Detection of β-lactamase and antibiotic susceptibility of clinical isolates of Staphylococcus aureus

Abstract Staphylococcus aureus (SA) is the pathogen of greatest concern in the clinical worldwide because of its intrinsic virulence and ability to cause the diverse array of life-threatening infections. In the last two decades or so, antimicrobial resistance has evolved into one of the most formidable problems of infectious diseases. Bacteria acquire resistance to β-lactam antibiotics, most commonly due to the production of enzymes called β-lactamase. This enzyme hydrolyzes the amide bond of β-lactam antibiotics to make them ineffective. In this study, screening of clinical isolates of SA showed β-lactamase activity in 11 clinical isolates (SA-1745, SA-2071, SA-2940, SA-3151, SA-4423, SA-4620, SA-4627, SA-4693, SA-4696, SA-10760, Methicillin-resistant Staphylococcus aureus (MRSA), and one wild- type strain (SA-96) by iodometric and nitrocefin disk methods. The antibiotic susceptibility profiles of 12 strains of SA were determined for four β-lactam antibiotics viz. penicillin, ampicillin, cefoxitin and oxacillin. All the clinical isolates showed resistant to these antibiotics. Further, minimum inhibitory concentration (MIC) of four β-lactam antibiotics was also determined by micro-broth dilution assay. The MIC of penicillin, ampicillin and oxacillin against wild-type strain of SA was 0.078 µg/ml, whereas the MIC of cefoxitin against wild-type strain was recorded as 1.25 µg/ml. The MIC of these antibiotics against 11 clinical isolates of SA including methicillin-resistant Staphylococcus aureus (MRSA) ranged between 1.95 µg/ml to 1000 µg/ml. All clinical isolates were highly resistant to penicillin and ampicillin followed by cefoxitin and oxacillin. The outcomes of the present study can be useful for the screening of other natural bioactive compounds against these virulent strains.