Gut microbiota has become an integral component of the host, and received increasing attention. However, for many domestic animals, information on the microbiota is insufficient and more effort should be exerted to manage the gastrointestinal bacterial community. Understanding the factors that influence the composition of microbial community in the host alimentary canal is essential to manage or improve the microbial community composition. In the present study, 16S rRNA gene sequence-based comparisons of the bacterial communities in the grass carp (Ctenopharyngodon idellus) intestinal contents and fish culture-associated environments are performed. The results show that the fish intestinal microbiota harbors many cellulose-decomposing bacteria, including sequences related to Anoxybacillus, Leuconostoc, Clostridium, Actinomyces, and Citrobacter. The most abundant bacterial operational taxonomic units (OTUs) in the grass carp intestinal content are those related to feed digestion. In addition, the potential pathogens and probiotics are important members of the intestinal microbiota. Further analyses show that grass carp intestine holds a core microbiota composed of Proteobacteria, Firmicutes, and Actinobacteria. The comparison analyses reveal that the bacterial community in the intestinal contents is most similar to those from the culture water and sediment. However, feed also plays significant influence on the composition of gut microbiota.

High-throughput sequencing approaches have enabled characterizations of the community composition of numerous gut microbial communities, which in turn has enhanced interest in their diversity and functional relationships in different groups of vertebrates. Although fishes represent the greatest taxonomic and ecological diversity of vertebrates, our understanding of their gut microbiota and its functional significance has lagged well behind that of terrestrial vertebrates. In order to highlight emerging issues, we provide an overview of research on fish gut microbiotas and the biology of their hosts. We conclude that microbial community composition must be viewed within an informed context of host ecology and physiology, and that this is of particular importance with respect to research planning and sampling design.

Daptomycin is the first of a new class of cyclic lipopeptide antibiotics used against multidrug-resistant, gram-positive pathogens. The proposed mechanism of action involves disruption of the functional integrity of the bacterial membrane in a Ca 2؉ -dependent manner. We have used transcriptional profiling to demonstrate that treatment of Bacillus subtilis with daptomycin strongly induces the lia operon including the autoregulatory LiaRS two-component system (homologous to Staphylococcus aureus VraSR). The lia operon protects against daptomycin, and deletion of liaH, encoding a phage-shock protein A (PspA)-like protein, leads to threefold increased susceptibility. Since daptomycin interacts with the membrane, we tested mutants with altered membrane composition for effects on susceptibility. Deletion mutations of mprF (lacking lysyl-phosphatidylglycerol) or des (lipid desaturase) increased daptomycin susceptibility, whereas overexpression of MprF decreased susceptibility. Conversely, depletion of the cell for the anionic lipid phosphatidylglycerol led to increased resistance. Fluorescently labeled daptomycin localized to the septa and in a helical pattern around the cell envelope and was delocalized upon the depletion of phosphatidylglycerol. Together, these results indicate that the daptomycin-Ca 2؉ complex interacts preferentially with regions enriched in anionic phospholipids and leads to membrane stresses that can be ameliorated by PspA family proteins.

Whereas most prokaryotes rely on binary fission for propagation, many species use alternative mechanisms, which include multiple offspring formation and budding, to reproduce. In some bacterial species, these eccentric reproductive strategies are essential for propagation, whereas in others the programmes are used conditionally. Although there are tantalizing images and morphological descriptions of these atypical developmental processes, none of these reproductive structures are characterized at the molecular genetic level. Now, with newly available analytical techniques, model systems to study these alternative reproductive programmes are being developed.

Cells rely on diffusion to move metabolites and biomolecules. Diffusion is highly efficient but only over short distances. Although eukaryotic cells have broken free of diffusion-dictated constraints on cell size, most bacteria and archaea are forced to remain small. Exceptions to this rule are found among the bacterial symbionts of surgeonfish; Epulopiscium spp. are cigar-shaped cells that reach lengths in excess of 600 m. A large Epulopiscium contains thousands of times more DNA than a bacterium such as Escherichia coli, but the composition of this DNA is not well understood. Here, we present evidence that Epulopiscium contains tens of thousands of copies of its genome. Using quantitative, single-cell PCR assays targeting single-copy genes, we have determined that copy number is positively correlated with Epulopiscium cell size. Although other bacteria are known to possess multiple genomes, polyploidy of the magnitude observed in Epulopiscium is unprecedented. The arrangement of genomes around the cell periphery may permit regional responses to local stimuli, thus allowing Epulopiscium to maintain its unusually large size. Surveys of the sequences of single-copy genes (dnaA, recA, and ftsZ) revealed genetic homogeneity within a cell consistent with only a small amount (Ϸ1%) of the parental DNA being transferred to the next generation. The results also suggest that the abundance of genome copies in Epulopiscium may allow for an unstable genetic feature, a long mononucleotide tract, in an essential gene. With the evolution of extreme polyploidy and large cell size, Epulopiscium has acquired some of the advantages of eukaryotic cells.dnaA ͉ Epulopiscium ͉ large bacteria ͉ mononucleotide repeat ͉ polyploid I t is well appreciated that many eukaryotes are orders of magnitude larger than all known members of the Bacterial and Archaeal domains. Eukaryotes have broken free of constraints on cell size by the development of sophisticated nutrient uptake systems, subcellular compartmentalization, and the use of a cytoskeleton and motor proteins to transport vesicles. With the further advance of multicellularity, cell and tissue specialization have allowed eukaryotes to attain tremendous dimensions (1, 2). Until recently (3, 4), bacterial (and archaeal) cells were considered simple, displaying little subcellular organization. Although we now know that bacterial cells are also highly organized, possessing motor and cytoskeletal proteins, and even extensive intracellular membranes in some instances (5), these cells are believed to rely on diffusion to access nutrients and other metabolically important chemicals. Diffusion coefficients of small molecules may impose time constraints on metabolite flux (6) that require bacterial cells to maintain very high surface-tovolume ratios. As a result, no part of the cytoplasm is very far from the external environment, and so exchange is rapid enough to sustain metabolic processes. Most large bacteria fit this paradigm and maintain a very thin cytoplasm; they are extremely long and sle...