[Diversity of antimicrobial bacteria associated with Siganus fuscescens from Naozhou island in Zhanjiang, China].

OBJECTIVE: Marine bacteria are a rich source of potentially useful antimicrobial molecules. The purpose of the study is to explore the diversity of bacteria with antimicrobial activity isolated from Siganus fuscescens gastrointestinal tract collected from Naozhou Island (20 degrees 52' N-20 degrees 56' N 110 degrees 33' E-110 degrees 38' E), Leizhou Bay, South China Sea. METHODS: We isolated bacteria from the gastrointestinal tract of fish sample using classical culturing technique, and determined antimicrobial activities of the isolates by Oxford cup method. We investigated diversity of antimicrobial isolates using phylogenetic comparative analysis of 16S rRNA gene sequences. RESULTS: According to the results of morphological observation and part of physiological and biochemical experiments, we isolated 68 strains from fish gastrointestinal tract. Among them, 19 strains with antimicrobial activities were acquired (27.9% of the isolates) and represented 19 different species, belonging to 12 genera (Rothia, Micrococcu, Brachybacterium, Brevibacterium, Psychrobacter, Paracoccus, Cobetia, Citrobacter, Pseudomonas, Bacillus, Lactobacillus and Staphylococcus) of 11 families (Microbacteriaceae, Dermabacteraceae, Brevibacteriaceae, Moraxellaceae, Rhodobacteraceae, Halomonadaceae, Enterobacteriaceae, Pseudomonadaceae, Bacillaceae, Lactobacillaceae and Staphylococcaceae) in three phyla (Actinobacteria, Proteobacteria and Firmicutes). Eight strains (42.1%), 7 strains (36.8%) and 4 strains (21.1%) were belonging to the phylum Firmicutes, Proteobacteria and Actinobacteria, respectively. The phylogenetic distance matrix results suggested that there were obvious genetic divergences between the majority of strains with antimicrobial activity and their phylogenetically most closely related typical strain, due to 16S rRNA gene sequences similarities ranging from 96.2 to 99.9%. In addition, 4 strains (ZJHD2-31, ZJHD5-23, ZJHD2-58 and M26) could represent potential new species, and identification of the novel strain M26 has been published in Antonie van Leeuwenhoek. CONCLUSION: There are abundant diversity for bacteria with antimicrobial activity and potentially more new species of microorganism in Siganus fuscescens gastrointestinal tract collected from Naozhou Island.

Paracoccus siganidrum sp. nov., isolated from fish gastrointestinal tract.

A bacterial strain, designated M26(T), was isolated from a fish gastrointestinal tract, collected from Zhanjiang Port, South China. 16S rRNA gene sequence analysis indicated that strain M26(T) belongs to the subclass α-Proteobacteria, being related to the genus Paracoccus, and sharing highest sequence similarity with Paracoccus alcaliphilus JCM 7364(T) (98.1 %), Paracoccus huijuniae FLN-7(T) (97.3 %), Paracoccus stylophorae KTW-16(T) (97.1 %) and Paracoccus seriniphilus DSM 14827(T) (96.9 %). The major quinone was determined to be ubiquinone Q-10, with Q-9 and Q-8 as minor components. The major fatty acid was identified as C18:1ω7c, with smaller amounts of C18:0 and C16:0. The G+C content of the genomic DNA was determined to be 64.3 mol%. The DNA hybridization value between strain M26(T) and the most closely related type strain, P. alcaliphilus, was 29.0 ± 1.0 %. The results of physiological and biochemical tests and low DNA-DNA relatedness showed that the strain could be readily distinguished from closely related species. On the basis of these phenotypic and genotypic data, strain M26(T) is concluded to represent a novel species of the genus Paracoccus, for which the name Paracoccus siganidrum sp. nov. is proposed. The type strain is M26(T) (=CCTCC AB 2012865(T) = DSM 26381(T)).

Effects of Rhodotorula mucilaginosa on the Immune Function and Gut Microbiota of Mice.

As a dominant species among marine yeasts, Rhodotorula benthica accounts for ~50% of all marine yeasts. Rhodotorula is rich in a variety of bioactive substances and commonly used in the production of carotenoids by microbial fermentation and is worth developing. Therefore, the present study used a strain of Rhodotorula mucilaginosa isolated from the coastal waters of the South China Sea as the target yeast to investigate its impact on the immune function and gut microbiota of mice. A total of 200 mice were randomly divided into gavage groups and control group and garaged for 30 consecutive days at different concentration. Samples were collected on day 15 and day 30 of gavage administration. The results showed that R. mucilaginosa ZTHY2 could increase the thymus and spleen indices of mice, and its effect on the thymus index was more significant after long-term gavage administration. Short-term (15 days) gavage administration of R. mucilaginosa suspension enhanced delayed hypersensitivity in mice, increased serum IgG, IgA, and IL-2. Long-term (30 days) gavage administration of R. mucilaginosa suspension significantly enhanced the phagocytosis of macrophages in mice and significantly increased serum TNF-α and INF-γ. R. mucilaginosa ZTHY2 altered the structure of the gut microbiota of mice at the phylum and genus levels, leading to an increased relative abundance of Firmicutes and Lactobacillus and a decreased relative abundance of Bacteroidetes. This strain increased the beneficial intestinal bacteria and reduced the harmful intestinal bacteria in mice. This study provides experimental evidence and lays the foundation for the future development and application of this strain as a microecological source of carotenoids.