Streptococcus mitis: Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes

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Introduction

Streptococcus mitis is a Gram-positive, alpha-hemolytic bacterium commonly found as a harmless commensal in the human oral cavity, but it can act as an opportunistic pathogen causing severe infections like infective endocarditis.

Streptococcus mitis colony characteristics on blood agar
Fig. Streptococcus mitis colony characteristics on blood agar
  • Classification: Belongs to the Viridans streptococci group.
  • Habitat: Inhabits the human oral cavity, throat, and nasopharynx.
  • Nature: Primarily a harmless commensal organism.
  • Risk: Causes severe opportunistic infections if it enters the bloodstream.

Morphology

Streptococcus mitis colony morphology on blood agar
Fig. Streptococcus mitis colony morphology on blood agar
  • Gram Stain: Gram-positive cocci.
  • Arrangement: Cells appear in chains or pairs.
Gram positive cocci of Streptococcus mitis
Fig. Gram-positive cocci of Streptococcus mitis
  • Shape: Oval or spherical cells.
  • Motility: Non-motile organism.
  • Spores: Non-spore-forming bacterium.
  • Capsule: Some strains possess a polysaccharide capsule.

Pathogenicity

  • Entry: Enters the bloodstream during dental procedures or vigorous brushing.
  • Endocarditis: Binds to damaged heart valves, forming biofilms.
  • Bacteremia: Causes bloodstream infections, especially in neutropenic cancer patients.
  • Virulence Factors: Utilizes surface adhesins to bind to host tissues.
  • Toxins: Produces pneumolysin-like cytolysins in certain strains.

Lab Diagnosis

  • Specimen: Blood, cerebrospinal fluid, or oral swabs.
  • Microscopy: Gram-positive cocci arranged in chains.
Oval or spherical cells Gram -positive cocci (GPC) in chains or pairs of Streptococcus mitis and few are resembling to Gram-negative due to older culture
Fig. Oval or spherical cells, Gram-positive cocci (GPC) in chains or pairs of Streptococcus mitis, and a few resemble Gram-negative due to older culture
Oval or spherical cells Gram -positive cocci (GPC) in chains or pairs of Streptococcus mitis at amgnification of 1000x with 8x optical zoom
Fig. Oval or spherical cells, Gram-positive cocci (GPC) in chains or pairs of Streptococcus mitis at magnification of 1000x with 8x optical zoom
  • Culture: Grows on Blood Agar under 5% CO₂.
  • Hemolysis: Displays alpha-hemolytic (green zone) colonies.
  • Optochin Test: Resistant to optochin, differentiating it from Streptococcus pneumoniae.
  • Bile Solubility: Insoluble in bile.
  • Identification: Confirmed via MALDI-TOF mass spectrometry or 16S rRNA sequencing.

Treatment

  • First Line: Penicillin G remains the preferred treatment.
  • Combination: Aminoglycosides (like gentamicin) are added for endocarditis.
  • Alternatives: Vancomycin or ceftriaxone for penicillin-resistant strains.
  • Susceptibility: Requires antibiotic susceptibility testing due to rising resistance.
Antimicrobial susceptibility testing (AST) result of Streptococcus mitis
Fig. Antimicrobial susceptibility testing (AST) results of Streptococcus mitis
Antibiogram of Streptococcus mitis
Fig. Antibiogram of Streptococcus mitis
Antibiotics susceptibility testing result of Streptococcus mitis
Fig. Antibiotic susceptibility testing results of Streptococcus mitis

Prevention

  • Hygiene: Maintain rigorous daily oral hygiene.
  • Prophylaxis: Administer prophylactic antibiotics before dental surgeries for high-risk cardiac patients.
  • Monitoring: Monitor absolute neutrophil counts in immunocompromised patients.

Keynotes

  • Group: Core member of the Viridans group.
  • Mimicry: Closely resembles Streptococcus pneumoniae genetically and phenotypically.
  • Target: Highly dangerous to patients with prosthetic heart valves.
  • Complication: Can cause “Viridans Streptococcal Shock Syndrome” in neutropenic patients.

Further Reading

  1. https://en.wikipedia.org/wiki/Streptococcus_mitis
  2. https://pmc.ncbi.nlm.nih.gov/articles/PMC7669753/
  3. https://pmc.ncbi.nlm.nih.gov/articles/PMC8407716/
  4. https://pmc.ncbi.nlm.nih.gov/articles/PMC9398851/
  5. https://pmc.ncbi.nlm.nih.gov/articles/PMC10376791/
  6. http://impactfactor.org/PDF/IJPCR/15/IJPCR,Vol15,Issue7,Article119.pdf
  7. https://www.dextrosetech.com/chennai/latest-update/staphylococcus-aureus-shape-morphology/21
  8. https://microbenotes.com/streptococcu-s-mitis/
  9. https://pmc.ncbi.nlm.nih.gov/articles/PMC6722419/
  10. https://pmc.ncbi.nlm.nih.gov/articles/PMC11272333/
  11. https://en.wikipedia.org/wiki/Streptococcal_pharyngitis

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