Although the Gram-negative bacterium Vibrio cholerae (right) is normally associated with human pathogenic disease, most V. cholerae cells spend their lives in an aquatic environment, and only a few of the many serotypes are able to cause disease. When strains acquire the right genetic makeup – such as the cholera toxin and toxin coregulated pilus associated with pathogenesis - they can cause cholera outbreaks through contamination of human food and water sources. The pandemic O1/O139 serogroups are the best-known of this species, responsible for worldwide pandemic cholera outbreaks (including the recent outbreak in Haiti), but non-O1/O139 serogroups can cause smaller outbreaks of diarrheal disease. These strains coexist in acquatic environments with other, nonpathogenic strains; what does this coexistence mean for population structure?
That was the question that a team of scientists led by Yan Boucher set out to answer. First author Paul Kirchberger and his coauthors collected 100-ml samples of water from brackish water near the coast of Massachusetts on two collection days separated by three weeks. Samples were filtered and plated on media selective for Vibrio species, after which the 480 isolates underwent multi-locus sequence typing (MLST) analysis. The authors used a large number of isolates from this single location to gain detailed understanding of the mixed populations, rather than a broad overview of a large geographic area. Their results are now available in Applied and Environmental Microbiology.
The team selected seven housekeeping genes for MLST analysis. Isolates with the same seven sequences were defined as the same sequence type (ST), and a difference in only one of seven loci was defined as the same clonal complex (CC). These parameters allowed the team to define 72 V. cholerae STs that could be grouped into 17 CCs (see figure, left).
Though their sample number was larger, the scientists observed less diversity than previous, similar studies had. A prior Mediterranean lagoon study of 109 isolates found 78 STs, only 14 of which grouped into 5 CCs, while a coastal California study of 156 isolates found 113 STs grouped into 8 CCs. The differences in diversity observed could be due to the smaller geographic range or timeframe of the current study, but may also provide a better ‘snapshot’ of the V. cholerae population is in a given waterway.
Collecting their samples at two different time points allowed the scientists to observe a population shift in the three-week separated samples. Only five of the 17 CCs were found in both samplings, while the remaining 12 were found in one or the other. The proportion of the five CCs identified in both samples changed from one sampling to another, suggesting that a new V. cholerae population, when present, can become dominant in a very short time frame (see figure, right). How does this happen? Vibrio populations are influenced by local phage and copepod populations, so a change in one of these populations, or another local condition, may quickly have repercussions in the dominant local bacteria.
The researchers looked at greater detail at some of the isolates’ genomic sequences. Sequencing from representatives of the five consistently-identified clades revealed differences in gene content, including clades with genes encoding a pilus and an ADP-ribosyltransferase toxin, which showed more similarity to the heat-labile enterotoxin LT-A of Escherichia coli than to the cholera toxin of the pandemic O1/O139 V. cholerae strains. The ability of Vibrio to exchange genes via plasmid transfer, phage or bacterial compentency means all V. cholerae are potential pathogens, given the right genetic combination.
The results show that diverse V. cholerae living together can rapidly change population proportions; in this report, the changes between the populations were among relatively harmless strains. Were a pathogenic strain lurking in low numbers, these data suggest it could become a major proportion of the local population in as short as a few weeks. The results emphasize the importance of understanding the population dynamics of local water environments, with real public health implications.
-- Julie Wolf