The Influence of Water Activity on Salmonella enterica Typhimurium Biofilm’s Thermal Resistance

Introduction: Frequent salmonellosis outbreaks and food recalls associated with low water activity (a_w) foods have suggested that they are susceptible to contamination with Salmonella enterica. Biofilms are complex bacterial structures characterized by cells embedded in a protective extracellular matrix. Because contamination of dry foods often occurs in processing plant environments, a better understanding of Salmonella biofilms is critical for its control.

Purpose: This work was conducted to determine the effect of low water activity on the ability of biofilms of S. enterica Typhimurium to tolerate heat exposure.

Methods: Biofilms were grown for 6 days in glass beads in non-selective media at 37°C. After rinsing, beads were dried at 38°C for 4 days and equilibrated at different water activities (0.11, 0.33, 0.53, 1.0) for 7 days. Biofilms were heated at 85 and 95°C and survivors were enumerated after plating in differential media. Scanning electronic microscopy was used to observe cells after thermal treatment.

Results: At low a_w values (0.11, 0.33, 0.53) slight reductions of viability of biofilms that were grown for 2, 4 and 6 days of less than 1 log CFU/g were observed at 95°C after 2 h. At 85°C the same level of viability loss was observed by 5 h. In contrast, the viability of biofilms equilibrated to a_w of 1.0 was reduced by at least 5 log CFU/g after 15 minutes at 85 and 95°C. The structure and morphology of low a_w biofilms was better preserved than that of biofilms exposed to 1.0 a_w as increased amounts of matrix were observed.

Significance: These findings are relevant because biofilms are believed to be the natural form of bacteria occurrence. This work may suggest that in a food processing setting, the exposure of plant environment biofilms to drier conditions may lead to greater persistence.