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Pasteurella multocida (Pm) and Mannheimia haemolytica (Mh) are major bacterial pathogens responsible for bovine respiratory diseases. However, the diversity of these two pathogens in transported calf populations remains poorly understood, which severely hinders the effective prevention and control of their infections. Objective To investigate the diversity of Pm and Mh in a group of fattening calves purchased from a calf trading market in Inner Mongolia and transported to a breeding farm in Hechuan, Chongqing. Methods After arrival, nasal swabs were collected from calves showing respiratory symptoms at four time points for bacterial isolation and culture. Suspected Pm and Mh colonies were selected based on colony morphology, hemolytic characteristics, and Wright-Giemsa staining results, followed by PCR identification and 16S rRNA gene sequencing for confirmation. Furthermore, the serotypes, biochemical and antibiotic resistance profiles, virulence genes, and resistance genes of the isolates were analyzed. Results A total of 23 strains of Pm serotype A, 10 strains of Mhserotype A6, and 1 strain of Mhserotype A2 were isolated from 68 nasal swabs collected at 4 different time points, and only 1 nasal swab harbored both Pm and Mh. Some isolates exhibited diversity in biochemical and antibiotic resistance profiles, which had no significant correlation with sampling time points. Antimicrobial susceptibility testing revealed that Pm and Mh isolates were resistant to most penicillins, aminoglycosides, and lincosamides but remained sensitive to cephalosporins and quinolones. Resistance gene detection showed that β-lactamase resistance (blaTEM ) genes were detected in 73.91% of Pm isolates and 90.91% of Mh isolates, while sulfonamide resistance (sul2) genes were found in 69.57% of Pm isolates and 18.18% of Mh isolates. Only one Mh isolate carried aminoglycoside resistance genes (aadA25 and aadB). Discrepancies were observed between resistance phenotype and the presence of selected resistance genes. All Pm and Mh isolates were pathogenic. Virulence gene analysis confirmed that Pm isolates consistently carried tonB, hsf-1, nanB, oma87, and tbpA, while Mh isolates showed the detection rates of 100% for gapA and dnaN, 82% for lktA, plpB, and tbpB, and 0 for ptfA. Conclusion These findings suggest that calves purchased from trading markets and transported over long distances to new farms harbor Pm and Mh strains exhibiting diversity in biochemical characteristics and drug resistance, which pose challenges for effective infection control. This study provides critical insights for developing prevention and control strategies against Pm and Mh infections in transported calves.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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