Distribution of genes encoding aminoglycoside modifying enzymes amongst methicillin resistant and methicillin sensitive Staphylococcus aureus isolates from Nigerian hospitals

Staphylococcus aureus has long been recognized as one of the major pathogenic organism of human which is responsible for a variety of infections including life threatening infections such as pneumonia. Aminoglycosides antibiotics play an important role in the therapy of staphylococcal infections despite the increase resistant to these drugs. This study aimed at determining the prevalence of genes encoding aminoglycoside modifying enzymes (AMEs) in clinical isolates of S. aureus from teaching hospitals in Nigeria. In this study, 86 culture collections of S. aureus obtained from 4 teaching hospitals in Nigeria were screened for the presence of genes encoding aminoglycoside resistant genes: (aac(6’)/aph(2”), aph(3’)-IIIa, ant(4’)-Ia) and mecA gene by polymerase chain reaction (PCR). Prior to this, antibiotic susceptibility testing was carried out on all the S. aureus strains against several antibiotics including gentamicin and cefoxitin. The prevalence of mecA gene was 44.2% (38 out of 86 S. aureus). Forty eight (55.8%) of the 86 S. aureus identified as gentamicin resistant phenotypically contained at least one of the 2 gentamicin resistant genes: aac(6’)/aph(2”) or aph(3’)-IIIa while ant(4’)-Ia gene was not detected in any of the isolates. The MIC50 and MIC90 for gentamicin resistant strains were 32 and >256 µg/ml, respectively, while the MIC50 and MIC90 for gentamicin sensitive strains were 1 and 8 µg/ml, respectively. The prevalence of gentamicin resistant genes was 29.3% for aac and 19.0% for aph. No ant gene was detected among the gentamicin resistant strains. SCCmec typing for all the gentamicin resistant methicillin resistant S. aureus strains showed diversity of the isolates with 4 (17.4%) out of the 23 were SCCmec II; 4 (17.4%) out of 23 were SCCmec III and 9 (39.1) out of 23 were SCCmec V while the remaining 8 (34.8%) were non-typeable. Using this scheme gentamicin-methicillin resistant, S. aureus strains were found to be widely distributed in all the four teaching hospitals studied. The study found an association between genes encoding AMEs and mecA on the genome of S. aureus isolates from Nigerian hospitals especially the gentamicin-methicillin resistant S. aureus, hence the need for establishment of effective infection control measures and antibiotic policies that will reduce the emergence of gentamicin methicillin resistant strain. Key words: aac(6’)/aph(2”), aph(3’)-IIIa, ant(4’)-Ia genes, gentamicin methicillin resistant Staphylococcus aureus, Nigeria.