Phenotypic characterization, molecular typing, and clonal relatedness within Staphylococcus isolates in healthy ocular conditions

This study aimed to explore the culturable bacterial community in healthy volunteers, with a focus on the staphylococcal population. First, 120 bacterial isolates were collected from conjunctival swabs of 49 healthy volunteers (aged 18 to 77 years). Isolates were characterized by phenotypic analyses, and 68 of them were confirmed as staphylococci by MALDI-TOF/MS and a multiplex PCR assay. The Staphylococcus strains (n = 68) were tested for resistance to the most relevant clinical antibiotics. Clonal relationships among isolates were evaluated by pulsed-field gel electrophoresis (PFGE), and the presence of mecA and mecC, as well as pvl genes, was investigated. Staphylococcus aureus (74%) was the major identified species, followed by Staphylococcus epidermidis (19%) and Staphylococcus xylosus (7%). A high incidence of resistance was detected against gentamicin (64%), erythromycin (62%), and penicillin (51%). Moreover, 33 isolates (48%) exhibited multidrug-resistant phenotypes, and the two S. epidermidis strains harbouring the mec genes exhibited the highest resistance. A marked heterogeneity among isolates, mostly for S. aureus and S. epidermidis, was revealed, clustering them into 20 and 4 genotypes, respectively. The prevalence of the same clones, isolated from different volunteers, across the three species, and the different phenotypic resistance patterns within the same PFGE type, was observed. These findings corroborate that staphylococci constitute a significant component of the ocular microbiota in healthy individuals; however, their exhibited antimicrobial resistance profiles are of concern.