Diffusion Information and Courses from MediaLab, Inc.
These are the MediaLab courses that cover Diffusion and links to relevant pages within the course.
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| The disk diffusion method of measuring antimicrobial sensitivity is also termed: | View Page |
| Which of the following media is commonly used when performing the Kirby-Bauer disk diffusion | View Page |
| The turbidity of the bacterial inoculum of a standard disc diffusion (Kirby-Bauer) susceptibility test is: | View Page |
| ESBL Activity Illustrated is the picture of the surface of a disk diffusion test including a ceftazidime disk (left) and a combintation ceftazidime/clavulanic acid disk (right).Observe in the photograph that the zone of inhibition around the the combination ceftazidime/clavulanic acid disk (right) is at least 5 mm larger than around the clavulanic acid disk (left).This observation that the presence of clavulanic acid, a beta-lactamase inhibitor, has resulted in such a large increase in the zone of inhibition indicates that an extended spectrum beta lactamase (ESBL)is being produced.When an organism is producing an ESBL, the susceptibility to individual cephalosporins cannot be predicted, thus requiring that each drug must be tested individually.It may be important to detect ESBL-producing stains of K. pneumoniae and E. coli as treatment failure may occur if the wrong cephalosporin is selected. | View Page |
| Vancomycin Resistance Vancomycin and ampicillin resistance among Enterococcus species, particularly E. faecium have been on a steady increase.The disk diffusion screening test is used in many laboratories to detect vancomycin resistant strains. Note in the upper photograph that no zone of inhibition is seen around either the vancomycin or the ampicillin disk, indicating resistance to both drugs.Vancomycin-resistant Enterococci (VRE) have been divided into three phenotypes--Van A, Van B, and Van C.Vancomycin-resistant strains of E. faecalis and E. faecium are commonly of the Van A phenotype, demonstrating high level resistance (MIC's higher than 64 ug/mL), as illustrated by total resistance of the test strain in the E test and the VA disk, as illustrated in the lower photograph.The strain shown in the lower photograph, however, is ampicillin susceptible at the level of 1 ug/ml (see lower set of yellow arrows), indicating that this drug may be effective in treating the urinary tract infection. | View Page |
| MRSA Disk Test The disk diffusion test can also be used in the detection of methicillin-resistant Staphylococcus aureus.Illustrated in the photograph is the surface of a Mueller-Hinton agar plate previously inoculated with a strain of S. aureus suspected of being methicillin-resistant. Although the zone of inhibition is at the borderline for resistance (18 mm); the presence of small colonies within the zone of inhibition (yellow arrows) indicates the presence of heteroresistant strains.The interpretation here, therefore, is "methicillin-resistant" staphylococci, even though the zone diameter appears to be adequate. The detection of the heteroresistant strains indicates that MIC studies are required. | View Page |