Bacterial disease has been a major problem in shrimp hatcheries throughout the world. Antibiotics are generally used to treat bacterial diseases in aquaculture. The use of antibiotics to combat bacterial disease has resulted in the development of resistant strains of bacteria. A promising alternate to antibiotics for control of pathogenic bacteria is phage therapy. In this study, bacteriophages of Vibrio harveyi were isolated from maturation tank water samples from shrimp hatcheries. Of a total of 76 lytic phages, twelve phages were selected based on broad spectrum of infectivity a large number of V. harveyi isolates. Stability of the phages was examined by exposing them to various physio-chemical conditions such as pH, temperature and chloroform.
The bacteriophages analyzed in the present work were able to withstand pH variation ranging from 6 to 11 and could tolerate 50 oC for 1 hr. None of the phages were affected by chloroform (5% v/v) treatment. Determination of optimal culture conditions such as composition and pH of the growth medium and incubation temperature was important to get higher titer of phages. The pH 7.5 to 8.5 of growth medium, incorporation of salts of calcium and magnesium and incubation temperature ranging between 25 to 30oC helped in increasing phage infectivity as higher number of plaques.
In adsorption assay, a very rapid adsorption of phages to its host cells was observed during the first 15 min, followed by a slower rate of adsorption thereafter. The number of unabsorbed phage particles was approximately 32% and below 20% within 15 and 30 min after infection, respectively.
One step growth curve revealed that the latent period of twelve phages ranged from 15 to 40 minutes and raise periods were observed to range from 20 to 80 min. The burst size was estimated to range between 45 to160 pfu per infected cell.
фVh03, фVh05 and фVh15 had a large burst size correlating to the large size of plaques on solid media. These measures were useful in maintenance of phages and mass production for biocontrol of pathogenic bacteria. The results of the study would be helpful in formulation of effective phage preparation to control the pathogenic bacterial host and for identification of the pathogenic bacteria using phage typing.