Typing and evaluating heat resistance of Bacillus cereus sensu stricto isolated from the processing environment of powdered infant formula

摘要

Bacillus cereus sensu lato is one of the most harmfulbacterial groups affecting the quality and safety ofpowdered infant formula (PIF). In this study, sampleswere collected from the raw materials and processingenvironments of PIF. A total of 84 isolates were identifiedas Bacillus cereus sensu stricto (B. cereus s. s.) by16S rRNA analysis, molecular typing technology, andphysiological and biochemical tests. The 84 B. cereuss. s. strains were assigned to panC group Ⅱ, group Ⅲ,and group Ⅳ. Then, the 7 housekeeping genes glpF,gmk, ilvD, pta, pur, pycA, and tpi were selected formultilocus sequence typing. Results showed that the84 isolates were clustered into 24 sequence types (ST),and 14 novel ST were detected. Among the 24 ST,ST999 (19/84, 22.62%) and ST1343 (13/84, 15.48%)predominated. The correlation between processing areasand ST showed that the processing environmentsof the production and packing areas were the mostsusceptible to contamination by B. cereus s. s. Sporesof these ST showed different heat resistance phenotypesevaluated by the analysis of D_T (time in minutes ofspore decimal reduction at each temperature) and Zvalues (temperature increase required to reduce the DTvalue to one-tenth of the original). Spores from groupⅢ according to panC gene analysis were the most heatresistant. These findings will help us to better understandB. cereus s. s. contamination and control in PIFprocessing environments.