Changes in culturability and virulence of Salmonella typhimurium during long-term starvation under desiccating conditions

The survival of Salmonella typhimurium under desiccation and starvation con ditions commonly associated with farm buildings was investigated in a desic cation model system: filtration onto polycarbonate membranes placed in a se aled desiccator with 0.0067 g/m(3) absolute humidity. Heterogeneities withi n bacterial populations in relation to time of desiccation were investigate d on a single-cell basis by epifluorescence microscopy coupled with an imag e analysis system in conjunction with fluorescent dyes Chemchrome V6 and DA PI. Changes in cellular states were compared to the results of plate counts (colony forming units, CFU) on selective (modified semi-solid Rappaport Va ssiliadis (MSRV)) and nonselective (nutrient agar (NA) and R2A agar) media, and to the measurements of infectivity and virulence using two animal mode ls (chicks and mice). During 9 weeks of experimental desiccation, total cel l counts (DAPI) of starved S. typhimurium remained stable, as did esterase activity (Chemchrome V6), but DAPI fluorescence intensity decreased slowly. Bacterial cells entered gradually into non-culturable states (decrease of CFU counts on MSRV, NA and R2A agar media) and the total loss of culturabil ity on NA (defined as probability of presence of 1 CFU on the membrane infe rior to 10(-6)) was obtained after 9 weeks. Loss of chick infectivity and m ice virulence in animal models occurred more rapidly, within three weeks of experimental desiccation. (C) 2000 Elsevier Science B.V. All rights reserv ed.