| In the present study, traditional culture-based techniques and denaturing gradient gel electrophoresis (DGGE) tequnique were used to study the gut microbiota of larvae, juvenile and two groups of young grouper Epinephelus cioides with different grow rate. In addition, the gut microbiota of E. cioides infected artificially with Vibrio alginolyticus was investigated. The general results are presented as follows:1. The aim of the study was to investigate the gut microbiota of the different stages from the eggs hatch to juvenile grouper. Larvae and juvenile samples were collected at day 2, 8, 22 and day 40 post hatch. The results showed that the total cultivable bacterial number increased from the larvae stage to juvenile stage, from 4.0×102 CFU to 1.4×107 CFU per fish. Lactic acid bacteria were only found in the 2 d larvae grouper and no Vibrio sp. observed. Vibrio was found in the 22 d larvae and 40 d juvenile grouper. Six bacterial species were found in the gut of 2 d larvae, but only three bacterial species were found in the gut of 8 d larvae. The cultivable bacterial species are similar between the larvae and inlet water, while those in 8 d and 22 d larvae are similar with there diets.DGGE results showed that the diversity of gut microbiota of 2 d, 8 d and 22 d of larvael grouper was similar. However, the DGGE profiles of the larvae and juvenile grouper showed an apparent difference. The DGGE profile of 22 d larvae grouper was similar to that of rotifer. There was a low similarity of the DGGE profile between 8 d larvae and its diets (fertilized eggs of the Pacific oyster Crassostrea gigas).2. The gut microbiota of two groups of juvenile grouper Epinephelus coioides, slow growing and fast growing grouper at 70 days post–hatch, were investigated by using traditional culture-based techniques and PCR/DGGE technique. Traditional culture-based techniques showed that the total bacterial number in the slow growing grouper ( 5.4×106 CFU/g ) was lower than those in the gut of fast growing grouper ( 9.0×106 CFU/g ). However the number of Vibrio and Lactic acid bacteria in the slow growing grouper ( 6.0×105 CFU/g and 1.5×105 CFU/g respectively ) were higher than those in the fast growing fish ( 3.4×105 CFU/g and 6.0×104 CFU/g ). The number of total bacteria ( 5.4×105 CFU/ml ), Vibrio ( 2.2×103 CFU/ml ) in the inlet water were lower than those in the gut of juvenile grouper, and no lactic acid bacteria observed in the inlet water. Four Vibrio species were isolated and comprised 12.3% of the total gut bacteria in slow growing grouper, whereas only two Vibrio species were isolated and comprised 3.6% of the total bacteria in fast growing grouper. Pseudomonas comprised 12.0% in the slow growing grouper, wheras only 5.6% in the fast growing grouper.Bacillus pumilus, Bacillus clausii and Psychrobacter sp. were only isolated and dominated in the gut of the fast growing fish. The three species showed antagonistic effect on pathogenic Vibrio, this may cause the lower number and less species of Vibrio in the gut of fast growing grouper and suggesting fast growing fish might harbor a favorable microbiota. The bacteria in the gut of grouper could be classified into three groups belonging toγ-proteobacteria,β-proteobacteria and Bacilli class.The results of DGGE analyses showed that the bacterial diversity in the gut of slow growing grouper was obvious lower than that in the fast growing grouper. Sequencing results showed that the fast growing grouper was dominated by Acinetobacter baumannii,Mycobacterium tuberculosis,Desulfotomaculum reducens and Petrotoga mobilis, however the slow growing grouper gut was dominated by Acinetobacter.3. The results of DGGE analyses showed the bacterial diversity in the gut microbiota of grouper after administered intragastrically with Vibrio alginolyticus . Sequencing results showed that the stomach,fore gut and hind gut samples were collected at day 1 to day 5 after administered intragastrically with Vibrio alginolyticus. The results showed that the total cultivable bacteria in the stomach, fore- and hind-gut of the experiment group were similar with the control group. The bacterial species in the stomach and hind gut of the experiment group were obvious less than the control group, however the species in the fore hand of experiment group were more than the control group.DGGE results showed that the bacterial diversity were obvious differences between the experiment group and control group. The microbiota of stomach was relative stable in the first three days after administered intragastrically with V. alginolyticus, whereas many bacteria disappeared in the fourth day, but appeared again in the fifth day. The fore gut bacterila diversity of the experiment group was no change in the first day compared to control group . The bacterial diversity was similar in the third and fourth day , with the lowest species. The hind gut bacterial diversity of the experiment group was much more in the first day , howere it reduced to the lowest in the fourth day. Sequencing results showed that Escherichia coli,Petrotoga mobilis and Vibrio furnissii were isolated from the stomch, fore and hind gut of both control group and experiment group . There were fewer bacterial species in the gut of experiment group in the fourth day, however, Shewanella sp. and Vibrio fischeri dominated.
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