This work was supported in part by a grant from the

Ellison Medical Foundation.

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chemotaxis in vibrio cholerae

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