Microbiology Flashcards
what is virulence based on?
the genetics of the organism
what is virulence?
the capacity of a microbe to cause damage to the host
what is adhesin?
enables binding of the organism to host tissue
what is invasin?
enables the organism to invade a host cell/tissue
what is impedin?
enables the organism to avoid host defenses i.e. barriers
what is aggressio?
causes damage to the host directly
what is modulin?
induces damage to the host indirectly i.e. induces host’s own immunity system to turn on the host
name the virulence factors
adhesin invasin impedin aggressin modulin
where is Staph. aureus found on the body as a commensal organism?
anterior nares & perineum
what is a virulence factor?
an ability of the organism which enables it to survive
what is the pathogenicity of staph. aureus?
can cause:
superficial lesions
systeimic problems
toxinoses
name two types of adhesins
fibrinogen-binding
collagen-binding
name an impedin
being a superantigen
PVL
what is a Panton-Valentine Leukocidin (PVL)?
a specific toxin for leukocytes associated with severe skin infections
assoc. with CA-MRSA
what two main conditions is PVL responsible for?
necrotising pneumonia & contagious severe skin infections
what are the clinical features of necrotising pneumonia?
necrotising haemorrhagic pneumonia
acute respiratory distress
refractory hypoxaemia
multi-organ failure
what type of symptoms usually precede necrotising pneumonia?
influenza like syndrome
how does narcotising pneumonia progress?
rapidly
name a suprantigen
TSST-1, causes a massive release of cytokines & an inappropriate immune response
what is the diagnostic criteria for toxic shock?
fever
diffuse macular rash & desquamation
hypotension
3+ organ systems involved
what does streptococcus pyogenes cause?
GAS
which skin infections does streptococcus pyogenes cause?
impetigo
cellulitis
necrotising faciitis
what is Lancefeild grouping?
a method of grouping catalase-negative, coagulase-negative bacteria based on the carbohydrate composition of bacterial antigens found on their cell walls
which Lancefeild group are streptococcus pyogenes in?
group A
how is group A of the lance field system further divided?
by the M protein antigens
what are the M1 & M3 protein antigens on streptococcus pyogenes?
the major serotype
what are the M3 & M18 protein antigens on streptococcus pyogenes?
severe invasive disease
what is impetigo?
usually found on the face, is an infection immediately below the surface (in the stratum corneum). highly contagious through contact with discharge on the face.
what is cellulitis?
a general term for infection deeper in the skin, in the dermis, not associated with necrosis
what is erysipelas?
a localised range of spreading subcutaenous skin infections with fever, rigours and nausea
how does necrotising fasciitis come about?
invasive strep. A strains penetrate mucous membranes & develop in lesions which rapidly destroy connective tissue
name a toxin responsible for tissue/cell destruction?
hemolysins streptolysin S (SLS)
how is SLS toxic to cells?
is a pore forming cytolysin
how does toxic shock like syndrome come about?
through pyrogenic exotoxins as a complication of invasive infections which can be caused by Strep. pyogenes
explain what transient flora are?
harmless organisms that live on the skin until it is washed & tired effectively
where are S. epidermidis found?
100% colonisation on skin & mucous membrane
when will S. epidermidis cause an infection?
in the immunocompromised, associated with foreign devices like catheters
what does a coagulase do?
promotes the formation of clots
what does fibrinolysin do?
promotes the dissolution of clots
what skin conditions can Staph. aureus cause?
rash folliculitis abscess carbuncles impetigo scalded skin syndrome
what is a carbuncle?
a multi-follicle abscess
what is community acquired Staph. aureus likely to cause?
rash
abscesses
carbuncles
what does the presence of collagen-binding protein in an organism mean?
that it is likely to affect specific sites such as bone
do most staph. aurues in the community have fibrinogen or collagen-binding properties?
fibrinogen-binding properties
what is a toxin?
a single protein component that has a specific affect on the body
why can’t necrotising pneumonia be supplemented with oxygen?
as the tissue needed to absorb the oxygen has already been destroyed
how does menstrual toss shock syndrome come about?
from the tampon becoming infected with S. aureus from the perineum being inserted into the body and then travelling into the bloodstream causing massive infection. S. aureus presumably got into the body via the perineum.
if an organism is very encapsulated, what would you expect to see in terms of virulence?
the organism to be very virulent
what does staphylococcus look like on gram stain?
gram positive cocci in clusters
which 2 types of staphylococcus are important?
staph. aureus
coagulase negative staph
how is staph aureus distinguished from all other staph. species?
is coagulase positive
i.e. produces enzymes including coagulase, an enzyme that clots plasma
where does staph. aureus cause infections?
can cause infections anywhere but specifically:
- wound
- skin
- joints
which antibiotic should be given in an staph. aureus infection?
flucloxacillin
which toxins do some strains of staph. aureus produce?
enterottoxin
SSSST
PVL
what does enterotoxin do to the body?
causes food poisoning
what is SSSST?
staph. scalded skin syndrome toxin
how does PVL work?
punches holes in the membranes of leukocytes
what does PVL usually cause in the UK?
abscesses
what are coagulase negative staphs usually?
skin commensals
what factor makes it difficult for white cells to attack coagulase negative staph?
they produce a slime
what does staph. saprophyticus cause?
urinary tract infection in women of child-bearing age
what do streptococcus species look like on gram stain?
gram positive cocci in chains
how are streptococcus species classified?
by hamolysis on blood agar
what is beta-haemolysis?
complete haemolysis
what is alpha-haemolysis?
partial haemolysis
what is gamma-hamolysis?
no haemolysis
are beta-haemolytic strep. pathogenic or not?
pathogenic organisms
name a toxin that beta-haemolytic strep. produce that damages tissue?
haemolysin
how are beta-haemolytic strep. further classified?
by antigenic structure on surface (serological grouping) - group A or B
give examples of infections caused by group a strep.
throat infections
severe skin infections
give an example of an infection caused by group b strep.
meningitis in neonates
wha are the 2 important categories of alpha-haemolytic strep?
strep. pneumoniae
strep. viridans
what is strep. pneumoniae?
a pathogen
the commonest cause of pneumonia
what are strep. viridans?
commensals of the mouth, throat & vagina
rarely cause infection
name a non-haemolytic strep species
enterococcus species
what do enterococcus species do?
commensal of bowel
common cause of UTIs
what does dry skin mean in terms of defence against infection?
causes desiccation of micro-organisms
what does sebum do in terms of defence against infection?
inhibits bacterial growth
what do gram positive organisms have which mean they are unlikely to dry up?
a very thick cell wall
what competitive bacterial flora are present on the skin?
staphylococcus epidermidis
corynebacterium sp.
proprionbacterium sp.
what bacterial skin infections are caused by staph aureus?
boils & carbuncles minor skin sepsis cellulitis infected eczema impetigo wound infection staphyloccal scalded skin syndrome