| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
,
,||,#
* School of Biological Sciences, University of Wollongong, Wollongong, New South Wales, Australia;
Department of Microbial Pathogenesis and Vaccine Development, German National Centre for Biotechnology, Braunschweig, Germany;
Life Sciences Institute, University of Michigan, Ann Arbor, Michigan, USA;
Howard Hughes Medical Institute,
|| Department of Internal Medicine and
# Department of Human Genetics, University of Michigan, Ann Arbor, Michigan, USA;
** Department of Molecular Science, University of Tennessee, Memphis, Tennessee, USA; and
Department of Pediatrics, UCSD, La Jolla, California, USA
1Correspondence: School of Biological Sciences, University of Wollongong, Wollongong, NSW, 2522, Australia. E-mail: mwalker{at}uow.edu.au
ABSTRACT
The globally disseminated Streptococcus pyogenes M1T1 clone causes a number of highly invasive human diseases. The transition from local to systemic infection occurs by an unknown mechanism; however invasive M1T1 clinical isolates are known to express significantly less cysteine protease SpeB than M1T1 isolates from local infections. Here, we show that in comparison to the M1T1 strain 5448, the isogenic mutant 
speB accumulated 75-fold more human plasmin activity on the bacterial surface following incubation in human plasma. Human plasminogen was an absolute requirement for M1T1 strain 5448 virulence following subcutaneous (s.c.) infection of humanized plasminogen transgenic mice. S. pyogenes M1T1 isolates from the blood of infected humanized plasminogen transgenic mice expressed reduced levels of SpeB in comparison with the parental 5448 used as inoculum. We propose that the human plasminogen system plays a critical role in group A streptococcal M1T1 systemic disease initiation. SpeB is required for S. pyogenes M1T1 survival at the site of local infection, however, SpeB also disrupts the interaction of S. pyogenes M1T1 with the human plasminogen activation system. Loss of SpeB activity in a subpopulation of S. pyogenes M1T1 at the site of infection results in accumulation of surface plasmin activity thus triggering systemic spread.—Cole, J. N., McArthur, J. D., McKay, F. C., Sanderson-Smith, M. L., Cork, A. J., Ranson, M., Rohde, M., Itzek, A., Sun, H., Ginsburg, D., Kotb, M., Nizet, V., Chhatwal, G. S., Walker, M. J. Trigger for group A streptococcal M1T1 invasive disease.
Key Words: Streptococcus pyogenes • SpeB • plasminogen
EPIDEMIC INVASIVE GROUP A Streptococcus (GAS, S. pyogenes) disease in the Western World since the mid-1980s has been paralleled by the emergence of a globally disseminated serotype M1T1 GAS clone (1)
. This M1T1 clone is associated with life-threatening infections such as necrotizing fasciitis and streptococcal toxic shock syndrome (2)
, while also causing uncomplicated infections in other patients (3)
. The mechanism that triggers the transition from local to invasive infection by GAS M1T1 is unknown, however, epidemiologic studies have documented an inverse relationship between human invasive disease and the expression of the streptococcal pyrogenic exotoxin B (SpeB) (4)
. SpeB is a secreted cysteine protease initially expressed as a 40 kDa zymogen, which is then converted to its 28 kDa active form by autocatalytic truncation (5)
. SpeB is known to cleave host proteins including extracellular matrix (ECM) components, cytokine precursors, immunoglobulins, and antimicrobial peptides (1
, 6
, 7)
and has been shown to play an important role in establishing localized skin infections (8)
. SpeB also degrades GAS virulence factors including the cell wall-associated fibrinogen-binding antiphagocytic M1 protein (9
, 10)
, various superantigens (11
, 12)
, and the secreted plasminogen activator streptokinase (13)
.
The capacity of GAS to assemble on its surface a trimolecular complex containing streptokinase, fibrinogen, and plasminogen has been correlated with a propensity for invasive infection (14)
. GAS is a highly specific human pathogen and the streptokinase produced by this organism displays a much greater affinity for human plasminogen than mouse plasminogen (15)
. A major advance in the understanding of GAS pathogenesis was attained by Sun et al. (15)
who established a humanized plasminogen transgenic mouse model. These researchers demonstrated that streptokinase and the activation of plasminogen to plasmin is critical for GAS dissemination in vivo, leading to the proposal that human plasminogen is hijacked for use as a virulence factor by GAS (15
, 16)
. In this study, we utilize the representative wild-type (WT) clonal M1T1 strain 5448 and the precise, in-frame allelic replacement mutant speB (11
, 17)
to investigate the interplay between SpeB and human plasminogen co-opted to the bacterial surface.
MATERIALS AND METHODS
Strains, media, and culture conditions
S. pyogenes strain 5448 (M1T1) and the isogenic speB mutant have been described previously (12)
. All S. pyogenes strains were routinely cultured at 37°C on horse-blood agar (BioMérieux, Sydney, NSW, Australia) or in static liquid cultures of Todd-Hewitt broth (Difco, Sydney, NSW, Australia) supplemented with 1% (w/v) yeast extract (THBY medium). When required, SpeB was inactivated by growth in the presence of cysteine protease inhibitor E64 (N-[N-(L-3-transcarboxyirane-2-carbonyl)-L-Leucyl]-agmatine) (Sigma, Sydney, NSW, Australia) at a final concentration of 28 µM.
Hyaluronic acid capsule determination
Overnight cultures of S. pyogenes were diluted 1:10 in fresh THBY media and grown to an optical density (OD) at 600 nm (OD600) of 0.5–0.6. The hyaluronic acid capsule was extracted and quantified by the stains-all method (18)
.
Preparation of culture supernatant proteins
An overnight culture of S. pyogenes was diluted 1:10 in fresh THBY medium and incubated at 37°C to late stationary phase (approx. 16 h). The culture supernatant was harvested by centrifugation at 8000 g for 20 min at 4°C and filter sterilized through a 0.22 µm filter (Millipore, Sydney, NSW, Australia) for use in SpeB activity assays. For sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, equal volumes of cell-free supernatant and 10% (v/v) trichloroacetic acid were mixed and incubated on ice for 1 h. Following centrifugation at 15,000 g for 20 min at 4°C, the pellet was washed with ice-cold 100% ethanol and centrifuged at 15,000 g for 15 min at 4°C. The ethanol was removed by aspiration, the pellet air-dried for 30 min and resuspended in 1 ml of 100 mM Tris-HCl (pH 7.6).
SpeB activity assays
Cysteine protease SpeB activity in S. pyogenes culture supernatants was determined according to the method of Hytönen et al. (19)
. To examine SpeB activity of large numbers of S. pyogenes colonies, the Columbia skim milk plate-based assay was used according to the method of Ashbaugh et al. (20)
.
Immunogold electron microscopy
S. pyogenes were fixed with 0.2% glutaraldehyde and 0.5% formaldehyde for 1 h on ice, washed with cacodylate buffer (0.1 M cacodylate, 0.9 M sucrose, 0.01 M MgCl2, 0.01 M CaCl2; pH 6.9) containing 10 mM glycine and dehydrated with ethanol following the progressive lowering of temperature protocol (10%, 30% ethanol on ice; 50% ethanol at –20°C; 70%, 90%, 100% ethanol at –30°C; each step for 30 min). Samples were infiltrated with the Lowicryl® resin K4M over 3 d before polymerization at –30°C with UV light (366 nm) for 2 d and 1 d at room temperature. Ultrathin sections were collected with formvar-coated cover-slips and incubated with SpeB specific IgG antibodies (75 µg/ml, 10 h at 4°C), washed with PBS and bound antibodies were made visible with protein A/G coated gold-particles (10 or 15 nm in size). Samples were counterstained with uranyl acetate for 1 min and air-dried before examination in a Zeiss transmission electron microscope EM910 at an acceleration voltage of 80 kV.
Plasminogen binding and cell surface plasmin activity
The binding of 125I-plasminogen was measured as reported elsewhere (14)
. Approximately 70 ng of 125I-plasminogen was added to 250 µl of cell suspension and incubated at room temperature for 45 min. The cells were harvested by centrifugation, the supernatant carefully aspirated and pellet-associated radioactivity measured using an automatid Wallac gamma counter (Perkin Elmer, Jugesheim, Germany). The results were expressed as a percentage of input radioactivity. All measurements were determined in triplicate.
Plasmin acquisition in human plasma by S. pyogenes isolates was determined essentially as described previously (21)
. Frozen plasma was purchased from the Red Cross Blood Bank (Sydney, NSW, Australia), defrosted on ice and pooled. Aliquots of pooled plasma were depleted of plasminogen by incubation at 4°C on ice with excess lysine-sepharose® 4B for 1–2 h with gentle agitation. The extent of plasminogen depletion and the plasmin activity of plasma was determined by chromogenic assay with Spectrozyme® PL (American Diagnostics Inc., Stamford, CT, USA), in the presence and absence of streptokinase, and by Western blot using a polyclonal rabbit anti-human plasminogen antibody (Calbiochem, Kilsyth, VIC, Australia).
GAS were cultured overnight as stationary cultures in 25 ml THY inoculated with 1 colony. GAS were pelleted by centrifugation at 800 g, washed in 50 ml of PBS, pH 7.4, prewarmed to 37°C, and resuspended to OD600 = 0.7 (corresponding to log phase, in which streptokinase secretion is induced). A 2 ml aliquot of this suspension was pelleted as above and resuspended in an equal vol of 100% plasma or plasminogen-depleted plasma at 37°C. GAS were incubated in plasma for 3 h at 37°C, pelleted by centrifugation and washed twice with 1 vol of ice-cold 0.01 M EDTA, 0.1% (w/v) gelatin in PBS, pH 7.4. GAS were resuspended in 0.1% (w/v) gelatin in PBS, pH 7.4 to OD600 = 0.75. Aliquots (100 µl) of this suspension were incubated in triplicate in the presence and absence of 20 µl Spectrozyme® PL, 2.5 mM at 37°C for 60 min in a 96-well plate. The reaction was quenched with 80 µl of 1.75 M acetic acid, the plates centrifuged at 800 g, and A405 of supernatants determined.
Plasmin activity was determined as the difference between A405 in the presence and absence of substrate, thus accounting for differences in the sedimentation efficiency of GAS isolates. Each isolate was assayed in at least two independent experiments. Plasmin equivalents and the linear range of the assay (A405=0–0.6) were determined using a standard curve of purified plasmin (Roche Diagnostics, GmbH, Mannheim, Germany).
Extraction of cell wall-associated proteins
Mutanolysin cell wall extracts were prepared from stationary phase cultures of S. pyogenes as reported elsewhere (22)
.
Immunological reagents
Affinity-purified rabbit anti-SpeB IgG was purchased from Toxin Technology Inc., Sarasota, FL, USA. Polyclonal rabbit antibodies were raised against purified recombinant S. pyogenes streptococcal enolase (SEN) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). A rabbit polyclonal antiserum was raised against Lancefield group C streptokinase (Sigma), which shares >90% identity with the streptokinase (Ska) protein of S. pyogenes. All rabbit immunizations were performed according to the method of Gillen et al. (23)
. The production of mouse polyclonal antiserum against M1 protein is described elsewhere (24)
.
Western blot analysis
SDS-PAGE was performed according to the method of Laemmli (25)
. Cell wall extracts or culture supernatant proteins were transferred to nitrocellulose membrane at 100 V for 1 h using the Mini Trans-Blot® (Bio-Rad) (26)
. The membranes were blocked in a solution of 5% (w/v) skim milk (Difco) in PBS (137 mM NaCl, 2.7 mM KCl, 7.9 mM Na2HPO4, 1.5 mM KH2PO4; pH 7.4) for 1 h at room temperature. After a 10 min wash with PBS, the membranes were incubated for 2 h with primary Ab diluted 1:1,000 in PBS. Following three washes for 10 min each with PBS, the membranes were incubated for 1 h with a 1:1,000 dilution of goat anti-rabbit IgG HRP conjugate (Bio-Rad) for SpeB, Ska, SEN, and GAPDH blots, or a 1:1,000 dilution of goat antimouse IgG HRP conjugate (Bio-Rad) for M1 protein blots. Excess secondary Ab was removed by three PBS washes for 10 min each and all blots developed in a solution of 100 mM Tris-HCl (pH 7.6) containing 1.4 mM diaminobenzidine and 0.06% (v/v) hydrogen peroxide.
Protease assays
SpeB protease (5 µg; Toxin Technology Inc.) was mixed with 5 µg of purified human plasminogen or fibrinogen (14)
, adjusted to final vol of 25 µl with PBS (pH 7.4) and incubated at 37°C for 3 h. Cleavage of substrate protein was determined by SDS-PAGE analysis. Positive controls containing 5 µg SpeB and 25 µg ß-casein (Sigma) were used in all assays. Negative controls containing protease or substrate only were also included.
Streptococcal infection model
AlbPLG1 transgenic mice (Tg+) (27)
heterozygous for the human plasminogen transgene were backcrossed greater than n = 6 with C57BL/J6 mice (Animal Resources Centre, Perth, Australia). Nontransgenic (Tg–) littermates were used as controls in all experiments. Streptococcal strains used for inocula were harvested at midlog phase (OD600 approx. 0.5). Bacteria were washed with an equal vol of sterile 0.7% saline and diluted to the required inoculum. The number of viable bacteria present was determined by counting colony-forming units (CFU) after plating a dilution series onto blood agar plates (BioMérieux). Groups of 10 Tg+ mice and Tg– littermates were infected with either 2.8 x 108 CFU (5448) or 1.35 x 108 CFU (speB mutant) via a s.c. injection into a shaved region of the right flank of each mouse. Mortality was recorded over 14 d.
For experiments examining the dissemination of streptococci during infection, a separate cohort of Tg+ mice was inoculated s.c. Wild-type 5448 was inoculated using 1.58 x 109 CFU (n=4; 1 mouse succumbed to infection on day 2) and 1.49 x 108 CFU (n=1). The 5448 mutant speB was inoculated using 1.71 x 109 CFU (mutant; n=4; 1 mouse succumbed to infection on day 2) and 4.5 x 107 CFU (n=1). On day 3 postinfection, mice were sacrificed by CO2 asphyxiation, and representative bacteria were isolated from the site of initial infection and bacterial loads present in the bloodstream were determined by plating serial dilutions onto blood agar. All experiments were conducted according to the Guidelines for the Care and Use of Laboratory Animals (National Health and Medical Research Council, Australia) and were approved by the University of Wollongong Animal Ethics Committee.
Statistical analyses
Average capsular hyaluronic acid, SpeB activity and plasminogen binding for 5448 and speB were compared using the two-tailed unpaired t test. Cell surface plasmin activity levels were evaluated using the Welch ANOVA (assuming unequal variances). Differences in survival of Tg+ and Tg– mice infected with 5448 and speB were determined by the log-rank test. Mean bacterial loads in the bloodstream (log10 CFU) of Tg+ mice infected with 5448 or speB were compared using the two-tailed unpaired t test. The t and log-rank tests were performed using GraphPad Prism version 4.02 (GraphPad Software Inc., San Diego, CA, USA). The Welch ANOVA was performed using JMP (SAS Institute, NC, USA).
RESULTS
Characterization of M1T1 strains 5448 and speB
Both WT and isogenic speB mutant strains expressed identical amounts of hyaluronic acid capsule (Fig. 1
a), excluding a pleiotropic mutation that confounded earlier studies of SpeB in GAS pathogenesis (20)
. Secreted cysteine protease activity was undetectable in the speB mutant (P
0.05; Fig. 1b
), and SpeB zymogen was detected only at the cell wall ExPortal microdomain (28)
of the parent M1T1 strain (Fig. 1c
). Nonetheless, both WT and speB mutant bound equivalent amounts of purified human plasminogen (Fig. 1d
). These data indicate that, like the GAS M5 strain characterized by Rosch and Caparon (28)
, the M1T1 clone also secretes SpeB zymogen via the ExPortal microdomain. The presence or absence of SpeB does not affect the capacity of M1T1 GAS to bind purified human plasminogen, suggesting that the surface plasminogen receptors 
-enolase (SEN) (29)
and GAPDH (30)
are not disabled by SpeB proteolysis. However, in comparison to the WT strain, the isogenic speB mutant accumulated 75-fold higher levels of human plasmin activity on the bacterial surface following incubation in human plasma for 3 h at 37°C (P0.05; Fig. 1e
).
|
Analysis of plasminogen activation system components
To elucidate the underlying reason(s) for the enhanced capacity of the cysteine protease-deficient GAS strain to accumulate surface plasmin activity, we compared WT and speB mutant strains for the expression of cell wall-associated plasminogen and fibrinogen receptors and for secretion of streptokinase. Active 28 kDa SpeB was identified only in supernatants of the WT strain; with addition of the SpeB inhibitor E64 just the 40 kDa SpeB zymogen form was detected (Fig. 2
a). Streptokinase was present only in supernatants of the speB mutant or the WT parent strain grown in the presence of E64 (Fig. 2a
). The fibrinogen-binding M1 protein was found in cell wall extracts only in the absence of SpeB activity, whereas plasminogen receptors SEN and GAPDH were detected in cell wall extracts of both WT and speB mutant (Fig. 2a
). SpeB was also observed to degrade plasminogen directly, as well as fibrinogen, which forms a trimolecular complex with streptokinase and plasminogen bound to the GAS surface (14
, 31)
(Fig. 2b
). Thus, SpeB interferes with the accumulation of GAS surface bound plasmin activity by cleaving four critical factors: plasminogen, the plasminogen activator streptokinase, fibrinogen and the fibrinogen-binding M1 protein (16)
.
|
Infection of humanized plasminogen transgenic mice
We next took advantage of heterozygous AlbPLG1 mice that express a human plasminogen transgene (Tg+) (15)
and littermate C57BL/J6 control mice (Tg–) to examine the virulence of WT M1T1 GAS and the isogenic speB mutant in a s.c. infection model. Only through use of this transgenic mouse line can one investigate the interaction of GAS with the plasminogen activation system, since this interaction is highly specific for human plasminogen (15
, 16)
. In comparison to infection of nontransgenic control mice, the virulence of WT M1T1 strain was significantly increased in Tg+ mice (P0.05; 10% vs. 80% mortality) (Fig. 3
a). This result indicates that the human plasminogen system plays a crucial role in the ability of M1T1 GAS to initiate a lethal systemic infection. In comparison to the WT GAS parent strain, the isogenic speB mutant was attenuated for virulence in the Tg+ plasminogen-humanized mice (P0.05; 80% vs. 20% mortality) (Fig. 3a, b
). Additionally, in comparison to GAS strain 5448, 72 h postinfection of Tg+ mice the isogenic speB mutant displayed significantly fewer bacterial counts in the blood (P0.05; Fig. 3c
). These data support a role for SpeB in GAS survival at the site of local infection in the skin, as proposed by Svensson et al. (8)
, and suggested this difference may translate into an overall decreased lethality on challenge with the speB mutant.
|
To further examine the interaction between the human plasminogen system and GAS cysteine protease SpeB, we examined the SpeB expression status of WT GAS isolated from the blood of the infected plasminogen-humanized Tg+ mice. The WT GAS used in the initial challenge uniformly expressed high levels of cysteine protease activity (100% of inoculum colonies tested; n=100). Bacteria isolated from the s.c. site of infection expressed a mixed phenotype (74%, 66%, 63%, and 31% of lesion recovered colonies SpeB-negative from 4 s.c. inoculated Tg+ mice). Bacteria recovered from the blood unvaryingly demonstrated very low SpeB activity (100% of blood isolated colonies examined SpeB-negative) (Fig. 3d
). This finding is consistent with the epidemiologic observation that M1T1 clinical isolates from human invasive disease express reduced levels of SpeB (4)
.
DISCUSSION
The mechanism utilized by GAS to switch from localized to systemic infection is presently unknown. However, subversion of the host plasminogen activation system is thought to play a key role (15
, 16)
. In this study, we have shown that the absence of cysteine protease SpeB activity is required for the accumulation of plasmin activity on the cell surface of M1T1 GAS. In a humanized plasminogen transgenic mouse model, the loss of SpeB activity at the site of infection triggers the systemic dissemination of M1T1 GAS in vivo. These data support the proposal that human plasminogen plays a critical role in the initiation of GAS invasive disease.
Plasminogen is the proenzyme form of plasmin, a blood clot-dissolving serine protease, which degrades ECM components and activates matrix metalloproteases (32)
. In S. pyogenes, two distinct pathways have been identified for the binding of plasminogen to the cell surface. The direct pathway is mediated by the plasminogen-binding GAS M-like protein (PAM) (33)
, streptococcal enolase (SEN) (29)
, and GAPDH (29
, 34)
. Indirect plasminogen-binding requires the formation of a trimolecular complex between plasminogen, streptokinase, and fibrinogen, which is bound to the GAS cell surface via plasminogen or fibrinogen receptors (14
, 31
, 35)
. Human plasminogen is converted to plasmin by the plasminogen activators urokinase (uPA) and tissue-type plasminogen activator (tPA), or by secreted microbial activators including the GAS virulence factor streptokinase (36)
. Here, we demonstrate that the presence of SpeB does not interfere with the binding of plasminogen to the GAS cell surface. However, SpeB degrades four key factors required for the accumulation of surface plasmin activity on the GAS surface: plasminogen, streptokinase, fibrinogen, and the fibrinogen-binding M1 protein.
Utilizing a speB-negative mutant and a humanized in vivo model for impetigo in which neonatal foreskin was engrafted onto the hind flanks of CB-17 scid mice, Svensson et al. (8)
determined that SpeB protease plays a critical role in the establishment of host tissue tropism by GAS. Our accumulated data suggest that SpeB, while contributing to localized infection, simultaneously mitigates the potential interaction of M1T1 GAS with the human plasminogen activation system. The ablation of SpeB expression in WT M1T1 GAS in the blood of plasminogen-humanized Tg+ mice indicates that the loss of SpeB activity in a subpopulation of bacteria occurs at the site of infection, allowing accumulation of surface plasmin activity. This subpopulation thus gain enhanced invasive propensity, favoring transition of GAS from the site of infection to the blood. In our model, vascular leakage induced by M protein complexing with fibrinogen to activate heparin binding protein release from neutrophils, may provide a source of plasminogen at the site of infection (37)
(Fig. 4
). The subpopulation of GAS undergoing a phase shift to eliminate SpeB expression gain not only the capacity to accumulate cell surface plasmin activity and spread systemically, but also elaborate greater levels of superantigens (12)
, the causative agents of streptococcal toxic shock.
|
The expression of the streptococcal cysteine protease SpeB and the human invasive disease severity are inversely related in M1T1 clonal isolates (4)
. The model we propose for GAS human systemic disease initiation accounts for this previously perplexing clinical observation and describes a mechanism by which systemic disease initiation occurs. The elucidation of the mechanism by which GAS causes human invasive disease will inform future treatment and prevention strategies.
ACKNOWLEDGMENTS
We thank K. S. Sriprakash (Queensland Institute of Medical Research, Queensland, Australia) for providing M1 protein antiserum. J. N. Cole, M. L. Sanderson-Smith, and A. J. Cork are the recipients of an Australian Postgraduate Award. D. Ginsburg is a Howard Hughes Medical Institute Investigator. This work was supported by the National Health and Medical Research Council of Australia and a National Institute of Health grant (PO1HL057346).
Received for publication January 20, 2006. Accepted for publication March 31, 2006.
REFERENCES
2-Macroglobulin-proteinase complexes protect Streptococcus pyogenes from killing by the antimicrobial peptide LL-37. J. Biol. Chem. 279,52820-52823-Enolase, a novel strong plasmin(ogen) binding protein on the surface of pathogenic streptococci. J. Biol. Chem. 273,14503-14515This article has been cited by other articles:
|
|
 |
|
  J. D. McArthur, F. C. McKay, V. Ramachandran, P. Shyam, A. J. Cork, M. L. Sanderson-Smith, J. N. Cole, U. Ringdahl, U. Sjobring, M. Ranson, et al. Allelic variants of streptokinase from Streptococcus pyogenes display functional differences in plasminogen activation FASEB J, September 1, 2008; 22(9): 3146 - 3153. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. L. Sanderson-Smith, K. Dinkla, J. N. Cole, A. J. Cork, P. G. Maamary, J. D. McArthur, G. S. Chhatwal, and M. J. Walker M protein-mediated plasminogen binding is essential for the virulence of an invasive Streptococcus pyogenes isolate FASEB J, August 1, 2008; 22(8): 2715 - 2722. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. Creti, M. Imperi, L. Baldassarri, M. Pataracchia, S. Recchia, G. Alfarone, and G. Orefici emm Types, Virulence Factors, and Antibiotic Resistance of Invasive Streptococcus pyogenes Isolates from Italy: What Has Changed in 11 Years? J. Clin. Microbiol., July 1, 2007; 45(7): 2249 - 2256. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. L. Sanderson-Smith, M. Dowton, M. Ranson, and M. J. Walker The Plasminogen-Binding Group A Streptococcal M Protein-Related Protein Prp Binds Plasminogen via Arginine and Histidine Residues J. Bacteriol., February 15, 2007; 189(4): 1435 - 1440. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. A. Kwinn, A. Khosravi, R. K. Aziz, A. M. Timmer, K. S. Doran, M. Kotb, and V. Nizet Genetic Characterization and Virulence Role of the RALP3/LSA Locus Upstream of the Streptolysin S Operon in Invasive M1T1 Group A Streptococcus J. Bacteriol., February 15, 2007; 189(4): 1322 - 1329. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. M. Nooh, R. K. Aziz, M. Kotb, A. Eroshkin, W.-J. Chuang, T. Proft, and R. Kansal Streptococcal Mitogenic Exotoxin, SmeZ, Is the Most Susceptible M1T1 Streptococcal Superantigen to Degradation by the Streptococcal Cysteine Protease, SpeB J. Biol. Chem., November 17, 2006; 281(46): 35281 - 35288. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
