Resistance of Enterobacter sakazakii (Cronobacter spp.) to environmental stresses.

AIM: To gain a better understanding of the survival and persistence of Enterobacter sakazakii in severe environments. METHODS AND RESULTS: We evaluated the resistance of Ent. sakazakii to various environmental stresses, including heating, drying, water activity (a(w)), and pH. The resistance of Ent. sakazakii to heat varies widely among strains. Most tested strains of Ent. sakazakii exhibited unusual resistance to dry stress, which depends on drying media. Growth of most strains occurred within 24 h at 37 degrees C when the initial a(w) of the medium was adjusted to 0.94 with sucrose or sodium chloride. The minimum pH for growth within 24 h at 37 degrees C was 3.9 or 4.1 for most strains tested. Additionally, there did not appear to be any relationship between resistance to stresses and biofilm-forming ability in Ent. sakazakii planktonic cells. CONCLUSIONS: These results indicate that Ent. sakazakii is much more resistant than other Enterobacteriaceae to environmental stresses. Moreover, it is likely that Ent. sakazakii has cross-resistance to dry and thermal stresses. SIGNIFICANCE AND IMPACT OF THE STUDY: The findings of this study will contribute to an improved understanding of the survival and behaviour of Ent. sakazakii, which will lead to improved strategies for preventing outbreaks of Ent. sakazakii infection.

Influences of milk components on biofilm formation of Cronobacter spp. (Enterobacter sakazakii).

AIM: To determine the critical component(s) of skim milk for biofilm formation of Cronobacter species. METHODS AND RESULTS: Biofilm forming ability of 72 Cronobacter strains in skim milk preparation was assayed by crystal violet staining. The results revealed that whey protein and casein are more important determinants of skim milk for biofilm formation than lactose, although there was a wide variation in biofilm forming ability. Biofilm structure and capsular material of six strains exhibiting different biofilm forming ability was investigated via electron microscopes. Scanning electron microscopy showed visually that while the strong biofilm formers (E27B, FSM 30 and 2.82) resulted in almost complete coagulation of skim milk, the weak biofilm formers (55, FSM 290 and 2.84) caused less coagulation. No capsule was clearly delineated in transmission electron micrographs of either strong or weak biofilm formers. CONCLUSION: These results indicate that, for biofilm formation of Cronobacter species in skim milk, nitrogen source is probably a more important determinant than carbohydrate, and that strong biofilm formers are responsible for substantial coagulation of skim milk. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides information for better understanding of the underlying mechanisms by which Cronobacter species form biofilm in infant formula milk.

Comparison of viability and heat resistance of Clostridium sporogenes stored at different temperatures.

The objective of this study was to determine the influence of storage temperature on the viability and heat resistance of Clostridium sporogenes spores. Spore suspension containing both spores and vegetative cells was divided into 3 groups to be stored at different temperatures of -20 (freezing), 4 (refrigerating), and 25 degrees C (ambient temperature). Samples stored for different times within the 2 mo were tested for viability by comparison of colony counts on plates and for heat resistance by determining D values at 121 degrees C. No significant differences were found in the viability of vegetative cells during the storage period, regardless of storage temperatures tested, while the viability of the spores stored for more than 4 wk was significantly higher at 4 degrees C than at -20 degrees C. The heat resistance of spores stored at 4 degrees C for more than 4 wk was remarkably higher than that at 25 degrees C, but similar to that at -20 degrees C throughout the storage period. Consequently, it turned out that a refrigerating temperature of 4 degrees C is satisfactory for storage of C. sporogenes spores in maintaining viability and heat resistance. This study suggests that storage temperature influences the viability and heat resistance of C. sporogenes spores.