Influence of marine water conditions on salmonella enterica serovar Thyphimurium survival

Abstract : Salmonella enterica serovar Typhimurium strains were isolated during a monitoring survey of the carpet shell clam (Ruditapes decussatus) collected from 23 stations along the Tunisian coast. The effect of starvation on Salmonella Typhimurium survival was investigated in vitro using microcosms composed of filtered (0.2 mu m) marine water under dark and ambient temperature conditions (25C for 30 days). Eventual changes in physiological, biochemical properties, serotyping, biotyping and antimicrobial sensitivity for the environmentally adapted Salmonella Typhimurium were monitored. In response to stress conditions, the Salmonella Typhimurium strains progressively lost culturability in the absence of notable changes in the total cell count (during all the period of 30 days). Strains appeared to enter into a viable but noncultureable state as determined by epifluorescence method. We have also observed that the T90 value (time required for the reduction of 90% of cells) for the five different Salmonella strains ranged between 25 and 30 h; this indicated the survivability of Salmonella under stress conditions. This state was also characterized by biochemical and antimicrobial changes. PRACTICAL APPLICATION This research highlighted the acquisition of the viable but nonculturable state of bacteria. This form of adaptation of certain pathogens does not really reflect their presence or absence in the marine environment and thus in foods of marine origin. Hence the use of conventional methods of bacterial identification must be completed more advanced research tools before declaring a healthy product or environment.