M4: Staphylococci and staphylococal infections Flashcards
Outline characteristics of the staphylococcus genus
- Gram-positive arranged in grape-like clusters
- Non-motile, non-spore forming
- Catalase-positive
- Aerobic and anaerobic metabolism
- Resistant to dry and high salt conditions
- Found mainly on skin and mucous membranes
Give examples of coagulase positive staphylococcus species
S. aureus
S. intermedius
Give examples of coagulase negative staphylococcus species
S. epidermidis
S. haemolyticus
S. lugdunensis
What are the main diagnostic features of staphylococcus aureus coagulase positive pathogens
- Colonies are pigmented (carotenoid pigments)
- Extra-cellular coagulase production
- Nuclease production that breaks down DNA
- Production of cell surface-associated enzyme clumping factors or bound coagulase
What are the pyogenic infections caused by staphylococci aureus
- Boils, carbuncles
- Wound (surgical( infection
- Abscesses
- Impetigo
- Mastitis
- Septicaemia
- Osteomyelitis
- Pneumonia
- Endocarditis
What are the toxin-mediated infections caused by staphylococci aureus
- Scalded skin syndrome
- Pemphigus neonatorum
- Toxic shock syndrome
- Food poisoning
What is the pathogenesis of staph. aureus
It is an opportunistic pathogen which causes infection at sites of lowered host resistance e.g. damaged skin or mucous membranes/ staphylococcal pneumonia which is a complication of influenza
What do enterotoxins/pyrogenic exotoxins produced by staph. aureus result in
These heat stable proteins induce nausea, vomiting and diarrhoea within a few hours and is most commonly due to contaminated foods e.g. processed meats, custard filled pastries, potato salads and ice cream
What do epidermolytic toxins/exfoliatins produced by staph. aureus result in
There are two types of toxins, A and B which cause
- Pemphigus neonatorum (distended blisters)
- Impetigo (flattened blisters)
- Scalded skin syndrome (extensive area of skin lost) - this is mainly seen in children which lack the neutralising antitoxin
Describe pustular impetigo
Vesicles are at different stages of development so some are pus-filled vesicles on an erythematous base, dry or crusted lesions
How does staph. aureus cause toxic shock syndrome
This is associated with highly absorbent tampons and vaginal colonisation with TSST-1 producing S. aureus in presence of tampon creates the conditions for bacterial multiplication and toxin production
What do haemolysins produced by staph. aureus result in
They are cytotoxic and result in lysis of RBCs and other phagocytic tissue cells
What do Pantones-valentine leucocidin produced by staph. aureus result in
Destruction of WBCs increasing resistance to phagocytosis and this is associated with community acquired MRSA
What do hyaluronidase produced by staph. aureus result in
Break down of intracellular ground substance (HA) of tissues
How do cell wall polymers act as virulence factors for staph. aureus
Peptidoglycans - inhibit inflammatory response, endotoxin-like activity
Lipoteichoic acid - interacts with toll-like receptors
How do cell surface proteins act as virulence factors for staph. aureus
Protein A - reacts with Fc region go IgG
Clumping factor - binds to fibrinogen
Fibronectin-binding protein - binds to fibronectin
What are the sources of infection for staph. aureus
- Infected lesions
- Healthy carriers
- Animals
Transferred by direct contact, air-borne dust and air-borne droplet nuclei
What samples can be taken for the laboratory diagnosis of staph. aureus
- Pus from abscesses, wounds and burns
- Sputum from cases of pneumonia
- Faeces or vomit from those with food poisoning
- Blood from patients with bacteraemia
- Mid-stream urine for those with cystitis or pyelonephritis
- Anterior nasal or perineal swabs from suspected carriers
How can patients with staph. aureus infections be treated
Sensitive to many antimicrobial agents but are resistant to benzyl penicillin
How does the production of penicillinase enzyme induce antibiotic resistance
Because it inactivates penicillin drug by opening the beta-lactam ring of penicillin molecule as well as ampicillin and amoxicillin
Which drugs are resistant to penicillinases
- Methicillin
- Cloxacillin
- Cephalosporin drugs
- Enzyme inhibitors clavulanic acid
How can a mutation induce antibiotic resistance
If the essential penicillin-binding protein found in bacterial cell envelope mutates
How can glycopeptides induce antibiotic resistance
GISA causes thickened cell walls
What drug is used in place of vancomycin
Synercid because people have vancomycin resistance
Describe coagulase-negative staphylococci
- Morphologically similar to S. aureus
- Non-pigmented
- Carried as part of the normal skin flora
- Don’t normally cause infection
What infections are caused by coagulase-negative staphylococci
Infected prostheses/implants Ventriculitis (shunt associated) Peritonitis Septicaemia Endocarditis (with prosthetic valves)
What treatment is given for coagulase-negative staphlococci
Resistant to; penicillin, gentamicin, erythromycin and chloramphenicol
Treated with; vancomycin
