Closing the Brief Case: Streptococcus intermedius Brain Abscesses in an Otherwise Healthy Young Man Diagnosed by 16S rRNA Gene Sequencing

ANSWERS TO SELF-ASSESSMENT QUESTIONS

• 1.What percentage of the S. anginosus group is susceptible to third-generation cephalosporins?
  • a.1 to 2%
  • b.33 to 34%
  • c.75 to 76%
  • d.97 to 98%

Answer: d. Ninety-seven to ninety-eight percent of the S. anginosus group is susceptible to third-generation cephalosporins, and over 90% is susceptible to penicillin. This contributes to the low culture yield when empirical antibiotics are started early in the course and prior to abscess sampling.

• 2.S. intermedius binds to which components of the extracellular matrix?
  • a.Fibronectin and laminin
  • b.Collagen and elastin
  • c.Heparan sulfate and chondroitin sulfate
  • d.Hyaluronic acid

Answer: a. Fibronectin and laminin. S. intermedius binds to fibronectin and laminin in the extracellular matrix and induces monocytes to release interleukin-8, resulting in neutrophil recruitment and proinflammatory cytokine activation and accumulation.

• 3.Challenges with traditional Sanger 16S rRNA sequencing in the diagnosis of a brain abscess include:
  • a.Inability to detect fastidious organisms
  • b.Protracted time and analysis for the assay compared to 16S next-generation sequencing
  • c.Limitations in identification of microbes using a software database of rRNA sequences
  • d.Inability to detect microbes after initiation of antibiotic courses

Answer: c. Answers a, b, and d are facilitated by 16S rRNA sequencing and provide these advantages over direct tissue culture methods. In contrast, rRNA sequence databases may differ, impacting sensitivity in the detection of different pathogens.

TAKE-HOME POINTS

• •Streptococci are the most common organisms isolated from brain abscesses worldwide.
• •Although brain abscesses can occur via contiguous spread, hematogenous spread, or direct inoculation, a source of infection remains unidentified in up to one-third of cases.
• •S. intermedius and the rest of the S. anginosus group are highly susceptible to penicillin and third-generation cephalosporins, often leading to negative culture results due to the early initiation of empiric antibiotics.
• •16S rRNA gene sequencing technology can be instrumental in identifying an infectious organism that fails to grow in culture and should be considered at the time of abscess sampling.
• •Some limitations of 16S rRNA gene sequencing technology include its dependence on available databases and its difficulty in identifying bacteria to the species level.