Biofilm forming multidrug resistant Staphylococcus aureus of dairy origin: molecular and evolutionary perspectives

Background: Biofilm production by Staphylococcus aureus is a prevailing cause of multidrug resistance. The evolutionary mechanisms of adaption with host and pathogenicity are poorly understood. Aims: The present study aimed to investigate the biofilm-forming potential, associated multidrug resistance, and the evolutionary analysis of S. aureus isolated from bovine subclinical mastitis. Methods: 122 S. aureus isolates were subjected to Congo red agar method (CRA), microtitre plate method (MTP), and PCR to check the biofilm-forming potential. The Kirby-Bauer disk diffusion method was used to evaluate the antibiotic resistance pattern. The icaA gene of isolates was subjected to molecular and evolutionary analysis using different bioinformatics tools. Results: The results showed that 63.93% of S. aureus isolates carried the icaA gene and the detection rate of CRA was higher (36.07%) compared to the MTP test (24.59%). A total of 78.21% and 56.41% of biofilm-positive isolates were methicillin-resistant S. aureus (MRSA) and vancomycin-resistant S. aureus (VRSA), respectively. All S. aureus isolates (100%) showed multidrug resistance. The molecular analysis showed an evolutionary link between isolates and revealed a strong codon bias, three different recombination events, and positive selection in some residues of the semi-conserved segments of the icaA gene. Conclusion: The study concluded that biofilm-positive isolates have a high tendency to exhibit methicillin, vancomycin, and multidrug resistance. The findings suggest that mutation and selection are the most likely causes of codon bias in the icaA gene sequences. The variations led by recombination events and positive selection are suggestive of bacterial strategy to combat antimicrobial effects and to escape the host's immune surveillance.