Rapid ID and Blood Cultures

Question 1

What does MALDI-TOF stand for?

Answer 1

Matrix assisted laser desorption ionization- time of flight

Question 2

What is the basic principle of MALDI-TOF? what is it using as a source of ID?

Answer 2

Protein biomarkers measured are highly expressed proteins responsible for housekeeping functions, such as ribosomal (16S) and transcription/translation factor proteins

Question 3

What is the basic procedure for MALDI?

Answer 3

1. Sample is mixed with excess matrix and dried on a MALDI target
2. Laser ionizes matrix molecules
3. Sample molecules are ionized by proton transfer from matrix:
   MH+ + A –> M + AH+.

Question 4

What is the “time of flight”

Answer 4

length of time it takes for the charged particle of protein to travel through the vacuum tube to the detector (with smaller ones travelling faster)

Question 5

What can MALDI be used to ID?

Answer 5

Gram positives and negatives, mycobacteria, yeast, filamentous fungi…

Question 6

What is done with the MALDI reading after it is obtained?

Answer 6

Compared to database of spectra for known species

Question 7

What is bacteremia ?

Answer 7

Presence of bacteria in the blood

Question 8

What two events usually are the cause of bacteremia?

Answer 8

1. Bacteria in the primary infection draining into the vascular system via the lymphatics
2. Direct entry of bacteria into the blood by needles, contaminated intravascular devices/graft material or trauma

Question 9

What are the 3 patterns/forms of bacteremia?

Answer 9

1. Transient
2. Intermittent
3. Continuous

Question 10

What are the most common primary foci for the bacteremia to have originated from?

Answer 10

intravascular devices, respiratory tract, urinary tract, and intra-abdominal infections

Question 11

What constitutes a transient bacteremia?

Answer 11

Lasting minutes to hours and usually resulting from physical damage to mucosal membranes (like tooth brushing)

Can be the onset of acute bacterial infections such as pneumonia, meningitis and septic arthritis.

Question 12

What is the usual cause of Intermittent bacteremia?

Answer 12

Undrained abscesses

Question 13

What is continuous bacteremia usually a reflection of ?

Answer 13

usually reflects an endovascular infection such as endocarditis or an infected aneurysm

Question 14

What are the 3 most common causes of Bacteremia?

Answer 14

1. Staph aureus
2. E. coli
3. Other coagulase negative staph

Question 15

Common blood isolates that always or nearly always (>90%) represent true infection include…

Answer 15

S. aureus, E. coli and other members of the Enterobacteriaceae, P. aeruginosa, S. pneumoniae, and Candida albicans

Question 16

Which blood isolates rarely (<5%) represent true bacteremia?

Answer 16

Corynebacterium, Bacillus spp., and Propionibacterium acnes

Question 17

Which groups of bacteria are problematic when it comes to diagnosing bacteremia? Why?

Answer 17

the viridans group streptococci, enterococci, and CNS, which represented true bacteremia 38, 78, and 15% of the time, respectively

Question 18

What is the proper protocol for blood collection?

Answer 18

You want two sets taken from different venipuncture sites
- each set contains two bottles (about 10mL each for adults)

Want to discard the first few mL of blood to avoid the skin plug

Make sure to disinfect the skin

Question 19

Why does so much blood need to be taken from each venipuncture site?

Answer 19

For adults, each additional ml of blood increases microbial recovery by up to 3% - To maximize recovery, 20 to 30 ml of blood should be cultured per venipuncture

Question 20

What should the blood to broth dilution be after collection and why?

Answer 20

Diluting blood into broth greater than 1:5 increases microbial recovery, probably by diluting antimicrobial agents and natural inhibitory factors in the blood

Question 21

What percentage of blood cultures are usually contaminated?

Answer 21

3-5%

Question 22

What does commercial blood culture media contain as an anticoagulant?

Answer 22

0.025 to 0.050 % SPS

Question 23

What is the purpose of ion exchange beads in the blood culture media ?

Answer 23

Present in anaerobic culture conditions to draw out antibiotics

Question 24

How long is the typical incubation period for a blood culture?

Answer 24

5 days

Question 25

What temperature does the blood culture need to be maintained at ?

Answer 25

35-37ºC

Question 26

How are positive tests detected in an automated blood culture machine

Answer 26

When bacteria grow they produce CO2 which changes the colour of the membrane on the bottom of the vial.
This is detected by sensors

Question 27

What is the procedure following identification of a positive culture

Answer 27

1. Aseptic aliquot taken to perform a gram stain
   - doctor notified immediately with the gram stain results
2. Blood is plated onto non-selective SBA or CHOC agar
   - would also plate onto MacConkey if you saw a gram neg in the stain

Question 28

What is the most common anaerobe causing bacteremia?

Answer 28

Bacteriodes spp.

- mainly B. fragilis