Our view of host-associated microbiota remains incomplete due to the presence of as yet uncultured constituents. The Bacteroidales family S24-7 is a prominent example of one of these groups. Marker gene surveys indicate that members of this family are highly localized to the gastrointestinal tracts of homeothermic animals and are increasingly being recognized as a numerically predominant member of the gut microbiota; however, little is known about the nature of their interactions with the host. Here, we provide the first whole genome exploration of this family, for which we propose the name “Candidatus Homeothermaceae,” using 30 population genomes extracted from fecal samples of four different animal hosts: human, mouse, koala, and guinea pig. We infer the core metabolism of “Ca. Homeothermaceae” to be that of fermentative or nanaerobic bacteria, resembling that of related Bacteroidales families. In addition, we describe three trophic guilds within the family, plant glycan (hemicellulose and pectin), host glycan, and α-glucan, each broadly defined by increased abundance of enzymes involved in the degradation of particular carbohydrates. “Ca. Homeothermaceae” representatives constitute a substantial component of the murine gut microbiota, as well as being present within the human gut, and this study provides important first insights into the nature of their residency. The presence of trophic guilds within the family indicates the potential for niche partitioning and specific roles for each guild in gut health and dysbiosis.

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Genomic characterization of the uncultured Bacteroidales family S24-7 inhabiting the guts of homeothermic animals

Antimicrobial peptides (AMPs): Ancient compounds that represent novel weapons in the fight against bacteria.

Publisher Summary This chapter provides an overview of the biology of Bacillus thuringiensis ( Bt ). The chapter examines how the recently solved X-ray crystal structure of one Bt toxin allows to model the mode of action of a whole family of related protein toxins, and how a knowledge of the physiology of the insect targets of Bt toxins is needed for a better understanding of the toxic mechanism. Bacillus thuringiensis ( Bt ), a family of bacteria which make insecticidal proteins, accounts for 90–95% of the insect biocontrol market. This chapter illustrates that Bt is the name given to a family of bacteria found throughout the world. Paradoxically, CryIIIA, the only Bt toxin whose three-dimensional structure has been solved, kills very small insects, and its insolubility at neutral pH makes it difficult to assay in vitro . The crystal structure of CryIIIA permits site-directed mutagenesis and segment swapping experiments to be designed in a rational manner in order to identify the functional domains of the toxins. This chapter explores one attractive prospect that can be the replacement of the specificity domain with a binding domain directed to a target of choice, opening up the possibility of “designer” pesticides or a new class of immunotoxins. Characterization of toxin receptors and investigation of the mechanism of toxin synergism may assist in strategies for resistance management, a vital concern if Bt is to maintain its important position as the most extensively used biological insecticide.

Mechanism of Action of Bacillus thuringiensis Insecticidal δ-Endotoxins

Inbreeding and loss of genetic diversity are predicted to decrease the resistance of species to disease. However, this issue is controversial and there is limited rigorous scientific evidence available. To test whether inbreeding and loss of genetic diversity affect a host''s resistance to disease, Drosophila melanogasterpopulations with different levels of inbreeding and genetic diversity were exposed separately to (a) thuringiensin, an insecticidal toxin produced by some strains of Bacillus thuringiensis, and (b) live Serratia marcescensbacteria. Inbreeding and loss of genetic diversity significantly reduced resistance of D. melanogasterto both the thuringiensin toxin and live Serratia marcescens. For both, the best fitting relationships between resistance and inbreeding were curvilinear. As expected, there was wide variation among replicate inbred populations in disease resistance. Lowered resistances to both the toxin and the pathogen in inbred populations were due to specific resistance alleles, rather than generalized inbreeding effects, as correlations between resistance and population fitness were low or negative. Wildlife managers should strive to minimise inbreeding and loss of genetic diversity within threatened populations and to minimise exposure of inbred populations to disease.

Does inbreeding and loss of genetic diversity decrease disease resistance

Publisher Summary This chapter discusses the structure and function of major groups of hemolymph proteins that are common to all insects, storage proteins, lipoproteins, vitellogenins, and inducible antibacterial proteins. It also discusses some proteins and peptides that are present in smaller amounts, sometimes occurring only in a few insect species. With the development of microsequencing techniques that can provide information about the sequence of amino acids at the N-terminal end of an intact protein on a sample of less than 20 picomoles, it has become feasible to use one- or two-dimensional polyacrylamide gel electrophoresis (PAGE) a the protein purification method of choice. In many cases, proteins can be transferred directly from gels to derivatized paper or other media and spots cut from the medium can be inserted directly into the sequencer. If necessary, proteins on the medium can be cleaved to peptides, which can be separated by high-pressure liquid chromatography (HPLC) or by PAGE.

Insect Haemolymph Proteins

The insect pathogen Bacillus thuringiensis produces an exoprotease, inhibitor A, at the beginning of the stationary growth phase. In vitro, the enzyme selectively destroys cecropins and attacins, two antibacterial proteins found in immune hemolymph from Hyalophora cecropia. The specificity of this enzyme was investigated using cecropin A(1–33) and HPLC for separation and characterization of the fragments obtained.



A maximum of 12 different peptides were produced and their positions in the known sequence of cecropin A(1–33) were deduced from their amino acid compositions. The enzyme did not show a stringent requirement for a specific amino acid sequence at the cleavage site but prefers a hydrophobic residue on the C-terminal side. The specificity of the enzyme is explained in terms of the open structure of the cecropins and a pronounced inability of inhibitor A to attack globular proteins.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1984.tb08000.x

Characterization of inhibitor A, a protease from Bacillus thuringiensis which degrades attacins and cecropins, two classes of antibacterial proteins in insects.

Gunnel Dalhammar

Papers

Characterization of inhibitor A, a protease from Bacillus thuringiensis which degrades attacins and cecropins, two classes of antibacterial proteins in insects.

Insect immunity: Inducible antibacterial activity in Drosophila