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Unit 2 Chapter 13

Antimicrobial Susceptibilty Testing

QuestionAnswer
Define MIC minimal inhibitory concentration - lowest concentration of antimicrobial agent that inhibits the growth of a bacterium
Define McFarland Turbidity standards used to quantify suspensions of latex particles or barium sulfate prepared at various densities to represent specific numbers of bacteria.
Explain Broth Macrodilution broth dilution MIC tests performed in test tubes are referred to as broth macrodilution MIC or tube dilution MIC tests. Generally, a twofold serial dilution series, each containing 1 to 2 mL of antimicrobial solutions
Kirby Bauer test *principle* thoroughly tested for its composition and its pH level; ensures that zones of inhibitions can be reproduced from the same organism; does not inhibit sulfonamides; ensures standardization and reproducibility.
Kirby Bauer test *reagents* Mueller Hinton agar plates; BBL BD Sensi Disks (various antimicrobials as in QC above); 37 C Incubator; Sterile polyester or cotton swabs; BBL Prompt inoculation system; Calipers, ruler, or template for measuring the diameters of inhibitory zones.
Kirby Bauer test *result reporting* determine the susceptibility/resistance of organism to each drug tested there are different charts for different organisms; for each drug indicate whether the zone size is susceptible (S), intermediate (I), or resistant (R) ; zone sizes are not reported
Kirby Bauer test *additional* culture medium is inoculated with test organism; filter paper discs, are placed on the medium. The organism will grow; the antibiotic inhibits the growth. If the organism is susceptible to a specific antibiotic, there will be no growth around the disc
Etest *principle* quantitative method that applies both the dilution of antibiotic and diffusion of antibiotic into the medium;consists of a predefined, continuous, exponential gradient of antibiotic concentrations immobilized along a rectangular plastic test strip.
Etest *reagents* Etest is supplied in a package of 100 or 30 (some reagents) test strips of one antimicrobial agent
Etest *result reporting* Read at point where ellipse intersects the scale. If a value between two twofold dilutions is seen, always round up; read value at complete inhibition of all growth; If the intersect differs on either side of the strip, read as the greater value.
Etest *additional* quantitative technique for determining antimicrobial susceptibility of Gram -/+ aerobic bacteria (En- terobacteriaceae, Pseudomonas, Staphylococcus and, Enterococcus;, N. gonorrhoeae, S. pneumoniae , Streptococcus and Haemophilus species)
beta-lactamase tests *principle* enzymes that chemically inactivate β-lactam molecules by disrupting the β-lactam ring component of the molecule
beta-lactamase tests *reagents* filter paper disks impregnanted with nitrocefin
beta-lactamase tests *result reporting* Nitrocefin is a cephalosproin, susceptible to most β-lactamases, that turns pink when hydroyzed.
beta-lactamase tests *additional* by competing for sites on transpeptidase β-lactam antibiotics prevent essential cross linking of the peptidoglycan
Define MBC minimal bactericidal concentration - lowest concentration of antimicrobial agent that kills 99.9% of the test bacteria
Define synergism occurs when the antimicrobial activity of a combination of antimicrobial agents is greater than the activity of the individual agents alone
Define antagonism occurs when the antimicrobial activity of combination of antimicrobial agents is less than the activity of the individual agent alone
Define indifference occurs when the antimicrobial activity of a combination of antimicrobial agents is equal to the activity of the individual agents.
Created by: luceroapril