Gut microbial status induced by antibiotic growth promoter alters the prebiotic effects of dietary DVAQUA® on Aeromonas hydrophila-infected tilapia: production, intestinal bacterial community and non-specific immunity.

The purpose of the present study was to investigate whether dietary antibiotic-induced changes in the fish intestinal microbiota altered host physiological responses to the infection with Aeromonas hydrophila in hybrid tilapia (Oreochromis niloticus ♀ × O. aureus ♂). After an 8-week induction period with an antibiotic-supplemented or antibiotic-non-supplemented diet, 160 hybrid tilapias in 16 tanks were each injected with phosphate buffered saline (PBS) or A. hydrophila at a dose of one-half of the LD(50). Then, all of the diets were changed to a prebiotic-supplemented one for the sequential 8-week response period. Parameters including production, gut microbial diversity and count, and non-specific immunity were determined at the end of the response period. Our results showed that A. hydrophila infection had no effects on the growth and diet conversion of tilapia, but it caused the decrease of the gut bacterial count, the number of visual bands, and the Shannon diversity and equitability indexes of gut bacteria in antibiotic-non-supplemented fish based on PCR-DGGE fingerprints. Infection with A. hydrophila reduced the gut bacterial evenness (lower Shannon equitability index), and slightly improved the gut bacterial richness (more visual bands) in antibiotic-supplemented tilapia. In addition, A. hydrophila infection affected non-specific immunity such as serum lysozyme activity and serum alternative complement pathway (C3 and C4) activities regardless of hybrid tilapia fed antibiotic-supplemented diets. These changes varied based on the intestinal microbial status of the fish before infection with A. hydrophila.