Antimicrobial-Resistance and Virulence-Associated Genes of Pasteurella multocida and Mannheimia haemolytica Isolated from Polish Dairy Calves with Symptoms of Bovine Respiratory Disease

Bovine respiratory disease causes significant economic losses in cattle farming due to mortality, treatment costs, and reduced productivity. It involves viral and bacterial infections, with <i>Pasteurella multocida</i> and <i>Mannheimia haemolytica</i> key bacterial pathogens. These bacteria contribute to severe pneumonia and are often found together. Poland has one of the highest levels of antimicrobial use in food-producing animals among European Union countries. A total of 70 bacterial strains were analyzed, 48 <i>P. multocida</i> and 22 <i>M. haemolytica</i>, collected from affected calves' respiratory tracts. The bacterial species were confirmed molecularly using PCR, which was also employed to detect antimicrobial resistance and virulence-associated genes. Antimicrobial susceptibility was determined using the broth microdilution method. Antimicrobial resistance varied between the two bacterial species studied. The highest resistance in <i>P. multocida</i> was to chlortetracycline 79.2% (38/48) and oxytetracycline 81.3% (39/48), while <i>M. haemolytica</i> showed 63.6% (14/22) resistance to penicillin and tilmicosin. The highest susceptibility was found for fluoroquinolones: <i>P. multocida</i> demonstrated 91.7% (44/48) susceptibility to enrofloxacin and 87.5% (42/48) to danofloxacin, while 77.3% (17/22) of <i>M. haemolytica</i> were susceptible to both tested fluoroquinolones. The <i>tetH</i> and <i>tetR</i> genes were observed only in <i>P. multocida</i>, at frequencies of 20.8% (10/48) and 16.7% (8/48), respectively. Both species carried the <i>mphE</i> and <i>msrE</i> genes, though at lower frequencies. All <i>M. haemolytica</i> contained the <i>lkt</i>, <i>gs60</i>, and <i>gcp</i> genes. All <i>P. multocida</i> carried the <i>sodA</i> gene, while the <i>hgbB</i> and <i>ompH</i> genes were present in 37.5% (18/48) and 20.8% (10/48) of strains, respectively. The highest resistance was observed against the most commonly used antibiotics in the European Union, although the resistance differed between the studied bacterial species and each strain exhibited the presence of at least one virulence gene.