Atypical multidrug-resistant serotype 1 Actinobacillus pleuropneumoniae with ApxIII in China.

Actinobacillus pleuropneumoniae (APP) is the primary etiological agent of porcine contagious pleuropneumonia and is responsible for substantial economic losses in the swine industry worldwide. In April 2023, an acute respiratory disease outbreak occurred on a pig farm in Longyan City, Fujian Province, China, resulting in a morbidity rate of 30% and a mortality rate of 56%. This study aimed to isolate and identify the causative pathogen and to characterize its major biological properties. An NAD-dependent bacterial strain was isolated from the lung tissues of deceased pigs and identified as APP serotype 1 through morphological characterization, biochemical testing, and serotype-specific PCR analysis. The isolate was designated APPFJLYC01. Toxin gene profiling revealed the presence of the ApxII, ApxIII, and ApxIV genes, whereas the ApxI gene was absent. This toxin gene pattern differs from the typical profile reported for serotype 1 strains (ApxI, ApxII, and ApxIV), suggesting potential genetic diversity in the distribution of toxin-associated genes among local APP populations. Antimicrobial susceptibility testing demonstrated that APPFJLYC01 was resistant to polymyxin, tetracycline, enrofloxacin, ampicillin, gentamicin, and amoxicillin/clavulanic acid, indicating a multidrug-resistant phenotype. Pathogenicity assessment in mice confirmed the virulence of the isolate, with a median lethal dose (LD50) of 7.91 × 108 CFU/mL. In conclusion, this study reports the isolation and characterization of a serotype 1 APP strain from Longyan, Fujian Province, exhibiting an atypical toxin gene profile and multidrug resistance. These findings provide valuable epidemiological data for understanding the genetic diversity and antimicrobial resistance characteristics of APP strains circulating in the region and may contribute to the development of effective prevention and control strategies for porcine pleuropneumonia.