Antibacterial Activity of Florfenicol and It′s Combinations with Doxycycline In vitro

Antibacterial activity,time-killing kinetic curve of florfenicol and it′s combinations with doxycycline in vitro were studied.The results revealed that minimal inhibitory concentration(MICs) of florfenicol against nine common standard strains were 0.2-1.6 mg/L,slightly lower than or equal to those of chloramphenicol and lower 2-80 times than those of thiamphenicol,and it′s MICs against clinical isolated strains resistant to chloramphenicol and thiamphenicol were 0.8-1.6 mg/L.The time-killing kinetic curves of florfenicol against E.coli and S.aureus,were similar to those of thiamphenicol,indicating the bacterial growth was inhibited at 1 or 2 MIC and gradually killed at both 4 and 8 MIC.The PAEs induced by antibiotics against E.coli and S.aureus were determined by means of viable counting procedure after the removal of drugs.The results demonstrated that PAEs duration induced by florfenicol was much longer than that of thiamphenicol and showed concentration dependent,PAEs duration induced by florfenicol against E.coli (1.45-2.07 h) was longer than that against S.aureus(0.48-1.18 h).The sensitivity test of florfenicol against four different organisms was conducted by “Chese-board” method when it was combined with doxycycline(1∶2),and the results illustrated that the combinations were indifference against the four tested organisms and frational inhibitory concentration(FICs) of florfenicol plus doxycycline(1∶2) against E.coli,S.pullorum,S.suis was 1.5,that is to say that MICs of these combinations were lower 2 times than those of florfenicol alone,when florfenicol was combined with doxycycline(1∶2),the killing rate of combination against E.coli and S.aureus increased as compared with it alone and the time-killing curve showed synergistic at 4 MIC of florfenicol or higher.Although the PAEs of the combinations were a little longer than that of singles against the same organism,they illustrated indifference.