The role of enteric bacteria in the Abalone, Haliotis Midae

Master Thesis


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University of Cape Town

The role of bacteria in the digestive tract of the abalone Haliotis midae was examined to determine whether bacteria aid hydrolysis of polysaccharides present in seaweeds which farmed abalone consume. The enteric bacteria were enumerated using culturable and total (DAPI) counts. The numbers of culturable bacteria fell between 3.5x10⁵ and 2.3x10⁸ cfu/g wet weight tissue. The DAPI counts were between 1.6x10⁹ and 5.1x10⁹ cells per gram of tissue. The numbers of bacteria differed between the crop, stomach and intestine. Electron microscopy showed that bacteria were present on the food and gut wall. No specialised structures, to aid adhesion of bacteria, were apparent on the gut wall. The isolated bacteria were identified to genus level using standard biochemical and morphological tests. The common genera identified were Vibrio, Alcaligenes, Flavobacteria, Pseudomonas and Aeromonas. The bacterial communities in each gut region varied, suggesting that both resident and transient bacterial populations are present in H. midae. Alcaligenes occurred mainly in the crop, while Vibrio species were predominant in the stomach and intestine. The bacterial isolates were tested for their ability to hydrolyse the polysaccharides alginate, laminarin, CMC, carrageenan and agarose. Bacteria able to utilise these polysaccharides belonged to the genera Flavobacteria, Pseudomonas, Vibrio, Alcaligenes, Bacillus and Enterobacteria. Many of the isolated bacteria were capable of utilising two or three of the substrates tested. Quantitative poly saccharase assays using the reducing sugar assays of Nelson (1944) and Somogyi (1952) and Gardner et al. (1988) were employed. These assays showed that bacteria from the crop exhibited the greatest degree of CMC and alginate hydrolysis. Bacteria from the intestine exhibited the greatest carrageenan and agarose hydrolysis. The endogenous enzymes produced by H. midae were examined using extracts of the hepatopancreas as it was found to be bacteria free. It was found that abalone synthesize a CMCase, laminarinase, alginase, carrageenanase and agarase. However, the synthesis of these enzymes was related to the diet of the abalone. Abalone fed Ecklonia (which contains relatively high concentrations of alginate and laminarin) exhibited significantly higher alginase and laminarinase activity than abalone fed Gracilaria. Similarly, abalone fed Gracilaria (which contains relatively high proportions of carrageenan and agar) produced significantly higher carrageenanase and agarase activity. Furthermore, these enzyme activities were found to be similar to those extracted from gnotobiotic abalone (obtained using the antibiotics chloramphenicol (250μg/ml), ampicillin (600μg/ml) and cefotaxime (250μg/ml)), indicating that polysaccharide synthesis occurs in the hepatopancreas of H. midae. Polysaccharase assays on gnotobiotic abalone were compared to assays on untreated abalone. Bacteria were found to significantly enhance the polysaccharase activity of alginase, laminarinase and agarase hydrolysis.

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