Antimicrobial Susceptibilities and Resistance Genes of Canadian Isolates of<i>Actinobacillus pleuropneumoniae</i>

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a severe and highly contagious respiratory disease responsible for economic losses in the swine industry worldwide. Although antimicrobial resistance in A. pleuropneumoniae has been recently reported in different countries, the current situation in Canada is unknown. The aim of the current study was to determine the antimicrobial susceptibilities of 43 strains of A. pleuropneumoniae isolated in Canada. In addition, antimicrobial resistance genes were detected with an oligonucleotide microarray. The impact of biofilm formation on susceptibility to antimicrobials was also evaluated. All isolates were susceptible to ceftiofur, florfenicol, enrofloxacin, erythromycin, clindamycin, trimethoprim/sulfamethoxazole, and tilmicosin. A low level of resistance was observed toward tiamulin, penicillin, and ampicillin as well as danofloxacin. We observed a high level of resistance to chlortetracycline (88.4%) and oxytetracycline (90.7%). The strains showing resistance to tetracycline antimicrobials contained at least one of the following tet genes: tetB, tetO, tetH, or tetC. Five isolates showed multiresistance to penicillins (blaROB-1), streptomycin [aph3′′ (strA)], sulfonamides (sulII), and tetracyclines (tetO) antimicrobials whereas three others showed multiresistance to streptomycin [aph3′′ (strA)], sulfonamides (sulII), and tetracyclines (tetB, tetO, or tetB/tetH) antimicrobials. To the best of our knowledge, this is the first description of tetC gene in Pasteurellaceae. Finally, cells of A. pleuropneumoniae in a biofilm were 100 to 30,000 times more resistant to antimicrobials than their planktonic counterparts.