12 topics with brief reviews: definitions, key facts, and seminar questions.
This section is dedicated to spherical bacteria — cocci. Here we examine their morphology, pathogenicity factors, and associated infections: ranging from common pyoderma and tonsillitis to life-threatening sepsis and meningitis.
The section provides general characteristics of the group and a detailed breakdown of key pathogens. You will study Gram-positive flora (Staphylococcus aureus, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae, enterococci) and Gram-negative Neisseria (meningococcus and gonococcus). We will also analyze the specifics of anaerobic cocci. Enterobacteriaceae are included in this block as well — although morphologically they are bacilli, they are often studied in parallel with pyogenic flora within this course due to similar principles of clinical diagnostics.
During colloquia and exams, instructors most frequently require thorough knowledge of the virulence factors of Staphylococcus aureus and Streptococcus pyogenes, algorithms for the differential diagnosis of pneumococci, and culture media for growing fastidious Neisseria. Mastering these topics is your essential foundation for clinical practice.
No. The cocci group is tinctorially heterogeneous: it includes both Gram-positive (e.g., staphylococci) and Gram-negative (e.g., Neisseria) bacteria.
What is the difference between coagulase-positive and coagulase-negative staphylococci?
Coagulase-positive staphylococci (primarily S. aureus) secrete the enzyme plasmocoagulase, which converts fibrinogen to fibrin, creating a protective shield against the immune system. Coagulase-negative species (S. epidermidis, S. saprophyticus) lack this enzyme and are generally less aggressive.
What is the difference between streptolysin O and streptolysin S?
Streptolysin O is oxygen-labile and stimulates antibody production (ASO), which is important for diagnostics. Streptolysin S is oxygen-stable, causes surface hemolysis on agar, but does not induce antibody production in the body.
Why is the capsule considered the primary virulence factor of this microorganism?
The capsule is polysaccharide in nature and effectively disguises the bacterium from immune cells, suppressing phagocytosis and allowing the pathogen to survive within the host organism.
What does Streptococcus pneumoniae look like on smear microscopy?
These are Gram-positive diplococci with a lancet or oval shape. A characteristic feature is the presence of a thick capsule surrounding the cells.
Why were enterococci historically classified as streptococci?
They were classified as Group D streptococci because their cell wall contains a group polysaccharide antigen shared with them.
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