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Abstract
S. aureus is a versatile and dangerous pathogen. The importance of S. aureus as a human pathogen, apart from its ability to cause a diverse range of life threatening infections, is its extra-ordinary potential to develop antimicrobial resistance.
Although emerging resistance amongst S. aureus is widespread, aminoglycosides still play an important role in anti-staphylococcal therapies.
The most common mechanism of resistance to aminoglycosides is enzymatic modification by aminoglycoside-modifying enzymes (AMEs); three families of enzymes are present acetyltransferases, adenyltransferases and phosphotransferases.
Large and diverse populations of AMEs exist and act at virtually every susceptible position on aminoglycoside structures. By testing the susceptibility of isolates against a range of clinically available and experimental aminoglycosides, a pattern of resistance emerges that is unique to a specific enzyme. This method has been referred as interpretative reading which aims to analyze the overall susceptibility pattern and so to predict the underlying mechanisms.
The aim of this work was to study the different phenotypic patterns of aminoglycosides resistant S. aureus in relation to the genotypic patterns detected by a multiplex PCR assay targeting the genes encoding AMEs.
This work was carried out in Microbiology and Immunology Department, Faculty of Medicine, Zagazig University and Zagazig University hospitals during the period from August, 2008 to June, 2009. In this work the 81 S. aureus isolates obtained from 170 clinical specimens were identified by Gram stain, catalase, tube coagulase and DNase test. This was followed by testing susceptibility of S. aureus isolates to aminoglycosides, oxacillin and other antibiotics using disc diffusion method.
All S. aureus isolates were subjected to a multiplex PCR assay targeting aac(6’ )-Ie+ aph(2” ), ant(4’ )-Ia and aph(3 )-IIIa genes encoding the AMEs.
The results of this study showed that S. aureus represents 81 (47.6%) from 170 collected samples. Thirty one S. aureus isolates showed resistance to at least one of the aminoglycosides tested, the highest resistance was to kanamycin where all isolates were resistant to it while the lowest resistance was to netilmicin.
The results of this study showed statistically significant agreement between aminoglycoside and methicillin resistance. There was also statistically high significant agreement (P<0.001) between methicillin resistance and the distribution of genes encoding AMEs.
As regard the results of the multiplex PCR assay, the aac(6’ )-Ie+ aph(2” ) gene encoding the bifunctional enzyme AAC(6’ )-Ie + APH(2” ) was the most common, then the ant(4′)-Ia gene encoding the ANT(4’ )-Ia enzyme and lastly the aph(3′)-IIIa gene encoding the APH(3’ )-IIIa enzyme. However, no gene could be detected in 8 isolates demonstrating phenotypic resistance to any one of aminoglycosides tested suggesting the presence of variant gene that cannot be detected within the primer or that new aminoglycoside resistance genes are circulating within the S. aureus population.
The results of this study showed that there was statistically high significant agreement (P<0.001) between aminoglycoside resistance and multiplex PCR results for aac(6’ )-Ie+ aph(2” ), aac(6’ )-Ie+ aph(2” ), ant(4’ )-Ia and aph(3 )-IIIa genes.