This study examined the biochemical properties of 39 strains of Haemophilus avium, a catalase-positive, nonpathogenic bacterium isolated from chickens. All strains produced acid from fructose, galactose, glucose, and mannose but not lactose. Variability in carbohydrate fermentation patterns and enzyme production (β-galactosidase and ornithine decarboxylase) allowed identification of 15 distinct biochemical biovars, including some previously unreported. The research resolved conflicting literature regarding maltose fermentation by demonstrating that chicken serum in test media affects acid production results. Only 25 of the 39 strains could be assigned to the three Pasteurella species recently proposed to replace H. avium classification. The remaining 14 strains exhibited phenotypic patterns not previously documented. This work emphasizes the significant biochemical heterogeneity within H. avium and suggests that further studies on pathogenic potential of arabinose-fermenting strains and unclassified biovars are warranted.

Key findings

• Fifteen biochemical biovars of H. avium were identified based on carbohydrate fermentation patterns and enzyme activities, with nine biovars previously unrecognized
• Chicken serum in fermentation media affects maltose test results; some strains produce acid from maltose only in the presence of chicken serum, explaining conflicting literature reports
• Only 64% of the 39 H. avium strains could be confidently assigned to the three proposed Pasteurella species (P. avium, P. volantium, and Pasteurella species A)
• Significant biochemical heterogeneity within H. avium prevents clear assignment of all strains to newly proposed Pasteurella taxa based on available phenotypic characteristics