. The morphological response of two strains of to ampicillin 10 μ/ml and erythromycin 10 μg/ml was studied by electronmicroscopy, MIC estimations and viable counts. In the presence of ampicillin, discrete lesions appeared in the bacterial cell walls through which cytoplasmic contents extruded and lysis occurred. A few spheroplasts, together with minicells of 0.15-μm diameter, and apparently normal cells were present after exposure to ampicillin for several hours. Conversely, erythromycin initially resulted in inhibition of division and the formation of filamentous organisms. The cell walls of these filaments were eventually disrupted with numerous small membranous vesicles appearing on their surfaces. On further erythromycin treatment, breakage of the cell wall at a restricted number of sites occurred, leading to cell lysis. In the presence of erythromycin, a few morphologically normal cells were present but no spheroplasts or minicells were observed. Viable counts demonstrated that ampicillin killed the bacteria faster than erythromycin. Regrowth did not occur in the continued presence of either antibiotic, but after their removal regrowth was observed.


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