LECTURE 4 (Gram +ve bacteria) Flashcards
(33 cards)
What are the properties of Viridans group of Streptococci?
- Can cause opportunistic infections -> can cause disease when they are protected from host defences
- Virulence is very low
Describe Enterococcus identification
Pin point colony -> Gram stain: Gram +ve cocci in chains -> Catalase test: -ve -> Streptococci -> Bile Esculin Test: +ve -> Growth in 6.5% NaCl -> Enterococcus -> Pyruvate Fermentation Test -> POSITIVE: Enterococcus Faecalis, NEGATIVE: Enterococcus faecium
What are the properties of Enterococcus?
- Facultative anaerobic, catalase -ve, gram +ve cocci
- Arranged individually, in pairs or short chains
- Has the group D antigen
- Normal inhabitants of the intestinal tract, female genital tract and oral cavity
- Intestinal inhabitants resist action of bile salts
Diseases in humans are associated with which Enterococcus?
Enterococcus faecalis & Enterococcus faecium
What is the clinical significance of Enterococcus?
- Upper and lower urinary tract infections
- Native valve and prosthetic valve endocarditis
- Intra-abdominal and pelvic infections
- Wound infections
- Septicemia and meningitis in neonates
Which is more antibiotic resistant? (E faecium/E faecalis)
E faecium
Describe Enterococcus
- ENVIRONMENTAL PERSISTENCE (capsule, biofilm, stress response)
- COLONISATION OF GI TRACT (stress response, aggregation substance, cytolysin, bile acid hydrolase, antibiotic resistance)
- TISSUE DAMAGE (cytolysin, gelatinase, extracellular superoxide production)
What is the mechanism of Enterococcus spread?
- Translocation through/breach of intestinal epithelium (cytolysin, extracellular superoxide production)
- External infection by shed organism (capsule, biofilm, stress response)
Describe the preliminary grouping of Gram +ve cocci
CATALASE: -ve = streptococcus group, +ve = salt tolerant? -> YES: staphylococcus/Micrococcus
What is Staphylococcus aureus?
A gram +ve bacterial infection that causes abscesses, various pyogenic infections (e.g endocarditis, osteomyelitis), food poisoning, scalded skin syndrome and toxic shock syndrome
- most common cause of hospital acquired pneumonia, septicaemia and surgical-wound infections
- most important cause of skin infections (cellulitis, impetigo etc)
- most common cause of bacterial conjunctivitis
What are the properties of Staphylococci?
- Gram +ve spherical cells
- Arranged in grape like irregular clusters
- Non-motile
- Do not form spores
What are the growth characteristics of Staphylococci?
- Grow readily under aerobic/microaerophilic conditions
- Grow rapidly at 37 degrees but form pigment best at room temp
- Colonies on solid media are round, smooth, raised and glistening
- S aureus = forms gray to deep golden yellow colonies
- S epidermidis = gray to white
What is the importance of Protein A?
Protein A is the major protein in the cell wall which binds to the Fc portion of IgG at the complement-binding site -> prevents activation of COMPLEMENT CASCADE -> no C3b is produced -> opsonisation and phagocytosis is greatly reduced
Do Coagulase-negative staphylococci produce protein A? (YES/NO)
YES
Describe the Staphylococci structure
- Protein A
- Teichoic acids = mediate adherence of staphylococci to mucosal cells
- Lipoteichoic acids = induce septic shock by inducing cytokines like IL-1 and TNF from macrophages
- Polysaccharide capsule = type 5 and 8 cause most infections, capsule is immunogenic -> makes producing an effective vaccine difficult
- Peptidoglycan = has endotoxin-like properties -> can stimulate macrophages to produce cytokines, activate the complement and coagulation cascades
What is important about bacteria that can produce catalase?
They can survive the killing effect of H2O2 within neutrophils
How is S. aureus distinguished from the others?
By coagulase production
Explanation: Coagulase is an enzyme that causes plasma to clot by activating prothrombin to form thrombin. Thrombin catalyses the activation of fibrinogen to form the fibrin clot.
What is the importance of Staphyloxanthin?
- Imparts a golden colour to its colonies
- Pigement enhances the pathogenicity of the organism by inactivating the microbicidal effect of superoxide and other reactive oxygen species within neutrophils
What are the different Hemolysins?
- α-toxin = pore-forming cytotoxin that lyses the cytoplasmic membranes by direct insertion into the lipid bilayer to form transmembrane pores
- β-toxin = toxic for many cells
- delta-toxin = disrupts biological membranes + may have a role in s. aureus diarrhoeal diseases
- gamma-hemolysin = leukocidin that lyses white blood cells
What are the properties of Enterotoxin?
- causes food poisoning characterised by prominent vomiting and watery, non-bloody diarrhoea
- acts in GI tract to stimulate release of IL-1 and IL-2 from macrophages and T helper cells
- resistant to heat, stomach acid and stomach and jejunum enzymes
What is Exfoliative?
- causes “scalded skin” syndrome in young children
- “epidermolytic” and acts as a protease that cleaves desmoglein in desmosomes -> leads to separation of epidermis at granular cell layer
What is P-V leukocidin?
A pore-forming toxin that kills cells (especially WBCs) by damaging cell membranes. The two subunits of the toxin assemble in the cell membrane to form a prom through which cell contents leak out.
What is Toxic shock syndrome toxin (TSST)?
A toxin that causes Toxic shock and enters the bloodstream causing a toxaemia. TSST is produced locally by S. aureus in the vagina, nose or other infected site.
What are the properties of coagulase-negative staphylococci?
- Do not produce exotoxins
- Do not cause food poisoning or toxic shock syndrome
- Cause pyogenic infections
