Systematic Bacteriology: Staphylococcus to Lactobacillus

Staphylococcus

• Staphylococcus species are Gram-positive, spherical bacteria arranged in grape-like clusters, and are facultative anaerobes and catalase-positive.
• The catalase test is important in distinguishing streptococci (catalase-negative) from staphylococci.
• The ability to produce staphylocoagulase divides the genus into two groups: "coagulase-positive" and "coagulase-negative".
• Staphylococcus aureus is coagulase-positive, has the greatest pathogenic potential, and is the most medically important member of the genus.
• Anterior nares and the skin are the main habitats for S. aureus and S. epidermidis, respectively.
• S. aureus grows on:
  • Nutrient agar, forming golden yellow pigmented colonies
  • Blood agar, producing colonies surrounded with a zone of ß-haemolysis (complete haemolysis)
• Important virulence factors of S. aureus include:
  • Staphylocoagulase: converts plasma fibrinogen to fibrin, leading to the formation of a fibrin barrier, protecting bacteria from phagocytes and immune defences, and localizing infection.

Streptococcus pyogenes

• Streptococcus pyogenes are Gram-positive cocci arranged in chains, and produce beta-haemolysis (complete haemolysis) on blood agar.
• Important virulence factors of S. pyogenes include:
  • M protein: enables the bacteria to colonize the skin and inhibits phagocytosis.
  • Streptococcal pyrogenic exotoxins (SPE-A, B, and C): pyrogenic exotoxin A is referred to as erythrogenic toxin or scarlet fever toxin.
  • Enzymes (e.g., streptokinase and hyaluronidase): contribute to the spreading nature of streptococcal infections.
• Important diseases caused by S. pyogenes include:
  • Pharyngitis (sore throat) and/or tonsillitis
  • Scarlet fever: characterized by the development of scarlet red rash, strawberry tongue, and a whitish coating on the back of the throat.
  • Skin and soft tissue infections (e.g., cellulitis)

Enterococcus

• Enterococcus species are Gram-positive cocci arranged in short chains, are catalase-negative, and are found normally in the oral cavity, human intestine, and female genital tract.
• The commonest two species are Enterococcus faecalis and Enterococcus faecium.
• Enterococci are:
  • Facultative anaerobes
  • Able to grow at 45°C
  • Able to grow in broth containing 6.5% NaCl
  • Able to grow on mitis salivarius agar, producing dark, blue-black colonies
  • Able to grow on bile esculin agar, giving black colonies
• Enterococci are responsible for oral conditions, including:
  • Periodontitis (chronic infection of the gingival and periodontal ligaments)
  • Failed root canal treatment
• Enterococcus faecalis is the species most commonly isolated from these infections.

Neisseria

• Neisseria species are Gram-negative cocci arranged in pairs with adjacent sides flattened to give a characteristic kidney-shape, and are aerobic and oxidase-positive.
• The genus includes:
  • Neisseria gonorrhoeae (gonococci): causes the sexually transmitted disease gonorrhoea.
  • Neisseria meningitidis (meningococci): causes meningitis.
  • Commensal Neisseria species: inhabitants of oro- and nasopharynx of healthy individuals (e.g., N. lactamica and N. sicca).
• Neisseria gonorrhoeae:
  • Is a fastidious species which grows on:
    • Chocolate agar (enriched medium)
    • Modified Thayer-Martin medium (MTM): a selective medium containing antibiotics, allowing easier isolation of the organism from specimens contaminated by other microbes.
  • Important virulence factors include:
    • Pili that mediate attachment to epithelial cells
    • Outer membrane proteins that contribute to invasion
    • IgA1 protease that may have a role in colonization
  • Important diseases caused by N. gonorrhoeae include:
    • Gonorrhoea: a sexually transmitted infection affecting males and females, causing acute urethritis with dysuria and purulent discharge in males, and cervicitis with a purulent cervical discharge in females.
    • Oral infection (oral gonorrhoea): usually occurs after orogenital exposure.