Flexibacter ovolyticus sp. nov., a Pathogen of Eggs and Larvae of Atlantic Halibut, Hippoglossus hippoglossus L.

Summary auto-generated

Hansen and colleagues describe Flexibacter ovolyticus sp. nov., a psychrotrophic bacterium isolated from the adherent bacterial layer on Atlantic halibut eggs and identified as an opportunistic pathogen causing high mortality in halibut eggs and larvae. The species was characterized through morphological and biochemical testing of 35 isolates, all showing consistent results: gram-negative, rod-shaped bacteria with gliding motility and pale yellow pigmentation. Key characteristics include positive reactions for Kovacs oxidase, catalase, and nitrate reductase; degradation of gelatin, tyrosine, DNA, and Tween 80; and inability to degrade starch, cellulose, or chitin. DNA analysis revealed G+C contents of 30.3-32.0 mol%, clearly separating F. ovolyticus from the closely related F. maritimus based on DNA-DNA hybridization showing only 30-42.7% relatedness. Notably, scanning electron microscopy revealed that F. ovolyticus possesses enzymatic capacity to dissolve both the chorion and zona radiata of egg shells, causing characteristic wounds and lesions. The bacterium exhibits an opportunistic pathogenic strategy, causing severe mortality when present in high numbers or when larval resistance is compromised. Strain EKD002 is designated the type strain.

Key findings

F. ovolyticus is a new Flexibacter species isolated from Atlantic halibut egg surfaces with enzymatic capacity to dissolve both the egg chorion and underlying zona radiata through exoenzymatic activity
DNA-DNA hybridization data (30.0-42.7% relatedness to F. maritimus) and G+C content analysis (30.3-32.0 mol%) demonstrate this is a distinct species from previously described Flexibacter species
The 35 isolates characterized formed a highly homogeneous group showing identical responses to morphological and biochemical tests, indicating a genetically uniform population
F. ovolyticus is an opportunistic pathogen causing dramatic increases in mortality during the hatching stage when it comprises the major portion of the egg bacterial epiflora

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Abstract

A psychrotrophic Flexibacter sp., Flexibacter ovolyticus sp. nov., was isolated from the adherent bacterial epiflora of Atlantic halibut (Hippoglossus hippoglossus L.) eggs and was shown to be an opportunistic pathogen for halibut eggs and larvae. The strains which we isolated had the enzymatic capacity to dissolve both the chorion and the zona radiata of the egg shells. A total of 35 isolates were characterized by using morphological and biochemical tests. These strains were rod shaped, gram negative, Kovacs oxidase positive, and pale yellow and exhibited gliding motility. They did not produce acid from any of the wide range of carbohydrates tested. Our isolates had the ability to degrade gelatin, tyrosine, DNA, and Tween 80. Starch, cellulose, and chitin were not degraded. The strains were catalase and nitrate reductase positive, did not produce H₂S, and did not grow under anaerobic conditions. F. ovolyticus resembles Flexibacter maritimus, but differs from the latter species in several biochemical and physiological characteristics. DNAs from F. ovolyticus strains had guanine-plus-cytosine contents which ranged from 30.3 to 32.0 mol% (strains EKC001, EKD002^T [T = type strain], and VKB004), and DNA-DNA hybridization studies revealed levels of relatedness between F. ovolyticus EKD002^T and F. maritimus NCMB 2154^T and NCMB 2153 of 42.7 and 30.0%, respectively. Compared with previously described Cytophaga and Flexibacter spp. with low guanine-plus-cytosine contents, F. ovolyticus constitutes a new species. Strain EKD002 (= NCIMB 13127) is the type strain of the new species.