Variation occurring in group A streptococci during human infection; progressive loss of M substance correlated with increasing susceptibility to bacteriostasis

Capsular Polysaccharide of Group A Streptococcus

Most clinical isolates of Streptococcus pyogenes elaborate a capsular polysaccharide, which is composed of hyaluronic acid, a high-molecular-mass polymer of alternating residues of N-acetyl glucosamine and glucuronic acid. Certain strains, particularly those of the M18 serotype, produce abundant amounts of capsule, resulting in formation of large, wet-appearing, translucent or "mucoid" colonies on solid media, whereas strains of M-types 4 and 22 produce none. Studies of acapsular mutant strains have provided evidence that the capsule enhances virulence in animal models of infection, an effect attributable, at least in part, to resistance to complement-mediated opsonophagocytic killing by leukocytes. The presence of the hyaluronic acid capsule may mask adhesins on the bacterial cell wall. However, the capsule itself can mediate bacterial attachment to host cells by binding to the hyaluronic-acid binding protein, CD44. Furthermore, binding of the S. pyogenes capsule to CD44 on host epithelial cells can trigger signaling events that disrupt cell-cell junctions and facilitate bacterial invasion into deep tissues. This article summarizes the biochemistry, genetics, regulation, and role in pathogenesis of this important virulence determinant.

Streptococcus pyogenes Sortase Mutants Are Highly Susceptible to Killing by Host Factors Due to Aberrant Envelope Physiology

Cell wall anchored virulence factors are critical for infection and colonization of the host by Gram-positive bacteria. Such proteins have an N-terminal leader sequence and a C-terminal sorting signal, composed of an LPXTG motif, a hydrophobic stretch, and a few positively charged amino acids. The sorting signal halts translocation across the membrane, allowing sortase to cleave the LPXTG motif, leading to surface anchoring. Deletion of sortase prevents the anchoring of virulence factors to the wall; the effects on bacterial physiology however, have not been thoroughly characterized. Here we show that deletion of Streptococcus pyogenes sortase A leads to accumulation of sorting intermediates, particularly at the septum, altering cellular morphology and physiology, and compromising membrane integrity. Such cells are highly sensitive to cathelicidin, and are rapidly killed in blood and plasma. These phenomena are not a loss-of-function effect caused by the absence of anchored surface proteins, but specifically result from the accumulation of sorting intermediates. Reduction in the level of sorting intermediates leads to a return of the sortase mutant to normal morphology, while expression of M protein with an altered LPXTG motif in wild type cells leads to toxicity in the host environment, similar to that observed in the sortase mutant. These unanticipated effects suggest that inhibition of sortase by small-molecule inhibitors could similarly lead to the rapid elimination of pathogens from an infected host, making such inhibitors much better anti-bacterial agents than previously believed.

Asymptomatic carriage of group A streptococcus is associated with elimination of capsule production

Humans commonly carry pathogenic bacteria asymptomatically, but despite decades of study, the underlying molecular contributors remain poorly understood. Here, we show that a group A streptococcus carriage strain contains a frameshift mutation in the hasA gene resulting in loss of hyaluronic acid capsule biosynthesis. This mutation was repaired by allelic replacement, resulting in restoration of capsule production in the isogenic derivative strain. The "repaired" isogenic strain was significantly more virulent than the carriage strain in a mouse model of necrotizing fasciitis and had enhanced growth ex vivo in human blood. Importantly, the repaired isogenic strain colonized the mouse oropharynx with significantly greater bacterial burden and had significantly reduced ability to internalize into cultured epithelial cells than the acapsular carriage strain. We conducted full-genome sequencing of 81 strains cultured serially from 19 epidemiologically unrelated human subjects and discovered the common theme that mutations negatively affecting capsule biosynthesis arise in vivo in the has operon. The significantly decreased capsule production is a key factor contributing to the molecular détente between pathogen and host. Our discoveries suggest a general model for bacterial pathogens in which mutations that downregulate or ablate virulence factor production contribute to carriage.

To harm or not to harm? On the evolution and expression of virulence in group A streptococci

Group A streptococci (GAS) cause three different types of infection (sensu lato) with distinct levels of virulence: asymptomatic colonization, superficial symptomatic infection, and invasive infection. To address why this pattern with several infection types has evolved, we combine mechanistic understanding from infection medicine with recent theory from evolutionary ecology. We propose that asymptomatic colonization and superficial symptomatic infection exploit different states of the host epithelium to maximize transmission between hosts in different epidemiological conditions, whereas the ability of the bacteria to cause invasive infection is a non-adaptive side effect of traits required for superficial symptomatic infection.

Natural variation in the promoter of the gene encoding the Mga regulator alters host-pathogen interactions in group a Streptococcus carrier strains

Humans commonly carry pathogenic bacteria asymptomatically, but the molecular factors underlying microbial asymptomatic carriage are poorly understood. We previously reported that two epidemiologically unassociated serotype M3 group A Streptococcus (GAS) carrier strains had an identical 12-bp deletion in the promoter of the gene encoding Mga, a global positive gene regulator. Herein, we report on studies designed to test the hypothesis that the identified 12-bp deletion in the mga promoter alters GAS virulence, thereby potentially contributing to the asymptomatic carrier phenotype. Using allelic exchange, we introduced the variant promoter into a serotype M3 invasive strain and the wild-type promoter into an asymptomatic carrier strain. Compared to strains with the wild-type mga promoter, we discovered that strains containing the promoter with the 12-bp deletion produced significantly fewer mga and Mga-regulated gene transcripts. Consistent with decreased mga transcripts, strains containing the variant mga promoter were also significantly less virulent in in vivo and ex vivo models of GAS disease. Further, we provide evidence that the pleiotropic regulator protein CodY binds to the mga promoter and that the 12-bp deletion in the mga promoter reduces CodY-mediated mga transcription. We conclude that the naturally occurring 12-bp deletion in the mga promoter significantly alters the pathogen-host interaction of these asymptomatic carrier strains. Our findings provide new insight into the molecular basis of the carrier state of an important human pathogen.

Genome-wide analysis of group a streptococci reveals a mutation that modulates global phenotype and disease specificity

Many human pathogens produce phenotypic variants as a means to circumvent the host immune system and enhance survival and, as a potential consequence, exhibit increased virulence. For example, it has been known for almost 90 y that clinical isolates of the human bacterial pathogen group A streptococci (GAS) have extensive phenotypic heterogeneity linked to variation in virulence. However, the complete underlying molecular mechanism(s) have not been defined. Expression microarray analysis of nine clinical isolates identified two fundamentally different transcriptomes, designated pharyngeal transcriptome profile (PTP) and invasive transcriptome profile (ITP). PTP and ITP GAS differed in approximately 10% of the transcriptome, including at least 23 proven or putative virulence factor genes. ITP organisms were recovered from skin lesions of mice infected subcutaneously with PTP GAS and were significantly more able to survive phagocytosis and killing by human polymorphonuclear leukocytes. Complete genome resequencing of a mouse-derived ITP GAS revealed that the organism differed from its precursor by only a 7-bp frameshift mutation in the gene (covS) encoding the sensor kinase component of a two-component signal transduction system implicated in virulence. Genetic complementation, and sequence analysis of covR/S in 42 GAS isolates confirmed the central role of covR/S in transcriptome, exoproteome, and virulence modulation. Genome-wide analysis provides a heretofore unattained understanding of phenotypic variation and disease specificity in microbial pathogens, resulting in new avenues for vaccine and therapeutics research.

Phenotypic heterogeneity within an infecting population is a strategy commonly used by bacterial pathogens to evade the host immune system and enhance survival. Such phenotypic variation has been observed for the human pathogen group A streptococci (GAS), which can cause a wide range of diseases with differing severity. However, the underlying mechanisms that control this variation, and the survival- and virulence-associated effects of this variation, have not been fully elucidated.

By assaying total gene expression the authors found that clinical GAS isolates from invasive and pharyngeal diseases had distinct gene expression patterns during growth in standard laboratory media. These two gene expression patterns conferred distinct virulence-associated attributes on the expressing GAS strain, as assessed using bacteremia and soft-tissue infection models of disease. Likewise, the ability to survive the bactericidal activity of human neutrophils was significantly different between GAS strains with the two distinct expression patterns. Transition from one gene expression pattern to the other required the mutation of the two-component signal transduction system CovRS (control of virulence R/S). The authors conclude that the ability of GAS to remodel its transcriptome plays a major contribution in its ability to colonize distinct niches of the human body and cause disease.

Preparation and properties of type-specific M antigen isolated from a group A, type 1 hemolytic streptococcus

Type-specific M antigen was extracted by heating type 1 group A streptococci at pH 2 in a boiling water bath. The protein was then purified by digestion with a preparation of crystalline ribonuclease which was free of proteolytic activity. It was further purified by fractional precipitation with (NH(4))(2)SO(4). Elementary chemical analysis of the preparation thus obtained showed an absence of phosphorus and a sulfur content of 2.46 per cent. In the ultraviolet the maximum absorption was at a wave length of 276 mmicro and the minimum at 255 mmicro. In electrophoresis experiments the preparation showed a single peak in the pH range of 3 to 9, but considerable boundary spreading was observed. The type 1 M antigen was isoelectric at pH 5.3 in sodium acetate buffer of ionic strength 0.1. The serological reactivity of the protein isolated was typical of type 1 M antigen. This protein induced the formation in rabbits of type-specific precipitins and protective antibodies. The absorption of type 1 antibacterial serum with the purified M antigen removed both the protective antibodies and the type-specific precipitins from the serum.

Rheumatic fever in the 21st century

In the first half of the twentieth century, the group A streptococcus (GAS) was established as the sole etiologic agent of acute rheumatic fever (ARF). In the century's latter half, the clinical importance of variation in the virulence of strains of GAS has become clearer. Although still obscure, the pathogenesis of ARF requires primary infection of the throat by highly virulent GAS strains. These contain very large hyaluronate capsules and M protein molecules. The latter contain epitopes that are cross-reactive with host tissues and also contain superantigenic toxic moieties. In settings where ARF has become rare, GAS pharyngitis continues to be common, although it is caused by GAS strains of relatively lower virulence. These strains, however, colonize the throat avidly and stubbornly. Molecularly distinct pyoderma strains may cause acute glomerulonephritis, but they are not rheumatogenic, even though they may secondarily colonize and infect the throat. Guidelines for the diagnosis, treatment, and prevention of GAS pharyngitis and ARF are reviewed with particular reference to the prevalence of the latter in the community.

Occurrence of R antigen specific for Group A type 3 streptococci

Approximately 50 per cent of Group A, Type 3 streptococci contain a hitherto undescribed antigen found only in Group A, Type 3 organisms. It is serologically distinct from Type 3 M antigen and is designated as 3 R antigen. Strains containing 3 R antigen but no Type 3 M antigen are "glossy," avirulent Type 3 variants. These strains can be obtained by repeated transfers of virulent M-containing streptococci in artificial media under unfavorable conditions of growth. These degraded streptococci recover Type 3 M antigen during serial passage through mice. The amount of 3 R antigen in a strain is not affected by a decrease or increase in M antigen. The 3 R antigen is unrelated to virulence. Antibodies to this antigen do not protect mice against infection or promote phagocytosis in bactericidal tests. The 3 R antigen-antibody system can give rise to confusion in M-precipitin reactions. In all these properties 3 R antigen is similar to 28 R antigen, although in certain other properties the two R antigens are not identical. They are serologically distinct. Virulence and ability to grow in normal human blood under the conditions of the bactericidal test are correlated with the presence of M antigen of Group A, Type 3 streptococci. Mouse protection and specific inhibition of growth of Type 3 streptococci by phagocytosis in bactericidal tests are associated with the presence of Type 3 anti-M antibodies.

Studies on the bacteriophages of hemolytic streptococci. II. Antigens released from the streptococcal cell wall by a phage-associated lysin

The lysis of cell walls of hemolytic streptococci by a phage-associated lysin has been described. A method is presented for preparing the lysin from Group C streptococcal phage lysates. Following lysis almost all of the cell wall carbohydrate is recovered in solution. This material has the serological reactivity, physical-chemical properties, and values for nitrogen, rhamnose, and glucosamine similar to those of the carbohydrate isolated from the cell walls by the Streptomyces albus enzyme. Group C carbohydrate isolated by either enzyme inactivates Group C bacteriophage. The protein liberated by the lysin from Group A Type 6 cell walls gives a type-specific precipitin reaction with homologous rabbit antiserum. Preliminary data are presented on the ammonium sulfate fractionation and the electrophoretic separation of a protein fraction with the serological reactivity of M protein.

Studies on the pathogenicity of group A streptococci. II. The antiphagocytic effects of the M protein and the capsular gel

A QUANTITATIVE STUDY OF THE COMBINED ANTIPHAGOCYTIC EFFECTS OF THE M PROTEIN AND THE HYALURONIC ACID CAPSULES OF FOUR STRAINS OF GROUP A STREPTOCOCCI REVEALED THE FOLLOWING FACTS RELATING TO THEIR INTRAPERITONEAL VIRULENCE IN MICE AND RATS: 1. The most virulent strain, S23M (matt), produced both a large hyaluronic acid capsule and a full complement of M protein, the combined effects of which rendered the organism highly resistant to surface phagocytosis. 2. The slightly less virulent strain, T14/46 (matt virulent) was somewhat more susceptible to surface phagocytosis owing to the fact that its smaller capsule was less antiphagocytic than that of the S23M organism. 3. The glossy variant of the S23 strain (S23G), which ranked third in virulence, was still more susceptible to surface phagocytosis because of its lack of detectable M substance. Its large hyaluronic acid capsule, however, was capable of protecting it against phagocytosis on glass. 4. The least virulent strain, T14 (matt avirulent), was the most susceptible of all to phagocytosis. Though it possessed both M substance and capsule, which together prevented its phagocytosis on glass, each of them was shown to be quantitatively and functionally deficient as compared to Strain S23M. The differences in phagocytability, which appear to be directly related to the pathogenicity of the organisms, could be adequately demonstrated in vitro only by phagocytic tests designed to measure surface phagocytosis in the absence of opsonins. This fact is in keeping with the observation, previously reported, that surface phagocytosis plays a critical role in the defense of the host, particularly during the earliest stages of experimental streptococcal infections. Its possible relation to suppuration during the later stages of infection is also discussed.

The occurrence of two M antigens in certain group A streptococci related to type 14

A strain of Group A streptococcus previously considered to belong to Type 14 was shown to have two immunologically distinct M antigens, designated Type 14 and Type 51. Most strains having the Type 14 M antigen were found to have also the Type 51 M antigen, and are considered to belong to Type 14–51. Four strains had the Type 51 M antigen without the Type 14 M antigen and one strain had the Type 14 M antigen without the Type 51 M antigen. The failure of a variant to produce the known M antigen of the parent strain does not necessarily mean that the strain is M^-, because a second M antigen may be present as was the case in several strains described here.

Structure of the has operon promoter and regulation of hyaluronic acid capsule expression in group A Streptococcus

Group A streptococcal strains vary widely in the amount of hyaluronic acid capsule they produce, although the has operon, which encodes the enzymes required for hyaluronic acid synthesis, is highly conserved. The three genes making up the has operon are transcribed from a single promoter located upstream of the first gene in the operon, hasA. To investigate transcriptional regulation of capsule synthesis, we studied the structure and function of the has operon promoter sequences from two strains of group A Streptococcus: a highly encapsulated M-type 18 strain and a poorly encapsulated M-type 3 strain. Transcriptional fusions of the has operon promoter to a promoterless chloramphenicol acetyltransferase gene were constructed in a temperature-sensitive shuttle vector. The influence of promoter structure on has operon transcription was reflected by chloramphenicol acetyl transferase activity in cell lysates of Escherichia coli harbouring the recombinant plasmids and in group A Streptococcus after integration of the promoter fusions into the streptococcal chromosome. Fusions including as few as 12 nucleotides upstream from the -35 site of the has promoter exhibited full activity, indicating that sequences further upstream do not affect has gene transcription. A transcriptional fusion of the has promoter from the highly encapsulated M-type 18 strain was threefold more active than a similar construct from the poorly encapsulated M-type 3 strain. Analysis of the promoter sequences for the two strains revealed differences in three nucleotides in the -35, -10 spacer region of the promoter and in four nucleotides in the +2 to +8 positions relative to the start site of hasA transcription. To determine the relative importance of the two groups of nucleotide substitutions, chimeric promoter sequences were constructed in which either of the two clusters of variant nucleotides from the M18 has promoter was substituted for the corresponding positions in the M3 has promoter. Analysis of these chimeric promoter fusions showed that sequence changes in both regions influenced promoter strength. These results define the limits of cis-acting chromosomal sequences that influence transcription of the has operon and indicate that the fine structure of the promoter is an important determinant of capsule gene expression in group A Streptococcus.

ELECTROPHORETIC SEPARATION OF CONSTITUENTS OF PARTIALLY PURIFIED M PROTEIN OF STREPTOCOCCUS PYOGENES

Pierce, William A., Jr. (Tulane University, New Orleans, La.). Electrophoretic separation of constituents of partially purified M protein of Streptococcus pyogenes. J. Bacteriol. 88:912-921. 1964.-Partially purified M protein of a group A, type 12 strain of Streptococcus pyogenes was studied by use of chemical, electrophoretic, and immunological techniques. It was demonstrated in immunodiffusion tests that the antigen contains multiple precipitating components. The type-specific antigen was identified, and evidence was presented that, in some instances at least, the cross-reactions observed between this type 12 M protein and heterologous antisera in immunodiffusion tests involve contaminating antigens rather than the component which precipitates with adsorbed homologous-typing antiserum. In passive hemagglutination tests where M protein was adsorbed to tanned sheep erythrocytes, it was found that antisera suitably adsorbed to show good specificity in capillary precipitin tests nevertheless still contain cross-reactive antibodies which are detectable by this more sensitive technique. Electrophoresis on starch paste separates some of the components of partially purified M protein, so that a fraction can be obtained which has fewer precipitating antigens, as determined in immunodiffusion tests, and which is less cross-reactive in passive hemagglutination tests with heterologous unadsorbed antistreptococcal antisera.

Penicillin for streptococcal pharyngitis: has anything changed?

It used to be simple: A single IM injection or 10 days of oral therapy would cure the sore throat and prevent rheumatic fever. Post-treatment carriage of group A does not seem to be adequate reason to switch strategies today.

Coregulation of type 12 M protein and streptococcal C5a peptidase genes in group A streptococci: evidence for a virulence regulon controlled by the virR locus

Group A streptococci express at least two surface-associated virulence factors, the antiphagocytic M protein and the antichemotactic streptococcal C5a peptidase (SCP). Preliminary evidence suggested that the biosynthesis of these two proteins is coordinately controlled and subject to simultaneous phase variation. To explore this possibility further, a series of phase-switching and phase-locked M- variants were assayed for SCP by enzyme-linked immunosorbent assay inhibition and for SCP-specific mRNA by dot blot hybridization. All M- cultures produced diminished amounts of SCP antigen and specific mRNA, whereas revertants produced quantities equivalent to those of the wild-type M+ culture. A phase-locked strain that harbors a deletion in a region upstream of the M12 and SCP genes, termed the virR locus, failed to produce SCP antigen or SCP-specific transcripts. The SCP-specific transcript produced by M+ bacteria was shown by Northern (RNA) blot hybridization to be 4 kilobases in size, distinguishing it from the transcript which encodes M protein. These data demonstrate that phase switching of both SCP and M12 proteins is at the transcriptional level and that expression is under the control of the upstream virR locus. We propose that the genetic determinants of these proteins and of colony morphology comprise a virulence regulon.

Passive acquired mucosal immunity to group A streptococci by secretory immunoglobulin A

We present a model in which animals are passively immunized at a mucosal site, allowing one to evaluate immunological protection at the mucosal level only. Affinity-purified, anti-M protein sIgA administered intranasally protected mice against systemic infection after intranasal challenge with group A streptococci. In contrast, anti-M protein serum Ig administered intranasally was not protective at this site, although it neutralized the antiphagocytic property of M protein and promoted phagocytosis. Protection by sIgA occurred despite the lower immunoreactivity of sIgA to purified M protein compared with serum Ig. The data suggest that sIgA can protect at the mucosa and may preclude the need for opsonic IgG in preventing streptococcal infection.

Penicillin and the marked decrease in morbidity and mortality from rheumatic fever in the United States

There has been a marked decline in mortality due to rheumatic fever in the United States. We present evidence for the important role of penicillin in changing the severity of rheumatic carditis, beginning about 1946. Since that year, mortality due to rheumatic carditis has rapidly decreased to zero at the hospital we studied (House of the Good Samaritan, Boston), the rate of loss of all murmurs in patients at the study hospital accelerated simultaneously and exceeded 40 percent by 1970, and the rates of decline in national mortality due to rheumatic carditis accelerated fourfold with the advent of antibiotics. These data, together with reports of recent outbreaks of rheumatic fever, emphasize the importance of continued efforts to diagnose and treat Group A streptococcal pharyngitis.

Streptococcus pyogenes type 12 M protein gene regulation by upstream sequences

A partial nucleotide sequence that included 1,693 base pairs of the M12 (emm12) gene of group A streptococci (strain CS24) and adjacent upstream DNA was determined. Type 12 M protein-specific mRNA of strain CS24 is transcribed from two promoters (P1 and P3) separated by 30 bases. The transcription start sites of the emm12 gene were located more than 400 bases downstream of a deletion that causes decreased M-protein gene transcription in strain CS64. Deletion analysis of M protein-expressing plasmids indicated that an upstream region greater than 1 kilobase is required for M-protein gene expression. The M-protein gene transcriptional unit appears to be monocistronic. Analysis of the emm12 DNA sequence revealed three major repeat regions. Two copies of each repeat, A and B, existed within the variable 5' end of the gene; repeat C demarcated the 5' end of the constant region shared by emm12 and emm6.

Expression of M type 12 protein by a group A streptococcus exhibits phaselike variation: evidence for coregulation of colony opacity determinants and M protein

Three major categories of colony opacity were observed for natural variants of the M type 12 (M12) group A streptococcus strain CS24. Colony opacity variants that switched between two alternative categories at significantly high frequencies were identified and are referred to as switching between more opaque (Op+) and less opaque (Op-) phenotypes. Twenty lineages of such variants were derived for analysis and were assessed for resistance to phagocytosis, acid-extractable M12 antigen, and M12 mRNA, criteria which define the M protein-positive phenotype (M+). Transition from the M+ to the M protein-negative phenotype (M-) correlated with a change from Op+ to Op-. Reversion to the Op+ phenotype was accompanied by reversion to the M+ state in all variants except one and occurred at a higher frequency than the forward M+ to M- switch. These data demonstrate the existence of M12 protein phaselike switching in the group A streptococcus strain CS24. The discovery of an Op+ M- revertant confirmed that colony opacity and M protein can be expressed independently and are distinct gene products. We suggest that coregulation of colony opacity and M protein expression accounts for their association among descendents of strain CS24. Southern blot hybridization analyses of digested genomic DNA from 27 M- variants and 15 M+ revertants were performed with DNA probes containing M12 protein and adjacent upstream sequences. DNA deletions were identified only in two stable M- variants, approximately 1.3 and 1.4 kilobases upstream from the M12 gene, respectively, whereas all unstable M- variants lacked detectable rearrangements. This suggests that deletions within or adjacent to the structural gene are unlikely to be responsible for the reversible switch in M protein expression. However, the association with the stable M- phenotype and the location of these deletions, as well as two other deletions, approximately 0.5 kilobase upstream from the M12 promoter in two previously described variants of strain CS24 suggests that a second gene product is required for full expression of M12 protein synthesis in this strain.

Repeated passage of freshly isolated group A streptococci on blood agar. II. Effect on adherence capacity

Group A streptococcal strains (three T-type 1, two T-type 2 and three T-type 4), freshly isolated from throat cultures, were subjected to 25 serial passages on blood agar. All strains changed their M protein production and/or opacity factor (OF)-activity during the passages. The capacity of each strain to adhere to a pool of buccal cells from six healthy individuals was studied both before and after passage. Five of six strains with decreased OF-activity/M protein production diminished significantly in adherence capacity, whereas one of two strains with increasing OF-activity adhered better to the epithelial cells. The results are discussed in relation to the clinical view of asymptomatic carriers of group A streptococci.

Small DNA deletions creating avirulence in Streptococcus pyogenes

The M protein is the antigen on the surface of group A streptococci that allows these bacteria to resist phagocytosis. DNA encoding the M12 protein was cloned into Escherichia coli and used as an isotopically labeled hybridization probe to compare genomic DNA's isolated from M+ and M- isogenic cultures in an effort to elucidate the genetic basis of this variation. DNA's from two spontaneous, independent M- variants contained small (approximately 50 base pairs) deletions which were mapped to identical restriction fragments within or adjacent to the M protein coding sequence. Taken together with the pleiotropic nature of these deletions, this suggests that they define a regulatory switch.

Repeated passage of freshly isolated group A streptococci on blood agar. I. Effect on M protein, opacity factor and IgG Fc-receptor activity

Seventy-six strains, 20 T-type 1, 20 T-type 2, 18 T-type 4 and 18 T-type 12 group A streptococci were isolated from throat cultures and subjected to 25 serial passages on blood agar. A single "glossy" colony was selected from each passage in order to diminish the M protein content of the strains. In accordance with previous results, the M protein synthesis, as estimated by an electro-immuno assay diminished in the T1 strains, in mean from 23.8 to 15.5% of a reference M protein preparation (p less than 0.01). Opacity factor (OF)-production was used as a measure for M protein in the T-type 2, the T-type 4 and the OF-positive T-type 12 strains. OF-excretion decreased significantly in the T-types 2 and decreased, though not significantly in the T-type 12 strains but increased (unexpectedly) in the T-type 4 isolates during subculture. However, irrespective of the changes in M protein/OF production these types all increased significantly in capacity to bind radiolabelled IgG via the Fc-fragment. It is known that streptococci isolated during convalescence resemble subcultured strains; it is suggested that IgG Fc-receptors are important for protection of the streptococci against phagocytosis in the asymptomatic carrier state.

Epidemiology of group A streptococcal infections--their changing frequency and severity

The frequency and severity of streptococcal infections and their sequelae have declined dramatically in the past century, yet the prevalence of streptococcal infections is still high. The reasons for this decline must be intimately related to host resistance, virulence of the agent, and environmental factors, especially crowding. Close examination of these fundamental influences does not reveal any evidence that humans have become less resistant to streptococcal infections, but they react less violently. There is some evidence that the agent may have lost a degree of its virulence. The decline in morbidity and mortality due to streptococcal infections began long before antibiotics, especially penicillin, were available. However, penicillin has proved to be an important factor in prevention of streptococcal infections, especially in rheumatic fever prophylaxis. There are certain indications that repeated streptococcal infections due to similar M types, occurring in young children over the past several decades, have resulted in some degree of immunity as well as the possible evolution of less virulent, but not less infectious, strains of group A streptococci. Also, a decrease in crowding would be expected to result in fewer streptococcal infections. Although there are more people in the world than at any other time in the history of man, urban population density in the western world, at least, is less than in the late 1800s and early 1900s.

Bacteriophage control of antiphagocytic determinants in group A streptococci

At least two genes have been shown to be required for the expression of the antiphagocytic M protein molecule in group A streptococci. Evidence for phage involvement in the expression of M protein is that: (a) M- cultures of bacteria can be converted to the M+ state (resistant to phagocytosis) upon lysogenization with appropriate bacteriophages; (b) without those bacteriophages the M- recipient culture could not be detected to revert to the M+ state, even under our most stringent selective conditions; and (c) stable M+ lysogens cured of their bacteriophages returned to the M- state. Immunochemical analysis of lysogenically converted M+ strains demonstrated that they contain precipitating and antiphagocytic determinants of the parental M-76 strain (CS110) rather than M-12 determinants expressed by the phage donor strain. This information strongly suggests that the M- strain CS112 possesses the structural gene for M protein, but that it remains predominantly unexpressed. Quantitation of the M antigen produced by these strains supports the observation that the M- phage-recipient strain possesses a small amount of extractable M antigen and that phage activates its synthesis by some unknown mechanism. Various possibilities to account for the phage requirement in M protein synthesis and its role in the transition between M+ and M- states are discussed.

Binding of aggregated IgG in the presence of fresh serum by group A streptococci producing pharyngeal infection: possible connection with types frequently involved in acute nephritis

109 streptococcal strains, belonging to diverse serological groups and types, were investigated as regards their capacity to bind IgG aggregates in the presence of fresh serum. Strains capable of such binding were not found in groups B,C,D,E,G,L,M or N. Such binding was restricted to a few types of group A streptococci: the potentially nephritogenic types 2, 6 and 12, and four strains belonging to type M 39, M 46 and M 22 or M 62, the nephritogenic capacity of which is unknown. Two of five strains isolated from patients with acute post-stretococcal glomerulonephritis (AGN) and 19/28 type T 12, SOR-strains, isolated during an epidemic in a kindergarten with associated cases of AGN, were found to bind aggregates. The findings suggest a possible association between capacity to bind aggregates in the presence of serum and the serological types of group A streptococci involved in acute nephritis following pharyngeal infection.

Some observations on the influence of the micro-environment on loss of M substance in strains of Streptococcus pyogenes

1. An M-producing strain of Streptococcus pyogenes type-12 was shown to be carried on the surface of apparently healthy tonsils for at least 3 years.

2. Loss of M substance in a strain of type-3 in a tonsillar carrier was shown to be by gradual replacement of matt forms by glossy variants. This pair of naturally occurring variants showed very different growth rates as mixtures in liquid media.

3. Under suitable conditions, glossy variants regularly appeared and replaced the M-producing strain from which they were derived in these and all other strains of type-3 and type-12 tested from a wide variety of sources.

4. It is suggested that the selection of variants by the differential nutrient value of the micro-environment is the deciding factor in carrier strains showing loss of M substance.