The Capsule and S-Layer: Two Independent and Yet Compatible Macromolecular Structures in Bacillus anthracis

170

181

SummaryEntry into intestinal epithelial cells is an essential feature in the pathogenicity of Salmonella typhi, which causes typhoid fever in humans. This process requires intact motility and secretion of the invasion-promoting Sip proteins, which are targets of the type III secretion machinery encoded by the inv, spa and prg loci. During our investigations into the entry of S. typhi into cultured epithelial cells, we observed that the secretion of Sip proteins and flagellin was impaired in Vi-expressing strains. We report here that the production of Sip proteins, flagellin and Vi antigen is differentially modulated by the RcsB-RcsC regulatory system and osmolarity. This regulation occurs at both transcriptional and post-translational levels. Under low-osmolarity conditions, the transcription of iagA, invF and sipB genes is negatively controlled by the RcsB regulator, which probably acts in association with the viaB locusencoded TviA protein. The cell surface-associated Vi polysaccharide, which was maximally produced under these growth conditions, prevented the secretion of Sip proteins and flagellin. As the NaCl concentration in the growth medium was increased, transcription of iagA, invF and sipB was found to be markedly increased, whereas transcription of genes involved in Vi antigen biosynthesis was greatly reduced. The expression of iagA, whose product is involved in invF and sipB transcription, occurred selectively during the exponential growth phase and was maximal in the presence of 300 mM NaCl. At this osmolarity, large amounts of Sips and flagellin were secreted in culture supernatants. As expected from these results, and given the essential role of Sip proteins and motility in entry, RcsB and osmolarity modulated the invasive capacity of S. typhi. Together, these findings might reflect the adaptive response of S. typhi to the environments encountered during the different stages of pathogenesis.

Section: Discussionmentioning

confidence: 99%

The RcsB–RcsC regulatory system of Salmonella typhi differentially modulates the expression of invasion proteins, flagellin and Vi antigen in response to osmolarity

Arricau

Hermant

Waxin

et al. 1998

170

181

“…The only surface component that can be excluded as the attachment site is the S-layer that covers the bacterium and is located between the PG and the capsule. Indeed, S-layer and capsule are two compatible and yet independent structures (Mesnage et al ., 1998).…”

Section: Introductionmentioning

confidence: 99%

Bacillus anthracis CapD, belonging to the γ‐glutamyltranspeptidase family, is required for the covalent anchoring of capsule to peptidoglycan

Candela

Fouet

2005

150

Summary Several examples of bacterial surface‐structure anchoring have been described, but they do not include polyglutamate capsule. Bacillus anthracis capsule, which is composed only of poly‐γ‐ d‐glutamate, is one of the two major virulence factors of the bacterium. We analysed its anchoring. We report that the polyglutamate is anchored directly to the peptidoglycan and that the bond is covalent. We constructed a capD mutant strain, capD being the fourth gene of the capsule biosynthetic operon. The mutant bacilli are surrounded by polyglutamate material that is not covalently anchored. Thus, CapD is required for the covalent anchoring of polyglutamate to the peptidoglycan. Sequence similarities suggest that CapD is a γ‐glutamyltranspeptidase. Furthermore, CapD is cleaved at the γ‐glutamyltranspeptidase consensus cleavage site, and the two subunits remain associated, as necessary for γ‐glutamyltranspeptidase activity. Other Gram‐positive γ‐glutamyltranspeptidases are secreted, but CapD is located at the Bacillus surface, associated both with the membrane and the peptidoglycan. Polyglutamate is hydrolysed by CapD indicating that it is a CapD substrate. We suggest that CapD catalyses the capsule anchoring reaction. Interestingly, the CapD– strain is far less virulent than the parental strain.

“…Thus, AtxA co-ordinates an important regulatory network, which might extend to chromosomal genes. Our laboratory has been studying the S-layer, a major protein network that covers the cell surface underneath the capsule (Mesnage et al ., 1998). Through studies of a plasmidless strain, we have shown previously that the composition of this structure is growth phase regulated, so that exponential phase cells are covered by the Sap protomers, whereas stationary phase cells are covered by the EA1 protomers (Mignot et al ., 2002).…”

Section: Introductionmentioning

confidence: 99%

A plasmid‐encoded regulator couples the synthesis of toxins and surface structures in Bacillus anthracis

Mignot

Mock

Fouet³

2003

SummaryTranscription of the major Bacillus anthracis virulence genes is triggered by CO 2 , a signal believed to reflect the host environment. A 180 kb plasmid, pXO1, carries the anthrax toxin genes and the genes responsible for their regulation, pagR and atxA ; the latter encodes a major trans -activator. It has long been known that pXO1 genes have major effects on the physiology of B. anthracis , probably through regulatory cross-talk between plasmid and chromosomal genes. Accordingly, we found that the chromosomal S-layer genes, sap and eag , are regulated by pXO1 genes so that only eag is significantly expressed in the presence of CO 2 . This effect results from the product of pagR acting as the most downstream element of a signalling cascade initiated by AtxA. In vitro evidence showed that PagR is a transcription factor that controls the S-layer genes by direct binding on their promoter regions. This work provides evidence that AtxA is a master regulator that co-ordinates the response to host signals by orchestrating positive and negative controls over genes located on all genetic elements.