The psychrotrophic bacterium, strain MFO, is more sensitive to the β-lactam mezlocillin at a low growth temperature (i.e. 8°C) than at a higher growth temperature (28°C). An early effect of this antibiotic at all temperatures is bacterial filamentation, but this occurs later at 8°C than at 28 °C, which suggests a lower permeability of the cell envelopes to mezlocillin at low growth temperature. β-Lactamase production is later induced by mezlocillin, but the level of this induction also depends on the growth temperature, the overall induction being much less efficient at 8 °C. It is hypothesized that the periplasmic concentration of the drug might be too low at 8 °C to allow efficient β-lactamase induction; this hypothesis was confirmed by the demonstration that β-lactamase production is drastically enhanced in cells cultivated at 8 °C permeabilized for 10 min by Na-EDTA. In addition, induction kinetic curves display a marked dependence upon growth temperature. A rapid saturation was evident when mezlocillin concentrations were increased at 8 °C; this was not seen at 28 T°C at up to 1000 μg mezlocillin ml. The results are discussed in terms of two different routes of drug permeation, depending on the growth temperature.


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