Ability of Shiga toxin-producing Escherichia coli and Salmonella spp. to survive in a desiccation model system and in dry foods

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 degrees 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 (515) 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 102 CFU/disk). After 22 to 24 months of subsequent storage at 4 degrees C, all of the selected salmonellae (4/4) and most of the selected STEC strains (12/15) survived, keeping the original populations (103 to 104 CFU/disk). In contrast to the case for storage at 4 degrees C, all of 15 selected strains (5 strains each of Salmonella spp., STEC O157, and STEC 026) died after 35 to 70 days of storage at 25 degrees C and 35 degrees 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 degrees 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 degrees C.