Salmonella spp. are affected by different levels of water activity in closed microcosms

Controlling water activity (a(w)) can significantly impact the growth of Salmonella in poultry litter and manure - a phenomenon that was studied quantitatively using two common serotypes of Salmonella. The quantitative effect of changes in levels of a(w) on Salmonella populations was determined using inoculated, frosted glass rectangles placed in closed chambers (microcosms). Glass rectangles with known concentrations of Salmonella enteritidis and S. brandenburg were placed in microcosms maintained at an a(w) level of 0.893 for 24 h at room temperature (RT) and then transferred to other microcosms maintained at the same temperature but with higher a(w) levels (0.932 and 0.987). Salmonella populations on the slides were quantified at 4, 18, 24, and 48 h. Slightly elevated levels of a(w) (<0.1, i.e., 10% equilibrium relative humidity) for 24 h resulted in a 100-fold increase in counts of Salmonella. The data also suggested that in vitro adaptation to dry environments may occur when the organisms are exposed to alternating levels of relatively high and low (0.987 and 0.893) levels of a(w). Any increased tolerance of Salmonella to reduced levels of a(w) could be the result of physico-chemical changes in the organism due to selective environmental pressure, formation of a protective biofilm, and/or entry into a dormant state. Results from this study are compatible with those from previously reported on-farm surveys, reinforcing the contention that maintaining a(w) below 0.85 in and around litter/manure surfaces in poultry or livestock bedding areas may be a critical factor in safe production of food.