Characterization of Biofilm Producing Coagulase-Negative Staphylococci Isolated from Bulk Tank Milk

Simple Summary Coagulase-negative staphylococci are considered less virulent than other variants. However, they have been increasingly recognized as an important cause of bovine mastitis. Moreover, the biofilm-forming ability appears to be important in CoNS pathogenicity, which leads more resistance to antimicrobials. This study investigated the pathogenic potential by assessing the biofilm-forming ability of CoNS isolated from bulk tank milk and analyzed the biogilm-associated resistance to antimicrobial agents. The results indicate that various CoNS isolated from bulk tank milk, not from bovine with mastitis, exhibited a high prevalence of biofilm-forming ability with a high prevalence of MDR, and also biofilm-associated genes with a high prevalence. Therefore, developing a strong monitoring and sanitation program for dairy factories is important to ensure hygienic milk production. Coagulase-negative staphylococci (CoNS) are considered less virulent as they do not produce a large number of toxic enzymes and toxins; however, they have been increasingly recognized as an important cause of bovine mastitis. In particular, the ability to form biofilms appears to be an important factor in CoNS pathogenicity, and it contributes more resistance to antimicrobials. The aim of this study was to investigate the pathogenic potential by assessing the biofilm-forming ability of CoNS isolated from normal bulk tank milk using the biofilm formation assay and to analyze the biofilm-associated resistance to antimicrobial agents using the disc diffusion method. One hundred and twenty-seven (78.4%) among 162 CoNS showed the ability of biofilm formation, and all species showed a significantly high ability of biofilm formation (p < 0.05). Although the prevalence of weak biofilm formers (39.1% to 80.0%) was significantly higher than that of other biofilm formers in all species (p < 0.05), the prevalence of strong biofilm formers was significantly higher in Staphylococcus haemolyticus (36.4%), Staphylococcus chromogenes (24.6%), and Staphylococcus saprophyticus (21.7%) (p < 0.05). Also, 4 (11.4%) among 35 non-biofilm formers did not harbor any biofilm-associated genes, whereas all 54 strong or moderate biofilm formers harbored 1 or more of these genes. The prevalence of MDR was significantly higher in biofilm formers (73.2%) than in non-formers (20.0%) (p < 0.05). Moreover, the distribution of MDR in strong or moderate biofilm formers was 81.5%, which was significantly higher than in weak (67.1%) and non-formers (20.0%) (p < 0.05). Our results indicated that various CoNS isolated from bulk tank milk, not from bovine with mastitis, have already showed a high ability to form biofilms, while also displaying a high prevalence of MDR.