Susceptibilities of Escherichia coli, Salmonella and Staphylococcus aureus isolated from animals to ofloxacin and commonly used antimicrobial agents

Susceptibilities of Escherichia coli, Salmonella and Staphylococcus aureus isolated from chickens, pigs and cattle to ofloxacin (OFLX) and commonly used antimicrobial agents were investigated. 1. E. coli (28 isolates) demonstrated the highest level of susceptibility of OFLX (MIC 0.10-0.39 micrograms/ml for all the isolates) among all the test drugs. Commonly used antimicrobial agents to which these isolates responded with relatively high susceptibilities (MIC50 0.78-6.25 micrograms/ml) included oxolinic acid (OXA), ampicillin (ABPC), kanamycin (KM) and chloramphenicol (CP) with their MIC50 values in the increasing order as above. Drugs to which these isolates responded with moderate to weak susceptibilities (MIC50 25 approximately greater than 800 micrograms/ml) were doxycycline (DOXY), streptomycin (SM), spectinomycin (SPCM) and sulfadimethoxine (SDMX) in the increasing order of MIC50. E. coli isolates with resistances to all the test drugs other than OFLX and OXA amounted to 7.1-57.1% of the isolates examined and 20 isolates (71.4%) in total. 2. Susceptibilities to OFLX and 4 existing pyridonecarboxylic acid derivatives of E. coli (48 samples) isolated recently from diarrheal pigs were compared. When evaluated in terms of MIC50, the values of OFLX and norfloxacin were both 0.10 micrograms/ml. The values increased by differences of 0.39-3.13 micrograms/ml in an order of OXA, pipemidic acid and nalidixic acid. 3. Salmonella (28 isolates) demonstrated the highest level of susceptibility to OFLX (MIC 0.20-0.39 micrograms/ml for all the isolates) among all the test drugs. The drugs to which these isolates responded with relatively high to moderate susceptibilities (MIC50 0.78-12.5 micrograms/ml) included ABPC, OXA, DOXY, KM, CP and SM with their MIC50 values increasing in this order. The drugs to which the isolates responded with low susceptibilities (MIC50 above 100 micrograms/ml) were SPCM and SDMX. Of all the 28 Salmonella isolates tested, 7.1-32.1% were resistant to all the test drugs other than OFLX and OXA. These resistant isolates amounted to a total of 12 isolates (42.9%). 4. S. aureus (28 isolates) were highly susceptible to OFLX (MIC50 and MIC90 were both 0.78 micrograms/ml). Commonly used antimicrobial agents to which the isolates responded with high to relatively high susceptibilities (MIC50 0.10-6.25 micrograms/ml) were, in the increasing order of MIC50: DOXY, ABPC, tylosin, tiamulin, KM, OXA and CP. Drugs with moderate to low bacterial susceptibilities (MIC50 12.5-100 microns/ml) were SD, SDMX and SPCM. Isolates resistant to all the test drugs except OFLX and SDMX amounted to 3.6-50% of the 28 isolates examined and they totalled 20 isolates (71.4%).(ABSTRACT TRUNCATED AT 400 WORDS)