Genetic mechanisms of persistence of the Culturable Vibrio cholerae in aquatic environment

Abstract

Cholera, an ancient disease caused by toxigenic strain of Vibrio cholerae, remains a human scourge killing millions of people worldwide in the past through pandemics and epidemics, and still continues to be a major public health threat to countries where sanitation and clean drinking water are not optimal. According to World Health Organization (WHO), 48 countries (mostly from Africa, Asia and South Americas) reported 2-3 million cases of cholera with 100-120 thousand deaths. Although V. cholerae is autochthonous to aquatic environments, the physiologic and genetic basis of persistence of the organism is not clearly understood despite decades of research. In other bacterial species, it has been reported that a subpopulation of a bacterium can shift to a “persister” phenotype by stochastic mechanism in response to environmental stressors, including antibiotic and nutrient-poor/limitation stresses. Furthermore, reports are also available that bacteria persisting in stationary phase can assume a growth advantage stationary phase (GASP) where a subpopulation of bacterium sustains positive selection mutation. Both persister and GASP phenotype are shown to play major role in the persistence of bacteria in adverse survival conditions. I hypothesis that, like other bacterial species, V. cholerae can assume both persister and GASP phenotypes and that such phenotypes can promote environmental persistence of V. cholerae. As microcosms mimic environmental conditions, particularly relative to nutrient-poor environmental conditions, I used filtered sterilized lake water microcosms (FSLW) to test my hypothesis. My data presented in this work have indeed proved that a subpopulation of V. cholerae can assume persister and GASP phenotype and that both phenotypes can response to supplementation of complex nutrients, including chitin and phosphate. My data also exhibited that GASP phenotype promotes a biofilm specific to FSLW but not in nutrient rich L-broth. In summary, I demonstrate that V. cholerae can survive in culturable form for years in nutrient-poor conditions and that, persister and GASP phenotypes underwent many changes, including morphologic, genetic and physiologic changes that presumably contributed to their persistence in microcosms.