Gram Positive Cocci Bacteria

Staphylococci Overview

• Staphylococci are Gram-positive cocci that grow in clusters.
• Common species include Staphylococcus aureus and some Staphylococcus epidermis.
• S. aureus is catalase and coagulase-positive, while S. epidermis is coagulase-negative.
• Found as commensals in the human body (nose, groin, colon).
• Can cause a range of infections, including soft tissue infections, impetigo, toxic shock syndrome (TSS), septicemia, and osteomyelitis.

Virulence Factors of Staphylococci

• Enzymes:

  • Coagulase: Forms blood clots, protecting bacteria from immune cells.
  • Hyaluronidase: Breaks down hyaluronic acid, allowing bacteria to spread.
  • Deoxyribonuclease (DNase): Degrades DNA, facilitating biofilm formation and spread.
  • Lipase: Digests lipids, aiding colonization of oily skin.
  • Beta-lactamases (penicillinase): Degrades penicillin, contributing to antibiotic resistance.
  • Staphylokinase: Dissolves fibrin clots, facilitating bacterial spread.

• Toxins:

  • Enterotoxins: Cause food poisoning, various serotypes (Alpha, Beta, Delta).
    • Alpha toxin (SEA): Pore-forming, damages cell membranes.
    • Beta toxin (SEB): Damages membranes, targeting macrophages.
    • Delta toxin (SED): Cytopathic, targets macrophages, causes cell death.
  • Superantigens (eg Toxic Shock Syndrome Toxin-1): Trigger massive immune responses, leading to septic shock.
  • Exfoliatin toxins (ETA, ETB): Cause scalded skin syndrome, separating epidermis layers.
  • Exotoxins (eg leukocidin): Target white blood cells.

• Immuno-evasive strategies:

  • Polysaccharide capsule (glycocalyx): Hides antigens, making bacteria less recognizable to the immune system.
  • Surface proteins:
    • Protein A: Disrupts opsonization and phagocytosis, preventing immune cells from engulfing bacteria.
    • Adhesins: Bind to host tissues, promoting bacterial attachment.
    • Clumping factors: Bind to fibrin/fibrinogen, contributing to bacterial aggregation and dissemination.

Methicillin-Resistant Staphylococcus aureus (MRSA)

• Emerged in the 1981.
• Significant increase in infections and mortality rates.
• Prevention and control measures have been implemented to reduce infections(mandatory surveillance, reduction targets, legal framework).
• Resistance is due to the mecA gene, acquired through horizontal gene transfer.
• mecA encodes a penicillin-binding protein 2a (PBP2a), which reduces affinity for beta-lactam antibiotics like methicillin and penicillin.
• Other resistance mechanisms may exist, including production of beta-lactamases.
• Diagnosis is required for appropriate treatments and preventing overuse of antibiotics.

Laboratory Diagnosis of Staphylococci

• Gram staining: Shows Gram-positive cocci in clusters.
• Colony Morphology: S. aureus often forms yellow colonies.
• Biochemical tests: Catalase and coagulase tests, as well as mannitol fermentation.
• Haemolysis: On blood agar, some strains cause haemolysis (beta or alpha).
• Molecular tests: PCR, MALDI-TOF MS, PFGE, and MLST for strain identification and antibiotic resistance determination.

Treatment of Staphylococcal Infections

• Decolonisation: Removal of bacteria from affected areas through cleaning and antibiotics.
• Antibiotics: Options vary based on the infection and antibiotic resistance (flucloxacillin, dicloxacillin, cephalosporins, clindamycin, vancomycin) and combinations of antibiotics.

Streptococcus Overview

• Streptococci are Gram-positive cocci in chains.
• Facultative anaerobes, oxidase-negative, catalase-negative, and fermentative bacteria.
• Distinguished by Lancefield groupings (based on surface carbohydrates).
• Common groups associated with human disease include A, B, C, D, and G.

Virulence Factors of Streptococci (S. pyogenes)

• Enzymes:
  • C5a peptidase: Inhibits complement activation.
  • Hyaluronidase: Breaks down hyaluronic acid, promotes bacterial spread.
  • Nucleases: Degrade DNA, promote bacterial spread
  • Proteinases and Streptokinases: Degrade soft tissue and fibrin clots.
• Toxins:
  • Streptolysin O: Oxygen-labile cytolysin (haemolytic).
  • Streptolysin S: Oxygen-stable cytolysin, less immunogenic.
  • Pyrogenic exotoxins: Superantigens causing fever and rash (scarlet fever).
• Immunoevasion:
  • Hyaluronic capsule: Mimics host tissue, preventing immune recognition.
  • Surface proteins (M protein, Lipoteichoic acids): Inhibit phagocytosis and complement activation.

Laboratory Diagnosis of Streptococci

• Gram staining: Shows Gram-positive cocci in chains.
• Colony morphology: may show mucoid colonies.
• Haemolysis: On blood agar (beta, alpha, gamma) to differentiate strains.
• Biochemical tests: Catalase, Bacitracin sensitivity
• Molecular tests: PCR to identify specific strains or genes.

Treatment of Streptococcal Infections

• Antibiotics: Penicillin, amoxicillin, erythromycin to treat, with consideration of antibiotic resistance profiles.
• Vaccines: Available for some streptococcal strains like S. pneumoniae.

This quiz covers essential knowledge about Staphylococci, including their characteristics, common species, and associated infections. Additionally, it delves into the various virulence factors that enable these bacteria to cause disease, highlighting their mechanisms of infection. Test your understanding of this important group of bacteria.