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Cocos Gram-positivos

Streptococcus mitis / oralis

Gram-positive cocci in chains

Identification & Growth

Identification

  • Alpha-hemolytic, optochin-resistant, bile solubility negative
  • MALDI-TOF may confuse them with S. pneumoniae — confirm with optochin/bile tests

Growth Conditions

  • Blood agar with 5% CO₂, 18–24 h

Clinical Significance

  • Subacute native valve endocarditis
  • Bacteremia and mucositis in neutropenic patients

Intrinsic Resistance

  • Aztreonam, colistin and nalidixic acid have no activity

Bench Alerts

  • Penicillin resistance is common: report the MIC, never the disk alone
  • Distinguishing from pneumococcus is mandatory before releasing the report

Clinical Notes

Clinical presentations

  • Leading cause of native valve infective endocarditis within the viridans group, especially following dental procedures.
  • Causes severe neutropenic bacteremia in patients with post-chemotherapy oral mucositis.
  • May be associated with streptococcal shock syndrome in the setting of febrile neutropenia.

Specimens and collection

  • Serial blood cultures prior to antibiotic therapy are essential for endocarditis diagnosis per Duke criteria.
  • Excised heart valve tissue should be sent for culture and, if available, 16S rRNA PCR.
  • Oral swabs are not useful for etiologic diagnosis of bacteremia given normal colonization status.

Epidemiology and at-risk populations

  • Colonizes the oral cavity as part of the normal microbiota, particularly tooth surfaces and mucosa.
  • Patients with prior valvulopathy or valve prostheses have increased endocarditis risk after transient bacteremia.
  • Neutropenic oncology patients with mucositis constitute a high-risk population for severe bacteremia.

Resistance and therapeutic implications

  • Penicillin resistance in mitis group streptococci is increasing globally and should be tested by MIC.
  • Isolates with reduced penicillin susceptibility require combination therapy with an aminoglycoside in endocarditis.
  • Vancomycin is an option in beta-lactam-allergic patients or resistant isolates.

Bench and reporting notes

  • Alpha-hemolysis on blood agar and optochin resistance help differentiate from S. pneumoniae.
  • Precise species identification within the mitis group is difficult by phenotypic methods; MALDI-TOF or gene sequencing is recommended.
  • Test penicillin susceptibility by a quantitative method (MIC) for every blood or CSF isolate.

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 mitis / oralis in the lab?

Streptococcus mitis / oralis is identified through alpha-hemolytic, optochin-resistant, bile solubility negative, maldi-tof may confuse them with s. pneumoniae — confirm with optochin/bile tests.

What are the intrinsic resistances of Streptococcus mitis / oralis?

This organism is naturally resistant to aztreonam, colistin and nalidixic acid have no activity. These drugs should not be reported as susceptible.

Where is Streptococcus mitis / oralis commonly found?

It is typically associated with subacute native valve endocarditis, bacteremia and mucositis in neutropenic patients.

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