Antibiotic resistance patterns of methicillin-susceptible Staphylococcus aureus and methicillin-resistant Staphylococcus aureus isolates in keratitis: Insights from an 11-year analysis.

PURPOSE: To determine the prevalence and antibiotic resistance trends in methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) keratitis over an 11-year study period.

DESIGN

Retrospective study.

METHODS

Microbiology records of patients with culture-positive Staphylococcus aureus (SA) keratitis presenting to a tertiary eye center over 11 years were retrospectively reviewed. Susceptibility data to commonly used topical antibiotics were obtained.

RESULTS

MSSA accounted for 58.4% of SA keratitis, while MRSA constituted 41.6%. No significant change in prevalence was observed for MRSA or MSSA keratitis over the study period. Both MSSA and MRSA had a higher proportion of isolates resistant to antibiotics in the fluoroquinolone group, compared to chloramphenicol, cefazolin, and vancomycin. A greater proportion of MRSA isolates were resistant to cefazolin (15.4%) compared to MSSA (0.5%). Almost all isolates were susceptible to vancomycin, except an isolated MRSA case in 2022. There was no significant increase in the proportion of MSSA and MRSA isolates resistant to antibiotics, except for the proportion of MSSA isolates resistant to chloramphenicol ( R2= 0.81), and MRSA isolates to cefazolin ( R2= 0.74).

CONCLUSION

No significant increase in the prevalence of MRSA keratitis over the study period was observed. A high proportion of resistance of MSSA isolates to commonly used topical antibiotics challenges the conventional notion that MRSA is more resistant to antibiotics than MSSA. The observed proportion of SA isolates resistant to fluoroquinolones and the increasing percentage of isolates resistant to cefazolin emphasize the need for continued surveillance to strategize empirical treatment policies. Minimal resistance to vancomycin supports its continued use as a reserve drug in cases of severe multi-drug resistant SA keratitis.