Diagnostic Microbiology

Question 1

Describe how MacConkey agar is both selective and differential.

Answer 1

Selective: dyes inhibit growth of Gram-positive and only allow Gram-neg
Diffential: pink lactose fermenting bacterium vs yellow non-fermenting bacterium

Question 2

Define the Rapid bench methods for bacterial ID.

Answer 2

1. Coagulase tubes: (+) for production of coagulase (usually pathogenic) vs the negative
2. Catalase test: production of H2O2 in (+) orgs (i.e. Staph) vs (-)) (i.e. Strept)

Question 3

Describe how API strips work in ID bacteria.

Answer 3

These are comparative results from an array of biochemical tests that can be matched up to a database/ control to ID the organism.

Question 4

Describe the process of the MALDI-TOF in 4 steps.

Answer 4

1. Bacteria is placed at the bottom in matrix (MA-)
2. Laser excites the organisms to burst them open (-LDI)
3. Liberated proteins fly down a vacuum tube (TOF) and time of flight is measured to impact.
4. Mass spectometry measures the data.
   * This bacterial ID method is rapid (5 minutes)!*

Question 5

When might DNA sequencing be used to ID bacteria?

Answer 5

DNA sequencing is recommended to distinguish very closely related microbes

Question 6

Describe the process and significance of the Direct Antigen test.

Answer 6

Direct antigen tests DIRECTLY ID bacteria from patient’s specimen

1. nasal/throat swab
2. inoculate in lateral flow device
3. ABs bind org in matrix
4. 2nd AB with color forms a line from reaction
   * This is the standard flu/strept detection test*

Question 7

What is the drawback for direct antigen detection? How is this clinically significant?

Answer 7

This test LACKS sensitivity! Docs want to make sure there’s a negative after a culture/PCR test to confirm. You can make this test more sensitive by acquiring more specimen BUT you risk a false positive.

Question 8

What is the significance of antimicrobial susceptibility testing?

Answer 8

Provides choices for optimal therapies, that should be narrow and less expensive. Can provide alternatives especially to patients allergic to certain antibiotics (i.e. penicillin)

Question 9

Define the Minimum inhibitory concentration

Answer 9

MIC = minimal amount of drug needed to completely inhibit the growth of the bacteria (esp. in broth dilution)

Question 10

Define zone of inhibition.

Answer 10

ZOI = Diameter of clear agar around antimicrobial is proportional to the MIC for a certain drug on agar plate.

Question 11

Describe the best way to determine MIC for an E-test.

Answer 11

Look for where the organism INTERSECTS the gradient distribution strip.

Question 12

What are the 3 categories used to interpret the MIC?

Answer 12

1. Susceptible (S) = if MIC is below, infection can be treated with this dose of AB
2. Intermediate (I) = imprecision of determined MIC
3. Resistant (R) = if MIC is higher and falls in a range where microbial resistance mechs are likely

Question 13

Define the therapeutic breakpoint. Describe the significance.

Answer 13

The concentration of antimicrobial that can get to site of infection. This is the MIC that distinguishes between S, I and R. If bug’s MIC is below < susceptible; equal = intermediate; above > resistant.

Question 14

What bacteria do not Gram stain? Provide 2 examples.

Answer 14

Atypical bacteria remain colorless after Gram-staining (Chlamydiaceae, Legionella, Mycoplasmatacaea)

1. Mycobacterium tuberculosis
2. Legionella pneumonia

Question 15

What medium is important for differentiating Gram-positive cocci from Gram-negative?

Answer 15

Minimal agar to which bacteria can grow with glucose and salts.

Question 16

What is the standard medium to grow M. tuberculosis?

Answer 16

Lowenstein-Jensen medium contains palmitic acid broth with growth detection system for rapid analysis.

Question 17

What diagnostic test differentiates S. aureus from S. epidermidis?

Answer 17

Coagulase test: S. aureus (+) and S. epidermidis (-)

Question 18

Describe what causes the distinctive color for alpha hemolytic bacteria on blood agar.

Answer 18

Breakdown of hemoglobin to bilirubin results in a greenish-brown halo around alpha hemolytic bacteria.