Date Published:
2024 Mar 06
Abstract:
Bacterial keratitis is a vision-threatening infection mainly caused by Gram-positive bacteria (GPB). Antimicrobial therapy is commonly empirical using broad-spectrum agents with efficacy increasingly compromised by the emergence of antimicrobial resistance. We used a combination of phenotypic tests and genome sequencing to identify the predominant lineages of GPB causing keratitis and to characterize their antimicrobial resistance patterns. A total of 161 isolates, including
Staphylococcus aureus (
n = 86), coagulase-negative staphylococci (CoNS;
n = 34),
Streptococcus spp. (
n = 34), and
Enterococcus faecalis (
n = 7), were included. The population of
S. aureus isolates consisted mainly of clonal complex 5 (CC5) (30.2%). Similarly, the population of
Staphylococcus epidermidis was homogenous with most of them belonging to CC2 (78.3%). Conversely, the genetic population of
Streptococcus pneumoniae was highly diverse. Resistance to first-line antibiotics was common among staphylococci, especially among CC5
S. aureus. Methicillin-resistant
S. aureus was commonly resistant to fluoroquinolones and azithromycin (78.6%) and tobramycin (57%). One-third of the CoNS were resistant to fluoroquinolones and 53% to azithromycin. Macrolide resistance was commonly caused by
erm genes in
S. aureus,
mphC and
msrA in CoNS, and
mefA and msr(D) in streptococci. Aminoglycoside resistance in staphylococci was mainly associated with genes commonly found in mobile genetic elements and that encode for nucleotidyltransferases like
ant(4')-Ib and
ant(9)-Ia. Fluroquinolone-resistant staphylococci carried from 1 to 4 quinolone resistance-determining region mutations, mainly in the
gyrA and
parC genes. We found that GPB causing keratitis are associated with strains commonly resistant to first-line topical therapies, especially staphylococcal isolates that are frequently multidrug-resistant and associated with major hospital-adapted epidemic lineages.
