Lecture 5 - Staphylococcus Aureus and Disease Flashcards
what does staphylococci mean?
bunch of grape granules
according to the greek reets
how many different staphylococci are there, and what are the three relevant ones?
30 different species
3 relevant ones are:
- Staphylococcus aureus
- staphylococcus epidermidis
- staphylococcus saprophyticus
what are the general characteristics of staphylococci?
- irregular cell clusters
- gram positive
- spherical cells
- non-motile
- resistance to dry conditions and up to 10% NaCl (halodurent)
what proves that S. aureus is an opportunistic pathogen?
the fact that 15% of normal healthy adults are persistant nasopharyngeal carriers without symptoms
S. aureus can colonise _____ ____ _____, the ________________, __ tract and _____________ tract
moist skin folds, oropharynx, GI, urogential
true or false: S. aureus is the major cause of hospital-acquired infections due to a high number of carriers amongst staff and other hospital users
true
what feature of S. aureus allows it to survive on dry surfaces for long periods of time?
capsule and its gram +ve morphology
how is S. aureus transmitted?
through our skin- but only where the skin’s barrier is breached. (e.g cut, splinter, surgery)
95% of S. aureus are resistant to which antibiotic?
penicillin, since S. aureus picked up the beta lectamase gene over the years
What are the types of drug resistant S. aureus?
- methicillin resistant staphylococcus aureus
- vancomycin resistant staphylococcus aureus (the superbug/killer bug)
- hospital acquired MRSA
- community acquired MRSA
what is the process of diagnostic we can use to find S. aureus?
- gram stain (shows gram +ve)
- catalase test (+ve, so must be a staphylococci)
- coagulase test (+ve means we have S. aureus)
- or mannitol salt agar (yellow and growth means +ve for staphylococci)
what makes some strains virulent, while others are opportunistic?
virulence factors
list the common virulence factors
- adhesins (promote attachment to host cells)
- invasins (help bacteria enter host cell)
- cytolysins
- spreading factors
- immunopathogenic factors (over stimulate immune response)
- mediate immune evasion
what are the adhesins in staph a?
MSCRAMMS (microbial surface components recognising adhesive matrix molecules)
- proteins in the bacterial cell wall which attach to the ECM proteins (e.g collagen) of the host cell
which virulence factor is important for tissue colonisation?
adhesins
do adhesins damage the host cell?
no, but they facilitate other virulence factors to do so
what are the two factors that damage the host cell?
- cytolysins
- exfoliative toxins
how does the alpha toxin (hemolysin) work?
it is a cytolysin that, when 7 proteins combine, forms a pore in the host cell and causes it to lyse
what does a hemolysin mean?
it can lyse red blood cells
how does the beta toxin work?
hydrolysis of membrane phospholipids and damages the membrane
how does the delta toxin work?
surfactant, detergent like action that dissolves the membrane
how does the gamma toxin and panton-valentine leukocidin work?
they are also pore forming toxins, but are rare
what does a cytolysin do?
causes the host cell to lyse in some form or another
- toxic for many types of cells
what are exfoliative toxins?
they are serine proteases that split desmosomes in the stratum granulosum epidermis
- no cytolysis or inflammation
- not found anywhere apart from 5-10% of staph aureus strains
what are the 5 main spreading factors?
- lipases
- nucleases
- hyaluronases
- proteases
- staphylokinase (unique to s. aureus) (fibrinolysin)
spreading factors are all?
enzymes
what is the main mechanism of bacterial spreading in staph aureus?
- when s. aureus enters the bloodstream, there is local inflammation and a clot forms, containing the bacteria with thrombosis
- S. aureus secreted staphylokinase, which activates plasminogen to dissolve the clot and allow S. aureus to spread
what are superantigens?
immunopathogenic factors
- they are a family of toxins produced by staphylococci that cause food poisoning
how do superantigens work in staphylococci?
they bridge the T cell receptor to the MHC II receptor and disrupts the presentation step.
- as a result, too many T cells are recruited, which causes overstimulation of the immune system
what are the 5 immune evasion factors for staph aureus?
- capsule
- slime layer
- catalase
- clumping factor CIF-A
- Protein A
how is a capsule an immune evasion factor?
- dense polysaccharide coat that inhibits phagocytosis (by preventing opsonisation)
- contains water so prevents bacteria from drying out
how is a slime layer and immune evasion factor?
- biofilm made from secreted polysaccharides that protects bacteria from immune factors and ABs
how is catalase an immune evasion factor?
- it detoxifies hydrogen peroxide
how is clumping factor CIF-A an immune evasion factor?
- aka bound coagulase
- prevents opsonization and therefore phagocytosis
how is protein A an immune evasion factor?
- unique to staph aureus
- binds IgG in the wrong orientation, which prevents opsonisation and phagocytosis
what three types of disease does S. aureus cause?
- superficial skin diseases
- invasive diseases
- toxigenic diseases
what are the skin diseases that S. aureus can cause?
- impetigo (localised cutaneous infection, mostly effects children)
- furuncles/boils (pus-filled cutaneous nodules)
- folliculitus (ingrown hair)
- carbuncles (multiple furuncles/boils that extend into subcutaneous tissue, can lead to systemic disease)
what is a corneal ulcer?
- deep infection of the cornea
- usually after abrasion such as contacts
- can be cause by S. aureus but also several other bacteria
what is pneumonia?
- caused by pneumococcas but also s. aureus
- necrotizing pneumonia is a severe version cause by S. aureus, where cytolysins are involved
what is osteomyelitis?
- mainly effects children
- metaphyseal infection of S. aureus
- can be treated with appropriate antibiotic
- can sometimes spread into joint and cause septic arthritis
what is septic arthritis?
mainly in children and young adults receiving joint injections, or hemotogenous spread from a localised infection.
- painful joint with pus in joint space
what is acute endocarditis?
- spread of bacteria into the bloodstream from a localised area
- most cases are hospital acquired
- can spread to the heart (endocarditis)
- s. aureus acute endocarditis has a mortality rate of around 50%
what are toxigenic diseases?
when toxins produced by staph aureus are solely responsible for the disease, so not an infection
what are the three most common toxigenic diseases produced by staph aureus?
- staphylococcal food poisoning
- toxic shock syndrome
- staphylococcal scalded skin syndrome SSSS
what is staphylococcal food poisoning?
- microbial intoxication, not an infection
- caused by heat-stable enterotoxins
- rapid onset and recovery, generally lasts for less than 24h
what is staphylococcal scaled skin syndrome?
- caused by exfoliative toxins
- common in very young children, development of immunity and low mortality rate
- cutaneous blisters followed by desquamation of epithelium
what is toxic shock syndrome?
- caused by superantigen-producing strain growing in a wound (strep can do this also)
- toxins enter the blood and cause systemic disease (lots of cytokines)
- multiple organ failure
what is menstrual toxic shock syndrome?
the growth of TSST-producing strain in vagina (toxic shock syndrome toxin is the only superantigen that can penetrate the mucosal barrier)
- caused by prolonged use of expandable tampon
- unique to staph aureus
in which circumstances is augmentin an appropriate AB to use against staph aureus?
if it is not an MRSA strain, as it will not work
in which circumstances is vancomycin an appropriate AB to use against staph aureus?
against an MRSA strain only
- as overuse can cause highly unwanted resistance
in which circumstances is flucoxacillin an appropriate AB to use against staph aureus?
if it is not an MRSA strain, as it will not work
what type of S. aureus disease are infants suceptible to?
scalded skin syndrome
what type of S. aureus disease are children with poor hygiene suceptible to?
impetigo, boils
what type of S. aureus disease are menstruating women suceptible to?
toxic shock syndrome
what type of S. aureus disease are patients with intravascular catheters suceptible to?
endocarditis, bacteremia
what type of S. aureus disease are patients with compromised pulmondary function suceptible to?
pneumonia and necrotizing pneumonia
what type of S. aureus disease are immunocompromised patients suceptible to?
unfortunately everything mentioned in this lecture
what are the risk factors for S aureus disease?
- presence of a foreign object eg splinter
- previous surgical procedure
- poor hygiene of hospital staff
- use of ABs that suppress the normal microbial flora
- lack of protective antibodies
The predominant source of Staphylococcus aureus as a cause of disease is?
anterior nares
