Sixty one strains of Pasteurella multocida and 35 strains of Actinobacillus pleuropneumoniae isolated from pneumonic lesions of porcine lungs during the period from 1985 to 1989 in Japan were tested for antibiotic susceptibility to chlortetracycline (CTC), thiamphenicol (TP), tylosin (TS), acetylisovaleryl-tylosin (AIV-TS), tilmicosin (TMS), mirosamycin (MRM). Most strains of both species were sensitive to CTC, TP and TMS. Growth of fifty-one strains (83.6%) and forty-six strains (75.4%) of P. multocida were inhibited with 3.13 /xg/ml of CTC and 0.78 fig/ml of TP, respectively. TS showed low activity against almost all strains (MIC S 6.25 xg/ml). Fifty-eight (95.1%), twenty-three (37.7%) and fifty (82%) of P. multocida showed MICs of g 6.25 iiglml against AIV-TS, TMS and MRM, respectively. The MICs of A. pleuropneumoniae against CTC were less than 1.56 jug/m/. Thirty-two strains (91.4%) and 33 strains (94.3%) of A. pleuropneumoniae were inhibited with 3.13 g/m/ of TP and TMS respectively. However, TS, AIV-TS and MRM showed low activity against all of A. pleuropneumoniae (MIC i? 6.25 |U,g/m/). Three different resistance patterns were observed in P. multocida and two in A. pleuropneumoniae isolates, respectively.
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