Bacteriological surveys were performed in 1995 and 1996 in three shrimp hatcheries located in the north of the People’s Republic of China. Samples were taken from routine productions of healthy Penaeus chinensis larvae, their environment and from diseased larvae. A total of 186 isolates from the dominant bacterial flora was characterized by Biolog metabolic fingerprinting and identified by comparison to a database of 850 Vibrio type- and reference strains. Representative Vibrio harveyi strains were further genotypically analyzed by AFLP fingerprinting of whole-genomes. An overwhelming predominance of V. alginolyticus and V. harveyi was observed in the larval developmental stages from zoae stage on. The flora associated with larvae is not very stable and is influenced by the bacterial flora of the administered food and by the environment. In the 1995 survey, the bacterial flora of successful P. chinensis larvae productions was mainly dominated by V. alginolyticus and unidentified Gram negative strains, while V. harveyi was absent. The bacterial numbers gradually increased from nauplii stage to post-larval stage, but few vibrios were isolated from nauplii stage. High V. harveyi numbers (up to 105 CFU/larva) in the larvae are correlated with weak larvae and mass mortalities. Between V. harveyi strains, isolated from healthy and diseased larvae, no phenotypic or genotypic differences were found. The presence of V. alginolyticus might influence the pathogenicity of V. harveyi or might have an impact on the resistance of larvae to bacterial pathogens.
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