Clinical Microbiology

Question 1

Microscopy is the preliminary method of bacterial identification - why?

Answer 1

Quick
Cheap
Easy

Question 2

Disadvantages of microscopy?

Answer 2

Little information
Impossible to distinguish pathogens from commensals
No info about antibiotic susceptibility

E.g. E. coli looks same as salmonella (gram -ve rods), and S. aureus looks same as S. epidermidis

Question 3

Colour - gram positive?

Answer 3

Purple

Question 4

Colour - gram negative?

Answer 4

Pink

Question 5

Are spiral organisms visible by gram stain or not?

Answer 5

No

E.g. mycobacteria, mycoplasma, chlamydia, coxiella, rickettsia

Question 6

How to stain mycomacteria?

Answer 6

Ziehl-Nielsen

Question 7

How to stain mycoplasma and chlamydia?

Answer 7

Trick question - don’t stain at all

Question 8

Why is culture useful?

Answer 8

Allows identification and sensitivity testing of vast majority of bacteria

Question 9

What are disadvantages of culture?

Answer 9

Slow - 24h minimum to grow bacteria

Hampered by prior antibiotic therapy

Some pathogens are difficult/impossible/dangerous to culture

Question 10

What is the coagulase test?

Answer 10

Test for staphylococcal identification

Detects production of coagulase by demonstrating conversion of fibrinogen to fibrin

Distinguishes S. aureus from other less virulent staphylococci

Question 11

Benefits of coagulase test?

Answer 11

Quick and simple

Question 12

How does the coagulase test work?

Answer 12

Loop of organism (i.e. some colonies) placed in test tube containing rabbit plasma

Tube incubated for 4h at 37 degrees

If clot - indicates coagulase production by organism - i.e. it is s. aureus

If no clot - not s. aureus

Question 13

What are coagulase +ve organisms?

Answer 13

S. aureus - highly virulent, variety of infections

Question 14

What are coagulase -ve organisms?

Answer 14

Several staphylococci - e.g. s. epidermidis (most common), s. haemolyticus

Includes skin commensal flora, blood culture contaminants, low virulence opportunistic pathogens, and prosthetic infection

Question 15

What are prosthetics?

Answer 15

Grafts
Long lines
Shunts
Pacemakers
Valves
Joints

Question 16

Disadvantage of biochemical idnetiifcation?

Answer 16

Takes 24h

Question 17

What does MALDI-TOF-MS stand for?

Answer 17

Matrix Assisted Laser Desorption Ionisation - Time of Flight - Mass Spectroscopy

Question 18

What is the key benefit of MALDI-TOF-MS?

Answer 18

Identification within seconds

Question 19

What are the principles of MALDI-TOF-MS?

Answer 19

Microorganisms loaded onto matrix

Laser hits loaded matrix and releases charged particles, which are accelerated across an electric field towards a detector.

Separation occurs by mass/charge ratio - computer generates spectral identity and matches this with a database, hence identifying the microorganism.

Question 20

What are non-cultural methods of identification?

Answer 20

Serology - antibody or antigen detection

Molecular biological techniques - PCR

Question 21

Staphylococci slide?

Answer 21

Arranged in clusters
Gram positive
Cocci

Question 22

What are the gram positive cocci?

Answer 22

Staphylococci - clusters
Streptococci - chains

Question 23

What are the gram positive bacilli?

Answer 23

Many different genera
Aerobic and anaerobic

Question 24

Gram positive rods can be?

Answer 24

Aerobic or anaerobic
Branched

Question 25

What are the aerobic gram positive rods?

Answer 25

Bacillus Corynebacteria Listeria Lactobacillus

Question 26

What are the anaerobic gram positive rods?

Answer 26

Clostridia (genus), includes: - C. tetani - C. botulinum - C. perfringens - C. difficile

Question 27

What is the paralysis in C. tetani and C. botulinum?

Answer 27

Flaccid - botulinum Spastic - tetanus

Question 28

What is pseudomembranous colitis?

Answer 28

Sever einfection with C. difficile Patient at risk of death from toxic megacolon and perforation

Question 29

What are the branched gram positive rods?

Answer 29

Nocardia - gram positive Actinomyces - gram negative