Probiotic efficiency of Bacillus sp. in Labeo rohita challenged by Aeromonas hydrophila: assessment of stress profile, haemato-biochemical parameters and immune responses Ankita Nandi 1 , Goutam Banerjee 1 , Suhas Kumar Dan 1 , Koushik Ghosh 2 & Arun Kumar Ray 1 1 Department of Zoology, Visva-Bharati University, Santiniketan, West Bengal, India 2 Department of Zoology, University of Burdwan, Burdwan, West Bengal, India Correspondence: Goutam Banerjee, Department of Zoology, Visva-Bharati University, Santiniketan 731235, India. E-mail: banerjee.goutam2@gmail.com Abstract Fish are always susceptible to a wide variety of deadly pathogens which cause a huge loss in aqua- culture industries. In this investigation, we have demonstrated the in vivo probiotic efficiency of Bacillus sp. MVF1 (GenBank Acc. No. KP256503) in Labeo rohita challenged with pathogenic strain of Aeromonas hydrophila (MTCC 1739). To check the probiotic potential of the selected bacterial strain, fish were divided into four groups: control, D1, D2 and D3. A total of 100 days (70 days pro- biotic feeding + 71th day sampling and 28 days challenged test + 29th day sampling) of feeding trial was conducted. To establish the probiotic potential of Bacillus sp. MVF1, certain haematolog- ical parameters (haemoglobin, total erythrocyte and leucocyte count), serum biochemical parame- ters (total protein, albumin and globulin), immune parameters (serum lysozyme and total IgM levels) and hepatic stress profile (malondialdehyde pro- duction, superoxide dismutase and catalase activ- ity) have been measured. Our results demonstrated that red blood cell number, white blood cell num- ber and haemoglobin content were much higher in D2 group fish compared to other groups and con- trol fish. Similarly, total protein contents, albumin concentration, globulin concentration, lysozyme activity and IgM production were also recorded to be highest in D2 group fish. This finding clearly indicated the probiotic potential of Bacillus sp. MVF1 in L. rohita. Furthermore, our results also demonstrated that 1 9 10 7 CFU g 1 feed (D2) pro- vides better immunity compared to 1 9 10 5 (D1) and 1 9 10 9 (D3). Due to beneficial effects, the bacterium Bacillus sp. MVF1 might be useful in aquaculture industries to reduce the disease sus- ceptibility. Keywords: Aeromonas hydrophila, Labeo rohita, probiotic, haematological parameters, stress pro- file, immune response Introduction Fish are always susceptible to a wide range of bac- terial, fungal, parasitic and viral diseases. Poor water quality, high stocking density and over feed- ing enhance the disease susceptibility in aquatic animals, including fish (Sihag & Sharma 2012; Banerjee, Nandi & Ray 2016). Conventionally, several types of antimicrobial agents such as oxoli- nic acid, oxytetracycline, potential sulphonamides (sulphadiazine and trimethoprim), furazolidone, chloramphenicol and amoxicillin are used to pre- vent the disease outbreak in aquaculture industries (Cruz, Ibanez, Hermosillo & Saad 2012; Romero, Feijo o & Navarrete 2012). However, random and unscientific uses of such drugs create several envi- ronmental problems (emergence of drug-resistant bacteria, destruction of aquatic ecosystem and altercation of gut symbiotic flora) and thus have been restricted to several countries (Ibrahem 2015; Rajkishore, Das, Tiwari & Kumar 2015). To address such issues, probiotics are introduced as a substitute of antibiotics in aquaculture sectors. © 2017 John Wiley & Sons Ltd 1 Aquaculture Research, 2017, 1–12 doi: 10.1111/are.13255