A metabolic switch is involved in lifestyle decisions in Photorhabdus luminescens.

Photorhabdus luminescens is a species of Gram-negative bacteria that is pathogenic to insects while also maintaining a mutualistic association with nematodes from the family Heterorhabditis. P. luminescens elaborates an extensive secondary metabolism during the post-exponential phase of growth that includes the production of an antibiotic called 3-5-dihydroxy-4-isopropylstilbene (ST), an anthraquinone pigment (AQ) and bioluminescence. In this study we identified a mutant that was unable to produce ST, AQ and light. This mutation was found to be in the mdh gene, encoding malate dehydrogenase, a key enzyme in the tricarboxylic acid (TCA) cycle. Interestingly the mdh mutant was unaffected in virulence but was unable to support nematode growth and development in vivo or in vitro. This clearly establishes that secondary metabolism in P. luminescens is required for the mutualistic interaction with the nematode. Furthermore, the construction of mutations in key genes in other central metabolic pathways confirmed the critical role for the TCA cycle in both secondary metabolism and mutualism, but not in virulence. Therefore, we conclude that the TCA cycle is required for the transition of P. luminescens from pathogen to mutualist implicating the involvement of a metabolic switch in the regulation of lifestyle decisions in this bacterium.

The Regulation of Secondary Metabolism and Mutualism in the Insect Pathogenic Bacterium Photorhabdus luminescens.

Photorhabdus is a genus of insect-pathogenic Gram-negative bacteria that also maintain a mutualistic interaction with nematodes from the family Heterorhabditis. This complex life cycle, involving different interactions with different invertebrate hosts, coupled with the amenability of the system to laboratory culture has resulted in the development of Photorhabdus as a model system for studying bacterial-host interactions. Photorhabdus is predicted to have an extensive secondary metabolism with the genetic potential to produce >20 different small secondary metabolites. Therefore, this system also presents us with a unique opportunity to study the contribution of secondary metabolism to the environmental fitness of the producing organism in its natural habitat (i.e., the insect and/or the nematode). In vivo and in vitro studies have revealed that the vast majority of the genetic loci in Photorhabdus predicted to be involved in the production of secondary metabolites appear to be cryptic and, to date, although several have been characterized, only three compounds have been studied in any great detail: 3,5-dihydroxy-4-isopropylstilbene, the ß-lactam antibiotic carbapenem, and an anthraquinone pigment. In this chapter, we describe how these compounds are made and the role (if any) that they have during the interactions between Photorhabdus and its invertebrate hosts. We will also outline recent work on the regulation of secondary metabolism in Photorhabdus and comment on how this has led to an increased understanding of mutualism in this bacterium.

Interaction of human complement with Sbi, a staphylococcal immunoglobulin-binding protein: indications of a novel mechanism of complement evasion by Staphylococcus aureus.

Staphylococcal immunoglobulin-binding protein, Sbi, is a 436-residue protein produced by many strains of Staphylococcus aureus. It was previously characterized as being cell surface-associated and having binding capacity for human IgG and beta(2)-glycoprotein I. Here we show using small angle x-ray scattering that the proposed extracellular region of Sbi (Sbi-E) is an elongated molecule consisting of four globular domains, two immunoglobulin-binding domains (I and II) and two novel domains (III and IV). We further show that together domains III and IV (Sbi-III-IV), as well as domain IV on its own (Sbi-IV), bind complement component C3 via contacts involving both the C3dg fragment and the C3a anaphylatoxin domain. Preincubation of human serum with either Sbi-E or Sbi-III-IV is inhibitory to all complement pathways, whereas domain IV specifically inhibits the alternative pathway. Monitoring C3 activation in serum incubated with Sbi fragments reveals that Sbi-E and Sbi-III-IV both activate the alternative pathway, leading to consumption of C3. By contrast, inhibition of this pathway by Sbi-IV does not involve C3 consumption. The observation that Sbi-E activates the alternative pathway is counterintuitive to intact Sbi being cell wall-associated, as recruiting complement to the surface of S. aureus would be deleterious to the bacterium. Upon re-examination of this issue, we found that Sbi was not associated with the cell wall fraction, but rather was found in the growth medium, consistent with it being an excreted protein. As such, our data suggest that Sbi helps mediate bacterial evasion of complement via a novel mechanism, namely futile fluid-phase consumption.