Diversity of Non-Flavobacterium columnare Bacteria Associated with Columnaris-like Diseased Fish

Abstract

Numerous yellow and pink pigmented bacterial isolates had been recovered from diseased fish exhibiting columnaris-like disease in tilapia (Oreochromis spp.) farms in Thailand. The predominant species had been identified as Flavobacterium columnare while taxonomic classification of the remaining isolates and their pathogenic potential remain undetermined. An additional yellow bacterial strain had also been obtained from a koi carp sample showing columnaris-like symptoms. To continue our previous work, this study described the identification of ten representatives of unknown culturable non-Flavobacterium columnare bacteria based on a combination of phenotypic characteristics and nucleotides homology of 16S rRNA gene and subsequently investigated their pathogenicity in Nile tilapia (Oreochromis niloticus) fingerlings. The majority of the yellow pigmented bacteria were identified as Chryseobacterium spp. while the remainders were identified as [Flexibacter] aurantiacus subsp. excathedrus and Flavobacterium indicum. The pink pigmented bacteria were identified as Flectobacillus roseus. Five representative species of the identified bacteria isolated from diseased tilapia were individually subjected to a pathogenicity test in healthy Nile tilapia fingerlings. The experimental challenge results within 14 days revealed that the tested bacteria exhibited low or no virulence to the fish (0-20% cumulative mortality). This suggests that the identified bacteria merely served as opportunistic pathogens that may require stressors for disease manifestation.