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 alternative programmes of gene expression, along with a transition into a dormant condition sometimes referred to as a viable non-culturable (VBNC) slate. Knowledge of how pathogenic species respond to nutrient limitation is therefore important for their detection and dissemination. This study used flow 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 VBNC state in this pathogen, contrary to reports from other workers. Analysis of rRNA and protein content identified a small proportion of cells in 110 day-old cultures that represented an active sub-population. This observation may provide an explanation for the long-term survival properties of this organism during prolonged exposure to nutrient limitation, as well as the high degree of heterogeneity observed in labelled cells.