Anti-Microbial Susceptibility Testing Flashcards
What three groups of bacteria have unique problems in terms of correlating in-vitro susceptibility results with clinical outcomes?
Streptococcus pneumoniae, Methicillin-resistant Staphylococcus, Gram-negative rods with evidence of unusual resistance mechanisms like extended spectrum beta-lactamases
How high does the maximum antibiotic level at the site of the infection have to be for the organism to be considered susceptible?
Twice the MIC (minimum inhibitory concentration) or more. If it is between the MIC and 2x the MIC it is considered intermediate or indeterminate
In what type of infections are urine antibiotic levels compared to the MIC instead of blood antibiotic levels, and how do urine levels usually compare to blood levels?
In bladder only infections (in kidney and blader infections, still use blood). Urine antibiotic concentrations usually higher than blood concentrations
Three body sites where antibiotic concentrations can be below the blood level
Large abscesses in soft tissues like the liver, bone, and the central nervous system
What medium is used for the Bauer Kirby Disk Diffusion Test?
Mueller Hinton agar
What is the Bauer Kirby Disk Diffusion Test used for?
It is not quite as accurate as the reference method, but used as a lower-tech backup for susceptibility testing
Two modern susceptibility tests commonly used in the US
Microtiter plate methods and Vitek method
What is the advantage of the Vitek method of susceptbility testing over the Microtiter plate and Bauer Kirby methods
Vitek is much quicker (6 hours vs 24 hours)
To what drugs do extended spectrum beta lactamases (ESBLs) confer resistance?
Penicillins, cephalosporins, and aztreonam (OFF: ESBLs are plasmid encoded)
What organisms most commonly have extended spectrum beta-lactamse (ESBL)?
Klebsiella pneumoniae, Klebsiella oxytocia, Escherichia coli, and sometimes other enterobacteriaceae
When a given condition is met in susceptibility testing, it is considered resistant to all penicillins, cephalosporins, and aztreonam regardless of other results. What is this condition?
Resistant to 4 uG/mL of cefpodoxime (in this case it is assumed to be an ESBL producing strain)
Two particular resistances that are difficult to detect with in-vitro susceptibility testing
Extended-spectrum Beta Lactamase and Inducible Beta Lactamase production. Special methods exist to check for these, findings preempt in-vitro findings
To what drugs does Inducible Beta Lactamase provide resistance and what organisms is this found in?
Penicillins and Cephalosporins, found in certain Gram-negative rods (citrobacter, enterobacter, aeromonas, pseudomonas, others)
In-vitro susceptibility testing can often miss inducible beta lactamase production. What is done to detect this enzyme?
Nothing presently. Fancy research labs can do it, but clinically we cant currently detect inducible beta lactamase in standard tests
What is Klebsiella pneumoniae carbapenemase and what is its clinical significance?
Confers resistance to certain drugs, found in Klebsiella and other enterobacteriaceae. Difficult to test for, suspected when Ertapenem resistance is found. Confimed with Modified Hodge test or by PCR
