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 degrees 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 degrees 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 degrees 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 degrees C, although these filaments tended to be shorter and less numerous. All strains also formed filamentous cells at 8 degrees C in retail milk or chicken meat extract. Filaments often exceeded 100 microm 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 degrees 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.

The Effects of Sodium Chloride on the Physiological Characteristics of Listeria monocytogenes

“…They found that filament formation with increased filament length became apparent in 5% sodium chloride and 7% potassium chloride (Zarei et al, 2012). These findings were also observed in Salmonella at low water activity and low temperature (Mattick et al, 2000(Mattick et al, , 2003.…”

Characterization of Osmotically Induced Filaments of Salmonella enterica

“…Different serovars of S. enterica filament during growth in hyperosmotic, acidic, alkali, and refrigerated environments (19,31,(34)(35)(36). Likewise, both E. coli and L. monocytogenes filament in similar suboptimal conditions (19,29,36,61).…”

Survival and Virulence of Salmonella enterica Serovar Enteritidis Filaments Induced by Reduced Water Activity

“…In addition to reduced-a w conditions, exposures to low or elevated temperatures and acid or alkaline pH levels lead to filament formation by Salmonella (19,24,25). Regardless, it appears to be a conserved response in several food-borne pathogens, including Listeria monocytogenes (1,9,16,17), E. coli O157:H7 (25), and Bacillus cereus (21).…”

“…Filamentous cells remained as filaments at both temperatures. We extended the incubation at 4°C to 19 days to determine whether prolonged cold stress would induce filamentation by the E40 control cells, as was observed by Mattick et al (24). No filamentation was observed.…”

“…For example, E. coli and Salmonella enterica subsp. enterica serovars Enteritidis and Typhimurium developed filaments without septa when exposed to refrigeration temperatures (24,27,31). Upon transfer to 30°C or 37°C, the filaments fragmented into typical-sized cells.…”