Purpose: This study investigated the resistance of LTS phase Salmonella enterica serovar Typhimurium ATCC 14028 to ultraviolet (UV) radiation in 0.85% (wt/vol) saline and in pasteurized apple juice. The extent of sub-lethal injury in LTS phase survivors was also investigated.
Methods: LTS phase Salmonella Typhimurium cells were cultured in tryptic soy broth with 0.6% yeast extract (TSBYE) at 35°C for 14 days. Exponential- and stationary-phase cells grown in TSBYE (35°C) for 4 h and 24 h, respectively, served as control. Cells (107 CFU/ml) from each physiological state were exposed to UV light in saline (80 µW/cm2) and apple juice (820 µW/cm2). The Salmonella Typhimurium survivors were enumerated by plating diluted (10-fold) samples on tryptic soy agar with 0.6% yeast extract or xylose lysine tergitol agar and counting bacterial colonies after incubation (35°C, 24 h).
Results: LTS phase cells consistently exhibited the highest UV resistance (P < 0.05). In saline, D-values of exponential, stationary, and LTS Salmonella Typhimurium were 0.35, 0.40 and 0.53 min, respectively; in apple juice D-values were 8.69, 8.81, and 10.12 min, respectively (P <0.05). UV radiation (80 µW/cm2) of Salmonella Typhimurium for 2.5 min in saline reduced the number of exponential- and stationary-phase cells by 7.19 and 6.30 log CFU/ml, respectively. In contrast, LTS cells were reduced by only 5.08 log. Within the three physiological states, LTS phase cells had the least sub-lethal injury in the surviving population (P < 0.05).
Significance: The results of present study indicate that the LTS state cross-protects Salmonella Typhimurium against UV radiation and should be considered when determining the UV radiation D-value for this pathogen.