Survival and filamentation of Salmonella enterica serovar Enteritidis PT4 and Salmonella enterica serovar Typhimurium DT104 at low water activity

In this study we investigated the long-term survival of and morphological c hanges in Salmonella strains at low water activity (a(w)). Salmonella enter ica serovar Enteritidis PT4 and Salmonella enterica serovar Typhimurium DT1 04 survived at low a, for long periods, but minimum humectant concentration s of 8% NaCl (a(w), 0.95), 96% sucrose (a(w), 0.94), and 32% glycerol (a(w) , 0.92) were bactericidal under most conditions. Salmonella rpoS mutants we re usually more sensitive to bactericidal levels of NaCl, sucrose, and glyc erol. At a lethal a(w), incubation at 37 degrees C resulted in more rapid l oss of viability. than incubation at 21 degrees C, At a(w) values of 0.93 t o 0.98, strains of S. enterica serovar Enteritidis and S. enterica serovar Typhimurium formed filaments, some of which were at least 200 mu m long. Fi lamentation was independent of rpoS expression, When the preparations were returned to high-a, conditions, the filaments formed septa, and division wa s complete within approximately 2 to 3 h. The variable survival of Salmonel la strains at low a(w) highlights the importance of strain choice when rese archers produce modelling data to simulate worst-case scenarios or conduct risk assessments based on laboratory data. The continued increase in Salmon ella biomass at low a(w) (without a concomitant increase in microbial count ) would not have been detected by traditional microbiological enumeration t ests if the tests had been performed immediately after low-a(w), storage. I f Salmonella strains form filaments in food products that have low a(w) val ues (0.92 to 0.98), there are significant implications for public health an d for designing methods for microbiological monitoring.