The genus Microbacterium: A bacterial community resistant to sanitisation processes in long-life milk

Microfiltration and pasteurization are treatments carried out to guarantee the safety of milk and to elongate the shelf life. These sanitization processes strongly modify the composition of the microbial population in raw milk. Microfiltered milk samples produced over three months and in two different industrial dairy plants were studied. The microbial analysis allowed us to identify Microbacterium sp. as dominant and other spoiling bacteria belonging to Bacillus, Acinetobacter, Micrococcus, Staphylococcus, Pantoea, and Escherichia. Microorganisms were identified after colony isolation and by 16S rRNA sequencing. To accurately identify Microbacterium species, gyrB gene similarity analysis was performed, and Microbacterium lacticum and Microbacterium paulum resulted being the two dominant species. The role played by Microbacterium sp. in milk is unknown, and a lab-scale experiment was carried out to evaluate the effects. Most Microbacterium isolated strains can grow in milk with moderate acidification activity. The strains were exposed to pasteurization treatment, and the isolated strains survived when a combination of time and temperature similar to industrial conditions was used. Antibiotic resistance was evaluated in the isolated Microbacterium strains, and resistance to clindamycin and ciprofloxacin was found.