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.
