<i>Salmonella</i> in Low Water Activity Foods: Physiological, genetic modification and control methods

The objective of this study was to evaluate the expression of fatty acid associated genes (fabA, fabD and cfa) of five serovars of Salmonella exposed to sugar over a 14-day period. Changes in the fatty acid composition of Salmonella Tennessee in glycerol solutions of different water activity (a_w) (1.0-0.6) and the relationship between survival and fatty acid modification (as altered by exogenously supplied fatty acids) at a_w 1.0-0.6 were also determined. Furthermore, the antimicrobial activities of carvacrol, cinnamaldehyde, and lauric arginate (LAE) alone or in combination against Salmonella Tennessee in a laboratory model of different a_w and their effectiveness in peanut paste with different fat levels over time was also evaluated.

Increased expression of the cfa gene over 14 days was associated with strains with a lower survival rate. The fabA gene was up-regulated for all strains for at least one sampling time and all time points tested for S. Tennessee ARI-33, suggesting its potential role in enhancing Salmonella survival in low a_w foods. The fatty acid composition of S. Tennessee cells did not differ (P>0.05) over time at a_w 1.0-0.6. Exogenous oleic and linoleic acids were both incorporated into S. Tennessee with a concomitant decrease of cyclopropane fatty acid (CFA, C17:0). However, the incorporation of these fatty acids from growth media had no effect on survival at a_w 1.0-0.6 over 14 days. Cells incubated in peanut oil had improved survival at a_w 0.8-0.6 as compared to controls. In the glycerol-sucrose model, all antimicrobials significantly reduced the population over time with LAE being the most effective compound. Cinnamaldehyde was more effective than carvacrol at a_w 0.5 and 0.3. In low fat peanut paste, none of the antimicrobials inhibited growth of the microorganism at a_w 1.0. However, inactivation was enhanced at reduced a_w. Cinnamaldehyde and LAE both reduced the population to undetectable levels on day 5 at the highest concentration tested (10 times higher than the glycerol-sucrose model). Fat content negatively impacted the efficacy of these antimicrobials. Inactivation efficacy of all antimicrobials was greatly decreased, but not eliminated, in 50% fat peanut paste.