Susceptibility of Hospital Staphylococcus aureus Isolates against Cephalosporins using Manual E-test

Abstract

Thirty clinical isolates of Staphylococcus aureus were collected from different hospitals in Khartoum State. In vitro studies were carried out by susceptibility disk diffusion method against cephalosporins, (cefazolin example of the 1st -generation, cefuroxime example for the 2nd, and cefotaxime example of 3rd –generation). The E-test was then used to perform the susceptibility tests on the isolated strains for the same antibiotics, which was performed manually and evaluated with standard organisms. Minimum inhibitory concentration (MIC) was determined for each isolated strain. The E-test results were compared with the susceptibility disk method. Comparison of the results was made after converting the data to qualitative categories (susceptible, intermediate, and resistant). Disk diffusion method results, showed that 17% of isolated strains were resistant, and 83% were sensitive to each cefazolin, cefuroxime and cefotaxime. E-test showed the same results with cefazolin, but it differs with cefuroxime since 73%,7% and 20% were susceptible, intermediate, and resistant respectively, and 76%,7%and 17%, were susceptible, intermediate, and resistant to cefotaxime respectively. The analysis of the results proved that there was a significant difference between the three generations in zone inhibition and MIC values (P value <0.05). The average of MIC values for each antibiotics cefazolin, cefuroxime, and cefotaxime were (0.9-3.5-4.7) μg/ml, respectively. Cefazolin was the most potent one showing significant antimicrobial activity against isolated S. aureus strains. The average of zone inhibition values for each antibiotics cefazolin, cefuroxime, and cefotaxime were (35.1- 32.1 -30) mm, respectively. Also the significant inversely weak relationship was demonstrated between zone diameter and MIC for each antibiotics (r= -0.54). The technique of applying several disks with different antibiotic concentrations at the same time demonstrated clearly the minimum inhibitory concentration of the antibiotic. E. test also proved its significance over the single disk method.