Gram positive bacteria Flashcards
Outline main features of: – Streptococcus – Staphylococcus – Corynebacterium How they cause disease – Their virulence factors Approaches to their control
Classification of bacteria
Obligate intracellular bacteria
Bacteria that may be cultured on artificial media (most)
-with or without a cell wall
–>growing as single cells or growing as filaments
—>rods or spirochaetes
—->gram positive or gram negative (or zn stain positive) then aerobic and anaerobic
Pain in shoulder, elevated temperature
• MRI scan, disc infection and osteomyelitis C6 &C7
• Blood cultures
– Staphylococcus aureus;
Gram positive cocci in clusters
Bone infection due to massive bacterial overload or trauma usually
Referred pain down the nerve from disc
Treat with flucloxacillin 3 months
Staphylococcus species
Currently at least 40 species
– Coagulase +ve or – ve
– Coagulase: enzyme produced by bacteria that clots
blood plasma. Fibrin clot formation around bacteria
may protect from phagocytosis.
– S.aureus most important (coag. +ve)
– Coagulase -ve species important in opportunistic
infections
Staphylococcus normal habitat
Nose and skin
Staph. aureus + spread
Means golden - forms yellow dots on agar
Spread by aerosol and touch
-carriers and shedders
Staph. aureus MRSA
Resistant to
- β-lactams
- gentamicin, erythromycin, tetracycline
Staph. aureus virulence factors
Pore-forming toxins (some strains) -α - haemolysin & Panton-Valentine Leucocidin (PVL) - causes haemmorhagic pneumonia Proteases -Exfoliatin Toxic Shock Syndrome toxin (TSST) - (stimulates cytokine release) Protein A (surface protein which binds Ig’s in wrong orientation)
Associated conditions with Staph aureus - pyogenic
Wound infections Abscesses (boils, carbuncles) Impetigo Septicaemia Osteomyelitis Pneumonia Endocarditis
Associated conditions with Staph aureus - toxin mediated
Scalded skin syndrome
Toxic shock syndrome
Food poisoning
Associated conditions with Staph aureus - coagulase negative
Infected implants
Endocarditis
Septicaemia
Coagulase-negative Staphylococci
S. epidermis
S. Saprophyticus
S. epidermidis
Infections in debilitated, prostheses, catheters
(opportunistic)
Main virulence factor -
ability to form persistent biofilms
S. saprophyticus
Acute cystitis
• haemagglutinin for
adhesion
• Urease – kidney stones
Wound received to shoulder 3 days later spreading inflammation – Fever, malaise Swab culture: Streptococcus pyogenes
Gram positive cocci in chains
β-haemolytic (group A)
Facultatively anaerobic
Penecillin sensitive
Haemolysis - 3 types
α - partial, greening -e.g. S.intermedius -H2O2 reacts with Hb β- complete lysis -e.g. S.pyogenes -Haemolysins O & S Non (or γ) - no lysis -e.g. some S.mutans
Streptococci classification
Classified in 3 ways
- Haemolysis
- Lancefield typing
- Biochemical properties
Sero-grouping: Lancefield grouping definition
Method of grouping catalase negative, coagulase negative bacteria based on bacterial carbohydrate cell surface antigens
Sero-grouping
Carbohydrate cell surface antigens -
Lancefield A-H and K-V
-antiserum to each group added
to a suspension of bacteria: clumping indicates
recognition
Not all equally important
-group A - S.pyogenes; important pathogen
-group B - S.agalactiae neonatal infections
Infections of S. pyogenes infections
Wound infections >> cellulitis Tonsillitis & pharyngitis Otitis media Impetigo Scarlet fever
Complications of S. pyogenes infections
Infection with any strain of S.pyogenes can give rise to immunologically mediated complications (prompt treatment needed)
Rheumatic fever
-inflammatory disease of heart, joints, skin, brain. Often
follows Strep. throat infection
-antibody cross reactivity
Glomerulonephritis
-inflammatory disease sometimes following
S.pyogenes infection
Complications of S. pyogenes infections - assessment of risk
Estimated from Anti-SLO titre
Anti-SLO titre
Anti-streptiolysin O (ASLO) titre
-ASLO – Antibody against SLO.
-SLO – beta heamolysis
Streptococcal exoenzymes are bound to
biologically inert latex particles
If streptococcal antibodies present in the test
sample, reaction occurs.
Presence of an ASO titer of >200 IU/mL in the
serum = agglutination of the latex particles
S. pyogenes virulence factors
SURFACE: Capsule - hyaluronic acid (protection) M protein - surface protein (encourages complement degradation) - cross reactivity? EXPORTED FACTORS Enzymes: -hyaluronidase - spreading -streptokinase - breaks down clots -C5a peptidase - reduces chemotaxis Toxins: -streptolysins O 7 s - binds cholesterol -erythrogenic toxin - SPeA - exaggerated response (streptococcal pyrogenic endotoxin A)
Heavy smoker noted nasal congestion and fever 2 days later cough and severe pain in chest “Rust-coloured” sputum Chest X-ray – consolidation Blood & sputum culture -“draughtsman” colonies -S.pneumoniae
Raised edge with little dimple in middle
Pneumonia
