Pattern of resistance to vancomycin and other antimicrobial agents in staphylococcal isolates in a university teaching hospital

BO Olayinka, AT Olayinka, JA Onaolapo, PF Olurinola



Multidrug resistance has been reported in clinical isolates of both coagulase-negative staphylococci (CoNS) and Staphylococcus aureus that are most often resistant to oxacillin/methicillin. Vancomycin, a glycopeptide is the drug of choice for infections caused by such multidrug resistant strains. This study determined the pattern of resistance to vancomycin and other antimicrobial agents in staphylococcal isolates from a University Teaching Hospital. Staphylococcal isolates from clinical specimens submitted to the diagnostic medical microbiology laboratory of the Ahmadu Bello University Teaching Hospital, Zaria (over a three-month period) were characterized using standard microbiological procedures and their susceptibility to vancomycin and other commonly used antimicrobial agents determined by Kirby-Bauer-NCCLS modified disc diffusion technique. A total of 56 of the 97 (57.7%) staphylococcal isolates characterized were resistant to vancomycin 30μg, showing a zone of inhibition less than 15mm. Most of these isolates were from urine (27.3%), wound (21.8%) and pleural aspirate (12.8%). The 56 staphylococcal isolates were made up of 75% (41/56) Staphylococcus aureus and 25% (14/56) coagulase-negative staphylococci. Majority of the isolates, 60.7% (34/56) produced β-lactamase enzyme. Resistance pattern to other antimicrobial agents was benzyl penicillin G (92.9%); tetracycline (69.6%); cefuroxime (60%); chloramphenicol (54.5%); oxacillin (49.1%); erythromycin (35.7%); gentamicin (25%) and ciprofloxacin (16.1%). Analysis of the multiple antibiotic resistance index (MARI) showed that majority (91.1%) were resistant to 3 to 7 of the other antimicrobial agents tested. No isolate was resistant to all the tested antimicrobial agents. A very high proportion of the staphylococcal isolates were resistant to vancomycin, a glycopeptide that is not commonly used in this environment. Ciprofloxacin and gentamicin appear to be the only agents that will be effective in treating infections by these isolates. The high proportion of isolates with MARI of 0.3 and above, suggest that the isolates originated from an environment where antibiotics are often used. There is need for constant, on-going antimicrobial resistance surveillance in important and commonly isolated clinically significant pathogens to form the basis for developing and implementing measures that will reduce the burden of antimicrobial resistance.

Key Words: vancomycin, methicillin resistance, Staphylococcus aureus, coagulase-negative staphylococci, antimicrobial agents

Afr. J. Clin. Exper. Microbiol. Vol.6(1) 2005: 21-27