Prevalence of Plesiomonas shigelloides in aquatic environments

Plesiomonas shigelloides is a gram negative, facultatively anaerobic rod that has been implicated in a number of outbreaks of sporadic and epidemic gastroenteritis, particularly in tropical and subtropical areas of the world. It was isolated with high frequency from aquatic environments particularly fresh water and even drinking water. The present study was pursued to determine the incidence of the microorganism in tap and raw water samples, to correlate its presence with residual chlorine levels and to evaluate the seasonal effect on incidence. This information about these parameters is not available for the city of Caracas, Venezuela. Samples were streaked directly on inositol-brilliant green-bile salts agar (IBB) and Plesiomonas agar (PL). In addition, at least 100 ml of each sample was concentrated through a 0.45 mum Millipore membrane filter, which was enriched on alkaline peptone water (pH 8.6) during 24 h at 35 C. Enrichments were streaked on IBB and PL agar plates. All plates were incubated at 35 C for 24 h. Isolates were characterized using biochemical tests. The microorganism was not isolated from tap water samples. On raw water samples, a higher incidence was observed during the warmer season. Results indicated that water is a possible microorganism reservoir but the observed residual chlorine levels appear to be effective in controlling public health risk due to the microorganism's presence in drinking water.