Isolation, identification, and drug resistance and potential pathogenicity evaluation of Photobacterium damselae subsp. damselae isolated from the gut of Litopenaeus vannamei

Abstract:

Background Photobacterium damselae is a resident bacterium in the shrimp gut and can cause various shrimp diseases. However, systematic research on the biological characteristics and potential pathogenicity of gut-derived P. damselae in shrimps remains lacking.Objective To provide a basis for prevention and utilization of this bacterium in shrimp aquaculture, we isolated, identified, and characterized P. damselae from the gut of the Pacific white shrimp (Litopenaeus vannamei).Methods Photobacterium strains were isolated and screened from the gut of L. vannamei sampled across six farming areas in Zhanjiang, Guangdong. Representative strains were identified based on morphological, physiological, and biochemical characteristics and ureC sequence. Their hemolytic activity and extracellular enzyme production were tested via the plate method. The resistance of the strains to 18 antimicrobial agents was evaluated via the disk diffusion method, and their virulence and potential pathogenicity were assessed by detecting typical virulence genes and conducting toxicity assays in Artemia franciscana larvae.Results P. damselae subsp. damselae (Pdd) was isolated from the guts of shrimps across different farming areas. Except differences in the utilization of sucrose, mannitol, cellobiose, and galactose, the physiological and biochemical characteristics of the 16 representative strains were consistent. Among the 16 strains, 7 showed α-hemolysis, while 9 exhibited β-hemolysis. All the strains produced extracellular lipase and lecithinase, with 11 of them also secreting chitinase. However, differences were observed in their enzymatic activities. The multidrug resistance rate of the 16 strains was as high as 87.5%. Moreover, at least 50.0% of the strains were resistant to ampicillin, amoxicillin, streptomycin, and furazolidone. All Pdd strains carried the phospholipase gene plpV, and the β-hemolytic strains all carried the hemolysin gene hlyAch. Additionally, four strains possessed the collagenase gene colP, while the virulence plasmid pPHDD1 and its related hemolysin genes dly and hlyApl were not detected in any of the strains. Except two Pdd strains that showed no obvious toxicity to A. franciscana larvae, the remaining 14 strains exhibited lethal effects (P<0.05).Conclusion Pdd is commonly found in the gut of L. vannamei, with the vast majority of strains exhibiting visible extracellular enzyme activity and multidrug resistance, as well as significant lethality to A. franciscana larvae, posing a potentially high pathogenic risk to L. vannamei.