Notes
Streptococci, Misc
Sections
Streptococcal agalactiae is beta-hemolytic; it is also referred to as Group B streptococcus.
Enterococci faecalis and Enterococci faecium, which were formerly classified as members of Group D streptococcus; they are alpha and gamma hemolytic.
Viridans group streptococci, include several alpha-hemolytic subgroups.
Streptococcus pneumoniae, aka, pneumococcus, which some authors categorize as members of the Viridans group.
Streptococcus pyogenes is discussed separately.
Group B streptococcus
Virulence factors:
Polysaccharide capsule inhibits phagocytosis.
Sialic acids in the capsule inhibit activation of the alternative complement pathway and neutrophilic activities, thus facilitating host immune evasion.
Colonization and Infection:
Group B streptococcus commonly colonizes the colon and genitourinary tracts of women.
Group B streptococcus can be passed to the neonate, particularly if labor is prolonged; administration of antibiotics to infected mothers can help prevent bacterial transmission.
Early-onset diseases occur within the first 7 days after birth, and include bacteremia, pneumonia, and meningitis. This is typically the result of transmission during labor and birth.
Late-onset diseases occur between 1 week and 3 months of age, and include bacteremia with meningitis.
In adults, Group B streptococcus is associated with post-partum endometritis, which is characterized by infiltration of plasma cells and inflammatory cells to the endometrium; it also causes wound and urinary tract infections.
Group B streptococcus is also associated with bacteremia leading to pneumonia and infections of the bones, joints, skin, and soft tissues; immunocompromised, diabetic, and alcoholic patients are at higher risk of Group B streptococcal infections.
Penicillin G is the preferred antibiotic to treat Group B streptococcal infections.
Enterococci faecalis and Enterococci faecium
Virulence factors:
Easily adhere to host surfaces via surface proteins and pili, which promote biofilm formation.
Resistant to most antibiotics; thus, although not as virulent as some other Gram-positive cocci, established infections are difficult to eradicate.
Infection:
Enterococci are particularly problematic in hospital settings, where they are associated with:
Urinary tract infections, Bacteremia, Endocarditis, and Peritonitis (inflammation of the peritoneum).
Viridans group streptococci
Subgroups:
S. mutans, S. salivarius, S. angionosus, S. mitis, and S. sanguinis.
Fortunately, this group is mostly vulnerable to antibiotic treatments, although S. mitis is an important exception.
Viridans group streptococci are associated with:
Deep tissue abscesses (S. sanguinis)
Bacteremia (S. mitis infection in cancer patients with neutropenia)
Subacute endocarditis (S. mitis and S. salivarius)
Dental caries (S. mutans).
Streptococcus pneumoniae
Virulence factors:
Polysaccharide capsule inhibits phagocytosis, and recombination of capsular genes facilitates antibiotic resistance.
Surface proteins bind respiratory tract epithelia, which facilitates respiratory infections.
IgA protease keeps the bacteria trapped within the mucin and inhibits clearance.
Pneumolysin lyses phagocytic cells and ciliated respiratory epithelium, which also prevents bacterial clearance.
Complement activation produces inflammation, and is triggered by pneumolysin, teichoic acid, and peptidoglycan fragments.
Hydrogen peroxide damages host tissues.
Phosphorylcholine facilitates entry into host cells, which protects bacteria from removal and promotes migration to deeper tissues.
Infections:
Pneumonia, particularly in the lower lobes: In the sample, we can see that bacteria, red blood cells, and white blood cells infiltrate the alveolar tissue. Symptoms include chest pain (aka, pleurisy) with fever and chills with coughing that produces rust-colored sputum. Be aware that patients with dysfunctional or absent spleens can have severe forms of pneumococcal pneumonia.
Otitis media and sinusitis can occur after viral infections that obstruct sinus drainage and allow bacteria to proliferate.
Pneumococcal meningitis can occur when Streptococcus pneumoniae spreads to the central nervous system. This can happen as a result of bacteremia or when trauma produces a connection between the nasopharynx and subarachnoid space.
Vaccine:
Given the severity of Streptococcus pneumoniae infections, the CDC recommends a pneumococcal vaccine for all children younger than 2 years old, all adults 65 years or older, and for anyone at increased risk of infection.
References
Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. Medical microbiology. Philadelphia: Elsevier/Saunders. (2013).
Levinson, W. E. Review of Medical Microbiology and Immunology. 14th Ed. Lange (2016)
Kumar, V., Abbas, A. K., & Aster, J. C. (2015). Robbins and Cotran pathologic basis of disease (Ninth edition.). Philadelphia, PA: Elsevier/Saunders.
Images
Endometritis (https://commons.wikimedia.org/wiki/File:Endometritis_-2-cropped-_very_high_mag.jpg).
Endocarditis (http://phil.cdc.gov/PHIL_Images/02122002/00007/PHIL_851_lores.jpg).
Blood smear (Braun, Mark. http://medsci.indiana.edu/c602web/602/c602web/virtual_nrml/nrml_lst_pad.htm).
Streptococcal meningitis (CDC/ Dr. Edwin P. Ewing, Jr.).
Dental decay (Wikipedia, Author Suyash.dwivedi).