STUDIES ON CHARACTERIZATION AND DEVELOPMENT OF RESISTANCE IN INDIGENOUS STRAINS OF STAPHYLOCOCCUS AUREUS AGAINST VARIOUS ANTIBIOTICS

Staphylococcus aureus has an exceptional ability to cause disease in human beings. The use of antibiotic has not only led to treatment of staphylococcal infections but also the emergence of antibiotic resistant strains. The development of multi-drug resistant Staphylococcus aureus is a serious global problem due to difficulty in treatment of multi-drug resistant strains.\ud \ud A prospective study on various strains, isolated from hospitalized and out door patients during January 1998 to December 1999 from Pakistan Institute of Medical Sciences Islamabad, Pakistan was conducted, to ascertain the prevalence of Staphylococcus aureus and pattern of antibiotic resistance. Among 12532 samples received in the pathology laboratory, 5069 samples gave bacterial growth and among these 2580 (51%) samples were Gram-positive cocci. Out of these Gram-positive cocci 1688 (65%) were Staphylococcus aureus.\ud \ud Among these Staphylococcus aureus, 56% were resistant to penicillin group, 27% were resistant to cephalosporin group, 22% were resistant to aminoglycoside group, 15% were resistant to quinolone group and 31 % were resistant to other antibiotics (cotrimaxazole, erythromycin, aztreonam, vancomycin, nitrofurantoin and meropenam).\ud \ud Antibiograms of Gram-positive cocci were determined against various antibiotics by disc diffusion method. The rate of resistance to most of the antibiotics, ampicillin, piperacillin, carbenicillin, penicillin, cephradine, cefotaxime, ceftriazone, amikacin, ofloxacin, pefloxacin, ciprofloxacin, cotrimexazole (septran), gentamicin, meropenem, ceftazidime, erythromycin, tobramycin, oxacillin, enoxacin, ceclor, was higher when tested against the isolates collected from pus as compared to those from blood and urine. Antibiotic resistant strains were prevalent in pus samples among all type of clinical samples.\ud \ud More than 90% of these Gram-positive isolates from blood, urine and pus were resistant to penicillin while 70%, 80% and 90% of the samples isolated from blood, urine and pus respectively were resistant to cotrimaxazole. These isolates were resistant to cephradine 23%, 36%, and 51 % in blood, urine and pus respectively. Where as bactericidal activity of vancomicin was found better than other antibiotics prescribed for these isolates.\ud \ud Out of total 2580 Gram-positive cocci 991 (38%) were multi-drug resistant. These strains were more prevalent in isolates from pus as compared to blood and urine isolates. Moreover, during two-year of study, MDR isolates from blood were from 29%, urine 45%, and from pus were 49%.\ud \ud In the prevalence study of 2580 Gram-positive cocci methicillin resistance and oxacillin resistant strains were 37%. Whereas in pus samples oxacillin resistant 35.56% in 1999 and 35.55% MRSA in1999. Blood samples had same pattern of resistance against methicillin and oxacillin (17% and 16%) in two years. Urine isolates had 10% oxacillin and 19% methicillin, resistant strains. Among 155 strains 147 were selected and found to be MDR.\ud \ud These strains showed resistance rate against ampicillin (92%), cephradine (60%), ciprofloxacin (59%), and gentamicin (58%). However intermediate resistance was found in case of van comic in (38%). These strains mostly resistant to oxacillin as well.\ud \ud The antibiotic sensitivity patterns of these Staphylococcus aureus were assessed against five groups of antibiotics. In the prevalence study of antibiotics given to hospitalized and non-hospitalized patients, among the five groups, the maximum prescribed groups were cephalosporin and penicillin. In the maximum prescribed groups the resistance rate was observed in penicillin group 56% and cephalosporin group showed 27% resistance. While the aminoglycoside and quinolone were 22% and 15% respectively during 1998-1999.\ud \ud MIC of the 155 clinical isolates of Staphylococcus aureus were determined by using break points of National Committee for Clinical Laboratory Standards (NCCLS). The MIC90 of ampicillin for the majority of clinical isolates were 8 to 2048 µgm/mL with ampicillin standard powder and standacillin. However, some isolates had inhibitory range of 8 to 1 024 µgm/mL with penbritin, ampicillin sodium and ampicillin, showing increased resistance in Staphylococcus aureus against these brands. Among all the 155 isolates 92% of the characterized and identified isolates were resistant to ampicillin.\ud \ud The activity of cephradine against Staphylococcus aureus showed a range of 8 to 2048 µgm/mL with standard powder and velosef. The cefatil, monocef and cefrinex had MIC90 range of 8 to 1024 µgm/mL. Cephrandine and its four brands were ineffective against 96% of the screened isolates. The resistance range of gentamicin standard powder and four brands was µgm/mL to 2048 µgm/mL. These 98% isolates were resistant to gentamicin standard powder and its four brands. The in-vitro inhibitory activity of ciprofloxacin standard powder and ciproxin ranged 1 to 128 µgm/mL. Novidat and vitoc, showed MIC90 resistance rates 1 to 64 µgm/mL. The 87% of tested isolates were resistant to ciprofloxacin standard powder, 93%, 91%, and 94% with its three brands (ciproxin, novidat and viloc) respectively. The resistance rate of vancomicin ranged 1 to 8 µgm/mL for both the brands. The 40 to 46% isolates were found resistant to vancocin and vancomicin. This indicates the development of resistance in Staphylococcus aureus against vancomycin. In view of these findings most of the isolates were resistant to the five groups of antibiotics and were multi-drug-resistant, only very few were in sensitive range. The p-values of MIC90 levels of ampicillin, cephradine, gentamicin, ciprofloxacin and vancomicin with their various brands did not vary significantly within each group against most of the isolates.\ud \ud In-vitro study of minimum bactericidal concentrations (MBC) of five groups of antibiotics against various isolates of Staphylococcus aureus revealed that 2 to 3 fold two-fold dilution higher value than in MIC of each group of antibiotic. Only MBC of vancomycin was in the range of 1 to 2 two-fold dilutions higher than their MICs. The results suggested that the determination of MBC of these resistant strains was quite essential in order to calculate the dose of antibiotic for patients affected with Staphylococcal infections.\ud \ud These data provide a prospective study on the increasing resistance in indigenous isolates of Staphylococcus aureus against various antibiotics and will form a basis for understanding and solution of this problem. In addition to these promptly preventive rather than curative measures should be adopted, such as better hygiene, as well as better hospital and post-operative care and in administration