COLD STRESS-INDUCED CHANGES IN THE AEROBIC HETEROTROPHIC GASTROINTESTINAL-TRACT BACTERIAL-FLORA OF RED HYBRID TILAPIA

Fifteen juvenile red hybrid tilapia Oreochromis mossambicus x O. macro chir averaging 50 g body weight were cold-stressed by immersion in 18 degrees C sea water; control fish (n=15) were kept at the acclimation temperature of 26 degrees C. Three fish from each group were killed 0, 24, 48 and 72 h after the start of the experiment. Gastrointestinal t racts were removed and dissected into the stomach, anterior gut and po sterior gut regions. Aerobic heterotrophic bacteria were identified an d enumerated relative to temperature, exposure time and gastrointestin al tract region. Gram negative genera included Alcaligenes, Flavobacte rium, Photobacterium, Pseudomonas and Vibrio; the latter three were pr edominant. Mean bacterial numbers and taxonomic composition of the mic roflora varied significantly (P<0.05) in response to the three test va riables. There were greater mean total bacterial numbers at IS than at 26 degrees C, due primarily to proliferation of Vibrio spp. Mean bact erial numbers after 24 h were greater than those at both the earlier a nd later sampling periods. Mean bacterial numbers in the stomach were less than those in the anterior and posterior gut. which were not sign ificantly different. The relative abundance of Vibrio spp. was negativ ely correlated with that of Flavobacterium, which may have reflected c ompetition between pathogenic and nonpathogenic species. Such stress-i nduced changes in the bacterial microflora may contribute to the onset of disease because several species of Vibrio are known primary or opp ortunistic fish pathogens. (C) 1997 The Fisheries Society of the Briti sh Isles.