Direct Regulons of AtxA, the Master Virulence Regulator of Bacillus anthracis

One Earth: The Equilibrium between the Human and the Bacterial Worlds

Misuse and abuse of antibiotics on humans, cattle, and crops have led to the selection of multi-resistant pathogenic bacteria, the most feared 'superbugs'. Infections caused by superbugs are progressively difficult to treat, with a subsequent increase in lethality: the toll on human lives is predicted to reach 10 million by 2050. Here we review three concepts linked to the growing resistance to antibiotics, namely (i) the Resistome, which refers to the collection of bacterial genes that confer resistance to antibiotics, (ii) the Mobilome, which includes all the mobile genetic elements that participate in the spreading of antibiotic resistance among bacteria by horizontal gene transfer processes, and (iii) the Nichome, which refers to the set of genes that are expressed when bacteria try to colonize new niches. We also discuss the strategies that can be used to tackle bacterial infections and propose an entente cordiale with the bacterial world so that instead of war and destruction of the 'fierce enemy' we can achieve a peaceful coexistence (the One Earth concept) between the human and the bacterial worlds. This, in turn, will contribute to microbial biodiversity, which is crucial in a globally changing climate due to anthropogenic activities.

Overlapping and Distinct Functions of the Paralogous PagR Regulators of Bacillus anthracis

The Bacillus anthracis pagA gene, encoding the protective antigen component of anthrax toxin, is part of a bicistronic operon on pXO1 that codes for its own repressor, PagR1. In addition to the pagAR1 operon, PagR1 regulates sap and eag, two chromosome genes encoding components of the surface layer, a mounting structure for surface proteins involved in virulence. Genomic studies have revealed a PagR1 paralog, PagR2, encoded by a gene on pXO2. The amino acid sequences of the paralogues are 71% identical and show similarity to the ArsR family of transcription regulators. We determined that the expression of either rPagR1 or rPagR2 in a ΔpagR1 pXO1⁺/pXO2^- (PagR1-PagR2) background repressed the expression of pagA, sap, eag, and a newly discovered target, atxA, encoding virulence activator AtxA. Despite the redundancy in PagR1 and PagR2 function, we determined that purified rPagR1 bound DNA corresponding to the control regions of all four target genes and existed as a dimer in cell lysates, whereas rPagR2 exhibited weak binding to the DNA of the pagA and atxA promoters, did not bind sap or eag promoter DNA, and did not appear as a dimer in cell lysates. A single amino acid change in PagR2, S81Y, designed to match the native Y81 of PagR1, allowed for DNA-binding to the sap and eag promoters. Moreover, the S81Y mutation allowed for the detection of PagR2 homomultimers in coaffinity purification experiments. Our results expand our knowledge of the roles of the paralogues in B. anthracis gene expression and provide a potential mechanistic basis for differences in the functions of these repressors. IMPORTANCE The protective antigen component of the anthrax toxin is essential for the delivery of the enzymatic components of the toxin into host target cells. The toxin genes and other virulence genes of B. anthracis are regulated by multiple trans-acting regulators that respond to a variety of host-related signals. PagR1, one such trans-acting regulator, connects the regulation of plasmid-encoded and chromosome-encoded virulence genes by controlling both protective antigen and surface layer protein expression. Whether PagR2, a paralog of PagR1, also functions as a trans-acting regulator was unknown. This work advances our knowledge of the complex model of virulence regulation in B. anthracis and furthers our understanding of the intriguing evolution of this pathogen.

Structural and Functional Analysis of Toxin and Small RNA Gene Promoter Regions in Bacillus anthracis

It was previously demonstrated that anthrax toxin activator (AtxA) binds directly to the σ^A-like promoter region of pagA (encoding protective antigen, PA) immediately upstream of the RNA polymerase binding site. In this study, using electrophoretic mobility shift assays and in vivo analyses, we identified AtxA-binding sites in the promoter regions of the lef and cya genes (encoding lethal and edema factors, respectively) and of two Bacillus anthracis small RNAs (XrrA and XrrB). Activities of all four newly studied promoters were enhanced in the presence of CO₂/bicarbonate and AtxA, as previously seen for the pagA promoter. Notably, the cya promoter was less activated by AtxA and CO₂/bicarbonate conditions. The putative promoter of a recently described third small RNA, XrrC, showed a negligible response to AtxA and CO₂/bicarbonate. RNA polymerase binding sites of the newly studied promoters show no consensus and differ from the σ^A-like promoter region of pagA. In silico analysis of the probable AtxA binding sites in the studied promoters revealed several palindromes. All the analyzed palindromes showed very little overlap with the σ^A-like pagA promoter. It remains unclear as to how AtxA and DNA-dependent RNA-polymerase identify such diverse DNA-sequences and differentially regulate promoter activation of the studied genes. IMPORTANCE Anthrax toxin activator (AtxA) is the major virulence regulator of Bacillus anthracis, the causative agent of anthrax. Understanding AtxA's mechanism of regulation could facilitate the development of therapeutics for B. anthracis infection. We provide evidence that AtxA binds to the promoters of the cya, lef, xrrA, and xrrB genes. In vivo assays confirmed the activities of all four promoters were enhanced in the presence of AtxA and CO₂/bicarbonate, as previously seen for the pagA promoter. The cya and lef genes encode important toxin components. The xrrA and xrrB genes encode sRNAs with a suggested function as cell physiology regulators. Our data provides further evidence for the direct regulatory role of AtxA that was previously shown with the pagA promoter.

Silkworm model for Bacillus anthracis infection and virulence determination

Bacillus anthracis is an obligate pathogen and a causative agent of anthrax. Its major virulence factors are plasmid-coded; however, recent studies have revealed chromosome-encoded virulence factors, indicating that the current understanding of its virulence mechanism is elusive and needs further investigation. In this study, we established a silkworm (Bombyx mori) infection model of B. anthracis. We showed that silkworms were killed by B. anthracis Sterne and cured of the infection when administered with antibiotics. We quantitatively determined the lethal dose of the bacteria that kills 50% larvae and effective doses of antibiotics that cure 50% infected larvae. Furthermore, we demonstrated that B. anthracis mutants with disruption in virulence genes such as pagA, lef, and atxA had attenuated silkworm-killing ability and reduced colonization in silkworm hemolymph. The silkworm infection model established in this study can be utilized in large-scale infection experiments to identify novel virulence determinants and develop novel therapeutic options against B. anthracis infections.