Intensive fish culture is a practice in aquaculture systems; however, it can produce negative environmental impacts due to the accumulation of organic nitrogen from feeding procedures, but also from fish wasting products. Recirculating aquaculture systems have emerged as one strategy to reduce these impacts, maintaining a healthy environment for fish. These reduce toxicity level through nitrifying biofilters, which use a microbial consortium to convert ammonia into nitrate. The main component of bacterial consortium is comprised by aerobic ammonia‐oxidizing bacteria and aerobic nitrite‐oxidizing bacteria, which are part of the whole nitrification process. These nitrifying bacteria grow in suspension or in fixed form, the latter characterized by a biofilm formation where bacteria adhere to a physical substrate in a self‐produced polymer matrix. The biofilm formation requires tight communication among bacteria to regulate the transcriptional circuits underlying the production of chemical signal molecules (e.g. N‐acetyl‐homoserine lactones) that control biofilm formation. This coordination is known as quorum sensing and can be considered as a mechanism that contributes to the coupling and maintenance of nitrification rate among bacteria by regulating expression levels of relevant genes associated with nitrification process. Therefore, the control of this process is crucial in recirculating aquaculture systems and its incorrect manipulation can produce a detriment of water quality. We here present an overview of the nitrification process in recirculating aquaculture systems. Subsequently, we describe nitrifying biofilters and nitrifying bacteria. Finally, we discuss how quorum sensing controls the efficiency of nitrifying bacteria and potential applications of nitrifying biofilters in intensive aquaculture systems.
The most important bacterial pathology currently occurring in Chilean freshwater salmon farming is the cold-water disease produced by the psychrotrophic bacteria Flavobacterium psychrophilum. The main aim of this study was to characterize the inhibitory activity of an antagonist strain on the formation of biofilms of a F. psychrophilum strain. The antagonistic strain Pseudomonas fluorescens FF48 was isolated from the sediment beneath the salmon cages of a freshwater Chilean salmon farm and was identified by using the 16S rRNA gene sequence analysis. The production of siderophores, mainly during the stationary phase of growth of the antagonist strain was demonstrated using the Chrome Azurol S method and through F. psychrophilum inhibition under iron saturation conditions. Subsequently, the effect of the antagonist supernatant on the formation of F. psychrophilum biofilm was tested using the crystal violet staining method observing an inhibition of the growth of F. psychrophilum, but no effect was observed when iron saturation concentrations were used. Furthermore, when the antagonist strain was previously deposited on the support, it completely inhibited the formation of F. psychrophilum biofilms, but when both bacteria were inoculated simultaneously no inhibitory effect was detected. In conclusion, it was demonstrated that FF48 strain is able to inhibit the formation of F. psychrophilum biofilms in vitro probably mediated by the siderophore production, suggesting its potential use as a biocontrol biofilm in freshwater fish rearing systems to prevent the persistence of biofilms of the fish pathogenic species F. psychrophilum.
Flavobacterium psyhrophilum affects farmed salmon and trout worldwide and is the etiological agent of bacterial cold water disease. The treatment of this disease is made with antibiotics. The emergence of resista nt strains and its ability to form biofilms have increased the resistance to antibiotics. Considering the diverse bioac tivity of Antarctic bacteria, we evaluated the inhibitory effect of compounds secreted by Antarctic bacterial strains on the biofilm formation of F. psychrophilum 19749. 26.9% of the Antarctic bacterial isolates inhibited strongly the biofilm, the greatest effect was shown by M19B. Proteins (48-56 kDa) secreted by M19B may be related to the inhibition of the biofilm formation of F. psychrophilum 19749.
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