Antimicrobial resistance and virulence gene patterns of <i>Staphylococcus aureus</i> in infectious mastitis: implications for inflammatory myopathies of the lactating breast

With the present work, we aimed to investigate antimicrobial resistance and virulence gene patterns of Staphylococcus aureus in lactating patients with infectious mastitis and evaluate their potential impact on inflammatory myopathies of the lactating breast. Between January 2021 and April 2024, 158 lactating patients with culture-confirmed infectious mastitis were treated at Hangzhou Linping District Maternal and Child Health Hospital. Among these, 119 isolates were identified as S. aureus (82 MRSA, 37 MSSA). Antimicrobial susceptibility and virulence genes were analyzed. Muscle involvement was inferred indirectly from clinical presentation, including marked local induration, tenderness extending to deeper breast tissue, and reduced breast mobility. No imaging or biopsy was performed to confirm myopathic changes directly. S. aureus was the predominant pathogen. Both MRSA and MSSA showed high resistance to penicillin G, erythromycin, and Clindamycin, while all isolates were susceptible to nitrofurantoin, linezolid, vancomycin, and rifampicin. MRSA exhibited higher resistance than MSSA (p<0.05). Frequent resistance genes included aac(6')/aph(2''), blaZ, mecA, aph(3')-III, and qacA/B. Virulence genes hla, clfA, clfB, and fnbA were common; pvl was less frequent in MRSA (p<0.05). MRSA infections were associated with stronger local inflammation and increased clinical markers possibly related to muscle involvement, raising the possibility of an association with myopathic changes in lactating breast tissue. S. aureus, particularly MRSA, is the main pathogen in lactating mastitis. Specific virulence genes may influence the severity of local inflammation and myopathic changes, highlighting implications for inflammatory myopathies in the lactating breast.