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

Abstract

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...

Thin Aggregative Fimbriae and Cellulose Enhance Long-Term Survival and Persistence ofSalmonella

“…The enhanced survival under conditions of desiccation would be advantageous in many natural environments and may also contribute to the contamination of dried food products (19). The finding that almost 10% of ATCC 14028 cells were alive after 9 months of storage illustrates the remarkable survival properties of Salmonella.…”

Comparative Genetics of the rdar Morphotype inSalmonella

“…Due to characteristic growth conditions (growth under 30°C, nutrient limitation, low osmolarity) (16,25) and increased resistance to desiccation (26,68) and antimicrobial agents (2,55,56,68), we hypothesize that the rdar morphotype has a fundamental role for Salmonella survival outside host environments. This may contribute to contamination of different food products (4,30) and passage between susceptible hosts.…”

SalmonellaProduces an O-Antigen Capsule Regulated by AgfD and Important for Environmental Persistence

“…It is possible the O-Ag capsule is a contributing factor to the survival of Salmonella spp. on desiccated foods (30). Furthermore, other capsules (58) and the extracellular matrix (26) are important in virulence.…”