Changes in viability and macromolecular content of long-term batch cultures of Salmonella typhimurium measured by flow cytometry

Exposure of many Gram-negative bacteria to prolonged starvation induces alt ernative programmes of gene expression, along with a transition into a dorm ant condition sometimes referred to as a viable non-culturable (VBNC) slate . Knowledge of how pathogenic species respond to nutrient limitation is the refore important for their detection and dissemination. This study used flo w cytometry, coupled with fluorescent dyes for viability and macromolecular content, to study the responses of the pathogen Salmonella typhimurium to prolonged batch culture. Statistical analysis of the flow cytometric data, together with total and culturable cell counts, failed to demonstrate a VBN C state in this pathogen, contrary to reports from other workers. Analysis of rRNA and protein content identified a small proportion of cells in 110 d ay-old cultures that represented an active sub-population. This observation may provide an explanation for the long-term survival properties of this o rganism during prolonged exposure to nutrient limitation, as well as the hi gh degree of heterogeneity observed in labelled cells.