Effect of different sub MIC concentrations of penicillin, vancomycin and ceftazidime on morphology and some biochemical properties of Staphylococcus aureus and Pseudomonas aeruginosa isolates

Sub inhibitory concentrations (sub-MICs) of antibiotics, although unable to kill bacteria, can modify their physiologic and biochemical integrity and may to some extent interfere with some bacterial functions. In this study the effect of penicillin, vancomycin, and ceftazidime were evaluated on two pathogenic and commonly encountered bacterial species in clinical practice. The test bacterial strains included Staphylococcus aureus ATCC 29213 and Pseudomonas aeruginosa ATCC 2783. In this study, coagulase and DNase activity, mannitol fermentation and morphologic change of S. aureus and oxidase activity, pigment production and morphologic change of P. aeruginosa were evaluated. Except for some changes in the morphology being apparent as enlarged and undivided cocci in 1/2 to 1/8 sub MIC of all above-mentioned antibiotics, S. aureus showed no change in any of its properties, like coagulase and DNase production and mannitol fermentation. In P.aeuginosa, except for some morphologic changes, i.e. elongation and filamentation, in 1/2 to 1/8 dilution of ceftazidime, no further changes were observed. Exposure of bacteria to sub MICs of antibiotic can produce some detectable morphologic changes without any alteration in other biochemical properties.