Cocos Gram-positivos

Streptococcus pyogenes (grupo A)

Gram-positive cocci in chains

Identification & Growth

Identification

Catalase-negative, PYR-positive
Susceptible to bacitracin (0.04 U disk)
Lancefield group A latex agglutination

Growth Conditions

Sheep blood agar: small, translucent colonies with wide zones of beta-hemolysis
35–37 °C with 5% CO₂, 18–24 h

Clinical Significance

Pharyngotonsillitis, scarlet fever, erysipelas, necrotizing fasciitis

Intrinsic Resistance

Aztreonam, colistin, aminoglycosides (low level) and sulfonamides have no clinical activity (EUCAST Expected Resistant Phenotypes v1.2)

Bench Alerts

No confirmed penicillin resistance has ever been reported — report as susceptible without testing
Test macrolides and clindamycin (with D-test) for penicillin-allergic patients

Clinical Notes

Clinical presentations

Causes acute pharyngotonsillitis, impetigo, erysipelas, and cellulitis, plus scarlet fever mediated by erythrogenic toxin.
Causes severe invasive infections, including necrotizing fasciitis and streptococcal toxic shock syndrome.
Associated with nonsuppurative post-infectious sequelae: rheumatic fever and post-streptococcal glomerulonephritis.

Specimens and collection

Oropharyngeal swab should touch both tonsils and the posterior pharyngeal wall, avoiding contact with the tongue and oral mucosa.
Rapid antigen detection tests have high specificity, but a negative result in a child should be confirmed by culture.
In suspected invasive infection, blood cultures and deep tissue culture (fasciitis) should be obtained before antibiotic therapy.

Epidemiology and at-risk populations

Streptococcal pharyngitis predominantly affects children aged 5–15 years, transmitted via respiratory droplets.
Severe invasive disease is more common in the elderly, diabetics, people who inject drugs, and those with recent varicella.
Outbreaks of invasive group A infection have been reported globally in recent years, with increased surveillance notifications.

Resistance and therapeutic implications

Penicillin resistance has never been described, and penicillin remains the treatment of choice for all clinical presentations.
Macrolide resistance (erythromycin/azithromycin) is region-variable and relevant for penicillin-allergic patients.
In severe invasive infection, clindamycin is added to penicillin for its toxin-production suppression effect (Eagle effect).

Bench and reporting notes

Complete beta-hemolysis on blood agar and bacitracin susceptibility (disk) support presumptive group A identification.
Latex agglutination for Lancefield group antigen confirms serologic classification when available.
Macrolide and clindamycin susceptibility testing, with D-test for inducible resistance, should be performed on invasive isolates.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Streptococcus pyogenes (grupo A) in the lab?

Streptococcus pyogenes (grupo A) is identified through catalase-negative, pyr-positive, susceptible to bacitracin (0.04 u disk), lancefield group a latex agglutination.

What are the intrinsic resistances of Streptococcus pyogenes (grupo A)?

This organism is naturally resistant to aztreonam, colistin, aminoglycosides (low level) and sulfonamides have no clinical activity (eucast expected resistant phenotypes v1.2). These drugs should not be reported as susceptible.

Where is Streptococcus pyogenes (grupo A) commonly found?

It is typically associated with pharyngotonsillitis, scarlet fever, erysipelas, necrotizing fasciitis.

Antibiogram InterpreterCheck BrCAST 2026 breakpoints Rugai InterpreterBiochemical identification tools