Effects of sublethal temperature stresses on the culturability and morphology of Staphylococcus aureus

Abstract

Temperature is one of the most important preservation methods in food industry. The objective of this study was to determine the effects of the sublethal temperature stresses on the culturability and morphology of Staphylococcus aureus ATCC 25923. The pure culture of S. aureus was grown in Tryptone Soya Broth before being subjected to three different temperature stresses (20°C, 40°C and 45°C) after reaching the log phase (6 hour) and stationary phase (16 hour). The survival of the bacteria in terms of cell injury on non-selective agar TSA and TSAS (TSA with added 4% NaCl) at these different temperature stresses were examined. Standard growth curve at optimal temperature (37°C) and the survival curves of S. aureus after subjected to different temperature stresses were obtained and the percentages of cell injury were also calculated. Results obtained showed that S. aureus is more heat resistant at 20°C during the log phase rather than at higher temperature stresses (40°C and 45°C). The apparent of biphasic curves were observed during the stationary phase of S. aureus. The percentage injury was higher at log phase rather than stationary phase where the highest percentage injury occurred after 9 hour of exposure at 20°C. The morphological changes observed under light microscope with an imaging system showed that the occurrence of clumping phenomenon during the stationary phase at 20°C and 45°C. The reported results described the culturability and morphology of pathogenic Gram-positive S. aureus under different temperature stresses.