Effects of sublethl temperature stresses on the culturability and morphology of Salmonella

Abstract

This study was related to the culturability and morphology of Salmonella in trypticase soy broth grown at 37°C before being subjected to three different sublethal temperature stresses ( 45°C, 40°C and 20°C) at exponential and stationary phases as inoculums using two-plating systems; TSA with and without 4% NaCl (TSAS). A standard growth curve of Salmonella at 37°C was established using plate counts showed that the exponential and stationary phase of the organism were achieved after 6 and 16 hours, respectively. When the bacteria were exposed to sublethal temperature stresses, the injury was occurred and the percentage injury was different between exponential phase and stationary phase. The presence of sodium chloride in the plating agar increased heat resistance of Salmonella. During the exponential phase cultures, the percentage sublethal injury at 45°C, 40°C and 20°C were ranged between 0.01 % to 15.81 %, 0.11 % to 3.40% and 0.23% to 7.47% respectively. However, the percentage of sublethal injury during stationary phase was lower than during exponential phase, which ranged between 0.12% to 13.72%, 0.11% to 2.02% and 0.11% to 3.03% for these following respective temperatures of 45°C, 40°C and 20°C. Sublethal temperature injury of Salmonella was developed more rapidly in log phase cells than in stationary phase cells. This is illustrated based on their percentage injury where the percentage injury during log phase was ranged between 0.01 % to 15 .81 %, which was higher than in stationary phase (0.11 % to 13. 72% ). Salmonella cells were able to survive throughout the sub lethal temperature stresses and undergone morphological changes to adapt to new temperatures. The study demonstrated cells of both exponential phase and stationary phase showed a significant variation of morphology. Cells of log phase were elongated while at stationary phase cells became coccoid. The stationary phase cells of Salmonella were more resistant than exponential phase cells exhibited by lower percentage injury and shorter their morphologies to become coccoid.