Purpose: To determine if there is a relationship between survival and fatty acid (FA) modification (as altered by exogenously supplied fatty acids) of Salmonella Tennessee at aw 1.0 - 0.6.
Methods: Glycerol solutions (aw 1.0 - 0.6) were inoculated with Salmonella Tennessee K4643 and incubated at 25°C. Total FA composition of parent strains on day 1 and 14 was determined by gas chromatography (GC). Salmonella Tennessee FA composition was modified by culturing in tryptic soy broth supplemented with 50 μg/ml oleic (C18:1) or linoleic acid (C18:2) or by incubating in peanut oil for 24 h. Modified Salmonella Tennessee cells were then inoculated in various water activity glycerol solutions and incubated at 25°C. Samples were taken over 14 days and plated on tryptic soy agar. Each experiment was repeated 3 times.
Results: The FA composition of the parent S. Tennessee cells did not differ (P > 0.05) at different aw and no change was found over time. Exogenous oleic and linoleic acids were both incorporated into Salmonella Tennessee with a concomitant decrease of cyclopropane fatty acid (CFA, C17). However, incorporation did not influence survival in glycerol solutions over 14 days. In peanut oil, there was a 2% increase of CFA (C17) and 4% decrease of oleic acid. Cells suspended in peanut oil were more resistant at aw 0.6 than parent cells (1.69 log vs. undetectable on day 14).
Significance: Reducing the aw to 0.6 did not change the FA composition of Salmonella Tennessee. Exogenous oleic and linoleic acids caused incorporation of these fatty acids from growth media but had no effect on survival at low aw. Cells incubated in peanut oil had improved survival at aw 0.6 compared to controls.