Studies on the mechanism of bacterial resistance to complement-mediated killing and on the mechanism of action of bactericidal antibody

A rhamnose-rich O-antigen of Paraburkholderia phymatum MP20 is required for symbiosis with Mimosa pudica

Paraburkholderia phymatum, a β-proteobacterium, forms a nitrogen-fixing symbiosis with many species of the large legume genus Mimosa as well as with common bean (Phaseolus vulgaris L.). Paraburkholderia are considered to have evolved nodulation independently from the well-studied α-proteobacteria symbionts of legumes. However, the detailed mechanisms important for β-rhizobia-legume symbiosis have not yet been determined. In this manuscript, we have sequenced the genome of P. phymatum MP20, a strain isolated from Mimosa pudica nodules, and utilized transposon mutagenesis to identify a mutant that showed delayed and ineffective nodulation of M. pudica. Further analysis revealed that the mutant strain produced an altered lipopolysaccharide lacking rhamnose containing O-antigen. Complementation with the wild-type gene restored the symbiosis. Microscopic analysis of the ineffective nodules showed that the mutant strain did not infect the cortical cells but was restricted to the endodermis. The results suggest that the O-antigen of P. phymatum is important for the bacterial infection of cortical cells and for nodule maturation. Further research will unveil the specific involvement of the glycosyltransferase gene in LPS biosynthesis and its impact on successful nodule formation by P. phymatum.IMPORTANCEThe nitrogen-fixing symbiosis between legumes and rhizobia is important for agricultural and environmental sustainability. The mechanisms of the symbiotic interactions are extensively studied using α-rhizobia. In contrast, mechanisms of symbiotic interactions important for β-rhizobia and their Caesalpinioid (mimosoid) legume hosts are not well known. Here, we describe the genome sequence of P. phymatum MP20, a β-rhizobia isolated from the nodules of M. pudica, and isolation and characterization of a transposon mutant defective in symbiosis. We demonstrate that the O-antigen of the LPS is required for nodulation and symbiotic nitrogen fixation. This study broadens our knowledge of symbiotic interactions in β-rhizobia and will lead to a better understanding of the wider rhizobial-legume symbiosis apart from the α-rhizobia.

The PmrAB system-inducing conditions control both lipid A remodeling and O-antigen length distribution, influencing the Salmonella Typhimurium-host interactions

The Salmonella enterica serovar Typhimurium lipopolysaccharide consisting of covalently linked lipid A, non-repeating core oligosaccharide, and the O-antigen polysaccharide is the most exposed component of the cell envelope. Previous studies demonstrated that all of these regions act against the host immunity barrier. The aim of this study was to define the role and interaction of PmrAB-dependent gene products required for the lipopolysaccharide component synthesis or modification mainly during the Salmonella infection. The PmrAB two-component system activation promotes a remodeling of lipid A and the core region by addition of 4-aminoarabinose and/or phosphoethanolamine. These PmrA-dependent activities are produced by activation of ugd, pbgPE, pmrC, cpta, and pmrG transcription. In addition, under PmrA regulator activation, the expression of wzz(fepE) and wzz(st) genes is induced, and their products are required to determine the O-antigen chain length. Here we report for the first time that Wzz(st) protein is necessary to maintain the balance of 4-aminoarabinose and phosphoethanolamine lipid A modifications. Moreover, we demonstrate that the interaction of the PmrA-dependent pbgE(2) and pbgE(3) gene products is important for the formation of the short O-antigen region. Our results establish that PmrAB is the global regulatory system that controls lipopolysaccharide modification, leading to a coordinate regulation of 4-aminoarabinose incorporation and O-antigen chain length to respond against the host defense mechanisms.

PagP activation in the outer membrane triggers R3 core oligosaccharide truncation in the cytoplasm of Escherichia coli O157:H7

The Escherichia coli outer membrane phospholipid:lipid A palmitoyltransferase PagP is normally a latent enzyme, but it can be directly activated in outer membranes by lipid redistribution associated with a breach in the permeability barrier. We now demonstrate that a lipid A myristate deficiency in an E. coli O157:H7 msbB mutant constitutively activates PagP in outer membranes. The lipid A myristate deficiency is associated with hydrophobic antibiotic sensitivity and, unexpectedly, with serum sensitivity, which resulted from O-antigen polysaccharide absence due to a cytoplasmically determined truncation at the first outer core glucose unit of the R3 core oligosaccharide. Mutational inactivation of pagP in the myristate-deficient lipid A background aggravated the hydrophobic antibiotic sensitivity as a result of losing a partially compensatory increase in lipid A palmitoylation while simultaneously restoring serum resistance and O-antigen attachment to intact lipopolysaccharide. Complementation with either wild-type pagP or catalytically inactive pagPSer77Ala alleles restored the R3 core truncation. However, the intact lipopolysaccharide was preserved after complementation with an internal deletion pagPDelta5-14 allele, which mostly eliminates a periplasmic amphipathic alpha-helical domain but fully supports cell surface lipid A palmitoylation. Our findings indicate that activation of PagP not only triggers lipid A palmitoylation in the outer membrane but also separately truncates the R3 core oligosaccharide in the cytoplasm. We discuss the implication that PagP might function as an apical sensory transducer, which can be activated by a breach in the outer membrane permeability barrier.

The PmrA/PmrB and RcsC/YojN/RcsB systems control expression of the Salmonella O-antigen chain length determinant

The lipopolysaccharide (LPS) is the outermost component of the cell envelope in Gram-negative bacteria. It consists of the hydrophobic lipid A, a short non-repeating core oligosaccharide and a distal polysaccharide termed O-antigen. We report here that the PmrA/PmrB and RcsC/YojN/RcsB two-component systems of Salmonella enterica serovar Typhimurium independently promote transcription of the wzzst gene, which encodes a protein that determines the chain length of the O-antigen. We show that the regulatory proteins PmrA and RcsB footprint partially overlapping regions of the wzzst promoter stimulating transcription from the same start site. Induction of the PmrA/PmrB or RcsC/YojN/RcsB systems increased the fraction of LPS molecules containing 16-35 O-antigen subunits, leading to heightened resistance to serum. The LPS of a rcsB null mutant exhibited an altered mobility in the O-antigen subunits attached to the lipid A-core region when separated on a SDS/PAGE gel, suggesting that RcsB may regulate additional LPS genes. Inactivation of the wzzst gene eliminated the enhanced swarming behaviour exhibited by the rcsB mutant. That multiple regulatory systems control wzzst expression suggests that the Wzzst protein is required under different environmental conditions.

Regulation of the mitogen-activated protein kinases by Brucella spp. expressing a smooth and rough phenotype: relationship to pathogen invasiveness

By comparing smooth wild-type Brucella spp. to their rough mutants, we show that the LPS O chain restricted the activation of the ERK1/2 and p38 mitogen-activated protein kinase (MAPK) pathways, thus preventing the synthesis of immune mediators that regulate host defense. We conclude that the MAPKs are a target for immune intervention by virulent smooth Brucella.

Different responses of macrophages to smooth and rough Brucella spp.: relationship to virulence

By comparing smooth wild-type Brucella strains to their rough mutants, we show that the lipopolysaccharide (LPS) O side chain of pathogenic Brucella has a dramatic impact on macrophage activation. It favors the development of virulent Brucella by preventing the synthesis of immune mediators, important for host defense. We conclude that this O chain property is firmly linked to Brucella virulence.

O-antigen structural variation: mechanisms and possible roles in animal/plant-microbe interactions

Current data from bacterial pathogens of animals and from bacterial symbionts of plants support some of the more general proposed functions for lipopolysaccharides (LPS) and underline the importance of LPS structural versatility and adaptability. Most of the structural heterogeneity of LPS molecules is found in the O-antigen polysaccharide. In this review, the role and mechanisms of this striking flexibility in molecular structure of the O-antigen in bacterial pathogens and symbionts are illustrated by some recent findings. The variation in O-antigen that gives rise to an enormous structural diversity of O-antigens lies in the sugar composition and the linkages between monosaccharides. The chemical composition and structure of the O-antigen is strain-specific (interstrain LPS heterogeneity) but can also vary within one bacterial strain (intrastrain LPS heterogeneity). Both LPS heterogeneities can be achieved through variations at different levels. First of all, O-polysaccharides can be modified non-stoichiometrically with sugar moieties, such as glucosyl and fucosyl residues. The addition of non-carbohydrate substituents, i.e. acetyl or methyl groups, to the O-antigen can also occur with regularity, but in most cases these modifications are again non-stoichiometric. Understanding LPS structural variation in bacterial pathogens is important because several studies have indicated that the composition or size of the O-antigen might be a reliable indicator of virulence potential and that these important features often differ within the same bacterial strain. In general, O-antigen modifications seem to play an important role at several (at least two) stages of the infection process, including the colonization (adherence) step and the ability to bypass or overcome host defense mechanisms. There are many reports of modifications of O-antigen in bacterial pathogens, resulting either from altered gene expression, from lysogenic conversion or from lateral gene transfer followed by recombination. In most cases, the mechanisms underlying these changes have not been resolved. However, in recent studies some progress in understanding has been made. Changes in O-antigen structure mediated by lateral gene transfer, O-antigen conversion and phase variation, including fucosylation, glucosylation, acetylation and changes in O-antigen size, will be discussed. In addition to the observed LPS heterogeneity in bacterial pathogens, the structure of LPS is also altered in bacterial symbionts in response to signals from the plant during symbiosis. It appears to be part of a molecular communication between bacterium and host plant. Experiments ex planta suggest that the bacterium in the rhizosphere prepares its LPS for its roles in symbiosis by refining the LPS structure in response to seed and root compounds and the lower pH at the root surface. Moreover, modifications in LPS induced by conditions associated with infection are another indication that specific structures are important. Also during the differentiation from bacterium to bacteroid, the LPS of Rhizobium undergoes changes in the composition of the O-antigen, presumably in response to the change of environment. Recent findings suggest that, during symbiotic bacteroid development, reduced oxygen tension induces structural modifications in LPS that cause a switch from predominantly hydrophilic to predominantly hydrophobic molecular forms. However, the genetic mechanisms by which the LPS epitope changes are regulated remain unclear. Finally, the possible roles of O-antigen variations in symbiosis will be discussed.

Serum resistance in an invasive, nontypeable Haemophilus influenzae strain

A common feature of many different organisms causing bacteremia is the ability to avoid the bactericidal effects of normal human serum. In Haemophilus influenzae encapsulated strains are particularly serum resistant; however, we found that a nonencapsulated strain (R2866) isolated from the blood of an immunocompetent child with meningitis who had been successfully immunized with H. influenzae type b conjugate vaccine was serum resistant. Since serum resistance usually involves circumventing the action of the complement system, we defined the deposition of various complement components on the surfaces of this H. influenzae strain (R2866), a nonencapsulated avirulent laboratory strain (Rd), and a virulent type b encapsulated strain (Eagan). Membrane attack complex (MAC) accumulation correlated with the loss of bacterial viability; correspondingly, the rates of MAC deposition on the serum-sensitive strain Rd and the serum-resistant strains differed. Analysis of cell-associated immunoglobulin G (IgG), C1q, C3b, and C5b indicated that serum-resistant H. influenzae prevents MAC accumulation by delaying the synthesis of C3b through the classical pathway. Among the initiators of the classical pathway, IgG deposition contributes most of the C3 convertase activity necessary to start the cascade ending with MAC deposition. Despite similar IgG binding, strain R2866 delays C3 convertase activity compared to strain Rd. We conclude that strain R2866 can persist in the bloodstream, in part by inhibiting or delaying C3 deposition on the cell surface, escaping complement mediated killing.

Genetics of O-antigen biosynthesis in Pseudomonas aeruginosa

Pathogenic bacteria produce an elaborate assortment of extracellular and cell-associated bacterial products that enable colonization and establishment of infection within a host. Lipopolysaccharide (LPS) molecules are cell surface factors that are typically known for their protective role against serum-mediated lysis and their endotoxic properties. The most heterogeneous portion of LPS is the O antigen or O polysaccharide, and it is this region which confers serum resistance to the organism. Pseudomonas aeruginosa is capable of concomitantly synthesizing two types of LPS referred to as A band and B band. The A-band LPS contains a conserved O polysaccharide region composed of D-rhamnose (homopolymer), while the B-band O-antigen (heteropolymer) structure varies among the 20 O serotypes of P. aeruginosa. The genes coding for the enzymes that direct the synthesis of these two O antigens are organized into two separate clusters situated at different chromosomal locations. In this review, we summarize the organization of these two gene clusters to discuss how A-band and B-band O antigens are synthesized and assembled by dedicated enzymes. Examples of unique proteins required for both A-band and B-band O-antigen synthesis and for the synthesis of both LPS and alginate are discussed. The recent identification of additional genes within the P. aeruginosa genome that are homologous to those in the A-band and B-band gene clusters are intriguing since some are able to influence O-antigen synthesis. These studies demonstrate that P. aeruginosa represents a unique model system, allowing studies of heteropolymeric and homopolymeric O-antigen synthesis, as well as permitting an examination of the interrelationship of the synthesis of LPS molecules and other virulence determinants.

Identification of the perosamine synthetase gene of Brucella melitensis 16M and involvement of lipopolysaccharide O side chain in Brucella survival in mice and in macrophages

Brucella organisms are facultative intracellular bacteria that may infect many species of animals as well as humans. The smooth lipopolysaccharide (S-LPS) has been reported to be an important virulence factor of these organisms, but the genetic basis of expression of the S-LPS O antigen has not yet been described. Likewise, the role of the O side chain of S-LPS in the survival of Brucella has not been clearly defined. A mini-Tn5 transposon mutant library of Brucella melitensis 16M was screened by enzyme-linked immunosorbent assay (ELISA) with monoclonal antibodies (MAbs) directed against the O side chain of Brucella. One mutant, designated B3B2, failed to express any O side chain as confirmed by ELISA, Western blot analysis, and colony coloration with crystal violet. Nucleotide sequence analysis demonstrated that the transposon disrupted an open reading frame with significant homology to the putative perosamine synthetase genes of Vibrio cholerae O1 and Escherichia coli O157:H7. The low G+C content of this DNA region suggests that this gene may have originated from a species other than a Brucella sp. The survival of B. melitensis mutant strain B3B2 in the mouse model and in bovine macrophages was examined. The results suggested that S-LPS or, more precisely, its O side chain is essential for survival in mice but not in macrophages.

Distribution of the rol gene encoding the regulator of lipopolysaccharide O-chain length in Escherichia coli and its influence on the expression of group I capsular K antigens

The rol (cld) gene encodes a protein involved in the expression of lipopolysaccharides in some members of the family Enterobacteriaceae. Rol interacts with one or more components of Rfc-dependent O-antigen biosynthetic complexes to regulate the chain length of lipopolysaccharide O antigens. The Rfc-Rol-dependent pathway for O-antigen synthesis is found in strains with heteropolysaccharide O antigens, and, consistent with this association, rol-homologous sequences were detected in chromosomal DNAs from 17 different serotypes with heteropolysaccharide O antigens. Homopolymer O antigens are synthesized by a pathway that does not involve either Rfc or Rol. It was therefore unexpected when a survey of Escherichia coli strains possessing mannose homopolymer O8 and O9 antigens showed that some strains contained rol. All 11 rol-positive strains coexpressed a group IB capsular K antigen with the O8 or O9 antigen. In contrast, 12 rol-negative strains all produced group IA K antigens in addition to the homopolymer O antigen. Previous research from this and other laboratories has shown that portions of the group I K antigens are attached to lipopolysaccharide lipid A-core, in a form that we have designated K(LPS). By constructing a hybrid strain with a deep rough rfa defect, it was shown that the K40 (group IB) K(LPS) antigen exists primarily as long chains. However, a significant amount of K40 antigen was surface expressed in a lipid A-core-independent pathway. The typical chain length distribution of the K40 antigen was altered by introduction of multicopy rol, suggesting that the K40 group IB K antigen is equivalent to a Rol-dependent O antigen. The prototype K30 (group IA) K antigen is expressed as short oligosaccharides (primarily single repeat units) in K(LPS), as well as a high-molecular-weight lipid A-core-independent form. Introduction of multicopy rol into the K30 strain generated a novel modal pattern of K(LPS) with longer polysaccharide chains. Collectively, these results suggested that group IA K(LPS) is also synthesized by a Rol-dependent pathway and that the typically short oligosaccharide K(LPS) results from the absence of Rol activity in these strains.

Cloning and expression of rfb genes from Vibrio anguillarum serotype O2 in Escherichia coli: evidence for cross-reactive epitopes

Vibrio ordalii and Vibrio anguillarum O2 express lipopolysaccharide (LPS) O antigens containing both specific and cross-reactive epitopes. The localization of these epitopes on the O antigen is not known. We have cloned and expressed the rfb gene cluster for O-antigen synthesis from V. anguillarum O2 (rfbVaO2) in Escherichia coli. E. coli DH5 alpha containing the recombinant plasmid pAM86 expressed O antigens which reacted with polyclonal antisera to V. ordalii and to V. anguillarum O2 LPS and with monoclonal antibody (MAb) 7B4, which is specific for V. anguillarum O2 O antigens. The recombinant strains were also protected from bactericidal killing by normal fish serum. Surprisingly, the LPS expressed from the cloned rfbVaO2 genes also reacted with MAb A16, which is specific for V. ordalii O antigens. Western immunoblot analysis revealed that MAb 7B4 reacted with recombinant LPS bearing shorter O-antigen repeat units, while MAb A16 reacted with the longer O antigens. Similar results were obtained when pAM86 was transformed into E. coli CLM4, which has a deletion spanning the sbcB-rfb region, indicating that the changes in antigenic profiles of O antigens from the recombinant strains were not due to genes within the E. coli rfb cluster. These data suggest that the epitope recognized by the MAb A16 is expressed by V. anguillarum O2 strains but it is apparently not accessible to the antibody in the native O polysaccharide. Cloning of the rfbVaO2 gene cluster resulted in expression of a novel O antigen. The modification(s) which leads to the alterations in antigenic profile of these recombinant LPS remains to be determined.

Lateral transfer of rfb genes: a mobilizable ColE1-type plasmid carries the rfbO:54 (O:54 antigen biosynthesis) gene cluster from Salmonella enterica serovar Borreze

Plasmid pWQ799 is a 6.9-kb plasmid isolated from Salmonella enterica serovar Borreze. Our previous studies have shown that the plasmid contains a functional biosynthetic gene cluster for the expression of the O:54 lipopolysaccharide O-antigen of this serovar. The minimal replicon functions of pWQ799 have been defined, and a comparison with nucleotide and protein databases revealed this replicon to be virtually identical to ColE1. This is the first report of involvement of ColE1-related plasmids in O-antigen expression. The replicon of pWQ799 is predicted to encode two RNA molecules, typical of other ColE1-type plasmids. RNAII, the putative replication primer from pWQ799, shares regions of homology with RNAII from ColE1. RNA1 is an antisense regulator of DNA replication in ColE1-related plasmids. The coding region for RNAI from pWQ799 shares no homology with any other known RNAI sequence but is predicted to adopt a secondary structure characteristic of RNAI molecules. pWQ799 may therefore represent a new incompatibility group within this family. pWQ799 also possesses cer, rom, and mob determinants, and these differ minimally from those of ColE1. The plasmid is mobilizable in the presence of either the broad-host-range helper plasmid pRK2013 or the IncI1 plasmid R64drd86. Mobilization and transfer of pWQ799 to other organisms provides the first defined mechanism for lateral transfer of O-antigen biosynthesis genes in S. enterica and explains both the distribution of related plasmids and coexpression of the O:54 factor with other O-factors in different Salmonella serovars. The base composition of the pWQ799 replicon sequences gives an average percent G+C value typical of Salmonella spp. In contrast, the percent G+C value is dramatically lower with rfb0:54, consistent with the possibility that the cluster was acquired from an organism with much lower G+C composition.

Relationships between rfb gene clusters required for biosynthesis of identical D-galactose-containing O antigens in Klebsiella pneumoniae serotype O1 and Serratia marcescens serotype O16

The lipopolysaccharide O antigens of Klebsiella pneumoniae serotype O1 and Serratia marcescens serotype O16 both contain a repeating unit disaccharide of [-->3)-beta-D-Galf-(1-->3)-alpha-D-Galp-(1-->]; the resulting polymer is known as D-galactan I. In K. pneumoniae serotype O1, the genes responsible for the synthesis of D-galactan I are found in the rfb gene cluster (rfbKpO1). We report here the cloning and analysis of the rfb cluster from S. marcescens serotype O16 (rfbSmO16). This is the first rfb gene cluster examined for the genus Serratia. Synthesis of D-galactan I is an rfe-dependent process for both K. pneumoniae serotype O1 and S. marcescens serotype O16. Hybridization experiments with probes derived from each of the six rfbKpO1 genes indicate that the cloned rfbSmO16 cluster contains homologous genes arranged in the same order. However, the degree of homology at the nucleotide sequence level was sufficiently low that hybridization was detected only under low-stringency conditions. rfbABSmO16 genes were subcloned and shown to encode an ABC-2 (ATP-binding cassette) transporter which is functionally identical to the one encoded by the corresponding rfb genes from K. pneumoniae serotype O1. The amino acid sequences of the predicted RfbA and RfbB homologs showed identities of 75.7% (87.9% total similarity) and 78.0% (86.5% total similarity), respectively. The last gene of the rfbKpO1 cluster, rfbFKpO1, encodes a bifunctional galactosyltransferase which initiates the formation of D-galactan I. RfbFKpO1 and RfbFSmO16 are 57.6% identical (with 71.1% total similarity), and both show similarity with RfpB, the galactosyltransferase involved in the synthesis of Shigella dysenteriae type I O-polysaccharide. The G+C contents of the rfbAB genes from each organism are quite similar, and values are lower than those typical for the species. However, the G+C content of rfbFSmO16 (47.6%) was much higher than that of rfbFKpO1 (37.3%), despite the fact that the average for each species (52 to 60%) falls within the same range.

Cloning and sequencing of a Bordetella pertussis serum resistance locus

We have characterized a new virulence factor in Bordetella pertussis: serum resistance. Compared with Escherichia coli HB101, wild-type B. pertussis was relatively resistant to classical-pathway, complement-dependent killing by normal human serum. However, a mutant of B. pertussis (BPM2041) which is less virulent in mice and which has Tn5 lac inserted in a previously uncharacterized bvg-regulated gene was found to be at least 10-fold more susceptible to serum killing than the wild type. We have named this locus brk, for Bordetella resistance to killing. We have cloned and sequenced the brk locus, and it encodes two divergently transcribed open reading frames (ORFs), termed BrkA and BrkB. Both ORFs are necessary for serum resistance. Within the 300 bases which separate the two ORFs and upstream of each ORF are putative sites for BvgA binding. BrkA shows 29% identity to pertactin and has two RGD motifs in addition to a conserved proteolytic processing site and an outer membrane targeting signal. Like pertactin, BrkA is involved in adherence and invasion. Despite the similarities, a pertactin mutant was found to be not as sensitive to serum killing as the BrkA or BrkB mutants. BrkB is similar to ORFs in E. coli and Mycobacterium leprae and displays domains of homology to various transporters. On the basis of its hydropathy profile, BrkB is predicted to be a cytoplasmic membrane protein. By Southern blot, brk sequences were found in Bordetella bronchiseptica and Bordetella parapertussis but not in Bordetella avium.

A plasmid-encoded rfbO:54 gene cluster is required for biosynthesis of the O:54 antigen in Salmonella enterica serovar Borreze

Previous studies demonstrated that the presence of a 7-8 kb plasmid is correlated with expression of the lipopolysaccharide (LPS) O:54 antigen in several Salmonella enterica serovars. In this study, a 6.7 kb plasmid from a field isolate of S. enterica serovar Borreze was shown to encode enzymes responsible for the synthesis of the O:54 polysaccharide. Curing the plasmid results in simultaneous loss of smooth O-polysaccharide-substituted LPS molecules and O:54 serotype. SDS-PAGE analysis of other O:54 isolates indicated that the O:54 O-polysaccharide can be co-expressed with an additional O-polysaccharide, likely encoded by chromosomal genes. The structure of the O:54 polysaccharide was determined by a combination of chemical and nuclear magnetic resonance (NMR) methods and was found to be an unusual homopolymer of N-acetylmannosamine (D-ManNAc) residues. The polysaccharide contained a disaccharide repeating unit with the structure:-->4)-beta-D-ManpNAc-(1-->3)-beta-D-ManpNAc-(1--> This structure does not resemble other O-polysaccharides in S. enterica. To examine the role played by plasmid functions in synthesis of the O:54 polysaccharide, the 6.7 kb plasmid was cloned to produce a hybrid plasmid (pWQ800) in pGEM-7Zf(+). In Escherichia coli K-12 delta rfb, pWQ800 directed the synthesis of authentic O:54 polysaccharide. Polymerized O:54 polysaccharide was also produced in S. enterica serovar Typhimurium rfb and rfc mutants. From these data, we conclude that pWQ800 carries the rfbO:54 gene cluster and synthesis of the O:54 polysaccharides does not require host chromosomal rfb functions. However, synthesis of the O:54 polysaccharide requires the function of the rfe and rffE genes which are part of the gene cluster encoding enzymes involved in biosynthesis of enterobacterial common antigen. The rffE gene product synthesizes the O:54 precursor, uridine diphospho-N-acetylmannosamine. This is the first description of a plasmid-encoded rfb gene cluster in Salmonella.

The effects of lactoferrin on gram-negative bacteria

Lactoferrin is an iron-binding protein found in human mucosal secretions as well as the specific granules of polymorphonuclear leukocytes. A variety of functions have been ascribed to the protein, and it appears to contribute to antimicrobial host defense. In particular, it has been shown to have direct effects on pathogenic microorganisms including bacteriostasis and the induction of microbial iron uptake systems. Still its overall physiologic role remains to be defined. It has appeared logical that antimicrobial activity of the protein arises from sequestration of environmental iron thereby causing nutritional deprivation in susceptible organisms. This argument is buttressed by the finding that selected highly virulent pathogens have evolved techniques to subvert this effect and use the protein as an iron source. However, recent observations indicate that the protein has additional properties that contribute to host defense. Work by several groups has shown that the protein synergistically interacts with immunoglobins, complement, and neutrophil cationic proteins against Gram-negative bacteria. Further, both the whole protein and a cationic N-terminus peptide fragment directly damage the outer membrane of Gram-negative bacteria suggesting a mechanism for the supplemental effects. This review will summarize these diverse observations with a consideration of how the in vitro work relates to the physiological role of the protein.

TnphoA-mediated disruption of K54 capsular polysaccharide genes in Escherichia coli confers serum sensitivity

To assess whether non-K1, group 2 capsular serotypes are important in conferring serum resistance to extraintestinal isolates of Escherichia coli, a K54 blood isolate (CP9) was evaluated as a model pathogen. Transposon mutagenesis (TnphoA) was used to generate isogenic capsule-negative mutants. CP9 was resistant to the bactericidal effects of serum, growing in 80% serum. In contrast, all of the capsule-negative mutants had an increased sensitivity to 80% normal human serum, undergoing a 2- to 3-log kill over 3 h when starting inocula of 10(4) to 10(7) CFU/ml were used. The killing of the capsule-negative strains was mediated through the alternative complement pathway and not by lysozyme or beta-lysins. The protective effect of the K54 capsule against the bactericidal activity of serum was not through inhibition of the complement cascade, nor did it appear to be through a difference in the binding of C3.

Mechanism of resistance to complement-mediated killing of bacteria encoded by the Salmonella typhimurium virulence plasmid gene rck

We find that pADEO16, a recombinant cosmid carrying the rck gene of the Salmonella typhimurium virulence plasmid, when cloned into either rough or smooth Escherichia coli and Salmonella strains, confers high level resistance to the bactericidal activity of pooled normal human serum. The rck gene encodes a 17-kD outer membrane protein that is homologous to a family of virulence-associated outer membrane proteins, including pagC and Ail. Complement depletion, C3 and C5 binding, and membrane-bound C3 cleavage products are similar in strains with and without rck. Although a large difference in C9 binding was not seen, trypsin cleaved 55.7% of bound 125I-C9 counts from rough S. typhimurium with pADEO16, whereas only 26.4% were released from S. typhimurium with K2011, containing a mutation in rck. The majority of C9 extracted from rck strain membranes sediments at a lower molecular weight than in strains without rck, suggesting less C9 polymerization. Furthermore, SDS-PAGE analysis of gradient peak fractions indicated that the slower sedimenting C9-containing complexes in rck strains did not contain polymerized C9 typical of the tubular membrane attack complex. These results indicate that complement resistance mediated by Rck is associated with a failure to form fully polymerized tubular membrane attack complexes.

Mechanism of YadA-mediated serum resistance of Yersinia enterocolitica serotype O3

Complement activation via the alternative pathway was analyzed with isogenic strains of Yersinia enterocolitica serotype O3 differing in plasmid content (p- or p+ strains) or selective lack of YadA expression (YadA- strain). The p+ strain was serum resistant, even after antibody-enhanced complement activation. Serum sensitivity was observed with the p- and YadA- strains but was more pronounced in the p- strain. The p+ strain deposited less C5b-9(m) complexes on its surface than the p- and YadA- strains. No size difference, however, was detected with solubilized C5b-9(m) complexes obtained from resistant and sensitive strains. At the C3 level, it became evident that surface-bound C3b was degraded faster into iC3b on the p+ strain than on the p- and YadA- strains. Our results demonstrate that YadA inhibits complement activation at the C3 and C9 level. As a result, reduced amounts of C5b-9(m) are generated on the surface of YadA-bearing bacteria. In addition, YadA seems to protect against the lytic action of those C5b-9(m) complexes whose deposition could not be prevented.

Beta-lactam antibiotics potentiate magainin 2 antimicrobial activity in vitro and in vivo

The ability of magainin 2 to augment antibiotic therapy was examined. Susceptibility to magainin 2 was determined on Escherichia coli incubated in the presence and absence of sublethal concentrations of antibiotics both in vitro and in vivo. Experiments in buffer and normal human serum revealed that E. coli exposed to sublethal amounts of cefepime, a beta-lactam antibiotic, was significantly more susceptible to the antimicrobial activity of magainin 2. Bacteria incubated with subinhibitory concentrations of other beta-lactam type antibiotics, but not amikacin (an aminoglycoside) or ciprofloxacin (a quinolone), were also more susceptible to magainin 2 in normal human serum. Bacteria were less susceptible to magainin 2 when they were examined in heat-inactivated serum. Complement was shown to be required for magainin 2 activity in serum by using C8-deficient sera. The combination of magainin 2 and cefepime was shown to be more antimicrobial in normal human serum for a variety of bacterial strains. Magainin 2 was completely inactive as a therapeutic agent when it was administered alone (2 mg per mouse) but significantly increased the survival of mice when it was administered with a low level of cefepime.

TraT lipoprotein, a plasmid-specified mediator of interactions between gram-negative bacteria and their environment

The TraT protein is a cell-surface-exposed, outer membrane lipoprotein specified by large, usually conjugative, F-like plasmids. Two biological activities have been associated with the protein: (i) prevention of self-mating of cells carrying identical or closely related conjugative plasmids, by blocking the formation of stable mating aggregates; and (ii) resistance to the bactericidal activities of serum, possibly by inhibiting the correct assembly or efficient functioning of the terminal membrane attack complex of complement. The protein therefore interacts not only with components of the outer membrane but also with specific external agents. In conjugative plasmids the traT gene lies within the region necessary for the conjugal transfer of DNA (tra), although its expression is not necessarily dependent on the expression of other tra genes. Recently, however, the gene has been discovered in isolation from other tra genes in nonconjugative virulence-associated plasmids, providing further evidence that the TraT protein may have a role in pathogenesis. The nucleotide sequences of several traT genes have been determined, and comparison of the corresponding amino acid sequences suggests that a central region of five amino acid residues flanked by hydrophobic domains determines the specificity of the protein in surface exclusion. Additionally, studies of mutants with different amino acid alterations within the hydrophobic domains have shown that insertion of charged residues disrupts normal outer membrane integrity. This review considers our current knowledge of the distribution, structure, and biological role(s) of the protein. Recent applications of the protein in studies of the unusual permeability properties of the outer membrane and for the transport of foreign antigenic determinants to the bacterial cell surface are also discussed.

Functions and relevance of the terminal complement sequence

The terminal complement sequence is initiated upon cleavage of C5 with liberation of C5a anaphylatoxin, and involves the assembly of macromolecular C5b-9 complexes either on cell surfaces or in plasma. Cell-bound C5b-9 complexes generate transmembrane pores that can cause cell death, or they can elicit secondary cellular reactions triggered, for example, by passive flux of calcium ions into the cells. In vivo functions of the fluid-phase SC5b-9 complex have not yet been defined, but the identity of S-protein with vitronectin (serum spreading factor) provokes the anticipation that significant biological functions of this complex do exist. The terminal complement sequence may fulfil protective functions when it is triggered on alien cells that are marked for destruction. Dysregulation in the complement sequence may, however, result in detrimental attack by C5b-9 on autologous cells. Examples include not only autoimmune disease states, but also the activation of complement on dead or dying cells, and bystander attack on blood cells during cardiopulmonary bypass. Methods for detecting and quantifying C5b-9 are outlined, and the potential usefulness of such assays in clinical research is discussed.

A third mechanism of serum resistance in Escherichia coli

15 serum-resistant strains of E. coli group III characterized by binding of both C3 and factor H in the immunofluorescence test were studied in respect of the mechanism on which serum resistance is based in these strains. Serum resistance in 7 strains were found to be established by one of the mechanisms first described by Joiner et al. or by Kubens et al. The classification of these strains should therefore be altered. The binding and consumption of C5 as well as the binding of C9 was investigated for the remaining 8 strains. All strains were found to bind the two complement components which are part of the membrane attack complex (MAC) without causing cell death. These results suggest that resistance in strains of group III is based on a third mechanism which shows similarities to data obtained for other species but has not yet been described for E. coli.

Common themes in microbial pathogenicity

A bacterial pathogen is a highly adapted microorganism which has the capacity to cause disease. The mechanisms used by pathogenic bacteria to cause infection and disease usually include an interactive group of virulence determinants, sometimes coregulated, which are suited for the interaction of a particular microorganism with a specific host. Because pathogens must overcome similar host barriers, common themes in microbial pathogenesis have evolved. However, these mechanisms are diverse between species and not necessarily conserved; instead, convergent evolution has developed several different mechanisms to overcome host barriers. The success of a bacterial pathogen can be measured by the degree with which it replicates after entering the host and reaching its specific niche. Successful microbial infection reflects persistence within a host and avoidance or neutralization of the specific and nonspecific defense mechanisms of the host. The degree of success of a pathogen is dependent upon the status of the host. As pathogens pass through a host, they are exposed to new environments. Highly adapted pathogenic organisms have developed biochemical sensors exquisitely designed to measure and respond to such environmental stimuli and accordingly to regulate a cascade of virulence determinants essential for life within the host. The pathogenic state is the product of dynamic selective pressures on microbial populations.