An in vitro evaluation of the combined antimicrobial activity of ampicillin and cloxacillin against ampicillin resistant Staphylococcus aureus using checker board technique (CBT)

The Checker Board Technique (CBT) was used to evaluate the in vitroantimicrobial activities of two penicillins; ampicillin and cloxacillin, against ampicillin resistant Staphylococcus aureus. Tubes of liquid medium were arranged in columns and rows, with the concentration of one antibiotic decreasing from left to right in each row and the concentration of the complementary antibiotic decreasing from top to bottom in each column. The tubes were then inoculated with the test microorganism and incubated. Growth is shown by turbidity in the medium and no growth by a clear medium. Minimum inhibitory concentration (MIC) was determined. Most of the combinations of both drugs exhibited synergistic activities against the test strain of Staphylococcus aureus, with the greatest activity produced by the 4:6 (ampicillin/cloxacillin) combination followed by the 3:7 ratio. The combination ratios 10:0 and 0:10 of ampicillin and cloxacillin showed no activity.