Identification and characterization of a second extracellular collagen-like protein made by group A Streptococcus: control of production at the level of translation

A recent study found that group A Streptococcus (GAS) expresses a cell surface protein with similarity to human collagen (S. Lukomski, K. Nakashima, I. Abdi, V. J. Cipriano, R. M. Ireland, S. R. Reid, G. G. Adams, and J. M. Musser, Infect. Immun. 68:6542-6553, 2000). This streptococcal collagen-like protein (Scl) contains a long region of Gly-X-X motifs and was produced by serotype M1 GAS strains. In the present study, a second member of the scl gene family was identified and designated scl2. The Scl2 protein also has a collagen-like region, which in M1 strains is composed of 38 contiguous Gly-X-X triplet motifs. The scl2 gene was present in all 50 genetically diverse GAS strains studied. The Scl2 protein is highly polymorphic, and the number of Gly-X-X motifs in the 50 strains studied ranged from 31 in one serotype M1 strain to 79 in serotype M28 and M77 isolates. The scl1 and scl2 genes were simultaneously transcribed in the exponential phase, and the Scl proteins were also produced. Scl1 and Scl2 were identified in a cell-associated form and free in culture supernatants. Production of Scl1 is regulated by Mga, a positive transcriptional regulator that controls expression of several GAS virulence factors. In contrast, production of Scl2 is controlled at the level of translation by variation in the number of short-sequence pentanucleotide repeats (CAAAA) located immediately downstream of the GTG (Val) start codon. Control of protein production by this molecular mechanism has not been identified previously in GAS. Together, the data indicate that GAS simultaneously produces two extracellular human collagen-like proteins in a regulated fashion.

Identification and characterization of the scl gene encoding a group A Streptococcus extracellular protein virulence factor with similarity to human collagen

Group A Streptococcus (GAS) expresses cell surface proteins that mediate important biological functions such as resistance to phagocytosis, adherence to plasma and extracellular matrix proteins, and degradation of host proteins. An open reading frame encoding a protein of 348 amino acid residues was identified by analysis of the genome sequence available for a serotype M1 strain. The protein has an LPATGE sequence located near the carboxy terminus that matches the consensus sequence (LPXTGX) present in many gram-positive cell wall-anchored molecules. Importantly, the central region of this protein contains 50 contiguous Gly-X-X triplet amino acid motifs characteristic of the structure of human collagen. The structural gene (designated scl for streptococcal collagen-like) was present in all 50 GAS isolates tested, which together express 21 different M protein types and represent the breadth of genomic diversity in the species. DNA sequence analysis of the gene in these 50 isolates found that the number of contiguous Gly-X-X motifs ranged from 14 in serotype M6 isolates to 62 in a serotype M41 organism. M1 and M18 organisms had the identical allele, which indicates very recent horizontal gene transfer. The gene was transcribed abundantly in the logarithmic but not stationary phase of growth, a result consistent with the occurrence of a DNA sequence with substantial homology with a consensus Mga binding site immediately upstream of the scl open reading frame. Two isogenic mutant M1 strains created by nonpolar mutagenesis of the scl structural gene were not attenuated for mouse virulence as assessed by intraperitoneal inoculation. In contrast, the isogenic mutant derivative made from the M1 strain representative of the subclone most frequently causing human infections was significantly less virulent when inoculated subcutaneously into mice. In addition, both isogenic mutant strains had significantly reduced adherence to human A549 epithelial cells grown in culture. These studies identify a new extracellular GAS virulence factor that is widely distributed in the species and participates in adherence to host cells and soft tissue pathology.

Identification and immunogenicity of group A Streptococcus culture supernatant proteins

Extracellular proteins made by group A Streptococcus (GAS) play critical roles in the pathogenesis of human infections caused by this bacterium. Although many extracellular GAS proteins have been identified and characterized, there has been no systematic analysis of culture supernatant proteins. Proteins present in the culture supernatant of strains of serotype M1 (MGAS 5005) and M3 (MGAS 315) mutants lacking production of the major extracellular cysteine protease were separated by two-dimensional gel electrophoresis and identified by amino-terminal amino acid sequencing and interrogation of available databases, including a serotype M1 genome sequence. In the aggregate, amino-terminal amino acid sequence data for 66 protein spots were generated, 53 unique sequences were obtained, and 44 distinct proteins were identified. Sixteen of the 44 proteins had apparent secretion signal sequences and 27 proteins did not. Eight of the 16 proteins with apparent secretion signal sequences have not been previously described for GAS. Antibodies against most of the apparently secreted proteins were present in sera from mice infected subcutaneously with MGAS 5005 or MGAS 315. Humans with documented GAS infections (pharyngitis, acute rheumatic fever, and severe invasive disease) also had serum antibodies reacting with many of the apparently secreted proteins, indicating that they were synthesized in the course of GAS-human interaction. The genes encoding four of the eight previously undescribed and apparently secreted culture supernatant proteins were cloned, and the proteins were overexpressed in Escherichia coli. Western blot analysis with these recombinant proteins and sera from GAS-infected mice and humans confirmed the immunogenicity of these proteins. Taken together, the data provide new information about the molecular aspects of GAS-host interactions.

Nonpolar inactivation of the hypervariable streptococcal inhibitor of complement gene (sic) in serotype M1 Streptococcus pyogenes significantly decreases mouse mucosal colonization

Group A Streptococcus (GAS) is a human pathogen that commonly infects the upper respiratory tract. GAS serotype M1 strains are frequently isolated from human infections and contain the gene encoding the hypervariable streptococcal inhibitor of complement protein (Sic). It was recently shown that Sic variants were rapidly selected on mucosal surfaces in epidemic waves caused by M1 strains, an observation suggesting that Sic participates in host-pathogen interactions on the mucosal surface (N. P. Hoe, K. Nakashima, S. Lukomski, D. Grigsby, M. Liu, P. Kordari, S.-J. Dou, X. Pan, J. Vuopio-Varkila, S. Salmelinna, A. McGeer, D. E. Low, B. Schwartz, A. Schuchat, S. Naidich, D. De Lorenzo, Y.-X. Fu, and J. M. Musser, Nat. Med. 5:924-929, 1999). To test this idea, a new nonpolar mutagenesis method employing a spectinomycin resistance cassette was used to inactivate the sic gene in an M1 GAS strain. The isogenic Sic-negative mutant strain was significantly (P < 0.019) impaired in ability to colonize the mouse mucosal surface after intranasal infection. These results support the hypothesis that the predominance of M1 strains in human infections is related, in part, to a Sic-mediated enhanced colonization ability.

Rapid selection of complement-inhibiting protein variants in group A Streptococcus epidemic waves

Serotype M1 group A Streptococcus strains cause epidemic waves of human infections long thought to be mono- or pauciclonal. The gene encoding an extracellular group A Streptococcus protein (streptococcal inhibitor of complement) that inhibits human complement was sequenced in 1,132 M1 strains recovered from population-based surveillance of infections in Canada, Finland and the United States. Epidemic waves are composed of strains expressing a remarkably heterogeneous array of variants of streptococcal inhibitor of complement that arise very rapidly by natural selection on mucosal surfaces. Thus, our results enhance the understanding of pathogen population dynamics in epidemic waves and infectious disease reemergence.

Extracellular cysteine protease produced by Streptococcus pyogenes participates in the pathogenesis of invasive skin infection and dissemination in mice

The role of an extracellular cysteine protease encoded by the speB gene in group A Streptococcus (GAS) skin infection was studied with a mouse model. Mice were injected subcutaneously with a wild-type GAS serotype M3 strain or a cysteine protease-inactivated isogenic derivative grown to stationary phase. The mortality rate of mice injected with the M3 speB mutant strain was significantly decreased (P < 0.0008) compared to that of animals injected with the wild-type parental organism. The abscesses formed in animals infected with the cysteine protease mutant strain were significantly smaller (P < 0.0001) than those caused by the wild-type organism and slowly regressed over 3 to 4 weeks. In striking contrast, infection with the wild-type GAS isolate generated necrotic lesions, and in some animals the GAS disseminated widely from the injection site and produced extensive cutaneous damage. All of these animals developed bacteremia and died. GAS dissemination was accompanied by severe tissue and blood vessel necrosis. Cysteine protease expression in the infected tissue was identified by immunogold electron microscopy. These data demonstrate that cysteine protease expression contributes to soft tissue pathology, including necrosis, and is required for efficient systemic dissemination of the organism from the initial site of skin inoculation.

Genetic inactivation of the extracellular cysteine protease enhances in vitro internalization of group A streptococci by human epithelial and endothelial cells

Group A Streptococcus (GAS) produces an extracellular cysteine protease (streptococcal pyrogenic exotoxin B) that participates in virulence. We examined two pairs of isogenic GAS strains (serotype M2 and M3) for ability to be internalized by human umbilical vein endothelial cells and A549 human lung fibroblasts. For both host cell types, the level of internalization by the cysteine protease-negative mutant strains was significantly greater than the wild type parent organisms. The data suggest that expression of the cysteine protease contributes to extracellular survival, an observation consistent with recent results from mouse infection studies (Lukomski et al., Infect immun 1998; 66: 771-6).

Genetic inactivation of an extracellular cysteine protease (SpeB) expressed by Streptococcus pyogenes decreases resistance to phagocytosis and dissemination to organs

Streptococcal pyrogenic exotoxin B (SpeB), a conserved cysteine protease expressed by virtually all Streptococcus pyogenes strains, has recently been shown to be an important virulence factor (S. Lukomski, S. Sreevatsan, C. Amberg, W. Reichardt, M. Woischnik, A. Podbielski, and J. M. Musser, J. Clin. Invest. 99:2574-2580, 1997). Genetic inactivation of SpeB significantly decreased the lethality of a serotype M49 strain for mice and abolished the lethality of a serotype M3 strain after intraperitoneal (i.p.) injection. In the present study, a wild-type M3 isolate and an M3 speB mutant derivative were used to investigate the mechanism responsible for altered virulence. Following i.p. injection, the mutant and wild-type strains induced virtually identical cellular inflammatory responses, characterized largely by an influx of polymorphonuclear leukocytes (PMNs). In addition, the mutant and wild-type strains rapidly entered the blood and were recovered from all organs examined. However, significantly fewer (P < 0.05) CFUs of the isogenic mutant derivative than of the wild-type parent strain were recovered from blood and organs. PMNs effectively cleared the M3 speB mutant from the peritoneum by 22 h, thereby sparing the host. In contrast, the wild-type M3 strain continued to replicate intraperitoneally and had the ability to kill phagocytes. This process allowed the wild-type strain to continuously disseminate, resulting in host death. Our results indicate that genetic inactivation of the cysteine protease decreased the resistance of the mutant to phagocytosis and impaired its subsequent dissemination to organs. These results provide insight into the detrimental effect of SpeB inactivation on virulence.

The simultaneous production of both Hly- and Hpm-like hemolysins is characteristic of the Proteus penneri species

Clinical isolates of Proteus penneri were tested for the presence of genes encoding hemolytic activity. Strains possessing DNA sequences similar to the hlyCABD genes in Escherichia coli were found. Each secreted a 110 kDa protein which reacted with a specific anti-HlyA antiserum. Southern blotting analysis revealed that the HindIII restriction fragment pattern for the hlyCABD genes of these strains was conserved. Similarly, the chromosomal location of these genes is relatively conserved based on the pattern of NotI digested DNA fragments separated by pulsed field gel electrophoresis. One strain carried an additional copy of the hlyCABD determinant which was mapped on a second NotI genomic fragment. All strains contained also chromosomally encoded sequences related to the hpmBA genes originally cloned from Proteus mirabilis. All strains produced a 166 kDa exoprotein detected in immunoblots with a specific antiserum raised against HpmA hemolysin. The hpmBA genes were located on other NotI fragments than hlyCABD genes. In contrast to the other Proteae, the simultaneous production of both hemolysins seems to be a common characteristics of Proteus penneri isolates.

Inactivation of Streptococcus pyogenes extracellular cysteine protease significantly decreases mouse lethality of serotype M3 and M49 strains

Cysteine proteases have been implicated as important virulence factors in a wide range of prokaryotic and eukaryotic pathogens, but little direct evidence has been presented to support this notion. Virtually all strains of the human bacterial pathogen Streptococcus pyogenes express a highly conserved extracellular cysteine protease known as streptococcal pyrogenic exotoxin B (SpeB). Two sets of isogenic strains deficient in SpeB cysteine protease activity were constructed by integrational mutagenesis using nonreplicating recombinant plasmids containing a truncated segment of the speB gene. Immunoblot analyses and enzyme assays confirmed that the mutant derivatives were deficient in expression of enzymatically active SpeB cysteine protease. To test the hypothesis that the cysteine protease participates in host mortality, we assessed the ability of serotype M3 and M49 wild-type strains and isogenic protease-negative mutants to cause death in outbred mice after intraperitoneal inoculation. Compared to wild-type parental organisms, the serotype M3 speB mutant lost virtually all ability to cause mouse death (P < 0.00001), and similarly, the virulence of the M49 mutant was detrimentally altered (P < 0.005). The data unambiguously demonstrate that the streptococcal enzyme is a virulence factor, and thereby provide additional evidence that microbial cysteine proteases are critical in host-pathogen interactions.

Identification of the O antigen polymerase (rfc) gene in Escherichia coli O4 by insertional mutagenesis using a nonpolar chloramphenicol resistance cassette

Computer analysis of the O4 polysaccharide gene cluster of Escherichia coli revealed the presence of two open reading frames (ORFs) encoding strongly hydrophobic polypeptides. O antigen polymerase, which is encoded by the rfc gene, is a potential membrane protein and therefore should be hydrophobic. To identify the rfc gene, these two ORFs were subjected to insertional mutagenesis. A chloramphenicol resistance cassette was designed which, when properly inserted, does not cause a polar effect in downstream genes. Each of two ORFs, cloned into a plasmid vector, was inactivated with this cassette. Two types of mutants bearing chromosomal insertions of the cassettes in each ORF were constructed by homologous recombination. These mutants were characterized by PCR, Southern blotting, and transverse-alternating-field electrophoresis. Only one class of mutants exhibited the expected O polymerase-deficient phenotype; they produced O4-specific, semirough lipopolysaccharide. Therefore, this ORF was identified as the rfc gene. The chromosomal rfc mutation was complemented in trans by the rfc gene expressed from a plasmid vector.

Left-handed Z-DNA and in vivo supercoil density in the Escherichia coli chromosome

A system for studying Z-DNA formation in the Escherichia coli chromosome was developed. Prior investigations in recombinant plasmids showed that alternating (Pur-Pyr) sequences can adopt a left-handed Z-DNA conformation both in vitro and in vivo. We constructed mobile, transposon-based cassettes carrying cloned (Pur-Pyr) sequences containing an EcoRI site in the center. These cassettes were subsequently inserted into different locations in the E. coli chromosome in a random fashion. A number of stable insertions were characterized by Southern analysis and pulsed-field gel electrophoresis mapping. A cloned temperature-sensitive MEcoRI methylase was expressed in trans as the probe to study Z-DNA formation in vivo. In this system, the control EcoRI sites were quickly methylated when cells were placed at the permissive temperature. Strong inhibition of the methylation was observed, however, only for the EcoRI sites embedded in a 56-bp run of (C-G). In contrast, the shorter sequence of 32 bp did not show this behavior. Prior in vitro determinations revealed that the longer tract required less energy to stabilize the Z-helix than the shorter block. We conclude that the observed inhibition of methylation is due to Z-DNA formation in the E. coli chromosome. In vitro, these sequences undergo the B- to Z-DNA transition at a supercoil density of -0.026 for the 56-bp insert and -0.032 for the 32-bp block. Since only the longer (C-G) tract but not the shorter run adopted the left-handed conformation in the chromosome, we propose that these densities establish the boundaries in the different chromosomal loci investigated; these boundaries are in good agreement with the extremes found in plasmids.

Analysis of antibiotic resistance determinants in Proteus penneri

The plasmid profiles of 65 strains of Proteus penneri were analyzed to determine whether resistance was determined chromosomally or by plasmids. Only seven strains harboured one to three plasmids, although these strains exhibited resistance to a wide range of antibiotics. Markers for ampicillin and tetracycline resistance could be transferred to Escherichia coli by transformation. Plasmids carried resistance to chloramphenicol in two strains and resistance to sulfonamides in one strain. The result showed that resistance is determined chromosomally rather than by plasmids, however the possibility that these bacteria may acquire resistance plasmids which change their antibiotic susceptibility pattern cannot be excluded.

Cell-free and cell-bound hemolytic activities of Proteus penneri determined by different Hly determinants

A collection of 45 Proteus penneri strains was characterized with respect to their hemolytic activity and representative cell-free or only cell-bound hemolysin possessing strains were chosen for further study. Extracellular Proteus penneri hemolysin, which was investigated earlier by hybridization, reacted with monospecific antiserum against alpha-hemolysin of Escherichia coli. In this paper we also show, using the colony hybridization technique, that the alpha-hemolysin-like determinant is widely distributed among Proteus penneri strains. Because one of the strains tested, which expressed a high activity of cell-bound hemolytic factor, did not carry such a Hly determinant, the presence of a second hemolysin is postulated. We cannot demonstrate any difference in hybridization patterns of alpha- and beta-hemolytic Proteus penneri strains and accumulation of the toxin molecule inside the cells was also not observed. The existence of another control mechanism, external to the hly operon, for hemolysin gene is suggested.

Extracellular haemolysin of Proteus penneri coded by chromosomal hly genes is similar to the alpha-haemolysin of Escherichia coli

Extracellular haemolysin of four Proteus penneri strains was characterized as a polypeptide of approximately 110 kD. The chromosomal DNAs of these strains cleaved with Hind III showed three fragments hybridizing with a DNA probe containing cloned haemolysin (hly) genes of Escherichia coli. The results presented here suggested that the haemolysin of P. penneri strains is chromosomally determined and similar to the alpha-haemolysin of E. coli.