Bacillus anthracis Overcomes an Amino Acid Auxotrophy by Cleaving Host Serum Proteins

Abstract

Bacteria sustain an infection by acquiring nutrients from the host to support replication. The host sequesters these nutrients as a growth-restricting strategy, a concept termed "nutritional immunity." Historically, the study of nutritional immunity has centered on iron uptake because many bacteria target hemoglobin, an abundant circulating protein, as an iron source. Left unresolved are the mechanisms that bacteria use to attain other nutrients from host sources, including amino acids. We employed a novel medium designed to mimic the chemical composition of human serum, and we show here that Bacillus anthracis, the causative agent of anthrax disease, proteolyzes human hemoglobin to liberate essential amino acids which enhance its growth. This property can be traced to the actions of InhA1, a secreted metalloprotease, and extends to at least three other serum proteins, including serum albumin. The results suggest that we must also consider proteolysis of key host proteins to be a way for bacterial pathogens to attain essential nutrients, and we provide an experimental framework to determine the host and bacterial factors involved in this process. IMPORTANCEThe mechanisms by which bacterial pathogens acquire nutrients during infection are poorly understood. Here we used a novel defined medium that approximates the chemical composition of human blood serum, blood serum mimic (BSM), to better model the nutritional environment that pathogens encounter during bacteremia. Removing essential amino acids from BSM revealed that two of the most abundant proteins in blood-hemoglobin and serum albumin-can satiate the amino acid requirement for Bacillus anthracis, the causative agent of anthrax. We further demonstrate that hemoglobin is proteolyzed by the secreted protease InhA1. These studies highlight that common blood proteins can be a nutrient source for bacteria. They also challenge the historical view that hemoglobin is solely an iron source for bacterial pathogens. There are three broad, necessary, and common elements inherent in most if not all bacterial infections of vertebrate hosts. They are (i) the initial entry of the bacterial invader and adherence to host tissues, (ii) the production of virulence factors or toxins (usually proteins) that subvert host innate and acquired immune defense mechanisms, and (iii) the acquisition and construction of biomolecules to drive cellular metabolism for the production of progeny. There has been intense investigation, and knowledge of the biochemistry, cell biology, and contribution to disease of a diverse array of bacterial adhesins and virulence factors (points i and ii identified above) is growing. However, our knowledge of nutrient acquisition during pathogenesis has lagged, perhaps because it is viewed as being a necessary and somewhat mundane process that may lack the elegant sophistication observed with virulence factors. Considering that all systemic infections are absolutely dependent on the replication of bacterial cells, identifying the mechanisms by which ba...

Progress toward the Development of a NEAT Protein Vaccine for Anthrax Disease

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The social network of microorganisms — how auxotrophies shape complex communities

“…Upon perturbation, the abundance of and composition in the gut commensal microbial community can change, creating an opportunity for pathogen overgrowth. During infection, amino-acid-auxotrophic pathogens, such as Staphylococcus aureus 71 , Bacillus anthracis 94 or Streptococcus pyogenes 4 (red ovals), compete with commensal bacteria for host amino acids (blue dots) and promote physiological changes in the host by releasing toxins (purple triangles), thus triggering inflammation. Antibiotic treatment leads to a decrease in microbial abundance that affects not only commensals but also pathogens.…”

Selective substrate uptake: The role of ATP-binding cassette (ABC) importers in pathogenesis

“…Interestingly, as mentioned above the PepT family also includes nikABCDF and cntABCDF , which are not known to import peptides, but share the overall structural similarity to the peptide transporters in this family [93]. Peptide transporters are necessary for the import of peptides derived from host proteins, like hemoglobin and proteasomal degraded proteins [94,95] as a source of nutrients, and for importing environmental cues for cellular functions, such as, chemotaxis, conjugation, and sporulation [96–98]. The di-peptide transporter, dppABCDF [99], has been implicated in virulence, but only the oppABCDF , sapABCDF and yejABEF systems are directly identified as virulence factors [100–102].…”