Abstract
We have studied the nature of ciprofloxacin resistance in methicillin sensitive and resistant Staphylococcus aureus among patients in a tertiary care hospital in Bengaluru, South India. All the isolates were highly resistant to ciprofloxacin. Molecular characterization of these samples performed using Staphylococcal Cassette Chromosome typing and multilocus sequence typing showed that 37.5% of total isolates and 59% of MRSA were sequence type (ST)772 and the rest were other STs. This indicates high prevalence of CA-MRSA in this tertiary care hospital serving the Indian community. Mutations responsible for ciprofloxacin resistance among these isolates in DNA gyrase (gyrA and gyrB) and topoisomerase IV (grlA and grlB) were analyzed by PCR amplification of specific fragments and sequencing. We found that for ST772 and five other STs present in this collection, single mutation in the gyrA gene, Ser-84→Leu, was sufficient for the high resistance. In vitro generation of ciprofloxacin resistance in two sensitive ST772 isolates by exposure to increasing antibiotic concentrations also resulted in the same single mutation of gyrA. The factors responsible for high ciprofloxacin resistance are varied and are dependent on the genetic background of the isolates and the environment. This is the first report on the mechanism of ciprofloxacin resistance among the most prevalent Indian CA-MRSA.
Highlights
Staphylococcus aureus is an important pathogen in the community and hospital, 30% of the population is colonized with the organism asymptomatically
This work demonstrates that a high percentage of patients in this tertiary care hospital carry CA-MRSA which is highly resistant to ciprofloxacin and a single mutation in gyrA gene is responsible for high resistance in ST772
Antibiotic susceptibility testing was performed for erythromycin, gentamicin, amikacin, tetracycline, chloramphenicol and ciprofloxacin by Kirby-Bauer disc diffusion according to the guidelines recommended by the Clinical Laboratory and Standards Institute on MuellerHinton agar plates (HIMEDIA, Mumbai, India) at 37 ̊C
Summary
Staphylococcus aureus is an important pathogen in the community and hospital, 30% of the population is colonized with the organism asymptomatically. Increasing use of systemic antibiotics in the hospitals has resulted in multi drug resistant S. aureus due to selective pressure. DNA gyrase subunits are common sites of resistant mutations in Gram negative organisms, while topoisomerase IV is most critical in case of Gram positive organisms there are few exceptions [4]. This study involves ciprofloxacin resistant MRSA and MSSA isolates, their genetic backgrounds and mutations important in causing the resistance as we have analyzed the clonal complexes present among Indian S. aureus isolates in an earlier study [11]. This work demonstrates that a high percentage of patients in this tertiary care hospital carry CA-MRSA which is highly resistant to ciprofloxacin and a single mutation in gyrA gene is responsible for high resistance in ST772. Mutations among S. aureus isolates from other genetic back grounds are examined
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