ABSTRACT
Due to the mounting awareness of the risks associated with the use of antibiotics in aquaculture, treatment with probiotics has recently emerged as the preferred environmental-friendly prophylactic approach in marine larviculture. However, the presence of unknown and variable microbiota in fish larvae makes it impossible to disentangle the efficacy of treatment with probiotics. In this respect, the recent development of a germ-free culture model for European sea bass (Dicentrarchus labrax L.) larvae opened the door for more controlled studies on the use of probiotics. In the present study, 206 bacterial isolates, retrieved from sea bass larvae and adults, were screened in vitro for haemolytic activity, bile tolerance and antagonistic activity against six sea bass pathogens. Subsequently, the harmlessness and the protective effect of the putative probiotic candidates against the sea bass pathogen Vibrio harveyi were evaluated in vivo adopting the previously developed germ-free sea bass larval model. An equivalence trial clearly showed that no harmful effect on larval survival was elicited by all three selected probiotic candidates: Bacillus sp. LT3, Vibrio lentus and Vibrio proteolyticus. Survival of Vibrio harveyi challenged larvae treated with V. lentus was superior in comparison with the untreated challenged group, whereas this was not the case for the larvae supplemented with Bacillus sp. LT3 and V. proteolyticus. In this respect, our results unmistakably revealed the protective effect of V. lentus against vibriosis caused by V. harveyi in gnotobiotic sea bass larvae, rendering this study the first in its kind.
Subject(s)
Antibiosis , Bass/microbiology , Fish Diseases/microbiology , Fish Diseases/prevention & control , Probiotics , Vibrio Infections/veterinary , Vibrio/physiology , Animals , Aquaculture , Bacillus/physiology , Germ-Free Life , Vibrio Infections/microbiology , Vibrio Infections/prevention & controlABSTRACT
Most bacteria recovered so far from sugar thick juice during storage represent strains of the species Tetragenococcus halophilus. Recently, several Gram-positive, non-motile, non-spore-forming cocci with other physiological and genetic traits were isolated from sugar thick juice samples from different origins. In this study, representative isolates were investigated using a polyphasic taxonomic approach. The 16S rRNA gene sequence similarity between these isolates and their closest relative, Tetragenococcus muriaticus, was 97.4%. The level of DNA-DNA relatedness between isolate T1(T), representing the newly found Tetragenococcus isolates, and T. muriaticus was 57%. Isolate T1(T) had a DNA G+C content of 36.7 mol%. Phylogenetic data and genomic and phenotypic features demonstrated that the isolates represent a novel species, for which the name Tetragenococcus osmophilus sp. nov. is proposed with T1(T) as the type strain (=LMG 26041(T) =DSM 23765(T)). Additionally, T. halophilus isolates from high-salt and high-sugar environments showed clear differences in several physiological and genetic characteristics like RAPD fingerprints and 16S rRNA gene sequences. DNA-DNA hybridizations, however, showed 79 to 80% relatedness between osmophilic and halophilic T. halophilus isolates, demonstrating that the different strains belong to the same species. Based on the phenotypic and genotypic differences observed, as well as the different origins of the strains and the industrial relevance of thick juice degradation, two subspecies of T. halophilus are described in this manuscript: T. halophilus subsp. halophilus subsp. nov. for the strains isolated from salt media and T. halophilus subsp. flandriensis subsp. nov. for the strains isolated from sugar-rich environments, which were first isolated in Flanders, Belgium. The type strains for the subspecies are IAM 1676(T) (=LMG 11490(T) =DSM 20339(T)) and T5(T) (=LMG 26042(T) =DSM 23766(T)), respectively.
