Microbiology
Summary
Streptococcus pneumoniae and the viridans streptococcal species are gram-positive cocci belonging to the Streptococcus genus, each with their own unique characteristics and roles in infectious diseases. Both species are alpha-hemolytic, meaning they exhibit partial hemolysis with a green hue surrounding the zone of hemolysis. Streptococcus pneumoniae, also known as Strep pneumo, has a polysaccharide capsule which acts as its major virulence factor. This encapsulated bacterium is optochin-sensitive and is characterized by its lancet-shaped, gram-positive diplococci when viewed under high-power microscopy. It is also known to be bile-soluble. Strep pneumo is the number one cause of community-acquired pneumonia in adults, often producing rust-colored sputum in lobar pneumonia. It also commonly causes infections such as meningitis, otitis media, and sinusitis (collectively referred to as MOPS). Strep pneumo has an IgA protease that helps it invade the immune system.
On the other hand, Strep viridans lacks a capsule and is optochin-resistant. Unlike Strep pneumo, it is bile-insoluble. Some strains, such as Strep mutans and Strep sanguinis, are associated with dental caries. Strep viridans can cause subacute endocarditis in damaged heart valves, most commonly affecting the mitral valve. The pathogenesis of Strep viridans infection involves the creation of dextrans from glucose, which aid in adherence to fibrin platelet aggregates at damaged tissue.
Lesson Outline
Don't stop here!
Get access to 128 more Microbiology lessons & 13 more medical school learning courses with one subscription!
FAQs
Streptococcus pneumoniae and viridans streptococci are both gram-positive cocci that are classified as alpha-hemolytic. However, Streptococcus pneumoniae is optochin-sensitive and bile-soluble, while viridans streptococci are optochin-resistant and bile-insoluble. Additionally, Streptococcus pneumoniae is more commonly associated with invasive infections such as pneumonia and meningitis, while viridans streptococci are often found in the oral flora and mainly cause dental caries and endocarditis.
Streptococcus pneumoniae can be distinguished from other alpha-hemolytic streptococci by its sensitivity to optochin, a chemical that inhibits its growth, as well as its solubility in bile. It also appears as lancet-shaped gram-positive diplococci under microscopic examination. These characteristics, combined with its morphology and typical clinical presentation, help in differentiating it from other alpha-hemolytic streptococci such as those in the viridans group.
Strep pneumo can cause a variety of infections, including pneumonia, meningitis, sinusitis, and otitis media (MOPS). It is also a common cause of community-acquired bacterial pneumonia and can result in bacteremia and sepsis. Less frequently, it can cause peritonitis, endocarditis, and joint infections. Infections with Strep pneumo can range from mild to severe, depending on the age of the patient, the immune status, and the specific clinical manifestation.
The pure polysaccharide vaccine contains the polysaccharide components from the bacteria's capsule and primarily stimulates an IgM immune response. It is administered to adults as a preventive measure. In contrast, the protein conjugated vaccine involves polysaccharides conjugated with a carrier protein, eliciting an IgG immune response. This vaccine is recommended for children under 2 years of age, as their immune systems are still developing and the IgG response is more effective in this age group.
