Gram positive organisms of clinical significance Flashcards
List some G+ cocci
Staphylococci Streptococci Enterococci Peptococci> anaerobic peptostreptococci> anaerobic
List some G+ rods
Clostridia> anaerobic
Corynebacteria (diphtheroids)
Listeria
Bacillus
What is the most common G+ organism of medical importance
Staphylococci
Streptococci
Characteristic of staphylococci
- Catalase-positive: good at dealing with oxidative defensive processes
- Hardy; survive well outside animal host
- Many are normally harmless commensals
- Infections can be relatively trivial or rapidly fatal
- often resistant to natural penicillin
What does staphylococcus aureus look like
Golden colonies
Characteristic of Staphylococcus aureus
-Most virulent staphylococcus
-Carried by healthy people
+in patients and healthcare staff
-Simple colonisation is not infection but can lead to infection
What are the surface protein and toon staphylococcus aureus
1) Protein A(cell wall factor): bid to IgG
- anti-phagocytic affect
2) Fibronectin-binding protein/FnBP(cell wall factor)
- Promoted binding to mucosal cells and tissue matrices
3) Cytolytic exotoxins
- Haemotoxins: PVL is most significant (Panton-Valentine Leucocidin) Attack mammalian cell membranes
4) Super antigen exotoxins
- Stimulate enhanced T cell response
- toxic shock syndrome
What are the significant mild/moderate infection caused by SU
Impetigo
Localised to hair follicles, sweat/sebaceous glands, subcutaneous abscesses
What are the significant moderate/severe infection caused by SU
- Localised skin infections (cellulitis)
- Dermis / subcutaneous fat (lymph system possibly involved)
- May respond to local therapy (if mild) but can lead to bacteraemia
- More rigorous treatment for moderate/severe infection
Major infection caused by SU
-Septicaemia / bacteraemia Often from starter lesion on skin wound, cannula, drain etc (ie often nosocomial) -Endocarditis (heart valves) IV drug users, or after bacteraemia. Often quite rapidly damaging infection -Pneumonia (often after ‘flu) Can be severe
Toxic shock syndrome symptoms
- Fever, rash, vomiting, diarrhoea, hypotension, multi-organ involvement
- Rapid, severe, caused by “instant” activation of T cell response
- Related to unremoved tampons, surgical dressings, nasal packs(much less common now that this has been recognised)
Major the clinically significant toxinoses cause by SU
- Toxic shock syndrome -Major
- Scalded skin syndrome- moderate
- Gastroenteritis- moderate
Scalded skin syndrome
Result of specific toxins Epithelial desquamation (skin peels)
Gastroenteritis
Ingestion of contaminated food
Nausea and vomiting
Treatment option for SU
Source control where possible: incision/drainage
Antibiotic: suppress toxin production as well as to kill
- choices limited by resistance
- beta- lactamase resistant lactams often used as first line
Hospital acquired MRSA
Has a PBP2a mutation so drugs are less effective
Vancomycin is the main antibiotic but VISA and VRSA exist
Characteristic of the coagulase-negative staphylococci (not S.aureus): Staphylococcus epidermidis
Part of normal flora
Lower virulence
Acquired drug resistance (b-lactams) is more common than in S. aureus
They are vancomycin resistive
Characteristic of the coagulase-negative staphylococci (not S.aureus): Staphylococcus saprophyricus (digest waste/decay)
- Part of the normal flora of female genital tract / perineum on both male/female
- Cystitis (infection of the bladder)
- especially in younger women(“honeymoon cystitis”) … within 24 h after intercourse
- Sensitive to most antibiotics
Describe the appearance of streptococci under the microscope
Long twisted chains
What are the types of haemolysis caused by the streptococci
Gamma: No haemolysis so red blood cells are fine
beta: Total destruction of red blood cells
Alpha: Chemical modification of the haem group making it look green
what is Haemolysin
it has a barrel like structure which can pass through red blood cells and cause it to lyse
It a a multimeric protein made of 7 subunits
What are the different classifications of streptococci?
1- Haemolytic properties
2- Serologic grouping (Lancefield)
What is serologic grouping
Antigen present in nearly all streptococcal cell walls in varying forms: antibodies can developed
Haemolytic streptococci fall into groups A-U so different antibodies will bind
Several fall outside the group including streptococci pneumonia
What are the characteristic of Group A beta-haemolytic streptococci (GAS)
- It is the most notable:
- Mostly carried in the nasopharyngeal
- Doesn’t survive well in environment
