In Argentina, as in other countries of Latin America, cholera has occurred in an epidemic pattern. Vibrio cholerae O1 is native to the aquatic environment, and it occurs in both culturable and viable but nonculturable (VNC) forms, the latter during interepidemic periods. This is the first report of the presence of VNC V. cholerae O1 in the estuarine and marine waters of the Río de la Plata and the Argentine shelf of the Atlantic Ocean, respectively. Employing immunofluorescence and PCR methods, we were able to detect reservoirs of V. cholerae O1 carrying the virulence-associated genes ctxA and tcpA. The VNC forms of V. cholerae O1 were identified in samples of water, phytoplankton, and zooplankton; the latter organisms were mainly the copepods Acartia tonsa, Diaptomus sp., Paracalanus crassirostris, and Paracalanus parvus. We found that under favorable conditions, the VNC form of V. cholerae can revert to the pathogenic, transmissible state. We concluded that V. cholerae O1 is a resident of Argentinean waters, as has been shown to be the case in other geographic regions of the world.Cholera reemerged in Latin America in 1991 after being absent from the continent for nearly a century. In Argentina, there have been seven epidemics since 1992, all of which were caused by El Tor biotype Vibrio cholerae O1. These cholera outbreaks occurred mainly during the summer months, and, consistent with reports from other geographic regions of the world, strains of V. cholerae O1 were isolated from water samples collected from rivers during epidemic periods. However, the microorganism could not be recovered from the environment during interepidemic periods (12).V. cholerae is a natural inhabitant of aquatic environments, and it has been found to survive for extended periods in estuarine and brackish waters (12,13,15). Furthermore, in response to extreme environmental conditions, the bacteria may enter a dormant state (7, 30), which has been designated the viable but nonculturable (VNC) state, since the organism cannot be recovered by using traditional culture media. It has been hypothesized that it is in the VNC state that V. cholerae survives in the environment during interepidemic periods. Also, interaction with plankton appears to play an important role in the ecology of the microorganism and to facilitate persistence, mainly in response to low temperatures and reduced nutrient concentrations (12,13,14,19,21).In Argentina, V. cholerae non-O1 was isolated from samples collected from the Río de la Plata in previous studies (5). Several facts, such as the seasonality of the outbreaks most likely associated with temperature shifts and plankton blooms, suggested that there are environmental reservoirs of V. cholerae. Additionally, in a recent study in which the genetic diversity of V. cholerae O1 strains isolated during the seven outbreaks that occurred in this country was analyzed, most of the cholera toxin (CT)-producing strains were found to belong to a single clonal group already recognized as the Latin American epidemic stra...
