Comparison of three different methods to detect the production of ?-lactamase enzyme by staphylococci

Abstract

Production ?-lactamase enzyme is the most common and important mode of exhibiting resistance to ?-lactam antibiotics. Manifestation of this enzyme is difficult to demonstrate in routine antibiotic sensitivity testing. Sensitivity to penicillin is not sufficient to indicate whether this antibiotic should be used clinically and main factor compromising the clinical efficacy of these drugs is the production of ?-lactamase enzyme. Only feasible method for determination of ?-lactam resistance is to demonstrate the physical presence of enzyme. Three different methods (acidometric method, iodometric method and chromogenic cephalosporin method) are available for this. The objective of present study is to compare these 3 method to detect the production of ?-lactamase enzyme in staphylococci isolated from various clinical samples. Susceptibility to penicillin, oxacillin and vancomycin was also determined. Out of total 150 isolates, 114(76%) shows ?-lactamase production by chromogenic cephalosporin method, 110(73.3%) by iodometric method and 108(72%) by acidometric method. Penicillin and oxacillin resistance was 74.6% and 53.3% respectively. All isolates are sensitive to vancomycin. Out of three different methods used, chromogenic cephalosporin method is more sensitive (76%) than iodometric method (73.3%) and acidometric method (72%). Results also correlates with penicillin sensitivity in ABST except 2 isolates which shows ?-lactamase production by chromogenic cephalosporin method but remains penicillin sensitive in disc diffusion method. In case of agar diffusion tests, there is often insufficient ?-lactamase production from the test organism before an inhibitory concentration of antibiotic has diffused from the disc. Only ABST can lead to therapeutic failure sometimes.