Current pattern of antibiotic resistance in Staphylococcus aureus clinical isolates and the emergence of vancomycin resistance.

OBJECTIVE: To determine the pattern of antibiotic resistance in the clinical isolates of Staphylococcus (S.) aureus, Methicillin Resistant S. aureus (MRSA) and define the possible emergence of Vancomycin resistant S. aureus (VRSA) in Karachi.

STUDY DESIGN: An observational study.

PLACE AND DURATION OF STUDY: Essa Laboratories and Department of Molecular Genetics, Ziauddin Hospital, from January to December 2009.

METHODOLOGY: Staphylococcal isolates from different clinical specimens, pus, urine, blood, high vaginal swab and other secretions received at Ziauddun laboratories and Dr.Essa laboratories were collected. The specimens were inoculated on blood agar, MacConkey agar and Chrom agar. Antibiotic susceptibility to conventional antibiotics was done by disc diffusion, and E-test. Methicillin resistance was tested by using Oxacillin and Methicillin disks and confirmed by gold standard PCR for presence of mecA gene. All MRSA strains were subjected in addition to Vancomycin screen agar test.

RESULTS: Out of the 450 S. aureus isolates 174 (38.6%) were found to be MRSA. In those isolates, high resistance was found to Cefixime (100%) Doxicycline (100%) Oxacillin (96.5%) Gentamicin, (96.3%), Timethoprim/Sulfametoxazole (95.6%) Chloramphenicol (93%) Tobramicin (81.03%), Ofloxacin (72.4%) and Ciprofloxacin (63.7%). Low resistance was found to Ceftazidine (36%), Amoxicillin/Clavulanate (32.7%), Fosfomycin (31%), Cefroxime (24%), Amikacin (17.2%) and Meropenem (13%). One isolate was found to be Vancomycin resistant (MIC 32 μg/ml). Four isolates had intermediate resistance, with two strains having MIC of 16μg/ml and two having MIC of 8μg/ml. These strains were also resistant to all the other tested antibiotics except Linezolid to which all isolates were susceptible.

CONCLUSION: Antibiotic resistance to all the conventionally used antibiotics was high in the tested isolates. All the strains were susceptible to Linezolid which is an expensive alternative with adverse side effects. Judicious use of antibiotics focused on the compliance and formation of antibiotic policy guide lines is highly recommended.