Biofilms are structured communities ofbacterial cells that are enclosed in a self-produced polymeric matrix andadhere to biotic and abiotic surfaces, an interface or each other.
Biofilmformation commences when planktonic bacteria attaches irreversibly to asurface. Free cell (FC) oral vaccines give poor and inconsistent immuneresponse and protection in fish mainly due to their destruction by enzymes instomach or foregut before reaching the immune responsive lymphoid organs.Biofilms (BF) are known for their resistant nature to antibiotics, antibodiesand phagocytic cells due to a protective glycocalyx layer. This paper iswritten to illustrate the biofilm oral vaccine which is believed to protect theantigens against gastric destruction and found highly encouraging results(antibody titre and protection upon challenge) in herbivore carps, omnivorecatfish (Clarias batrachus), carnivorefish (Channa striatus) and tiger shrimp (Penaeusmonodon). IntroductionAquaculture is a steadily growing industryworld over.
The number species and new technologies are increasing inaquaculture industry for rising production. Intensification has come up as aboon to meet the increasing food demand. However, diseases are stumbling block,causing huge economic loss in millions each year world over.
To controlbacterial and parasitic diseases, antibiotics and drugs were usedindiscriminately. Antibiotic has already raised lots of criticism over itsnegative impact on living biota may lead to antibiotic-resistant pathogen. In this prospect, vaccinations against the bacterial and viral diseases play avital role in finfish and shellfish aquafarming. Importanceof oral vaccines in aquacultureDepending on the age and size of the fish,commercial vaccines are administered either orally (by mixing with the feed),by immersion (dip or bath) or by injection through the intraperitoneal (i.p.
)or intramuscular (i.m.) route (Embregts and Forlenza, 2016). Vaccine byinjection route although very effective in terms of immune response and longterm protection but it has some side effects including tissue inflammation,adhesion and necrosis. On the other immersion vaccination required highproduction costs. Oral vaccines are anattractive alternative to reduce the stress on the fish during immunization(Gudding et.
al., 1999). Due to theease, simplicity and practical applicability oral vaccination became the choiceof antigen delivery. However, attempts to orally vaccinate against differentbacterial diseases have either yielded mild and short lived or inadequateresponses. One of the important factors for the inconsistency and poor responseto oral vaccination is the digestive degradation of antigens in the foregut,before the vaccine reaches immune-responsive areas in the hind-gut and otherlymphoid organs (Johnson and Amend, 1983; Rombout et. al., 1985).
Needfor biofilm oral vaccines in aquacultureStrategies evaluated for prevention ofvaccine destruction in gut, such as delivering vaccines in encapsulated antigenmicrospheres (EAM), enteric coated vaccine and bioencapsulation of vaccine inlive feed which complex, costly and not practical. In this respect biofilmvaccines would be the best alternative way for oral route of vaccination.Biofilm cell produce adhesive exopolymeric substance, which is calledglycocalyx, offer protection to antigen from gastric destructionDesigning of biofilm-based vaccinesTo mimic in vivo conditions, invitro bacteria have to be grown in liquid media by providing surface (suchas bentonite clay, microsperes, chitin flakes, etc.) depleting nutrients andadding iron chelators and growing for longer periods. This leads to slow growthof the organisms forming biofilms, which can express some novel proteins andensure maximum EPS production. Efficacy of biofilm vaccines in aquaculture Over the 20 years, the vaccine research hasbeen oriented towards safer and more effective vaccine preparation from anormal inactivated bacterial vaccine to the biofilm vaccine (Azad et.al., 1997).
Various researchesconducted in the preparation of biofilm of Aeromonashydrophila and Vibrio alginolyticusand its use as oral vaccine under the direct supervision of renowned Indianscientist and ICAR (Indian Council of Agriculture Research) Emeritus ProfessorDr. K.M Shankar at the Laboratory of Aquatic Health Management, College ofFisheries, Mangalore have shown promising outcome in teleost fishes like catla,rohu, common carp (Azad et.al., 1997,1999, 2000), catfishes like C. batrachus(Nayak et. al., 2004), C.
striatus (Siriyappagouder et. al., 2014) and also from crustaceanstiger shrimp (P. monodon) (Krupesh et. al., 2010). The hypothesis wasproposed and substantiated first by Azad et al.
, (1997, 1999). They promoted a virulent A. hydrophila isolate to form biofilm on chitin flakes andsuccessfully utilized it as oral vaccine in catla, rohu and common carp.Biofilm vaccinated carps had significantly higher antibody titre and protectionthan free cell vaccinated fishes. Advantage of biofilm and free cell vaccines when studied by antigen localizationemploying monoclonal antibody based immunefluorescence, indicated that biofilmantigen compared to free cell antigen retained for longer time in largerquantities in gut and other lymphoid organs like kidney and spleen (Azad et al., 2000). The protective nature ofglycocalx was believed to be protecting biofilm from gastric hydrolysis as freecells showed a quicker clearance from lumen of hindgut within 6h of vaccineuptake compared to biofilm, which remained for 48h following oral delivery. Inaddition to that, biofilm might represent a more immunogenic material as seento have induced higher antibody and protective immunity (Azad et al.
, 1997, 1999) altogether making itas a better vaccine carrier in oral immunization regime, keeping thisbackground biofilm of A.hydrophila was evaluated fororal vaccination of walking catfish (Clarius batrachus) by Nayak et al., 2004. Serum agglutinating antibody titre and relativepercent survival (RPS) following challenge were found to be significantlyhigher in catfish fed with BF vaccinecompared to that with free cell vaccine. Another study by Siriyappagouder et al., 2014, with C.striatus, a carnivorous fish model, fed with biofilm (BF) and free cell(FC) of A. hydrophila with the samedose and duration of Nayak et.
al.,2004. They observed BF vaccinated fish upon challenge with A. hydrophila at 109 cfu/ml had significantly higherrelative per cent survival (88) than that with FC (29.6).
Krupeshet al., (2010) used biofilm of V. alginolyticus to study preliminaryimmune response in tiger shrimp (Penaeusmonodon) and the study showed that biofilm cells were superior to free cellin stimulating the non-specific immune response of Penaeus monodon. Conclusion Biofilm cells probed to be effective as oralvaccination of fin and shellfishes.
It can be considered as simple, cheap andideal oral vaccination technique for bulk administration. Production of Biofilmvaccines and its efficacy has been proven in institutional research experimentsand it is expected that the biofilm vaccine will available in the market by2018 as commercial production was undertaken by the aqua drug company toaddress the infectious diseases of Indian aquaculture.