Isolation and characterization of Avibacterium paragallinarum with different nicotinamide adenine dinucleotide requirements

•Three different types of A. paragallinarum based on growth requirement were isolated from commercial chicken flocks in South Korea.•This study is the first description of variant strains of A. paragallinarum showing NAD-independent and unusual growth requirement characteristics in Asia.•Variant strains showing unusual growth characteristics were more resistant to antibiotics than the typical NAD-dependent strains. Twenty field isolates of Avibacterium paragallinarum were obtained from chickens in South Korea during 2011–2015. The isolates were identified by a HPG-2 PCR assay specific for A. paragallinarum and by biochemical tests. Growth requirements, Page serovars, carbohydrate fermentation patterns, and antimicrobial susceptibility were also examined. Most isolates (16/20) showed the typical requirement for nicotinamide adenine dinucleotide (NAD) and an enriched CO2 atmosphere for growth. One isolate needed increased levels of NAD and serum for good growth. Three isolates showed NAD-independent growth on blood agar under aerobic conditions. In terms of carbohydrate fermentation patterns, three biochemical biovars were recognized; these varied with respect to acid production from maltose and D-xylose. The 16 typical NAD-dependent isolates were serovar A while the variants, both NAD-independent isolates and the isolate with increased NAD dependency were non-typeable. All isolates were sensitive to amoxicillin-clavulanic acid, ceftiofur, gentamicin, and spectinomycin. High rates of resistance, including intermediate resistance, to lincomycin (100%), cloxacillin (75%), and erythromycin (70%) were observed. The four variant strains (the three NAD-independent isolates and the isolate showing unusual growth requirements) were more resistant to antibiotics than the typical NAD-dependent strains. The finding of NAD-independent forms of A. paragallinarum extends the known distribution of this form, previously only reported in South Africa, Mexico and Peru. There is clearly a need for increased caution in the diagnosis and, possibly, the control of infectious coryza.