Effects of woody forages on biodiversity and bioactivity of aerobic culturable gut bacteria of tilapia (Oreochromis niloticus)

because it is relatively costly [4, 5] and it can lead to water pollution [6]. Therefore, finding some new protein sources used in aquaculture feed has become a focus of aquaculture research [7]. Plant-derived forage has advantages of being obtained from a wide variety of sources and being low cost. With respect to animal-derived feed, plant-derived feed broadens aquaculture feed resources and reduces feed costs [8]. Moreover, some plant-derived forages with high nutritional value and that are rich in amino acids have been widely developed and utilized. Recently, cotton meal [9], soybean meal and peanut meal [10, 11] have been widely used in aquaculture feed and are very valuable. Some woody plants with nutritional value and low price are ideal new feed for aquatic ani- mals [12]. Woody materials like Moringa oleifera Lam (MOL), the dry leaves of which contain 30.3% crude protein [13], and Folium mori (FM) and Broussonetia papyrifera fruits containing some antioxidant substances [14, 15]. Furthermore, the woody plants have great resource advantages, and their utilization needs to be improved in China. In previous studies, Rugang Han [16] used MOL leaf meal as aquatic feed, while Kaviraj et al. [17] used fermented FM to feed the Indian major carp Labeo rohita. On the other hand, Lim et al. [18] and Zaineldin et al. [19] have shown that the rational use of microbial fermentation technology can produce fer- mented feeds that can improve feed digestibility, feed intake and immunity of aquatic animals.

These recent studies have indicated that the application of woody forages, even after their fer- mentation, in aquatic animal feed has broad application. In recent years, studies have shown that woody forage has a great impact on fish intestinal microbes [20]. After feeding the golden pompano Trachinotus ovatus woody forage or refer- ence feed, the intestinal bacterial communities of the two groups was different at the phylum level. The most abundant phyla in fish fed the reference feed were Fusobacteria (30.2%), Firmi- cutes (29.7%), Proteobacteria (27.3%) and Bacteroidetes (9.5%), whereas Proteobacteria (39.1, 23.8, 16.4 and 46.1%), Firmicutes (17.5, 46.7, 55.9 and 18.2%), Fusobacteria (26.7, 14.8, 17.1 and 14.9%) and Bacteroidetes (8.2, 10.7, 9.0 and 7.7%) were the dominant phyla in the fish fed FM, MOL, Neolamarckia cadamba and Broussonetia papyrifera, respectively. The observed dif- ferences might be due to the presence of bacteria in the intestinal wall, mucosa and contents of tilapia [21]. Insights into the microbial diversity present in the intestines of fish may increase our understanding of the metabolic potential of intestinal microorganisms, the roles of some feed additives and the host health [22, 23]. In addition, because fish are exposed to a very high microbial load in the aquatic environments, this close contact with the surrounding water likely affects early gut colonization [24]. Water appears to affect the fish gut microbiota from the mouth onwards [25], but feed has important effects on the fish gut microbiome at a later

development stage [26, 27]. Intestinal microorganisms play important roles in nutrient metab- olism secreting number of digestive enzymes, and growth factors that promote the metabolic activities of animals [28]. Few studies have investigated the effect of woody feed on the aerobic culturable intestinal bacteria of aquatic animals. Thus, the first objective of the present study was to explore the effects of four woody for- ages, including MOL leaf meal, FM meal, fermented MOL leaf meal and fermented FM meal, on the aerobic culturable intestinal bacteria present in tilapia to gain a theoretical basis for the application of the woody forages in the feed of tilapia and other types of fish. The increased interest during the last decade on bacteria in the gut of fish is related to the fact that some bacteria often produce bacteriocins and other chemical compounds that may inhibit colonization of pathogenic bacteria in the gut. Therefore, the second aim of the present study was whether it is possible to find bacteria that inhibit pathogens in vitro through cultur- able methods. PLOS ONE Effects of wood forages on gut bacterial biodiversity and bioactivity in tilapia PLOS ONE | https://doi.org/10.1371/journal.pone.0235560 July 2, 2020 2 / 17