Abstract
Unpasteurized milk is used to produce aged artisanal cheeses, which presents a safety concern due to possible contamination with foodborne pathogens, especially Listeria monocytogenes. The objective of this study was to examine the composition of the bacterial community in unpasteurized milk used to prepare Gouda cheese artificially contaminated with L. monocytogenes (~1 log CFU/ml) and assess the community dynamics and their potential interaction with L. monocytogenes during a 90-day ripening period using targeted 16S rRNA sequencing. The diversity of bacterial taxa in three batches of unpasteurized milk was not significantly different, and the microbiomes were dominated by species of Lactococcus, Streptomyces, Staphylococcus, and Pseudomonas. The highest relative abundances were observed for Pseudomonas fluorescens (31.84–78.80%) and unidentified operational taxonomic units (OTUs) of Pseudomonas (7.56–45.27%). After manufacture, both with and without L. monocytogenes-contaminated unpasteurized milk, Gouda cheese was dominated by starter culture bacteria (including Lactococcus lactis subsp. cremoris, lactis, lactis bv. diacetylactis, and Streptococcus thermophilus), in addition to unassigned members in the taxa L. lactis and Streptococcus. During ripening there was an overall decrease in L. lactis abundance and an increase in the number of taxa with relative abundances >0.1%. After 90-day ripening, a total of 82 and 81 taxa were identified in the Gouda cheese with and without L. monocytogenes, respectively. Of the identified taxa after ripening, 31 (Gouda cheese with L. monocytogenes) and 56 (Gouda cheese without L. monocytogenes) taxa had relative abundances >0.1%; 31 were shared between the two types of Gouda cheese, and 25 were unique to the Gouda cheese without added L. monocytogenes. No unique taxa were identified in the Gouda cheese with the added L. monocytogenes. This study provides information on the dynamics of the bacterial community in Gouda cheese during ripening, both with and without the addition of L. monocytogenes.
Highlights
Microbes and cheesemaking have been intertwined for hundreds of years, with the first description of cheese microorganisms dating back to 1665 (Donnelly, 2014)
A total of 11 different taxa were identified with relative abundances >1% in at least one of the three milk batches and included L. lactis, Pseudomonas fluorescens, P. koreensis, P. moraviensis, Polynucleobacter necessarius, Staphylococcus aureus, Bacillus thuringiensis, Klebsiella pneumoniae, and Komagataeibacter rhaeticus, as well as unclassified members in the genera of Streptomyces and Pseudomonas
Differences in the relative abundances of some taxa were more pronounced than others in the three batches of milk (Figures 1, TABLE 1 | Observed operational taxonomic units (OTUs) and alpha diversity metrics of the three batches of unpasteurized milk (AB, CD, and EF) used for Gouda cheese manufacture
Summary
Microbes and cheesemaking have been intertwined for hundreds of years, with the first description of cheese microorganisms dating back to 1665 (Donnelly, 2014). The knowledge gained from these studies has resulted in a more controlled, standardized, and safe process for producing many of these cheese types (Donnelly, 2014). Cheeses obtain their unique flavors from defined starter cultures and aging (ripening) conditions in an industrial setting that yields a predictable and replicable result. Pathogenic E. coli contamination of Gouda cheese and/or the unpasteurized milk that was used for manufacture has led to multiple outbreaks between 1998 and 2011 (Gould et al, 2014), and in 2013, it was responsible for 23 illnesses, five hospitalizations, and one death (Currie et al, 2018)
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