SummaryRickettsia conorii, an obligate intracellular tickborne pathogen and the causative agent of Mediterranean spotted fever, binds to and invades non-phagocytic mammalian cells. Previous work identified Ku70 as a mammalian receptor involved in the invasion process and identified the rickettsial autotransporter protein, rOmpB, as a ligand; however, little is known about the role of Ku70-rOmpB interactions in the bacterial invasion process. Using an Escherichia coli heterologous expression system, we show here that rOmpB mediates attachment to mammalian cells and entry in a Ku70-dependent process. A purified recombinant peptide corresponding to the rOmpB passenger domain interacts with Ku70 and serves as a competitive inhibitor of adherence. We observe that rOmpB-mediated infection culminates in actin recruitment at the bacterial foci, and that this entry process relies in part on actin polymerization likely imparted through protein tyrosine kinase and phosphoinositide 3-kinase-dependent activities and microtubule stability. Small-interfering RNA studies targeting components of the endocytic pathway reveal that entry by rOmpB is dependent on c-Cbl, clathrin and caveolin-2. Together, these results illustrate that rOmpB is sufficient to mediate Ku70-dependent invasion of mammalian cells and that clathrin-and caveolin-dependent endocytic events likely contribute to the internalization process.

cThe LytR-CpsA-Psr (LCP) proteins are thought to transfer bactoprenol-linked biosynthetic intermediates of wall teichoic acid (WTA) to the peptidoglycan of Gram-positive bacteria. In Bacillus subtilis, mutants lacking all three LCP enzymes do not deposit WTA in the envelope, while Staphylococcus aureus ⌬lcp mutants display impaired growth and reduced levels of envelope phosphate. We show here that the S. aureus ⌬lcp mutant synthesized WTA yet released ribitol phosphate polymers into the extracellular medium. Further, ⌬lcp mutant staphylococci no longer restricted the deposition of LysM-type murein hydrolases to cell division sites, which was associated with defects in cell shape and increased autolysis. Mutations in S. aureus WTA synthesis genes (tagB, tarF, or tarJ2) inhibit growth, which is attributed to the depletion of bactoprenol, an essential component of peptidoglycan synthesis (lipid II). The growth defect of S. aureus tagB and tarFJ mutants was alleviated by inhibition of WTA synthesis with tunicamycin, whereas the growth defect of the ⌬lcp mutant was not relieved by tunicamycin treatment or by mutation of tagO, whose product catalyzes the first committed step of WTA synthesis. Further, sortase A-mediated anchoring of proteins to peptidoglycan, which also involves bactoprenol and lipid II, was not impaired in the ⌬lcp mutant. We propose a model whereby the S. aureus ⌬lcp mutant, defective in tethering WTA to the cell wall, cleaves WTA synthesis intermediates, releasing ribitol phosphate into the medium and recycling bactoprenol for peptidoglycan synthesis.

The pathogenesis of spotted fever group (SFG) Rickettsia species, including R. conorii and R. rickettsii, is acutely dependent on adherence to and invasion of host cells, including cells of the mammalian endothelial system. Bioinformatic analyses of several rickettsia genomes revealed the presence of a cohort of genes designated sca genes that are predicted to encode proteins with homology to autotransporter proteins of Gram-negative bacteria. Previous work demonstrated that three members of this family, rOmpA (Sca0), Sca2, and rOmpB (Sca5) are involved in the interaction with mammalian cells; however, very little was known about the function of other conserved rickettsial Sca proteins. Here we demonstrate that sca1, a gene present in nearly all SFG rickettsia genomes, is actively transcribed and expressed in R. conorii cells. Alignment of Sca1 sequences from geographically diverse SFG Rickettsia species showed that there are high degrees of sequence identity and conservation of these sequences, suggesting that Sca1 may have a conserved function. Using a heterologous expression system, we demonstrated that production of R. conorii Sca1 in the Escherichia coli outer membrane is sufficient to mediate attachment to but not invasion of a panel of cultured mammalian epithelial and endothelial cells. Furthermore, preincubation of a recombinant Sca1 peptide with host cells blocked R. conorii cell association. Together, these results demonstrate that attachment to mammalian cells can be uncoupled from the entry process and that Sca1 is involved in the adherence of R. conorii to host cells.

Pathogenic rickettsiae are the causative agents of Rocky Mountain spotted fever, typhus, and other human diseases with high mortality and an important impact on society. Although survivors of rickettsial infections are considered immune to disease, the molecular basis of this immunity or the identification of protective antigens that enable vaccine development was hitherto not known. By exploring the molecular pathogenesis of Rickettsia conorii, the agent of Mediterranean spotted fever, we report here that the autotransporter protein, rickettsial outer membrane protein B (rOmpB), constitutes a protective antigen for this group of pathogens. A recombinant, purified rOmpB passenger domain fragment comprised of amino acids 36 to 1334 is sufficient to elicit humoral immune responses that protect animals against lethal disease. Protective immunity requires folded antigen and production of antibodies that recognize conformational epitopes on the rickettsial surface. Monoclonal antibodies (MAbs) 5C7.27 and 5C7.31, which specifically recognize a conformation present in the folded, intact rOmpB passenger domain, are sufficient to confer immunity in vivo. Analyses in vitro indicate this protection involves a mechanism of complement-mediated killing in mammalian blood, a means of rickettsial clearance that has not been previously described. Considering the evolutionary conservation of rOmpB and its crucial contribution to bacterial invasion of host cells, we propose that rOmpB antibody-mediated killing confers immunity to rickettsial infection.

Background: Staphylococcus aureus LcpABC attach wall teichoic acids (WTA) to peptidoglycan. Results: S. aureus capsular polysaccharide (CP5) is linked to peptidoglycan in a manner requiring lcpABC genes. Conclusion: Unlike WTA, CP5 attachment is mediated preferentially by LcpC. Significance: LCP proteins display substrate preferences for the transfer of undecaprenyl-bound polymers to peptidoglycan.

The envelope of Staphylococcus aureus is comprised of peptidoglycan and its attached secondary polymers, teichoic acid, capsular polysaccharide, and protein. Peptidoglycan synthesis involves polymerization of lipid II precursors into glycan strands that are cross-linked at wall peptides. It is not clear whether peptidoglycan structure is principally determined during polymerization or whether processive enzymes affect cell wall structure and function, for example, by generating conduits for protein secretion. We show here that S. aureus lacking SagB, a membrane-associated N-acetylglucosaminidase, displays growth and cell-morphological defects caused by the exaggerated length of peptidoglycan strands. SagB cleaves polymerized glycan strands to their physiological length and modulates antibiotic resistance in methicillin-resistant S. aureus (MRSA). Deletion of sagB perturbs protein trafficking into and across the envelope, conferring defects in cell wall anchoring and secretion, as well as aberrant excretion of cytoplasmic proteins. IMPORTANCEStaphylococcus aureus is thought to secrete proteins across the plasma membrane via the Sec pathway; however, protein transport across the cell wall envelope has heretofore not been studied. We report that S. aureus sagB mutants generate elongated peptidoglycan strands and display defects in protein secretion as well as aberrant excretion of cytoplasmic proteins. These results suggest that the thick peptidoglycan layer of staphylococci presents a barrier for protein secretion and that SagB appears to extend the Sec pathway across the cell wall envelope. Staphylococcus aureus, a Gram-positive bacterial pathogen, replicates via septal assembly of membranes and peptidoglycan into the cross wall compartment (1, 2). The peptidoglycan of the cross wall is split by murein hydrolases, separating daughter cells that assume a spherical shape (3). Earlier work identified three murein hydrolases with cross-wall-splitting activities: Atl (autolysin), Sle1, and LytN (3-5). Atl and Sle1 are secreted into the extracellular milieu and subsequently cleave septal peptidoglycan at the cross wall but not elsewhere as access is restricted by teichoic acid modification of peptidoglycan (6-8). LytN, on the other hand, is secreted into the cross wall compartment (5). S. aureus Atl is synthesized as a preproenzyme with an N-terminal signal peptide and prodomain (9, 10). Secreted pro-Atl is processed to generate Atl N-acetylmuramoyl-L-Ala-amidase (Atl AM ) and Atl Nacetylglucosaminidase (Atl GL ), and each binds via GW domains to lipoteichoic acids (10-12). Earlier work demonstrated that Atl functions as an endo-␤-N-acetylglucosaminidase (12, 13). Although initially designated autolysin (Atl), the S. aureus atl mutant does not display an autolysis phenotype yet forms large clusters of incompletely separated bacteria and is defective for penicillin-induced killing (14). The LysM domains of Sle1 promote its binding to cross wall peptidoglycan, and sle1 mutants also form clusters of incompletely s...

The pathogenic lifecycle of obligate intracellular bacteria presents a superb opportunity to develop understanding of the interaction between the bacteria and host under the pretext that disruption of these processes will likely lead to death of the pathogen and prevention of associated disease. Species of the genus Rickettsia contain some of the most hazardous of the obligate intracellular bacteria, including Rickettsia rickettsii and R. conorii the causative agents of Rocky Mountain and Mediterranean spotted fevers, respectively. Spotted fever group Rickettsia species commonly invade and thrive within cells of the host circulatory system whereby the endothelial cells are severely perturbed. The subsequent disruption of circulatory continuity results in much of the severe morbidity and mortality associated with these diseases, including macropapular dermal rash, interstitial pneumonia, acute renal failure, pulmonary edema, and other multisystem manifestations. This review describes current knowledge of the essential pathogenic processes of adherence to and invasion of host cells, efforts to disrupt these processes, and potential for disease prevention through vaccination with recently identified bacterial adherence and invasion proteins. A more complete understanding of these bacterial proteins will provide an opportunity for prevention and treatment of spotted fever group Rickettsia infections.

bBacillus anthracis, the causative agent of anthrax, replicates as chains of vegetative cells by regulating the separation of septal peptidoglycan. Surface (S)-layer proteins and associated proteins (BSLs) function as chain length determinants and bind to the secondary cell wall polysaccharide (SCWP). In this study, we identified the B. anthracis lcpD mutant, which displays increased chain length and S-layer assembly defects due to diminished SCWP attachment to peptidoglycan. In contrast, the B. anthracis lcpB3 variant displayed reduced cell size and chain length, which could be attributed to increased deposition of BSLs. In other bacteria, LytR-CpsA-Psr (LCP) proteins attach wall teichoic acid (WTA) and polysaccharide capsule to peptidoglycan. B. anthracis does not synthesize these polymers, yet its genome encodes six LCP homologues, which, when expressed in S. aureus, promote WTA attachment. We propose a model whereby B. anthracis LCPs promote attachment of SCWP precursors to discrete locations in the peptidoglycan, enabling BSL assembly and regulated separation of septal peptidoglycan. Bacillus anthracis, the causative agent of anthrax, is a sporeforming, Gram-positive bacterium that germinates in host infected tissues and replicates as elongated chains of vegetative forms (1, 2). This unique growth pattern appears to be caused by the regulated separation of septal peptidoglycan, which generates bacterial chains whose mere size precludes clearance by host phagocytes (3-5). The genetic determinants for B. anthracis chain formation are conserved among pathogenic species of the Bacillus cereus group and, as demonstrated with B. cereus G9241, likely contribute to disease pathogenesis (6, 7). Hallmarks of pathogenic Bacillus species are virulence plasmids, pXO-1 and pXO-2 in B. anthracis (8, 9), providing for toxin production and capsulation (10, 11) as well as the chromosomally encoded surface (S)-layer locus (12). Two S-layer proteins of B. anthracis, Sap and EA1, are endowed with S-layer homology (SLH) domains, which retain these proteins in the bacterial envelope by binding to the secondary cell wall polysaccharide (SCWP) (13-15). S-layer protein crystallization domains, responsible for the spontaneous assembly of these polypeptides into a paracrystalline array (16), form a twodimensional lattice that can be thought of as bacterial integument (17)(18)(19).The structural genes of S-layer proteins, sap and eag, are flanked by genes encoding determinants for S-layer protein secretion (secA2 and slaP) (12) or pyruvylation (csaB) (14) as well as acetylation of the SCWP (patA1/2 and patB1/2) (20). CsaB-mediated pyruvylation of the terminal N-acetylmannosamine (ManNAc) of the SCWP (21), with the repeat struc- (22), is a prerequisite for the assembly of S-layer proteins and 22 B. anthracis S-layer-associated proteins (BSLs) (14). PatAB1/2-mediated acetylation of SCWP molecules affects the assembly of EA1 and of some but not all BSLs (20). Recent studies have begun to identify genes for SCWP synthesis, which, unlike py...