Laboratory Support for Outbreak Investigation of Invasive Group A Streptococcal Disease

The National Centre for Streptococcus (NCS) provides laboratory support for the investigation of clusters or outbreaks of invasive group A Streptococcus (GAS) disease. The decision to initiate this type of investigation rests with the local public health agencies. The investigation team should coordinate the shipment of isolates and required information to the NCS through its local provincial laboratory or designate laboratory. The local outbreak investigation team is asked to provide a brief written description of the event, and this should be forwarded, via the appropriate provincial laboratory or designated laboratory, to the NCS in advance of isolate submission. This facilitates a timely laboratory response. Isolates associated with outbreak investigation are given priority testing status, and reports are phoned or faxed to the submitters as soon as testing is complete. Preliminary results for isolates from an outbreak investigation should be available within 1 week.

Contact the NCS at: Phone: (780) 407-8977; (780) 407-8937 Fax: (780) 407-8984 Email:;

If the investigation is to include testing of contacts (i.e. characterization of non-invasive GAS isolates), arrangements for the primary culture of these specimens should be made through the local microbiology laboratories and/or the provincial laboratories. Isolates may then be forwarded to the NCS for further investigation. Submitters are encouraged to use the NCS submission form for all isolates. This document is available through the NCS Web site at (Partners\National Centres\National Centre for Streptococcus\NCS Requisition Forms).

Characterization of Streptococcus pyogenes (GAS)

The analysis of GAS includes serologic and molecular techniques. The strain profile includes the identification of the M protein type and T protein, and anti-opacity factor testing for serum opacity factor-positive strains.

M typing

M protein is a significant virulence factor produced by GAS. It is a surface protein antigen that gives the organism the ability to resist phagocytosis as a way of evading the human immune response to infection. Traditional serologic characterization of M protein relies on an antigen-antibody reaction between the organism and M type specific antisera. The M antisera are inherently difficult to prepare (commercial reagents are not available), and consequently this specialized testing is offered in only six reference laboratories worldwide. There are 86 M protein types that have been officially classified by this serologic method. The testing is performed by immunodiffusion and, especially for less common M types, several steps may be required to classify the strain. The production of M protein is coded by the emm gene, and molecular analysis is able to classify a rapidly expanding number of emm types. The emm type corresponds to the M type (e.g. M1 = emm 1) when the strain belongs to one of the internationally recognized M types. However emm typing is able to classify a large number of strains for which traditional M antisera are not available. This makes emm typing a more specific tool.

T typing

T protein is not a virulence factor, but it provides an additional serologic "marker" with which to differentiate strains. A GAS strain may carry one or more T antigens, and the same T pattern may be shared by several M or emm types. T typing is therefore a less specific typing method than M or emm typing; however, there is an association between the T pattern and the specific M or emm type. T typing is performed by observing the antigen-antibody reaction with specific T typing antisera.

SOF typing

Serum opacity factor (SOF) is an enzyme that is produced by some M/emm types. It is an apoproteinase that is named for its ability to produce opacity when an extract of the strain is mixed with mammalian serum (horse serum is typically used). Some M types are known to be SOF positive and others are typically SOF negative (e.g. M1 is always SOF negative, and M22 is typically SOF positive). The opacity factor of SOF-positive strains may be typed by neutralization of the reaction using specific antisera. This is called anti-opacity factor (AOF) typing and, with few exceptions, the AOF type is consistent with the M/emm type.

sic gene typing of M1 strains

Traditional fingerprinting techniques (e.g. pulsed field gel electrophoresis) are not specific enough to differentiate strains of the same M/emm type. However, for M1/emm 1 strains, which account for 20% to 30% of the invasive disease-causing strains each year in Canada, variation within the sic (streptococcal inhibitor of complement) gene may be used to provide further analysis of isolates associated with an outbreak. Utilization of this testing for specific investigations must be discussed with the NCS before isolates are sent.

Writers: Marguerite Lovgren and Gregory J. Tyrrell

Selected Readings

  1. Beall B, Gherardi G, Lovgren M et al. emm and sof gene sequence variation in relation to serological typing of opacity-factor positive group A streptococci. Microbiology 2000;146:1195-209.
  2. Beall B, Facklam RR, Elliott JA et al. Streptococcal emm types associated with T-agglutination types and the use of conserved emm gene restriction fragment patterns for subtyping of group A streptococci. JMed Microbiol 1998;47:893-8.
  3. Facklam RF, Martin DR, Lovgren M et al. Extension of the Lancefield classification for group A streptococci by addition of 22 new M protein gene sequence types for clinical isolates: emm 103 to emm 124. Clin Infect Dis 2002;43:28-38.
  4. Facklam R, Beall B, Efstratiou et al. emm typing and validation of provisional M types for group A streptococci. Emerg Infect Dis 1999;5:247-53.
  5. Tyrrell GJ, Lovgren M, Forwick B et al. M types of group A streptococcal isolates submitted to the National Centre for Streptococcus (Canada) from 1993-1999. J Clin Microbiol 2002;40:4466-71.
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