Antibacterial metabolites in the microbial and phytoplankton flora of the "green water" culture of shrimp (Penaeus monodon): Part I. Anti-luminous vibrio factors associated with the "green water" grow-out culture of shrimp (Penaeus monodon)

Abstract

The ability of the “green water” grow-out culture of black tiger shrimp (Penaeus monodon) to prevent outbreaks of luminous vibriosis was investigated by screening associated isolates of bacteria, fungi, phytoplankton and fish skin mucus for anti-luminous Vibrio metabolites. Among the 87 bacterial isolates tested, 62 (71%) caused inhibition (+~+++) of the pathogen after 24-48 hr co-cultivation. The variation in growth inhibition rates of +, ++, and +++ were demonstrated by 14 (16%), 13 (15%), and 28 (32%) isolates, respectively, 24 hr after treatment. Eight bacterial isolates showed consistently sustained maximum inhibition (+++/+++) of luminous Vibrio after 24-48 hr exposure. Majority of these luminous Vibrio inhibiting bacterial isolates were obtained from tilapia mucus and gut. In tests with fungi, 4 of 20 (20%) yeast isolates showed intracellular metabolites inhibitory to luminous Vibrio. Among filamentous fungi, 5 of 45 (11%) isolates yielded intracellular metabolites while 3 of 41 (7%) isolates had extracellular metabolites inhibitory to luminous Vibrio. These fungal isolates were identified as Rhodotorula sp., Saccharomyces sp., Candida sp., Penicillium sp., mycelia sterilia and two unidentified species. The microalgae, Chaetoceros calcitrans and Nitzchia sp. consistently demonstrated complete inhibition of luminous Vibrio from 24 and 48 hr post-exposure, respectively, and during the 7 day experiment. Leptolyngbia sp. caused a 94-100% reduction of the luminous Vibrio population from 10⁴ to 10¹ cfu/mL 24 hr post-exposure which was sustained throughout the 10-day observation period. In contrast, the inhibitory effects of Skeletonema costatum on luminous Vibrio was bacteriostatic throughout the 7 days exposure while Nannochlorum sp. did not significantly inhibit luminous Vibrio. The skin mucus of jewel tilapia Tilapia hornorum had no resident luminous bacteria and inhibited this bacterial pathogen in 6-48 hr proportionate to the 10³ and 10⁵ cfu/mL test concentrations of luminous Vibrio. This study provides scientific explanation that the effectivity of the “green water” culture of tiger shrimp in preventing outbreaks of luminous vibriosis among P. monodon juveniles in grow-out ponds can be attributed to the presence of anti-luminous Vibrio factors in the bacterial, fungal, phytoplankton flora and the skin mucus of tilapia associated with this novel technique of shrimp culture.