Molecular basis of group A streptococcal pyrogenic exotoxin B

Comparative analysis of emm types, superantigen gene profiles and antibiotic resistance genes among Streptococcus pyogenes isolates from ocular infections, pharyngitis and asymptomatic children in south India

Group A Streptococcus (Streptococcus pyogenes) is responsible for a wide array of infections and incidence is high in developing countries like India. Although distribution of emm types of S. pyogenes in India has been described, its association with the virulence genes and ocular isolates is less concentrated. In the present study emm type surveillance as well as its association with toxin gene profile was analyzed. Ocular infected cases such as lacrimal abscess, corneal ulcers, mucocoele showed the presence of 20 S. pyogenes isolates. For noninvasive isolates, we screened 370 pharyngitis cases and 400 asymptomatic school children and recovered 33 pharyngitis and 14 carrier isolates respectively. 14 Emm type distributions were observed in ocular isolates, 11 emm types each in pharyngitis and asymptomatic carrier isolates. The two dominant emm types, emm49 and emm63 were accounted for 33% of the total S. pyogenes isolates. Among ocular isolates, slo, smeZ, speB and speG were found in >50% of isolates, in pharyngitis smeZ (48%), speB (45%) and speG (42%) genes were found to be prevalent. Alarmingly, carrier isolates showed more prevalence to virulence genes than the ocular and pharyngitis isolates with speF (79%), speB, speG (64%), slo and sil (64%). Among the three groups, pharyngitis isolates harbored more prtF1 (33%) and prtF2 (94%) than the asymptomatic carriers (28% and 71%) and the ocular isolates (45% and 40%). 450bp Size band in prtF1 and 350bp size band in prtF2 showed dominance. Among the three groups tested, the distribution of ermB and mefA was high in pharyngitis isolates (30%) where 10 isolates showed the presence of both genes. None of the isolates showed the presence of ermA and tetO genes. Dendrogram generated based on the virulence and antibiotic resistance gene profiles revealed that except one cluster, all other clusters showed some correlation with ocular, pharyngitis and asymptomatic carrier isolates, irrespective of their emm types.

The role of nephritis-associated plasmin receptor (NAPlr) in glomerulonephritis associated with streptococcal infection

It is well known that glomerulonephritis can occur after streptococcal infection, which is classically referred to as acute poststreptococcal glomerulonephritis (APSGN). The pathogenic mechanism of APSGN has been described by so-called immune complex theory, which involves glomerular deposition of nephritogenic streptococcal antigen and subsequent formation of immune complexes in situ and/or the deposition of circulating antigen-antibody complexes. However, the exact entity of the causative antigen has remained a matter of debate. We isolated a nephritogenic antigen for APSGN from the cytoplasmic fractions of group A streptococcus (GAS) depending on the affinity for IgG of APSGN patients. The amino acid and the nucleotide sequences of the isolated protein revealed to be highly identical to those of reported plasmin(ogen) receptor of GAS. Thus, we termed this antigen nephritis-associated plasmin receptor (NAPlr). Immunofluorescence staining of the renal biopsy tissues with anti-NAPlr antibody revealed glomerular NAPlr deposition in essentially all patients with early-phase APSGN. Furthermore, glomerular plasmin activity was detected by in situ zymography in the distribution almost identical to NAPlr deposition in renal biopsy tissues of APSGN patients. These data suggest that NAPlr has a direct, nonimmunologic function as a plasmin receptor and may contribute to the pathogenesis of APSGN by maintaining plasmin activity.

Dispersal of Group A streptococcal biofilms by the cysteine protease SpeB leads to increased disease severity in a murine model

Group A Streptococcus (GAS) is a Gram-positive human pathogen best known for causing pharyngeal and mild skin infections. However, in the 1980's there was an increase in severe GAS infections including cellulitis and deeper tissue infections like necrotizing fasciitis. Particularly striking about this elevation in the incidence of severe disease was that those most often affected were previously healthy individuals. Several groups have shown that changes in gene content or regulation, as with proteases, may contribute to severe disease; yet strains harboring these proteases continue to cause mild disease as well. We and others have shown that group A streptococci (MGAS5005) reside within biofilms both in vitro and in vivo. That is to say that the organism colonizes a host surface and forms a 3-dimensional community encased in a protective matrix of extracellular protein, DNA and polysaccharide(s). However, the mechanism of assembly or dispersal of these structures is unclear, as is the relationship of these structures to disease outcome. Recently we reported that allelic replacement of the streptococcal regulator srv resulted in constitutive production of the streptococcal cysteine protease SpeB. We further showed that the constitutive production of SpeB significantly decreased MGAS5005Δsrv biofilm formation in vitro. Here we show that mice infected with MGAS5005Δsrv had significantly larger lesion development than wild-type infected animals. Histopathology, Gram-staining and immunofluorescence link the increased lesion development with lack of disease containment, lack of biofilm formation, and readily detectable levels of SpeB in the tissue. Treatment of MGAS5005Δsrv infected lesions with a chemical inhibitor of SpeB significantly reduced lesion formation and disease spread to wild-type levels. Furthermore, inactivation of speB in the MGAS5005Δsrv background reduced lesion formation to wild-type levels. Taken together, these data suggest a mechanism by which GAS disease may transition from mild to severe through the Srv mediated dispersal of GAS biofilms.

An insert in the covS gene distinguishes a pharyngeal and a blood isolate of Streptococcus pyogenes found in the same individual

Expression of the extensive arsenal of virulence factors by Streptococcus pyogenes is controlled by many regulators, of which CovRS is one of the best characterized and can influence ∼15 % of the genome. Animal models have established that mutants of covRS arise spontaneously in vivo resulting in highly invasive organisms. We analysed a pharyngeal and a blood isolate of S. pyogenes recovered from the same individual 13 days apart. The two isolates varied in many phenotypic properties including SpeB production, which were reflected in transcriptomic analyses. PFGE, multilocus sequence typing and partial sequencing of some key genes failed to show any differences except for an 11 bp insert in the covS gene in the blood isolate which caused a premature termination of transcription. Complementation of a fully functional covS gene into the blood isolate resulted in high expression of CovS and expression of speB. These results, showing a pharyngeal and a blood isolate from a single individual differing by a simple insertion, provide evidence for the model that regulatory gene mutations allow S. pyogenes to invade different niches in the body.