No abstract

The smear of surface-ripened cheese harbors complex microbiota mainly composed of typical Gram-positive aerobic bacteria and yeast. Gram-negative bacteria are usually classified as unwanted contaminants. In order to investigate the abundance and impact of Gram-negative bacteria naturally occurring in the smear of surface-ripened cheese, we performed a culture-based analysis of smear samples from 15 semi-hard surface-ripened cheese varieties. The quantity, diversity and species distribution of Proteobacteria in the surface smear of the analyzed cheese varieties were unexpectedly high, and comprised a total of 22 different species. Proteus and Morganella predominated most of the analyzed cheese varieties, while Enterobacter, Citrobacter, Hafnia and Serratia were also found frequently. Further physiological characterization of Proteus isolates revealed strong proteolytic activity, and the analysis of volatiles in the smear cheese surface headspace suggested that Enterobacterales produce volatile organic flavor compounds that contribute to the organoleptic properties of surface-ripened cheese. Autochthonous members of Enterobacterales were found in 12 of the 15 smear samples from surface-ripened cheeses, suggesting that they are part of the typical house microbiota that shape the organoleptic properties of the cheese rather than represent unwanted contaminants. However, further investigation on safety issues of the individual species should be performed in order to manage the health risk for consumers.