Correlation by scanning electron microscopy of in vitro and in vivo effects of amoxicillin and ampicillin on the morphology of Escherichia coli.

The effect of in vitro incubation with subinhibitory and inhibitory doses of amoxicillin or ampicillin on the morphology of E. coli 257 was evaluated by scanning electron microscopy (SEM). No effect, with either penicillin was seen at 1/16 X MIC (0.39 microgram/ml). Filament formation was the major effect seen with both agents at 1/8, 1/4 and 1/2 X MIC but only with ampicillin at the MIC (6.25 microgram/ml). Cells incubated with amoxicillin at the MIC were primarily short rods with bulbous swellings. This morphological difference between the two antibiotics was dose-related since ampicillin, at 2 X MIC, also caused the formation of short rods with bulbous swellings. SEM examination of omentum from amoxicillin and ampicillin treated mice 2 h after infection with E. coli 257 indicated a correlation of morphological effects seen in vitro and in vivo with the respective antibiotics. For example, following a single subcutaneous treatment with amoxicillin and ampicillin at 250 mg/kg, the plasma concentrations 2 h later (the time of infection) were 8.2 and 7.7 microgram/ml respectively but short rods with bulbous swellings were seen in amoxicillin-treated mice whereas filaments were found in the ampicillin-treated mice.