Rapid ID and Blood Cultures Flashcards
What does MALDI-TOF stand for?
Matrix assisted laser desorption ionization- time of flight
What is the basic principle of MALDI-TOF? what is it using as a source of ID?
Protein biomarkers measured are highly expressed proteins responsible for housekeeping functions, such as ribosomal (16S) and transcription/translation factor proteins
What is the basic procedure for MALDI?
- Sample is mixed with excess matrix and dried on a MALDI target
- Laser ionizes matrix molecules
- Sample molecules are ionized by proton transfer from matrix:
MH+ + A –> M + AH+.
What is the “time of flight”
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)
What can MALDI be used to ID?
Gram positives and negatives, mycobacteria, yeast, filamentous fungi…
What is done with the MALDI reading after it is obtained?
Compared to database of spectra for known species
What is bacteremia ?
Presence of bacteria in the blood
What two events usually are the cause of bacteremia?
- Bacteria in the primary infection draining into the vascular system via the lymphatics
- Direct entry of bacteria into the blood by needles, contaminated intravascular devices/graft material or trauma
What are the 3 patterns/forms of bacteremia?
- Transient
- Intermittent
- Continuous
What are the most common primary foci for the bacteremia to have originated from?
intravascular devices, respiratory tract, urinary tract, and intra-abdominal infections
What constitutes a transient bacteremia?
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.
What is the usual cause of Intermittent bacteremia?
Undrained abscesses
What is continuous bacteremia usually a reflection of ?
usually reflects an endovascular infection such as endocarditis or an infected aneurysm
What are the 3 most common causes of Bacteremia?
- Staph aureus
- E. coli
- Other coagulase negative staph
Common blood isolates that always or nearly always (>90%) represent true infection include…
S. aureus, E. coli and other members of the Enterobacteriaceae, P. aeruginosa, S. pneumoniae, and Candida albicans
Which blood isolates rarely (<5%) represent true bacteremia?
Corynebacterium, Bacillus spp., and Propionibacterium acnes
Which groups of bacteria are problematic when it comes to diagnosing bacteremia? Why?
the viridans group streptococci, enterococci, and CNS, which represented true bacteremia 38, 78, and 15% of the time, respectively
What is the proper protocol for blood collection?
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
Why does so much blood need to be taken from each venipuncture site?
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
What should the blood to broth dilution be after collection and why?
Diluting blood into broth greater than 1:5 increases microbial recovery, probably by diluting antimicrobial agents and natural inhibitory factors in the blood
What percentage of blood cultures are usually contaminated?
3-5%
What does commercial blood culture media contain as an anticoagulant?
0.025 to 0.050 % SPS
What is the purpose of ion exchange beads in the blood culture media ?
Present in anaerobic culture conditions to draw out antibiotics
How long is the typical incubation period for a blood culture?
5 days
What temperature does the blood culture need to be maintained at ?
35-37ºC
How are positive tests detected in an automated blood culture machine
When bacteria grow they produce CO2 which changes the colour of the membrane on the bottom of the vial.
This is detected by sensors
What is the procedure following identification of a positive culture
- Aseptic aliquot taken to perform a gram stain
- doctor notified immediately with the gram stain results - Blood is plated onto non-selective SBA or CHOC agar
- would also plate onto MacConkey if you saw a gram neg in the stain
What is the most common anaerobe causing bacteremia?
Bacteriodes spp.
- mainly B. fragilis
