Abstract
Processed cheese is a commercial product characterized by high microbiological stability and extended shelf life obtained through the application of severe heat treatment. However, spore-forming bacteria can survive through thermal processes. Among them, microorganisms belonging to Bacillus genus have been reported. In this study, we examined the microbiological population of the first hours’ production of processed cheeses in an Italian dairy plant during two seasons, between June and October 2020. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used to identify bacteria colonies, allowing the isolation of Bacillus cereus and Bacillus subtilis strains. These results were further confirmed by amplification and sequencing of 16 rRNA bacterial region. A multi-locus sequence type (MLST) analysis was performed to assess the genetic similarity among a selection of isolates. The fourteen B. cereus strains showed two sequence types: ST-32 was observed in only one strain and the ST-371 in the remaining thirteen isolates. On the contrary, all twenty-one B. subtlis strains, included in the study, showed a new allelic profile for the pycA gene, resulting in a new sequence type: ST-249. For B. cereus strains, analysis of toxin genes was performed. All isolates were positive for nheABC, entFM, and cytK, while hblABCD, bceT, and ces were not detected. Moreover, the biofilm-forming ability of B. cereus and B. subtilis strains was assessed, and all selected isolates proved to be biofilm formers (most of them were stronger producers). Considering the genetical similarity between isolates, jointly with the capacity to produce biofilm, the presence of a recurring Bacillus population could be hypothesized.
Highlights
Processed cheeses are produced by melting into a homogeneous molten blend, one or more cheeses with other ingredients and usually emulsifying salts [1,2]
Cultures in PCA showed total bacterial load of vegetative cells varied over time, with a highest density observed in summer, whereas lowest levels occurred in autumn with statical significance differences assessed during the two seasons (p < 0.001) (Figure 2)
Results showed the presence of microorganisms of Bacillus genus, in particular B. subtilis and B. cereus (Table 1)
Summary
Processed cheeses are produced by melting into a homogeneous molten blend, one or more cheeses with other ingredients (cream, butter, whey) and usually emulsifying salts [1,2]. Processing may be carried out in batch cookers in which temperature–time treatment varies (e.g., 70–95 ◦C for 4–15 min), by obtaining pasteurized processed cheese or in continuous cookers in which the blend is typically heated with direct steam injection to ∼140 ◦C for ∼10 s, or by manufacturing sterilized high-moisture processed cheese [1]. Spore-forming bacteria are ubiquitous, founded mainly in the soil, but they are able to colonize the gastrointestinal tract of insects and animals [4]. They belong to the Firmicutes phylum and, among them, Bacilli and Clostridia are the most relevant in the dairy industry [5]. Due to their ubiquity in nature, it is a real challenge to determine the true source of contamination in a dairy farm, which could be related to inadequate handling of raw materials [4], to cross contamination in the dairy plant [6], or for production of spores during food processing [7]
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