Effect of phage on the infectivity of Vibrio cholerae and emergence of genetic variants.
ABSTRACT Seasonal epidemics of cholera in Bangladesh are self-limited in nature, presumably due to phage predation of the causative Vibrio cholerae during the late stage of an epidemic, when cholera patients excrete large quantities of phage in their stools. To further understand the mechanisms involved, we studied the effect of phage on the infectivity and survival of V. cholerae shed in stools. The 50% infectious dose of stool vibrios in infant mice was approximately 10-fold higher when the stools contained a phage (1.8 x 10(3) to 5.7 x 10(6) PFU/ml) than when stools did not contain a detectable phage. In competition assays in mice using a reference strain and phage-negative cholera stools, the infectivity of biofilm-like clumped cells was 3.9- to 115.9-fold higher than that of the corresponding planktonic cells. However, the difference in infectivity of these two cell populations in phage-positive stools was significantly less than that in phage-negative stools (P = 0.0006). Coculture of a phage and V. cholerae or dilutions of phage-positive cholera stools in nutrient medium, but not in environmental water, caused rapid emergence of phage-resistant derivatives of the bacteria, and these derivatives lost their O1 antigen. In cholera stools and in intestinal contents of mice prechallenged with a mixture of V. cholerae and phage, the bacteria remained completely phage susceptible, suggesting that the intestinal environment did not favor the emergence of phage-resistant derivatives that lost the O1 antigen. Our results indicate that phages lead to the collapse of epidemics by modulating the required infectious dose of the bacteria. Furthermore, the dominance of phage-resistant variants due to the bactericidal selective mechanism occurs rarely in natural settings, and the emerging variants are thus unable to sustain the ongoing epidemic.
Article: Characterization of the gene encoding the major secreted lysophospholipase A of Legionella pneumophila and its role in detoxification of lysophosphatidylcholine.[show abstract] [hide abstract]
ABSTRACT: We previously showed that Legionella pneumophila secretes, via its type II secretion system, phospholipase A activities that are distinguished by their specificity for certain phospholipids. In this study, we identified and characterized plaA, a gene encoding a phospholipase A that cleaves fatty acids from lysophospholipids. The plaA gene encoded a 309-amino-acid protein (PlaA) which had homology to a group of lipolytic enzymes containing the catalytic signature GDSL. In Escherichia coli, the cloned gene conferred trypsin-resistant hydrolysis of lysophosphatidylcholine and lysophosphatidylglycerol. An L. pneumophila plaA mutant was generated by allelic exchange. Although the mutant grew normally in standard buffered yeast extract broth, its culture supernatants lost greater than 80% of their ability to release fatty acids from lysophosphatidylcholine and lysophosphatidylglycerol, implying that PlaA is the major secreted lysophospholipase A of L. pneumophila. The mutant's reduced lipolytic activity was confirmed by growth on egg yolk agar and thin layer chromatography and was complemented by reintroduction of an intact copy of plaA. Overexpression of plaA completely protected L. pneumophila from the toxic effects of lysophosphatidylcholine, suggesting a role for PlaA in bacterial detoxification of lysophospholipids. The plaA mutant grew like the wild type in U937 cell macrophages and Hartmannella vermiformis amoebae, indicating that PlaA is not essential for intracellular infection of L. pneumophila. In the course of characterizing plaA, we discovered that wild-type legionellae secrete a phospholipid cholesterol acyltransferase activity, highlighting the spectrum of lipolytic enzymes produced by L. pneumophila.Infection and Immunity 12/2002; 70(11):6094-106. · 4.16 Impact Factor
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ABSTRACT: During an epidemic of cholera we simultaneously cultured water from 30 important ponds, tanks and rivers of Dhaka city, to determine the role of surface water as a reservoir of Vibrio cholerae and nonagglutinating (NAG) vibrios and whether their presence or absence can be used as an indicator of a cholera epidemic in the community. Out of 4.016 samples 1216 (30.3%) were positive for NAG vibrios and one (0.025%) for Vibrio cholerae. Ponds showed a higher rate of NAG isolation (31.6%) than did rivers (21.9%). NAG group II constituted 87% and group V 10% of all positives. There were mild-to-moderate seasonal variations in isolation rates. The NAG isolation rate from water was related to the incidence of hospitalized NAG diarrhoea cases, but not to simultaneous cholera cases. Surface water does not appear to be an efficient natural reservoir of Vibrio cholerae, as it is for NAG vibrios. The absence of V. cholerae or presence of NAG vibrios in public surface water sources has no relationship with the extent of cholera cases present in a community.Tropical and geographical medicine 01/1985; 36(4):335-40.