Antibiotic sensitivity of enterobacteriaceae by disc diffusion and rapid fermentation sensitivity test methods

A total of 400 strains of Gram negative bacilli were examined both by conventional disc diffusion (DD) and by rapid fermentation sensitivity test (RFST) methods. Various strains which were examined included Esch. coli Kleb. aerogenes, Kleb. pneumoniae, Prot. vulgaris, Prot. mirabilis, Prot. morgani, Prot. rittgeri and Providencia. The antibiotics used were streptomycin, chloramphenicol, kanamycin, gentamycin and ampicillin. The correlation between the two methods was found to vary from 75 to 98 per cent. By RFST method the results are obtained within six hours. The limitations of this method are (i) it cannot be used for Pseudomonas aeruginosa as it does not ferment glucose, (ii) it cannot be used for antibiotics active only at acidic pH like tetracycline and (iii) partial or intermediate sensitivity cannot be tested by this method. On the other hand, it is a simple, inexpensive, rapid and reliable method feasible in small laboratories also. The results are not affected by the size of inoculum or quality of medium (agar) used. Thus, it is very helpful especially when immediate antibiotic sensitivity is required.