Shrimp culture industry has often been affected by infectious diseases mainly of bacterial and viral etiology causing loss of production [1]. As there are limitations in using antibiotics or vaccines for management of diseases in shrimp culture, there is growing interest in using immunostimulants. A range of preparations such as live and killed bacteria, glucans, peptidoglycans and lipopolysaccharide (LPS) have been used to stimulate the immune system of shrimp [2]. Recently, scope for vaccination of shrimp has also been demonstrated [3]. Bacterin prepared from Vibrio harveyi was found to enhance levels of hemolymph agglutinins in Penaeus monodon [4]. Use of Vibrio alginolyticus probiotic in Penaeus vannamei culture could effectively improve the survival and growth by competitively excluding potential pathogenic bacteria [5].
Route of administration determines the success of immunostimulation [6]. Oral route was reported to be most beneficial for administration of immunostimulants in shrimp [7]. Gut destruction of vaccines administered orally is a common hindrance in fish leading to poor response [8]. Therefore, to withstand hostile conditions of stomach or foregut, a gut resistant form of immunostimulant or vaccine is always required. In general, compared to free cells (FC), biofilm (BF) of bacteria is resistant to action of antibiotics [9], antibodies [10] and disinfectants [11]. The resistant nature of BF of Aeromonas hydrophila has been used for successful oral vaccination of carp [8]. The better performance of A. hydrophila BF vaccine in carp by superior antigen delivery to lymphoid tissues has been demonstrated by antigen localization using monoclonal antibodies [12]. Furthermore, as shrimp require continuous immune stimulation with antigenic preparations due to the absence of immune memory [6], a slow and steady releasing form of immunostimulant is needed. Against this background, the present study was undertaken to evaluate the immune response and resistance to experimental microbial infection in P. monodon fed with BF of V. alginolyticus.