With 50 strains of methicillin-and cephem-resistant
S.aureus (MRSA) isolated from clinical materials as test organism, the MIC and bactericidal effect of β-lactam antibiotics against MRSA cultured at varying temperature were determined.The results were as follows:
1.The MIC values of various β-lactam antibiotics were measured under the following conditions: 1) inoculum size 10
6/ml, incubation temperature at 30°C, 2) inoculum size 10
6/ml, incubation temperature at 30°C. inoculum size 10
6/ml.incubation temperature at 37°C, 4) inoculum size 10
6/ml, incubation temperature at 37°C. The experiment revealed that MICs greatly varied with change of the incubation temperature, even if the inoculum size of MRSA was constant. Namely, compared with MICs at 30°C, MICs at 37°C generally shifted towards at a lower concentration range with 8-to 32-fold increase in sensitivity.
2.The bactericidal effect of each β-lactam antibiotic against MRSA was determined under the same conditions of culture as in the measurement of MICs.Some concentrations of antibiotics which showed no bactericidal effect at 30°C clearly recognized bactericidal effect at 37°C.
3.The fact that the bactericidal effect varied with change of incubation temperature was confirmed by the observation of morphological changes of MRSA through a phase-contrast microscope.When the organisms were exposed to the antibiotic, swollen cells inhibiting septal wall synthesis began to be observed at low concentrations.Whereas cell lysis was almost not detected even when the antibiotic concentration was increased considerably high in organisms cultured at 30°C. the cell lysis were observed together with swollen cells while the antibiotic concentration was still at a relatively low level in organisms cultured at 37°C.
4.The discrepancy of MICs and bactericidal effect through change of incubation temperature was observed in derivative strain eliminated PCase plasmid but retaining resistance to methicillin and cephem too.
5.It was investigated that the above phenomenon occurred because penicillin-binding protein-2' itself is temperature sensitive, so the optimal temperature for its production was at 30°C than at 37°C.
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