Catalase pos GPC Flashcards
What is the difference between the normal oxidase test and the modified oxidase test? How does the latter work? Why use it? *
Normal: detects cytochrome c on surface* (differentiates Gram Neg)
Modified: oxidase rgt + DMSO > dissolve lipid = release enzyme out of cell (differentiate micrococcus*)
List the 5 virulence factors of S. aureus and give one example of why or how each one acts to assist in the ability of the organism to cause infection?
- Capsular polysaccharides: adherence, protect from immune resp
- Protein A: prevent detection of phagocytes
- Enzymes: B-lactamases resistance to penicilin or other B-lactam antibiotics;
- Haemolysins: toxic to host cells (a - toxic to PMN, RBC & neurotoxic effect [myelin])
- Toxins: enterotoxins (A->D, E, H, I= heat resitant)
Why are all isolates of S. aureus reported with sensitivities even though it could be part of the normal flora of a patient?
S. aureus is significant in noscomial infections (hospital settings)
Describe how you would identify an unknown catalase positive GPC to genus and species level (assume it could be one of the three main staphylococci)?
Coagulase: Pos = S. aureus
Novobiocin (on urine): R = S. saprophiticus. S = S. epidermidis
(MALDITOF)
What test divides all staphylococci into two groups? How do both versions of it work?
Coagulase tes: Slide & tube
- SLIDE: bound coagulase (Cwall) aka “clumping factor”. Fibrinogen in plasma clumps w/ coagulase on Cwalls of cells=> white clumps
- TUBE: free coagulase (secreted). Incubate for 4 hrs in plasma & let bact. secrete coagulase => clots w/ fibrinogen
Why is it necessary to do confirmatory tests on an isolate suspected of being S. aureus?
Bc there are other Staph. sp. that are coagulase pos, so need to do other tests to confirm S.aureus
Name two other species of Staphylococcus that could give a positive tube coagulase result? What about two species that could give a positive clumping factor result?
TUBE: S. schleiferi subsp. coagulans and S. schleiferi subsp. schleiferi (V)
SLIDE: S. schleiferi subsp. schleiferi and S. sciuri subsp. rodentium
What type of infection is S. saprophyticus normally associated with? Is it Novobiocin resistant or susceptible?
Community acquired UTI (esp in young sex active women)
R to Novobiocin
What type of clinically significant infections is S. epidermidis normally associated with? Why? (2 reasons with explanations)
a) noscomial infections
b) Surgery, implant medical devices = allow invasion of organism
Describe the pathogenesis (processes/virulence factors used by the organism to cause infection) of a nosocomial infection with S. epidermidis.
- produce biofilm= resistant to antibiotic & immune resp. & adhere to plastic surfaces
- readily acquires resistance from other bact. (pass plasmids)
What factors would help you to determine the clinical significance of isolating a coagulase negative staphylococci (CoNS)? i.e. should the isolate be regarded as pathogenic and reported with an ID and susceptibility profile?
- Seen in gram smear
- Purity of growth
- quantity of growth
- site of infection
- clinical history
- check previous MALDI-TOF results
- More it ‘ticks’ list = more significant
What does MRSA stand for?
Methicillin resistant Staph. aureus
If a laboratory relied solely on identifying S. aureus using both a slide and tube coagulase test, which three other species of Staphylococcus could
conceivably produce the same results? i.e. be misidentified as S. aureus? Would a DNAse test help to differentiate between them, why/why not?
- S. schleiferi subsp. coagulans
- S. schleiferi subsp. schleiferi (V)
- S. intermedius
- DNAse test doesn’t help differentiating between them bc have same result as coagulase
How could you typically tell if either Micrococcus luteus or Kocuria rosea were growing in culture?
- Micrococcus. luteus: bright yellow colonies
- Kocuria rosea: red/orange (on skin swab - not mistake for S. marcescens)
Features of Micrococcus, Kocuria, Kytococcus
- GPC
- Catalase pos
- Obligate anaerobe
- microdase pos (aka modified oxidase test)
