Comparison of disk-diffusion and E-test methods for in-vitro susceptibility of Streptococcus pneumoniae isolates to oxacillin and ceftriaxone

Susceptibility of Streptococcus pneumoniae (pneumococci) to oxacillin and ceftriaxone was compared using disk diffusion and E-test methods. A total of 206 children attending Gertrude’s Children’s Hospital (GCH) were recruited. Sterile Copan Flocked Swabs were used to obtain nasopharyngeal swabs. Samples were inoculated on gentamicin blood agar and initial identification was done on the basis of colony morphology. Optochin test was performed to definitively identify the isolates as pneumococci. Antibiotic testing was done using disk diffusion and E-test methods on Muller Hinton agar enriched with 5% defibrinated sheep blood. A total of 42 (20%) isolates were recovered from the samples. Based on disk diffusion method, 74 and 40% of the isolates were resistant to oxacillin and ceftriaxone while; on the basis of E-test, 45 and 14% of the isolates were resistant to oxacillin and ceftriaxone, respectively. About 40% (n=17) of isolates that had zone diameters ≥ 20 mm which is considered susceptible to oxacillin by disk diffusion, had MICs ≤0.06 µg/ml correspondingly susceptible by E-test. Most isolates deemed susceptible to (≤0.5 µg/ml) ceftriaxone by E-test (72%, n=30) also exhibited susceptibility (≥27mm) to the antimicrobial agent by disk diffusion. E-test presents more sensitive results compared to disk diffusion. Isolates that exhibit resistance to penicillin and ceftriaxone by disk diffusion should be confirmed by E-test before being reported as resistant.