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What is MIC and how do you test for it for an organism?
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MIC = Minimum Inhibitory Concentration - what is the min. amount of antibiotic applied to stop growth of an organism. You produce many different cultures and range the antibiotics. The first plate that shows NO growth is the MIC.
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What are a few organisms that are difficult to get Susceptibility Test results?
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MRSA. Beta-Lactamase producing bacteria. Any fastidious (tough to grow) organisms.
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What is the advisable ratio of the Amount of Antibiotic available at the body site of infection TO the MIC of the organism?
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4:1. You should plan on having 4x the amount of drug bioavailable at the body site of infection than the MIC calls for.
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After susceptibility testing, the drug availability blood level is below the MIC - what is your conclusion? What if the 2 values are equal?
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Organism is RESISTANT (blood level below MIC). If equal the susceptibility is INTERMEDIATE.
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What is the method of the Bauer Kirby Disk Diffusion Test?
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Use a special agar(Mueller Hinton agar plate). Streak swabs of the organism across the plate. Get little disks that are impregnated with the antibiotic of choice and place them on the plate. Incubate OVERNIGHT. If there is no growth near the disks, this means the bacteria is susceptible to the organism.
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What is the concept of Breakpoints in either MIC testing or Disk Diffusion testing?
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Breakpoints are the transition from Susceptible to Intermediate to Resistant. Known organisms that are susceptible or resistant are used to gauge values.
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What are the 2 problems with BK diffusion testing?
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Overnight Incubation necessary. ONly 92% accurate.
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If a ESBL (Extended Spectrum beta-lactamase) organism is shown to be susceptible to a Cephalosporin, what should you do?
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Ignore the Test. Assume that ESBL Enterobacteriaciae are RESISTANT to ALL Cephalosporins, Peniciliins..
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After screening for an ESBL, how do you confirm that the organism is an ESBL?
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Grow the organism on a plate with 2 antibiotic disks present - 1 with a Beta-Lactamase Inhibitor and one without it - if the organism grows near the inhibitor and doesn't without it - confirmed ESBL producing organism.
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What is an Inducible Beta-Lactamase organism and how can you detect it?
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Inducible means it is a slowly-producing Beta-lactamase that will slowly confer resistance to a drug. It is MISSED easily in susceptibility testing. There is NO good way to find these.
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What are 2 orgnisms (1organism, 1 class) that show inducible beta-lactamases?
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Klebsiella and Enterobacteriaciae family.
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What 1 gene on the inducible beta-lactamase just became testable in labs?
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AmpC beta-lactamase.
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What enzyme confers resistance to the Carbapenem antibiotic family?
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KPC - Klebsiella Pneumoniae Carbapenemase
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If you place Klebsiella pneumoniae on an agar plate with the 3 Carbapenem drugs, what would you see it is a Klebsiella Pneumoniae Carbapenemase?
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YOu would see a mixture of resistance, intermediate, and susceptible to the 3 drugs.
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What is heteroresistance, and what strain and organism can exhibit this?
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Heteroresistance is when only some colonies in a culture of infection show resistance. This is seen with MRSA - only some colonies carry the Mec gene that causes abnormal PBPs and confers resistance.
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