Characterization of Raw Milk Bacteria via MALDI-TOF-MS and Resistance Screening

This study investigated antibiotic resistance among bacteria isolated from raw milk and assessed their potential public health significance. A total of 188 bacterial isolates were recovered by culture, of which 145 were purified and identified using a mass spectrometry-based identification method. Staphylococcus species were the most common bacteria (64.8%), followed by Bacillus species (8.2%), Aerococcus viridans (6.8%), and Enterococcus species (6.2%). None of the tested Staphylococcus isolates carried genes associated with methicillin resistance. Multidrug resistance was observed in all Bacillus cereus and Enterococcus gallinarum isolates, as well as in 66.7% of Staphylococcus sciuri, 57.2% of Staphylococcus xylosus, 25% of Staphylococcus haemolyticus, 25% of Enterococcus faecium, 23.8% of Staphylococcus aureus, and 20% of Aerococcus viridans isolates. Multiple antibiotic resistance index values were highest for Bacillus cereus (0.750), Escherichia coli (0.600), and Enterococcus species (0.560). These findings indicate that raw milk may contain bacteria with important antibiotic-resistance characteristics and may therefore represent a potential public health concern.