Lactococcosis is a disease that causes fish spoilage and has economic impacts. The disease is caused by Lactococcus garvieae. It can affect both marine and freshwater species and is mainly active during the summer months. It has been associated with high water temperatures. The disease's symptoms consist of haemorrhagic septicaemia and meningoen cephalitis. L.garvieae can also affect humans i.e. it is a zoonotic (Meaning it can affect humans as well as animals). Disease outbreaks in fish are often treated with antibiotics their prolific use has led to antibiotic resistance in some cases. Vaccinations have also been used to manage fish disease contraction, however vaccinations are not effective on immature fish and the length of effective prevention is short lived. Probiotics are feed supplements that beneficially improve the health of the host by direct or indirect modulation of the gut microbiota. The benefits of favourable bacteria in the gastrointestinal tract are: production of inhibitory compounds, inhibition of virulent gene expression and enhancement to immune responses, improved gastric morphology and improvement to digestion.
Cytokine molecules are thought to contribute to the immune defence mechanisms of Oncorhgnchus mykiss particular to fish are: pro-inflammatory cytokines e.g. interleukin-1β (1L-1β), interleukin-8 (IL-8), tumour necrosis factor α (TNF-α) and anti-inflammatory cytokine IL-10. It is believed these cytokines respond to bacterial colonisation and invasion. The aim of Sanchez et al.'s research was to determine the protective effects of three species of lactic acid bacteria against L.garvieae and to assess the impacts of probiotics on fish immune systems and organs.
The species used as a probiotic combined with fish feed were: Lactobacillus plantarum, Lactococcus lactis and Leoconostoc mesenteroides. They were gathered from cells from the gills of healthy rainbow trout. These strains were known to be antagonistic to L.garvieae. They were then grown in agar. Fish were obtained and tanks were set up with one for each treatment: a control group, cohabitation group, and treatment tanks, each tank contained 45 fish. The tanks were supplemented with different treatments of probiotics. The experiment was run for 36 days. 21 days after the start of the feeding period fish were infected with L.garvieae. Organs and tissues were taken for analysis which consisted of molecular techniques: PCR-DGGE, RNA extraction, cDNA synthesis and real time PCR. Statistical analysis was then carried out.
The results indicate that fish fed the probiotic diet of L.plantarum had significantly lower mortality than diets containing L.lactis and L.mesenteroides and the control. Examination of the dead fish confirmed mortality was due to L.garvieae infection.
Conversely to what might be intuitively instinctive molecular techniques revealed that the intestinal mucosa of all fish were devoid of L.garvieae. It was also found that no probiotic populations were present in the intestinal mucosal tissue. Other studies by Brunt and Austin (2005) showed that a probiotic containing Aeromonas sobria also reduced L.garvieae infection. Sanchez et al.'s research confirms that probiotics can have a beneficial role in the reduction of L.garvieae induced mortalities.
PCR Examination of immune related cytokine genes indicated that they were significantly upregulated in the organs of fish fed L.plantarum in contrast to the control. The relationship here is thought to be explained by the theory “that the presence of resident mucosal bacteria in the intestine of fish may modulate the local immune system by increasing the secretion of cytokines and various immune regulatory substances”. Therefore the presences of beneficial bacteria induce immune responses and upregulate cytokines and thus provide protection. This research also presented molecular evidence that L.plantarum populations were primarily luminal and did not establish resident populations on the epithelium itself. The authors conclude that the host's immunological responses in gene expression i.e. upregulated cytokines are linked to the presence of probiotics and contribute to increased disease resistance in L.plantarum fed fish.
This research is important for society as it aids food production without the excessive use of antibiotics which can cause selection of resistant strains of bacteria and manifest in multiple consequences. Probiotic methods could help produce higher yields of fish in environments predisposed to the onset of Lactococcosis. It may also be beneficial in the prevention of zoonotic infection therefore also providing protection for humans as well as fish.
Reference:
Tania Pérez-Sánchez, José Luis Balcázar, Daniel L. Merrifield, Oliana Carnevali, Giorgia Gioacchini, Ignacio de Blas, Imanol Ruiz-Zarzuela, Expression of immune-related genes in rainbow trout (Oncorhynchus mykiss) induced by probiotic bacteria during Lactococcus garvieae infection, Fish & Shellfish Immunology, Volume 31, Issue 2, August 2011, Pages 196-201
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