27 - Staphylococcus Flashcards
Staphylococci
- Gram positive cocci
- Non motile, non spore forming
- Facultative anaerobes
- Mesophiles (8-45ºC)
- Cells divide in multiple planes to form irregular clumps
- Catalase positive, oxidase negative
- High salt and drying tolerance
Main reservoir
Mammals, not generally found in nature or the environment
Divided into two major groups
- Coagulase positive (mostly pathogenic, e.g. S. aureus)
- Coagulase negative (relatively non pathogenic, e.g. S. epidermidis)
Coagulase positive staphylococci
- S. intermedius
- S. delphini
- S. aureus (most important human pathogen)
Staphylococcus aureus
- Produces golden pigment
- Carried by population transiently or persistently
- Nostrils is most common site
S. aureus diseases
- Skin and soft tissue infections (e.g. impetigo)
- Food poisoning (ingestion of pre formed enterotoxin)
Cell associated S. aureus virulence factors
- Adhesins
- Microcapsule (adhesion & immune evasion)
- Cell wall teichoic acids
Life threatening diseases of S. aureus
- Osteomyelitis
- Sepsis
- Acute endocarditis
- Pneumonia
- Toxic shock
Excreted S. aureus virulence factors
- Enzymes such as coagulase, protease, lipase (digest host tissue to allow spread)
- Exotoxins (some superantigens, may be carried on plasmids or phages)
Coagulase
- Causes blood plasma to clot
- Coagulase enzyme reacts with prothrombin
- Forms complex (staphylothrombin)
- May be free or bound
Staphylothrombin complex
- Can cleave fibrinogen
- Causes formation of fibrin clot
- Fibrinogen is soluble whereas fibrin is not
Fibrin clot
- Protects S. aureus from phagocytosis and other host defences
- Fibrin may also deposit onto surface of S. aureus
Bound coagulase
Attached to cell walls
Free coagulase
Released extracellularly
Toxins produced by most strains of S. aureus
Cytotoxins (haemolysins)
Haemolysins
- Alpha, beta, delta, gamma
- Not all strains have all haemolysins
- Cytolytic, damage membranes
- Involved in tissue invasion
Toxins produced by some strains of S. aureus
- Exfoliative toxins A and B
- Toxic Shock Syndrome toxin
- Enterotoxins
- Panton-Valentine leucocidin (PVL)
Alpha and gamma haemolysins
Forms beta barrel transmembrane pores
Beta haemolysins
- Does not form pores
- Hydrolyses sphingomyelin (enzyme)
Delta haemolysins
- Phenol-soluble modulins
- non-specific membrane damage or short-lived pores
Enterotoxins
- Cause emesis after ingestion
- More than 20, A to E most common
- Heat and acid stable
- Genes located on mobile elements (plasmids, bacteriophages)
A and E toxin location
Prophage
B and C toxin location
Pathogenicity island
D toxin location
Plasmid
Superantigens
- SEA and SEB are superantigens
- Cause non-specific T cell activation
- Binds directly to Class II MHC
- Leads to massive cytokine release and shock
Toxic shock syndrome
- Rare complication of S. aureus
- Associated with vaginal products
- As bacteria grow, Toxic Shock Syndrome Toxin released and enters bloodstream
Epidemiology of S. aureus
- Hospital or community acquired
- Risk factors different according to specific disease (e.g. age, female, surgery)
- May be commensal
S. aureus antibiotic susceptibility
- Originally penicillin
- Now Methicillin (MRSA emerged through mecA gene)
- Vancomycin then used (VRSA emerged through vanA and B genes)
vanA
- encoded on transposon
- S. aureus either retains VRE conjugative plasmid or transposon transferred into S. aureus
S. aureus vancomycin resistance
- VANC binds D-ala-
d-alla - Disrupts peptidoglycan assembly
- Resistance achieved by altering cell wall intermediates
Coagulase negative staphylococci
- Commensal flora of human skin
- Mostly opportunistic pathogens
- May also contaminate clinical specimens
Examples of coagulase negative staphylococci
- S. epidermidis, produces slime and biofilm
- S. lugdunensis, Endocarditis, high mortality rate
S. epidermidis infections
- Major nosocomial pathogen (causes infections of indwelling devices)
- Leads to endocarditis, bacteraemia
- Unlikely to cause infection in health yhost
S. epidermidis virulence factors
- Biofilm formation
- Slime production
- Produces phenol soluble modulins (pro-inflammatory, sometimes cytolytic)
Slime
- Viscous, unorganised, extracellular polysaccharide polymer
- Loosely-adhered to bacterial cell surface, easily washed off
Role of slime in infection
- Allows bacteria to adhere to smooth surfaces
- Inhibits neutrophil chemotaxis
- Reduces antibiotic effectiveness
S. epidermidis antibiotic susceptibility
- Susceptible to vancomycin
- Methicillin resistance encoded on mobile genetic element (SCCmec)
Staphylococcal cassette chromosome mec (SCCmec) in S. aureus originating from S. epidermidis
- S. epidermidis is a commensal therefore exposed to many different antibiotics
- S. epidermidis may be a reservoir for antibiotic resistance genes
- S. epidermidis transfers mobile genetic elements to S. aureus, but reverse does not occur (due to CRISPR)
