Bacteria Recovered from Aquaculture in Oman, with emphasis on Aeromonas Spp.

Abstract

Aquaculture is being seriously considered as a promising sustainable industry in the Sultanate of Oman. Fish farming commenced in Oman in 1986, but it was only in 2011 that it became a more commercially driven sector. While worldwide aquaculture production is expected to rise to meet the shortage in capture fisheries, there is a parallel requirement to identify potential threats to the health and welfare of existing aquatic farmed stocks and to take appropriate steps to mitigate them. As aquaculture in Oman is in an early stage of development, it is important to acquire baseline data on the existence and prevalence of aquatic diseases and pathogens to help the Government make policy decisions to develop health management regimes applicable for Omani aquaculture. Therefore, this study was conducted to evaluate current farming practices of tilapia in Oman, to investigate the bacterial species composition and distribution from different sites in some of the economically important fish species, and to study the characteristics and pathogenicity of Aeromonas species. The current practices were studied for 9 Nile tilapia (Oreochromis niloticus) farms from four areas (Al Batinah, Ad Dhahirah, Ad Dakhiliyah and Ash Sharqiyah North) during the period of September to November 2012 by using questionnaires and interviews with the farm owners and staff. In total 417 fish representing 5 target species were chosen on the basis of the commercial importance and their potential for aquaculture in Oman, including red spot emperor (Lethrinus lentjan), king soldier bream (Argyrops spinifer), white spotted rabbit fish (Siganus canaliculatus), abalone (Haliotis mariae) and tilapia (Oreochromis niloticus). The fish were collected from 5 main sampling areas in Oman (Muscat, Mudhaibi, Manah, Sohar and Salalah) based on the Atlas of suitable sites for aquaculture in Oman to investigate the bacterial species composition and distribution. The animals were examined for clinical signs of disease prior to routine bacteriology. Bacterial isolates were recovered using traditional methods and identified to species level using phenotypic and molecular approaches using 16S rDNA, 16S rDNA RFLP and 16S rDNA sequencing. Experimental fish challenge studies were also conducted using both live bacterial cells and ECP protein to investigate the pathogenicity of Aeromonas isolates. In addition, the presence of some virulence factors was investigated using both phenotypic and genotypic methods. The results of this study showed that, the most farms in the Oman follow very similar farming practices. The major proportion of the tilapia is consumed within the local communities. A number of farmers have experienced mortalities, which were considered to be attributable to poor water quality, overcrowding or due to excessive feeding. Farmers facing fish mortalities tended not to record the problems due to a lack of understanding of the concept of fish farm management. There is a regulation about aquaculture and related quality control, but it has not yet been implemented in an appropriate manner in Oman. From the diverse group of bacteria recovered from wild and farmed fish, 83% of the total isolates comprised Gram negative, rod-shaped bacteria. The most frequently isolated groups from marine and cultured fish were Aeromonas spp., Vibrio spp., Sphingobacterium spp., Micrococcus spp. and Staphylococcus spp., with Aeromonas spp. being the predominant group representing 25% of the isolates recovered in this study. Identification of the Aeromonas spp. showed 57% agreement between the results of phenotypic and genotypic methodologies, and determined 6 species as the dominant organisms, i.e. A. veronii, A. jandaei, A. caviae, A. trota, A. encheleia and A. salmonicida. 65% of the iso-lates shared 99% 16S rDNA sequence similarity with the closest sequences in GenBank, and the dominant species was A. veronii. In conclusion, the Aeromonas isolates recovered from fish with clinical signs of disease showed heterogeneity in their identification profiles and their pathogenicity.