Background: Staphylococcus aureus is a frequent cause of eye infections, with isolates exhibiting increased antimicrobial resistance to commonly prescribed antibiotics. The study aimed to analyse the genomic differences in resistance genes, mutations, sequence types, core and pan genes present in S. aureus strains causing ocular infections (keratitis and conjunctivitis) from USA and Australia.
Methods: Minimum inhibitory concentration (MIC) was used to find the antibiotic susceptibility of S. aureus ocular isolates: 9 from Australia (7 keratitis and 2 conjunctivitis) and 10 from USA (5 keratitis and 5 conjunctivitis). These also underwent whole genome sequence analysis. Resfinder, snippy and multi locus sequence type (MLST) were used to find acquired resistance genes, mutations, and sequence types. Roary and Parsnp were used to analyse core and pangenome phylogenies.
Results: Phenotypically keratitis strains from USA were more resistant than Australian strains and more commonly possessed the acquired resistance genes ant (9)-Ia (80 vs 20%), aadD (60 vs 0%) erm(A) (80 vs 20%) and tetK (40 vs 20%). Similarly, 40% of conjunctivitis USA strains possessed ant (6)-la, aph (3’)-III and erm(A), msr(A) erm(C), mph(C) whereas none of these genes were detected in the Australian conjunctivitis strains. Only strains from the USA (keratitis=16%, conjunctivitis=21%) possessed the methicillin resistance gene mecA. However, larger numbers of mutations in endogenous genes that may encode for resistance were found in keratitis strains from Australia than USA. Four clones, ST5, ST105, ST8 (keratitis USA) and CC1 (keratitis AUS) were detected. The average core and pan genes were larger in USA keratitis (2330, 3288) and conjunctivitis strains (2374, 3641) than keratitis (2272, 3118) and conjunctivitis (2326, 3069) strains from Australia.
Conclusions: Resistance to antibiotics was associated with possession of acquired resistance genes rather than mutations in genes. The clones that were found in ocular isolates were the same as those previously reported to be circulating in non-ocular S. aureus strains in USA and Australia.