Phenotypic Antimicrobial Susceptibility and in Silico Target-Based Analysis of Drug Resistance in Avibacterium paragallinarum

Avibacterium paragallinarum is responsible for infectious coryza, an economically important respiratory disease of chickens that leads to antimicrobial resistance and poses difficulties due to the lack of well-defined species-specific interpretive criteria. In this work we assessed phenotypic antibiotic susceptibility by Kirby–Bauer disc diffusion using eight A. paragallinarum isolates and in silico binding analysis for selected antibiotics with a bacterial target protein. Fourteen antibiotics were tested: oxytetracycline, penicillin-G, streptomycin, fosfomycin, neomycin, ceftiofur, enrofloxacin, doxycycline, ciprofloxacin, tetracycline, gentamicin, erythromycin, amoxicillin, and ampicillin. Amoxicillin and ampicillin demonstrated the greatest apparent activity, with 100% of isolates being categorized as susceptible, whereas penicillin-G demonstrated 100% resistance, and oxytetracycline and neomycin each demonstrated 87.5% resistance. Ceftioufur (75%), ciprofloxacin (62.5%), streptomycin (62.5%), tetracycline (50%), and enrofloxacin (50%) also demonstrated high resistance. Docking analysis revealed that oxytetracycline had the highest predicted binding energy (−8.8 kcal/mol), followed by ampicillin (−8.1 kcal/mol), amoxicillin (−7.9 kcal/mol), and penicillin (−7.2 kcal/mol). The discrepancy between the top predicted binding energy and the high phenotypic resistance to oxytetracycline highlights that docking scores should not be considered evidence of antimicrobial efficacy. These findings demonstrate that high in silico binding affinity (e.g., oxytetracycline) does not correlate with whole-cell antimicrobial efficacy in clinical A. paragallinarum serovar A field isolates. Consequently, computational docking scores must be interpreted with caution and rigorously validated against standardized phenotypic susceptibility testing to avoid flawed therapeutic predictions in poultry disease management.