Clinical Microbiology

Question 1

Differentiate between pathogens, colonisers & contaminants.

Answer 1

1) Pathogens: Capable of damaging host tissues & elicit host response, producing signs & symptoms of infection
2) Colonisers: Presence of normal flora & pathogenic organisms w/o eliciting host response
3) Contaminants: Typically acquired during collections/processing of host specimens w/o eliciting host response

Question 2

Name all of the normally sterile sites in our human body.

Answer 2

1) Genitourinary tract (except urethra & vagina)
2) Central nervous system
3) Lower respiratory tract (lungs, alveoli, bronchioles)
4) Cardiovascular system
5) Bones & joints

Question 3

Why is it important to conduct microbial identification & antimicrobial susceptibility testing (AST)?

Answer 3

• Assist clinicians in identifying clinically significant isolates from appropriately collected specimens
• Subsequently guiding the selection of appropriate targeted antibiotic therapy in order to optimise clinical outcomes.
• Reduce infection-related & overall mortality when pt. are treated expeditiously with antibiotics to which organism is susceptible, not antibiotics which there is possible resistance against.

Question 4

List the methods used for the identification of bacteria in the clinical microbiology laboratory.

Answer 4

1) Microscopy (w/ differential stains - Gram/acid-fast stain)
2) Culture (w/ agar / broth / differential media)
3) Biochemistry (presence of specific enzymes and/or nutrients)
4) Serologic / Immunologic diagnostics
5) Molecular / Nucleic acid-based diagnostics (PCR)
6) Mass spectroscopy (MALDI-TOF)

Question 5

How long does it take to obtain the results of gram staining?

Answer 5

24 hours

Question 6

Describe the morphology & gram stain of staphylococcus aureus.

Answer 6

Gram-positive cocci in clusters

Question 7

Cocci are observed to be in chains under a microscope that remains purple after Gram staining. What is its likely identity?

Answer 7

Streptococcus spp.

Question 8

Diplococcus are observed under a microscope that remains purple after Gram staining. What is its likely identity?

Answer 8

Streptococcus pneumoniae

Question 9

Describe the morphology & gram stain of Escherichia coli.

Answer 9

Gram-negative rods

Question 10

Briefly describe the methods used to determine the minimum inhibitory concentration (MIC) of antibiotics.

Answer 10

1) Agar / broth dilution method
- Quantitative
- Increasing concentrations of antimicrobial are added to each test tube containing the culture broth & microbe
- Lowest concentration of antimicrobial used that prevents visible growth of microbes = MIC
2) Kirby-Bauer disk diffusion method
- Qualitative
- Filter paper discs containing set concentrations of antimicrobials –> antimicrobials diffuse outwards
- Zone (diameter) of inhibition corresponds with antimicrobial activity
3) E-test
- Agar-based testing methods w/ graduated concentrations layered on plastic strip
- MIC = growth intersects w/ plastic strip
- Use higher value if MIC does not lie on distinct markings

Question 11

Define ‘breakpoints’.

Answer 11

Critical concentrations predicting susceptibility & resistance to predict therapeutic response.
- MIC & diameter of zones of inhibition are often used as breakpoints.

Question 12

MICs of different Abx against a particular microorganism are directly comparable. True or false?

Answer 12

False.

Different Abx has different MW & thus different concentrations are required to meet MIC of microbes.

Question 13

Briefly explain what ‘susceptible’, ‘intermediate’ & ‘resistant’ means on an AST report.

Answer 13

1) Susceptible (S):
- implies infection due to isolate may be appropriately treated w/ recommended dosage of Abx
- likely therapeutic success
2) Intermediate (I):
- implies infection due to isolate may be appropriately treated with a high dose of Abx / physically concentrated in specific body sites
- uncertain response
- indicates a buffer zone that should prevent small, uncontrolled technical factors from causing major discrepancies in interpretations
3) Resistant (R):
- implies isolates are not inhibited by usually achievable concentrations of Abx w/ normal dosage schedules
- likely therapeutic failure

Question 14

The Abx with the lowest MIC reported on an AST report is considered to be the best treatment option recommended. True or false?

Answer 14

False.

MICs are only in vitro estimate of antimicrobial activity, thus may not necessarily lead to therapeutic success in vivo.

Question 15

Briefly explain the factors affecting in vivo activity of Abx.

Answer 15

1) Patient’s immune system
2) Protein binding of Abx affecting available free Abx circulating
3) Ability of Abx to reach site of infection
4) Drug-drug interactions
5) Ability to drain / remove infected foci
6) Expression of in vivo only enzymes that inhibit Abx activity

Question 16

What are ‘antibiograms’?

Answer 16

Cumulative susceptibility results that tabulates prevailing antimicrobial susceptibility of common bacterial isolates in hospitals / institutions.

• Results are institution-dependent
• Abx w/ susceptibility of < 50% are not used.

Question 17

When are antibiograms used?

Answer 17

• To assess local susceptibility rates of common microorganisms
• Monitor resistance trend over time
• Guide selection of empiric Abx Tx when AST & cultures results are not available

Question 18

When empirically treating an infection of a patient who recently travelled overseas, we can use local antibiograms to guide Tx selection even if we suspect he was infected overseas. True or false?

Answer 18

False.

Use appropriate antibiograms based on likely location of infection, since antibiograms are institution-dependent!

Question 19

Which class(es) of antibiotic can be used to treat Pseudomonas aeruginosa via PO administration?

Answer 19

Fluoroquinolones

Question 20

Which classes of antibiotics should be AVOIDED when treating infections caused by beta-hemolytic streptococcus infections?

Answer 20

1) Fluoroquinolones
2) Tetracyclines
3) Cotrimoxazole

Question 21

Which classes of antibiotics are generally not used as monotherapy in Singapore due to prevailing high rates of Abx resistance?

Answer 21

Macrolides & tetracyclines

Question 22

Which classes of antibiotics are NOT active against Enterococcus spp.?

Answer 22

1) Cephalosporins
2) Cotrimoxazole
3) Clindamycin
4) Ertapenem

Question 23

Which gram-negative microorganisms are commonly referred to as “non-fermenters”?

Answer 23

1) Pseudomonas aeruginosa

2) Acinetobacter baumannii