Virulence and genetic determinants of Staphylococcus spp. in human mastitis: a prevalence study

Lactational mastitis is a common inflammatory breast disease affecting breastfeeding women and is associated with Staphylococcus spp. Increasing antimicrobial resistance and virulence among these pathogens complicate treatment and emphasize the need for accurate laboratory diagnostic methods. This study investigated the microbiological profile, antimicrobial resistance, and virulence characteristics of Staphylococcus spp. isolated from lactational mastitis and evaluated the correlation between phenotypic and molecular detection of major virulence and methicillin-resistance determinants.

A cross-sectional study was conducted in central Gujarat, India, using 180 human milk samples collected from 150 women with lactational mastitis and 30 healthy lactating controls. Somatic cell count, bacterial isolation, biochemical identification, antimicrobial susceptibility testing, haemolysis, slime production, biofilm assays, polymerase chain reaction detection of the hla, icaA, and mecA genes, and phylogenetic analysis were performed. All mastitis samples exhibited somatic cell counts exceeding 1,000 cells/µL, whereas healthy controls remained below this threshold (p < 0.001). Staphylococcus spp. accounted for 89.4% of isolates. The icaA, hla, and mecA genes were detected in 76.39%, 54.65%, and 46.58% of isolates, respectively. Phenotypically, biofilm formation, alpha-haemolysis, and cefoxitin resistance were observed in 72.67%, 52.79%, and 37.88% of isolates, respectively. Excellent agreement was observed between the corresponding phenotypic and molecular findings, with κ values of 0.89 for icaA/biofilm formation, 0.97 for hla/alpha-haemolysis, and 0.89 for mecA/cefoxitin resistance.

Elevated somatic cell count effectively distinguished mastitis from healthy controls. The high prevalence of virulent and antimicrobial-resistant Staphylococcus spp., particularly mecA-positive isolates, underscores the need for continuous surveillance, targeted antimicrobial therapy, and cost-effective phenotypic screening to support routine clinical diagnosis and epidemiological monitoring.