Sources of Bacillus cereus contamination in a pasteurized zucchini purée processing line, differentiated by two PCR-based methods.
In previous work, raw materials used for processing pasteurized zucchini purée (zucchini, milk proteins and starch) and cultivation soil of zucchinis were found to be potential sources of stored product contamination with Bacillus cereus. 134 B. cereus strains originating from these sources and from the stored product were typed using coliphage M13 sequence-based polymerase chain reaction (M13-PCR) and DNA amplification fingerprinting. Combined patterns from the two methods were compared using Dice's coefficient and the unweighted pair group method with average cluster analysis. 16 combined profile groups and six unclustered strains were formed at 75% similarity level. Representative strains of each group were subsequently examined for growth at low temperature (4 degrees C, 7 degrees C and 10 degrees C). Based on strain relatedness, five major groups and two minor groups among the nine groups containing zucchini strains were common to both zucchini and soil origins, indicating that the soil was the main initial source of contamination for zucchinis. Strains from zucchinis and soil were heterogeneous compared with strains from dehydrated ingredients (milk proteins and starch), and were mainly composed of psychrotrophic strains. Convergent results from molecular typing and growth at low temperature showed that psychrotrophic contaminants of the stored product originated from zucchinis, whereas non-psychrotrophic contaminants originated from milk proteins. The number of genetic groups decreased during the purée processing, suggesting selection of strains during zucchini washing operations, heat treatment and storage.