Heat Resistance of Salmonella weltevreden in Low-Moisture Environments

170

In order to determine desiccation tolerances of bacterial strains, the survival of 58 diarrheagenic strains (18 salmonellae, 35 Shiga toxin-producing Escherichia coli [STEC], and 5 shigellae) and of 15 nonpathogenic E. coli strains was determined after drying at 35°C for 24 h in paper disks. At an inoculum level of 10 7 CFU/disk, most of the salmonellae (14/18) and the STEC strains (31/35) survived with a population of 10 3 to 10 4 CFU/disk, whereas all of the shigellae (5/5) and the majority of the nonpathogenic E. coli strains (9/15) did not survive (the population was decreased to less than the detection limit of 10 2 CFU/disk). After 22 to 24 months of subsequent storage at 4°C, all of the selected salmonellae (4/4) and most of the selected STEC strains (12/15) survived, keeping the original populations (10 3 to 10 4 CFU/disk). In contrast to the case for storage at 4°C, all of 15 selected strains (5 strains each of Salmonella spp., STEC O157, and STEC O26) died after 35 to 70 days of storage at 25°C and 35°C. The survival rates of all of these 15 strains in paper disks after the 24 h of drying were substantially increased (10 to 79 times) by the presence of sucrose (12% to 36%). All of these 15 desiccated strains in paper disks survived after exposure to 70°C for 5 h. The populations of these 15 strains inoculated in dried foods containing sucrose and/or fat (e.g., chocolate) were 100 times higher than those in the dried paper disks after drying for 24 h at 25°C.A nationwide outbreak of gastroenteritis due to consumption of dried squid chips (water activity, 0.5 to 0.6) contaminated with Salmonella enterica serovars Oranienburg and Chester occurred in Japan in 1999, in which a total of 1,634 cases were reported from all over the country (46/47 prefectures) (25). Even though this outbreak was the first one caused by contaminated dry foods in Japan, a number of outbreaks of gastroenteritis caused by dry foods such as dried milk, chocolate, potato chips, and almonds contaminated mainly by salmonellae have been reported in the United States and Europe since the 1960s (5,12,16,18). Since the middle of the 1990s, outbreaks of Shiga toxin-producing Escherichia coli (STEC) O157 infections have also been reported to be associated with dry foods such as deer jerky and salami in the United States (8,14,19,24). Many outbreaks of gastroenteritis associated with dry foods contaminated with salmonellae and STEC strains have been reported all over the world, even though the bacterial growth was inhibited by low water activity (a w ) in these dry foods. Accordingly, we tried to investigate the ability of these desiccated strains to survive under dried conditions, using salmonellae and STEC O157, O26, and O111 strains in paper disks with a w s of 0.5 to 0.6 and in selected dry foods. In order to study the effect of bacterial environments on the survival of bacteria during a process of being made "desiccated" bacteria and of storage conditions on the survival of desiccated bacteria produced after 24 h of drying, we examined t...

Section: Discussionsupporting

confidence: 50%

Section: Discussionmentioning

confidence: 99%

Ability of Shiga Toxin-Producing Escherichia coli and Salmonella spp. To Survive in a Desiccation Model System and in Dry Foods

Hiramatsu

Matsumoto

Sakae

et al. 2005

170

“…This makes the assumption that all bacterial cells within a population have the same heat sensitivity, but significant deviations from linearity have been reported elsewhere (5,9,26,33,40,60). In such cases, curve fitting will give more accurate descriptions of the data than will D values because shoulders or tailing can be incorporated.…”

mentioning

confidence: 97%

Effect of Challenge Temperature and Solute Type on Heat Tolerance of Salmonella Serovars at Low Water Activity

Mattick

Jørgensen

Wang³

et al. 2001

135

Salmonella spp. are reported to have an increased heat tolerance at low water activity (a w ; measured by relative vapor pressure [rvp]), achieved either by drying or by incorporating solutes. Much of the published data, however, cover only a narrow treatment range and have been analyzed by assuming first-order death kinetics. In this study, the death of Salmonella enterica serovar Typhimurium DT104 when exposed to 54 combinations of temperature (55 to 80°C) and a w (rvp 0.65 to 0.90, reduced using glucose-fructose) was investigated. The Weibull model (LogS ‫؍‬ ؊bt n ) was used to describe microbial inactivation, and surface response models were developed to predict death rates for serovar Typhimurium at all points within the design surface. The models were evaluated with data generated by using six different Salmonella strains in place of serovar Typhimurium DT104 strain 30, two different solutes in place of glucose-fructose to reduce a w , or six low-a w foods artificially contaminated with Salmonella in place of the sugar broths. The data demonstrate that, at temperatures of >70°C, Salmonella cells at low a w were more heat tolerant than those at a higher a w but below 65°C the reverse was true. The same patterns were generated when sucrose (rvp 0.80 compared with 0.90) or NaCl (0.75 compared with 0.90) was used to reduce a w , but the extent of the protection afforded varied with solute type. The predictions of thermal death rates in the low-a w foods were usually fail-safe, but the few exceptions highlight the importance of validating models with specific foods that may have additional factors affecting survival.

“…Considering the fact that the vast majority of these products undergo thermal processing before packaging, it has been suggested that there is a correlation between the low water content of the product and the increased thermal resistance of the pathogen (9,21,24,39,43). Using laboratory models, it was demonstrated that reducing water activity either by desiccation (24,31) or by addition of solutes (2,12,30,37,38) resulted in increased thermal tolerance.…”

mentioning

confidence: 99%

Effect of Desiccation on Tolerance of Salmonella enterica to Multiple Stresses

Gruzdev

Pinto

Sela

2011

198

116

Reducing the available water in food is a long-established method for controlling bacterial growth in the food industry. Nevertheless, food-borne outbreaks of salmonellosis due to consumption of dry foods have been continuously reported. Previous studies showed that dried Salmonella cells acquire high tolerance to heat and ethanol. In order to examine if dehydration also induces tolerance to other stressors, dried Salmonella enterica serotype Typhimurium cells were exposed to multiple stresses, and their viability was assessed. Indeed, desiccated S. Typhimurium acquired higher tolerance to multiple stressors than nondesiccated cells. The dried cells were significantly more resistant to most stressors, including ethanol (10 to 30%, 5 min), sodium hypochlorite (10 to 100 ppm, 10 min), didecyl dimethyl ammonium chloride (0.05 to 0.25%, 5 min), hydrogen peroxide (0.5 to 2.0%, 30 min), NaCl (0.1 to 1 M, 2 h), bile salts (1 to 10%, 2 h), dry heat (100°C, 1 h), and UV irradiation (125 W/cm 2 , 25 min). In contrast, exposure of Salmonella to acetic and citric acids reduced the survival of the dried cells (1.5 log) compared to that of nondesiccated cells (0.5 log). Three other S. enterica serotypes, S. Enteritidis, S. Newport, and S. Infantis, had similar stress responses as S. Typhimurium, while S. Hadar was much more susceptible and gained tolerance to only a few stressors. Our findings indicate that dehydration induces cross-tolerance to multiple stresses in S. enterica, demonstrating the limitations of current chemical and physical treatments utilized by the food industry to inactivate food-borne pathogens.