Genotypic Comparison of Pasteurella multocida from Healthy Animals at Entry to the Feedlots with That and from Bovine Respiratory Disease-Affected Animals during the Fattening Period

SIMPLE SUMMARY: Pasteurella multocida can be found as a commensal resident in the nasopharynx of healthy calves but is considered one of the major bacterial pathogens contributing to bovine respiratory disease (BRD). We compared the genetic characteristics of P. multocida recovered from apparently healthy animals (n = 74) at the time of entry to the feedlot with isolates from animals affected by BRD during the fattening period (n = 32) using different molecular techniques (capsular and LPS typing, as well as PFGE). The genomes of a subset of 14 P. multocida isolates identified in clinically and non-clinically affected calves were sequenced, and in silico analysis of multilocus sequence types, virulence-associated genes, and antimicrobial resistance genes was carried out. These isolates of P. multocida belonged mainly to sequence types ST79 and ST13, but ST80 was also identified. Isolates were further compared with the genome sequences available in the NCBI genome database of 53 P. multocida isolates of ST13, ST79, and ST80 from different geographical locations for a global comparison. ABSTRACT: The aim of this study was to investigate the possible genotypic differences between commensal Pasteurella multocida isolates from apparently healthy animals (AHA) at the time of entry to feedlots and those from BRD-affected animals (BRD-AA). A total of 20 batches of beef calves in seven feedlots were followed-up during the fattening period. P. multocida was isolated from 28.1% of AHA and 22.9% of BRD-AA. All isolates belonged to the A: L3 genotype. Most isolates from clinical cases (81.0%) grouped into a PFGE cluster were significantly associated with BRD cases (OR, 24.9; 95% CI, 6.4–96.2). The whole genomes of 14 isolates representative of the pulsotypes most frequently detected in BRD-AA and AHA were sequenced and compared with 53 bovine genomes belonging to the identified ST13, ST79, and ST80 genotypes for a global comparison. No differences were found in the virulence-associated gene content between sequence types (STs) globally or between BRD-AA and AHA isolates in this study. Significantly, ST79 isolates harbored ARGs, conferring resistance to different antimicrobials, including macrolides and tetracyclines, which are commonly used for the treatment of BRD. Two Spanish ST79 isolates carried an ICE highly similar to ICE Tn7407, which was recently detected in Germany, suggesting that ST79 P. multocida isolates in Europe and North America may be associated with different ICEs.