Streptococcus intermedius: PCR Detection and Clinical Overview

Streptococcus intermedius belongs to the phylum Firmicutes, class Bacilli, order Lactobacillales, family Streptococcaceae, and genus Streptococcus.

It is part of the Streptococcus anginosus group (formerly known as the “Streptococcus milleri group”), which also includes S. anginosus and S. constellatus.

This organism is a Gram-positive coccus, typically 0.5–1.0 μm in diameter, arranged in pairs or short chains. On blood agar, it commonly exhibits α-hemolysis (greenish discoloration), though β-hemolysis may occasionally occur.

Growth requires enriched media supplemented with blood or serum and is optimal at 35–37°C under CO₂-enriched conditions.

Streptococcus intermedius is a common inhabitant of the human oral cavity and upper respiratory tract. It is frequently found in dental plaque, gingival crevices, tongue surfaces, and tonsillar crypts.

It may also be isolated from the gastrointestinal tract and female genital tract, though less frequently.

In healthy individuals, it contributes to biofilm formation and participates in maintaining microbial balance without causing disease.

The pathogenic potential of Streptococcus intermedius is associated with several virulence mechanisms.

Biofilm formation: Enables strong adherence to host tissues and medical devices, enhancing resistance to immune responses and antimicrobial agents.

Extracellular enzymes: Production of hyaluronidase, DNase, and proteases facilitates tissue invasion and dissemination.

Anti-phagocytic properties: Surface structures may inhibit phagocytosis, allowing persistence within infected tissues.

Synergistic infections: Frequently participates in polymicrobial infections, particularly with anaerobic bacteria, increasing overall pathogenicity.

Streptococcus intermedius is strongly associated with purulent infections, especially deep abscess formation.

Common clinical presentations include:

Oral and maxillofacial infections: Periodontal abscesses, periapical infections, and deep space infections of the jaw and neck.

Central nervous system infections: It is one of the most frequently isolated streptococci in brain abscesses, often presenting with fever, headache, and neurological deficits.

Hepatic abscess: May arise via hematogenous spread, presenting with fever and right upper quadrant pain.

Pulmonary infections: Lung abscess and empyema, often associated with aspiration.

Other infections may include sinusitis, otitis media, intra-abdominal infections, and, less commonly, infective endocarditis.

Conventional culture methods require 48–72 hours and may have limited ability to differentiate species within the Streptococcus anginosus group.

MALDI-TOF mass spectrometry provides rapid and accurate species-level identification and is widely used in clinical laboratories.

Real-time PCR (qPCR) offers high sensitivity and specificity, particularly useful in culture-negative cases or when prior antibiotic use affects bacterial recovery.

Probe-based qPCR kits targeting Streptococcus intermedius enable qualitative and quantitative detection by monitoring fluorescence signals during amplification.

Streptococcus intermedius is generally susceptible to β-lactam antibiotics, including penicillins and cephalosporins (e.g., ceftriaxone), as well as carbapenems and vancomycin.

It is intrinsically resistant to metronidazole, a characteristic shared by streptococci.

For deep-seated infections such as brain or liver abscesses, prolonged antibiotic therapy (typically 4–6 weeks) combined with surgical drainage is often required.

Early diagnosis and appropriate intervention are critical for favorable outcomes. Delayed treatment may lead to severe complications or mortality.