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Medical Microbiology > MRSA > Flashcards

MRSA Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

What are S.aureus?

A

Gram-positive cocci
Commensal bacteria colonising mammals and birds

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2
Q

Describe S.aureus

A

25% of healthy humans carry S.aureus in their nose
Common cause of minor infections not requiring antibiotic treatment
Common cause of hospital-associated infection in immunocompromised patients

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3
Q

What do S.aureus colonies look like?

A

Golden on blood agar

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4
Q

What does S.aureus produce?

A

Dozens of virulence factors - toxins, immune evasion mechanisms, bind to host tissues, etc

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5
Q

What selective media is used to diagnose S.aureus?

A

Mannitol Salt agar (MSA)

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6
Q

What other bacteria can be cultured on MSA?

A

Staph epidermidis
Micrococcus luteus

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7
Q

What are Methicillin resistant staphylococcus aureus?

A

S.aureus that carry mecA gene on SCCmec mobile genetic element

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8
Q

What are MRSA resistant to?

A

All beta-lactam antibiotics
Inc. flucloxacillin, carbepenems, cephalosporins

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9
Q

Why are beta lactams best to use?

A

Inexpensive
Orally administered
Few side effects

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10
Q

What is the main class of drug used to treat and prevent MRSA?

A

Glycopeptides

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11
Q

How does MRSA impact healthcare?

A

MRSA infection incidence adds to S.aureus burden
No vaccine for S.aureus or MRSA
Patients with MRSA infection treated with inferior antibiotics

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12
Q

What happens if MRSA in the hospital is common?

A

Beta lactams cannot be used for prophylaxis or empirical therapy

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13
Q

What are the reservoirs of MRSA?

A

Normally live in/on host predominantly in nose, throat, armpit and groin
Each host niche is colonised by ecosystems of microbes (microbiome)

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14
Q

What do colonising bacteria act as?

A

Often they are the reservoir of infecting bacteria particularly in immunocompromised hosts
>80% of patients become infected with their own S. aureus colonising bacteria

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15
Q

What are the types of MRSA?

A

Hospital associated
Community associated
Livestock associated

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16
Q

How do S.aureus vary?

A

Mobile genetic elements

17
Q

What define lineages/clonal complexes?

A

Unique combinations of hundreds of genes, especially surface proteins and their regulators

18
Q

What is the S-aureus gene structure?

A

Core - found in all strains, >97% conserved, gene order
Core variable - define lineages, widely distibuted
Mobile genetic elements - inc many virulence factors, host adaptation factors and antimicrobial resistance genes

19
Q

What influences invasive disease from S.aureus?

A

Host factors

20
Q

What are the differences between human and animal isolates of S.aureus?

A

Bovine isolates belong to unique lineages
Horse isolates belong to either bovine or human lineages

21
Q

What are the human specific S.aureus genes?

A

scn - complement inhib protein, human specific
chip - chemotaxis inhib protein, human specific
sak - staphylokinase, plasminogen activation and invasive spread, human specifc
sea - enterotoxin A, food poisoning

22
Q

What can happen if MGEs are transferred?

A

New DNA can lead to a strain with enhanced virulence or resistance

23
Q

What is encoded on MGEs?

A

Most of the AMRs, toxins and host-specificity factors

24
Q

Name the MGEs of S. aureus

A

Staphylococcal cassette chromosomes (SCCs)
Bacteriphage
S. aureus pathogenicity islands (SaPI)
Plasmids
Transposons

25
Q

What does SCC encode?

A

Resistance to methicillin, kanamycin etc.
Gene for mucoid capsule

26
Q

What does the bacteriophage encode?

A

Panton-Valentine leukocidin
Enterotoxin A
Exfoliative toxin

27
Q

What does SaPI encode?

A

Enterotoxins B/C
Toxic shock toxin

28
Q

What do plasmids encode?

A

Resistance to b-lactams, tetracyclines, aminoglycosides, macrolides, fusidic acid, heavy metals
Exfoliative toxin

29
Q

What do transposons encode?

A

Resistance to macrolides, b-lactams, vancomycin

30
Q

What have successful antimicrobial resistant HA-MRSA accumulated on MGEs?

A

mecA on SCCmec
Phi3 scn, chp, sak
Bacteriophage
vanA on conjugative plasmid/transposon from Enterococci (vancomycin resistance)

31
Q

Describe CA-MRSA

A

Severe skin and soft tissue infection in healthy people
Outbreaks in schools, sports teams, military, prisons (close contact environments)
Prevalent in US and Asia

32
Q

What is the marker of CA-MRSA?

A

Panton Valentine leukocidin (PVL) toxin gene carried on bacteriophage

33
Q

What have successful AMR CA-MRSA accumulated on MGEs?

A

mecA on SCCmec
phi3 scn, chp, sak
Bacteriophage
Virulence/PVL toxin on bacteriophage

34
Q

Describe LA-MRSA

A

Pig farms have become heavily colonised with LA-MRSA of lineage CC398
Humans that come into contact with these farms become colonised
Small proportion of humans become infected with their colonising isolates

35
Q

What have successful AMR LA-MRSA accumulated on MGEs?

A

mecA on SCCmec
Tetracycline resistance on plasmids/transposons

36
Q

What is the mechanism of gene transfer?

A

Generalised transduction via endogenous bacteriophage

37
Q

What blocks horizontal transfer?

A

Restriction modification

Medical Microbiology (23 decks)

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