Filament Formation by Salmonella spp. Inoculated into Liquid Food Matrices at Refrigeration Temperatures, and Growth Patterns When Warmed

Abstract

In this study, the formation of multicellular filamentous Salmonella cells in response to low temperatures was investigated by using isolates of Salmonella enterica serovar Enteritidis PT4 and S. enterica serovar Typhimurium DT104 as the inocula. The formation of filamentous cells in two liquid food matrices at the recommended maximum temperature for refrigeration (8°C) was monitored and compared with that in tryptone soya broth. Giemsa staining was performed to locate nuclear material within the filaments. Single filaments were warmed on agar at 37°C, and the subsequent rate of septation was quantified. For all strains tested, >70% of the Salmonella cells inoculated had become filamentous after 4 days in media at 8°C, indicating that filamentation could occur during the shelf life of most refrigerated foods. Strains with impaired RpoS expression were able to form filaments at 8°C, although these filaments tended to be shorter and less numerous. All strains also formed filamentous cells at 8°C in retail milk or chicken meat extract. Filaments often exceeded 100 μm in length and appeared straight-sided under the microscope in media and in foods, and Giemsa staining demonstrated that regularly spaced nucleoids were present. This phenotype indicates that an early block in cell septation is probably responsible for filamentation. When filaments were warmed on agar at 37°C, there was a rapid completion of septation, and for one filament, a >200-fold increase in cell number was observed within 4 h. There are clear public health implications associated with the filamentation of Salmonella in contaminated foods at refrigeration temperatures, especially when the possibility of rapid septation of filamentous cells upon warming is considered.