Forty-six isolates of Enterobacteriaceae screened by the resistant to 50 /ml nalidixic acid(NA) were examined by minimal inhibitory concentration(MIC) for in vitro susceptibility to the quinolone derivatives, norfloxacin(Nf), enoxacin(En),
Abott-56619(A9), Abott-56620(A0). ofloxacin(Of), ciprofloxacin(Ci), oxolinic acid(Ox), rosoxacin(Ro) and pefloxacin(Pe).
Ci was the most active agents against all Enterobacteriaceae tested, followed by Nf, AO, A9, Pe and En.
The MICs required to inhibit at least 90% of the strains(MIC90) ranged from 0.125 to 8 g/ml for Ci and from 0.25 to 8 g/ml for Cf. Ci was about four times more active than Nf. Ox, old analog of NA, and Ro were lesser active than other
fluoroquinolones, Cross resistance was observed between NA and Ox.
The MICs of quinolone derivatives correlated with the MIC levels of NA. According to the MIC levels of NA, MICs of all quinolone derivatives were changed, The MICs of all quinolone derivatives were raised by stepwise increases of
MIC of
NA. Although stepwise increases in MIC of fluoroquinolone were seen with Enterobacteriaceae tested, significant increases of MIC did not showed.
Compared to the isolated strains, the selected variants strains with resistant to NA as a whole were more susceptible to all quinolone derivatives tested. The MICs of selected variants were raised by stepwise increases of MIC of
NA.
NA
sensitive strains which were selected were at least two-fold more susceptible to En, A0, A9, Pe, Ro, Ox and Of than were the NA resistant strains.
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