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PR3151 Principles of Antimicrobial Use > eL Clinical Microbiology > Flashcards

eL Clinical Microbiology Flashcards

1
Q

List some normally sterile sites

A
  • CNS
  • CVS
  • Lower respiratory tract
  • Bone, joint
  • Genitourinary tract (except urethra and vagina)

Closed systems

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2
Q

Where is normal flora derived from?

A
  • Maternal genital tract
  • Skin of attendents
  • Mouth and pharynx of close contacts
  • Air-borne organisms
  • Environment and food

*These ares exposed to external environment and acquire microbes

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3
Q

What is the timeline from collection to identification, culture, and AST results?

A

Day 1: gram stain
Day 2-3: culture bacteria (on agar)
Day 3-4: further tests, AST

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4
Q

What is used to classify microscopic appearance of the culture?

A
  • Gram stain
  • Acid-fast stain
  • Morphology - rods, cocci, curbed, spiral
  • Colonial morphology (spatial arrangement) - diplococcus, cocci in clusters/chains
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5
Q

Laboratory diagnostics: Biochemistry and Culture
What can they tell us about the bacteria?

A

Biochemistry:
- Specific nutrients: fermenters vs non-fermenters (of carbohydrates)
- Carbon sources for growth
- Presence of enzymes: catalase, coagulase, oxidase

Culture:
- Growth environment: aerobic vs anaerobic
- General enriched agar (blood, chocolate)
- Selective media (MacConkey agar for enteric gram negative)

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6
Q

Besides microscopy, culture, and biochemistry, what other methods can be used for laboratory diagnostics?

A
  • Serology/immunology diagnostics (detect antigens/antibodies)
  • Molecular/nucleic acid-based diagnostics (nucleic acid probes, PCR)
  • Mass spectrometry (MALDI-TOF, compare mass spectrum profile to library)
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7
Q

What cultures are used for macroscopic appearance?

A
  1. MacConkey’s Agar
  2. Blood Agar
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8
Q

What can MacConkey’s Agar identify?

Explain how it works and how it appears?

A

Gram-negative enteric rods (and whether they are lactose or non-lactose fermenting)

How it works:
MacConkey agar has bile salts required for survival and growth of enteric gram-negative bacteria

Macroscopic Appearance:
Lactose fermenting: RED/PINK, surrounded by zone of acid precipitated bile
Non-lactose fermenting: COLORLESS/YELLOW

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9
Q

What are the 3 lactose-fermenting enteric gram negative rods?

A
  1. Enterobacter spp.
  2. Escherichia coli.
  3. Klebsiella spp.
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10
Q

What are some examples of non-lactose fermenting gram-negative rods?

A
  1. Salmonella
  2. Shigella
  3. Proteus
  4. Yersinia
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11
Q

What can blood agar identify? Describe the appearance.

A

Macroscopic Appearance:
Blue-green colonies + grape-like odor: Pseudomonas aeruginosa
Golden colonies: Staphylococcus aureus

Macroscopic Appearance of HEMOLYSIS - identifies diff categories of streptococcus species

  1. Alpha-hemolytic Strep - partial hemolysis (GREEN-BROWN bacteria growth)
  2. Beta-hemolytic Strep - complete hemolysis (LIGHTENED AGAR around GREEN-BROWN bacteria growth)
  3. Gamma-hemolytic Strep - no hemolysis (no media color change)
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12
Q

Gram stain is used to identify microscopic appearance. Describe gram stain.

A

Gram-positive gram stain: PURPLE
Gram-negative gram stain: PINK

  1. Primary stain (crystal violet) bind to peptidoglycan and stains cell purple (thick peptodoglycan in gram positive)
  2. Iodine bind, form crystal violet-iodine complex
  3. Alcohol/acetone decolorize the complex on gram-negative
  4. Counter stain (Safranin) stains gram-negative pink
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13
Q

Acid-fast stain is used to identify microscopic appearance. Describe acid-fast stain.

A

Thick layer of mycolic acid in cell wall of acid-fast positive bacilli: RED/PURPLE

Acid-fast negative: BLUE

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14
Q

Why acid-fast might be needed? (why gram stain does not work for AFB?)

A

AFB have thick layer of mycolic acid + glycolipids, hence crystal violet primary stain unable to penetrate to peptidoglycan layer.

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15
Q

What biochemistry enzyme tests are used to determine growth and metabolic characteristics of the bacteria?

A
  1. Catalase Test
  2. Coagulase Test
  3. Oxidase Test
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16
Q

Explain how Catalase test works?

A

Catalase is an enzyme that catalyzes conversion of H2O2 to H2O and O2.

Catalase positive colony placed in H2O2 => BUBBLING due to O2 production

E.g., Catalase positive: Staphylococcus
Catalase negative: Streptococcus, Enterococcus

17
Q

Explain how Coagulase test works.

A

Coagulase enzyme causes plasma to clot.

Coagulase positive bacteria cells clump tgt in coagulase reagent, but remain dispersed in water control.

E.g.,
Coagulase postive: Staph. Auerus
Coagulase negative: Staph. Epidermidis

18
Q

Explain how oxidase test works.

A

Oxidase will convert the reagent from reduced to oxidized state (color change to PURPLE)

E.g.,
Oxidase positive: Pseudomonas Aeruginosa GNR
Oxidase negative: E. Coli GNR, and other enterobacterales

19
Q

MacConkey’s Agar identifies ____________ and differentiates them into __________ and __________ fermenters.

Both fermenters (all enterobacterales) are ___________.

A

Enteric gram-negative rods (Enterobacterales)

Lactose (RED/PINK) and non-lactose fermenters (COLORLESS/YELLOW).

Both are OXIDASE-negative (no color change in oxidase test)

20
Q

What are non-fermenters?

A

Non-fermenters have no ability to ferment sugar/carbohydrates.

E.g., P. aeruginosa, Acinetobacter baumanii

21
Q

When is AST performed?

A
  1. When there is clinically significant isolates
  2. When specimens are appropriately collected
  3. When there is possible resistance to primary therapeutic agents
  4. When standard performance method and interpretive criteria are established
22
Q

What are the 3 methods for antimicrobial susceptibility testing (AST)?

A
  1. Agar or broth dilution method
  2. Kirby-Bauer disk diffusion method
  3. E-test
23
Q

How is MIC and MBC determined using the agar/broth dilution method?

A

MIC is determined by the first clear petri dish after 18-24h of dilution.

All petri dish without visible growth is plated on agar plates and incubated again. MBC is determined by the first petri dish with no growth or 99.9% decrease.

24
Q

Disk diffusion method measures _______

A

Diameter of zone inhibition

*Filter paper discs containing set conc. of antibiotics placed on bacteria agar plate

25
Q

How is E-test conducted to measure MIC?

A

E-test strip contains graduated antibiotic conc.
Strip is placed onto inoculated agar plate, and incubated.
Antibiotic from strip is transferred/diffused to the agar in a stable continuous gradient

MIC is read where the growth intersects the plastic strip

26
Q

Apart from AST, what are some factors that affect choice of antibiotic?

A
  • Immune function
  • Protein binding of drug
  • Ability of drug to reach site of infection
  • Drainage/removal of infected foci
  • Drug interactions
  • Resistance (enzymes that inhibit the antibiotic e.g., B lactamase)
27
Q

What are breakpoints?

A

Critical conc. which predict susceptibility/resistance (determined by standards)

MIC breakpoint:
Susceptible when MIC =< S breakpoint
Resistant when MIC > R breakpoint
Intermediate when >S, =< R

Zone diameter breakpoint:
Susceptible when diameter >=S
Resistant when diameter

28
Q

An agent is bactericidal when:

A

MBC within one 2-fold dilution of MIC

MBC =< 2MIC

Once MBC > 2MIC: agent is not bactericidal

29
Q

What is an antibiogram?

A

Antibiogram shows cumulative susceptibility results that is tabulated from common bacterial isolates collected in hospital/institution.

Shows how many % of the isolate was susceptible to a certain antibiotic

30
Q

What is the usefulness of antibiogram?

A
  • Assess local susceptibility rates
  • Monitor resistance trend over time
  • Guide selection of antibiotic tx for empiric treatment

PR3151 Principles of Antimicrobial Use (3 decks)

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