Phaeobacter gallaeciensis Reduces Vibrio anguillarum in Cultures of Microalgae and Rotifers, and Prevents Vibriosis in Cod Larvae

Abstract

Phaeobacter gallaeciensis can antagonize fish-pathogenic bacteria in vitro, and the purpose of this study was to evaluate the organism as a probiont for marine fish larvae and their feed cultures. An in vivo mechanism of action of the antagonistic probiotic bacterium is suggested using a non-antagonistic mutant. P. gallaeciensis was readily established in axenic cultures of the two microalgae Tetraselmis suecica and Nannochloropsis oculata, and of the rotifer Brachionus plicatilis. P. gallaeciensis reached densities of 107 cfu/ml and did not adversely affect growth of algae or rotifers. Vibrio anguillarum was significantly reduced by wild-type P. gallaeciensis, when introduced into these cultures. A P. gallaeciensis mutant that did not produce the antibacterial compound tropodithietic acid (TDA) did not reduce V. anguillarum numbers, suggesting that production of the antibacterial compound is important for the antagonistic properties of P. gallaeciensis. The ability of P. gallaeciensis to protect fish larvae from vibriosis was determined in a bath challenge experiment using a multidish system with 1 larva per well. Unchallenged larvae reached 40% accumulated mortality which increased to 100% when infected with V. anguillarum. P. gallaeciensis reduced the mortality of challenged cod larvae (Gadus morhua) to 10%, significantly below the levels of both the challenged and the unchallenged larvae. The TDA mutant reduced mortality of the cod larvae in some of the replicates, although to a much lesser extent than the wild type. It is concluded that P. gallaeciensis is a promising probiont in marine larviculture and that TDA production likely contributes to its probiotic effect.

Effect of TDA‐producing Phaeobacter inhibens on the fish pathogen Vibrio anguillarum in non‐axenic algae and copepod systems

“…Several co‐culture studies of P. inhibens and V. anguillarum in axenic systems have shown significant inhibition of vibrios, resulting in lowered vibrio counts relative to the inoculum level (D'Alvise et al ., 2010, 2012; Grotkjær et al ., 2016b). In contrast, P. inhibens kept V. anguillarum at the inoculum level in both the non‐axenic co‐culture experiments conducted in the present study and in similar experiments with non‐axenic Tetraselmis and Artemia (Grotkjær et al ., 2016a).…”

“…Furthermore, Phaeobacter spp. can protect larvae of turbot and cod in challenge trials (Planas et al ., 2006; D'Alvise et al ., 2012, 2013). Phaeobacter spp.…”

“…An alternative to antibiotics in such systems is probiotics, that is live microorganisms which exert beneficial effects on the host health (FAO and WHO, 2001), for example by inhibiting growth of pathogenic bacteria (Gatesoupe, 1999; Skjermo and Vadstein, 1999; Kesarcodi‐Watson et al ., 2008). Members of the genera Phaeobacter and Ruegeria which produce the broad‐spectrum antimicrobial compound tropodithietic acid (TDA) have proven probiotic properties (Ruiz‐Ponte et al ., 1999; Hjelm et al ., 2004; Planas et al ., 2006; D'Alvise et al ., 2012). Phaeobacter spp.…”

“…Phaeobacter spp. inhibit the growth of pathogenic vibrios and protect live feed such as rotifers and Artemia (D'Alvise et al ., 2012; Grotkjær et al ., 2016a,b). Furthermore, Phaeobacter spp.…”

“…and Ruegeria spp. have been carried out in axenic live feed or fish larvae systems (Planas et al ., 2006; D'Alvise et al ., 2012, 2013; Grotkjær et al ., 2016b). Although non‐axenic algae and Artemia systems have been tested (Grotkjær et al ., 2016a), the probiotic effect of Phaeobacter spp.…”