Recent trend of aminoglycoside resistance among Staphylococcus aureus isolates in tertiary care hospital

Aminoglycosides still play an important role in antistaphylococcal therapies, although emerging resistance amongst staphylococci is widespread. The objective of the present study was to know the percentage of aminoglycoside resistance among Staphylococcus aureus isolates. A total of 250 isolates of S. aureus were studied from different clinical specimens like blood, pus, wound swabs, sputum, ear swabs and body fluids. All the isolates were tested for their susceptibility to four aminoglycosides namely gentamicin, amikacin, tobramycin and netilmicin by Kirby Bauer disc diffusion method using criteria of standard zone of inhibition. Methicillin resistant S. aureus (MRSA) detection was done by cefoxitin disc diffusion method. Out of the 250 S. aureus isolates, 66 (26.4%) isolates demonstrated resistance to at least one of the four aminoglycosides tested. Four isolates were resistant to all the four aminoglycosides tested. All these four were MRSA. The most active antimicrobial agent against S. aureus was found to be netilmicin followed by amikacin. Continued surveillance at both the genotypic and phenotypic levels as well as adherence to well-designed antibiotic and infection control policies are necessary to understand and limit further rise of resistant isolates.


INTRODUCTION
Staphylococcus aureus is a major cause of hospital and community-acquired infections, and can result in serious consequences.The importance of S. aureus as a human pathogen, apart from its ability to cause a diverse range of life-threatening infections, is its extraordinary potential to develop antimicrobial resistance (Lowy, 2003).
One of the classes of antibiotics playing an important role in the therapy of serious staphylococcal infections is aminoglycosides.Although aminoglycosides are predominantly used for the treatment of Gram-negative infections, they are also known to have antistaphylococcal activity (Jana and Deb, 2006).This becomes an important consideration when patients are treated empirically for suspected sepsis with an aminoglycoside and  ureidopenicillin, other penicillins, or cephalosporins susceptible to staphylococcal beta-lactamases.In addition, the emergence of methicillin-resistant S. aureus is increasing the clinical importance of the antistaphylococcal activity of aminoglycosides (Hammerberg et al., 1986).Increased resistance to these drugs have been reported from many countries (Hauschild et al., 2008) and little data is available on their resistance pattern in staphylococci from this part of India.So this study was undertaken to gain some insight into the susceptibility pattern of aminoglycosides among S. aureus in our tertiary care hospital.

MATERIALS AND METHODS
This study was performed between November 2010 and April 2012 in the Department of Microbiology at our tertiary care hospital, India.A total of 250 isolates of S. aureus were isolated from different clinical specimens like blood, pus, wound swabs, sputum, ear swabs and body fluids.Only one isolate per patient was included in the study.All the isolates were tested for their susceptibility to gentamicin (10 μg), amikacin (30 μg), tobramycin (10 μg), netilmicin (30 μg) by Kirby Bauer disc diffusion method using criteria of standard zone of inhibition (CLSI, 2010).S. aureus ATCC 29213 was used as quality control strain for in vitro susceptibility testing.MRSA detection was done by cefoxitin disc diffusion method.

RESULTS
Of the 250 S. aureus isolates included in this study, 66 (26.4%) were resistant to at least one of the four aminoglycosides tested (Table 1).04 (1.6%) S. aureus isolates were resistant to all the four aminoglycosides tested.All these four were MRSA.The most active antimicrobial agent against S. aureus was netilmicin.21 (19.6%)MRSA isolates were resistant to gentamicin, tobramycin and amikacin and showed susceptibility only to netilmicin.35 (32.7%)MRSA isolates showed resistance to gentamicin and tobramycin.Of the 107 MRSA isolates, 61 (57.0%) were resistant to at least one of the four aminoglycosides tested.

DISCUSSION
Aminoglycosides are potent bactericidal agents, inhibiting protein synthesis by binding to the 30S ribosomal subunit.Gentamicin and tobramycin are the most active against staphylococci and are often used in combination with either a β-lactam or a glycopeptide, especially in the treatment of staphylococcal endocarditis, as these drugs act synergically (Schmitz et al., 1999).Bacterial resistance to aminoglycosides is widely recognized as a serious health threat.The major mechanism of aminoglycoside resistance in staphylococci is drug inactivation by cellular aminoglycoside-modifying enzymes such as aminoglycoside acetyltransferases (AAC), aminoglycoside adenyltransferases (ANT) or aminoglycoside phosphotransferases (APH) activity (Jana and Deb, 2006).
Since the first report on gentamicin resistance among staphylococci, strains resistant to both methicillin and gentamicin have been the cause of serious infections and extensive outbreaks.Many of these organisms are often resistant to a number of other antibiotics (Anupurba et al., 2003).In 1969, the first clinical gentamicin-resistant MRSA (GR-MRSA) strain was isolated.By 1980s GR-MRSA had become epidemic in Australia, the United States and Europe (Cafferkey et al., 1983).In fact many strains of MRSA exhibit resistance to both β-lactams and aminoglycosides (Anupurba et al., 2003;Tiwari et al., 2008).
In the present study, 66 (26.4%) strains of S. aureus and 61 (57.0%)MRSA were resistant to gentamicin which correlates with the study from North India (Tiwari et al., 2008) which reported 55.8% gentamicin resistance among MRSA.Aminoglycoside resistance reported in S. aureus isolates from different countries, and especially gentamicin resistance, is of clinical importance because it can compromise the therapeutic effectiveness of these antibacterial agents (Hauschild et al., 2008).
In this study, 62 (24.8%) S .aureusisolates were resistant to tobramycin which correlates well with the report from Europe (Schmitz et al., 1999) which reported 29.0 % resistance among S. aureus to tobramycin.A study from South Maharashtra region of India reported that more than 90% S. aureus isolates were resistant to gentamicin and tobramycin (Kandle et al., 2003).
In this study, 04 (1.6%) and 26(10.4%)S. aureus isolates were resistant to netilmicin and amikacin respecttively.Another study from Poland reported 24.4% S. aureus strains were resistant to amikacin and that no strain of S. aureus was found resistant to netilmicin (Hauschild et al., 2008).Of the 250 S. aureus isolates, 66 (26.4%) were resistant to at least one of the four aminoglycosides tested which is somewhat lower than the finding of the authors (Hauschild et al., 2008) who reported that 38.1% of their isolates of S. aureus were resistant to one of the aminoglycosides tested.
In our study, members of aminoglycoside group were tested separately because in vitro testing of one member, however, may not predict in vitro result for other members in the group.Further genotypic studies are needed for the isolates demonstrating phenotypic resistance to any of the member of this class.It will help in the identification of the known or any new gene encoding aminoglycoside modifying enzyme in staphylococcal isolates which could account for the phenotype.
In summary, among the aminoglycosides tested, maximum susceptibility was found for netilmicin (98.4%) followed by amikacin (89.6%).Highest resistance was observed for gentamicin (26.4%).Continued surveillance at both the genotypic and phenotypic levels as well as adherence to well-designed antibiotic and infection control policies are necessary to understand and limit further rise of resistant isolates.

Table 1 .
Phenotypic resistance patterns of S. aureus (MRSA and MSSA) for four aminoglycosides.
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