Medical Microbiology: Streptococcus & Enterococcus

Study Notes

Greek origin of "Streptus" means “easily bent or twisted,” which reflects the chain-like formation of these bacteria.
Streptococcus belongs to the family Streptococcaceae, while Enterococcus is classified under Enterococcaceae.

Classification is based on:
- Serological (Lancefield) typing
- Hemolysis pattern on blood agar (BA)
- Biochemical tests
Streptococcus has over 100 identified species; Enterococcus consists of more than 54 species.

Both are Gram-positive cocci, typically found in pairs or chains, measuring 0.5 - 2 µm.
They are facultative anaerobes, with some being capnophilic (notably Streptococcus pneumoniae).
Require enriched media for growth, except non-fastidious Enterococcus, and have an optimal incubation temperature of 35-37°C.
May display hemolytic properties: α-, β-, or non-hemolytic (γ-hemolysis).
They are catalase-negative, fermentative, non-spore-forming, and non-motile.

Major causes of human infections are within three Lancefield groups:
- Group A (Streptococcus pyogenes)
- Group B (Streptococcus agalactiae)
- Group C (varied streptococcal species)
Rare infections can arise from Groups E, F, G, and L, with Group D representing the Enterococcus genus which was previously categorized under Streptococcus.

Gram-positive cocci arranged in chains, measuring 1-2 µm.
Form β-hemolytic colonies on blood agar, which are white, convex, and ≤ 1-2 mm in diameter.
Notably susceptible to bacitracin and negative in the pyrrolidonyl arylamidase (PYR) test.

Group C, G, and F are commonly associated with infections:
- Important species include Streptococcus dysgalactiae (Groups C and G) and Anginosus group (Groups A, C, F, G or non-group).
Small colony size in Anginosus group indicates variations based on group affiliation.

Infants are recommended to receive the 13-valent conjugated pneumococcal vaccine at ages 2, 4, 6, and 12-15 months.
Vaccine effectiveness depends on prevalent serotypes responsible for invasive disease and is notably poor in certain populations (asplenia, sickle cell disease, etc.).

Early onset infections occur within the first week of life, often presenting with respiratory distress, fever, lethargy, and hypotension.
Late onset infections manifest between 7 days to 3 months post-birth.
Streptococcus pneumoniae is a significant cause of bacterial pneumonia in children (under 5 years), individuals over 50, and those with weakened immune defenses.
Clinical symptoms of pneumonia include sudden onset chills, high fever, cough with rusty to pink sputum, and pleuritic chest pain.