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Perhaps the most efficient means for detecting methicillin-resistant staphylococci in clinical laboratories is the use of the agar dilution screening test. Illustrated in the image is a Mueller-Hinton agar plate containing 6 ug/mL of oxicillin, previously inoculated with a strain of Staphylococcus aureus. Oxacillin is used as a marker for methicillin resistance because it is more stable in the agar medium. Growth on this screening medium is presumptive for methicillin resistance. Thus, in the presence of growth, as shown here, a follow-up minimum inhibitory concentration (MIC) test must be performed to determine the exact level of resistance.

CultureBacterial culture, utilizing selective/differential media, is an effective method for recovering Clostridium difficile. Its drawbacks are the length of time required (up to four days), as well as the inability to distinguish toxigenic strains from non-toxigenic strains. Positive cultures require follow-up testing for the ability to produce toxin.Cell Cytotoxicity Neutralization Assay (CCNA) This assay detects the presence of C. difficile toxin in fecal samples. A filtrate of stool sample is prepared and inoculated onto sensitive tissue culture cells. Typically human fibroblast cells are utilized; if toxin is present in the filtrate, it causes the fibroblasts to round up in a characteristic cytopathic effect. To verify that the cytopathic effect is caused by C. difficile toxin (and not some other toxic component or viral agent), the filtrate is also inoculated in parallel onto a second set of tissue culture cells, to which C. difficile specific anti-toxin has been added. Absence of cytopathic effect in the second set of cell cultures provides evidence that the cellular changes in the first set were caused by C. difficile toxin. Although CCNA is considered a gold standard for the detection of C. difficile toxin, it is labor intensive, requires the use of cell cultures, and requires at least 48 hours of incubation.

Stool culture is very effective in detecting C. difficile. Unfortunately, non-toxigenic strains will also grow, requiring strains to be tested for toxin production. The greatest disadvantage to culture is the length of time that is needed before results are available, which may be up to four days. However, antibiotic sensitivity testing following culture is useful for strain-typing that would provide necessary epidemiological information during nosocomial outbreaks.Colonies of C. difficile will appear white, flat, and spreading on blood agar (see top image on the right). Cycloserin- cefoxitin-fructose agar(CCFA) is a selective media that is used for isolation of C. difficile. There is however, no distinction between pathogenic and commensal strains, which all produce yellow colonies with a characteristic "ground glass" appearance. as shown in the bottom image on the right. The characteristic odor of "horse manure" aids in identification of C. difficile. Stool samples are directly inoculated onto CCFA and incubated in an anaerobic atmosphere at 37°C for 48 hours. Large, thin, gram-positive bacilli with spores will be observed on a Gram stain of a typical colony, as shown below.

Many hospitals have chosen to implement screening programs to identify patients who are colonized with vancomycin resistant enterococci (VRE). Screening patients provides information about potential source of illness and allows staff to implement appropriate infection control measures. These measures decrease transimission and reduce the number of patients who become infected with VRE.Peri-rectal or anal swabs, as well as stool specimens, are inoculated onto selective media. One medium utilized is bile esculin azide agar containing 6µg/ml of vancomycin. Black colonies that grow on this medium are identified as enterococcus to the species level and further confirmed as vancomycin-resistant by an appropriate susceptibility testing method.Chromogenic agars specific for VRE are also commercially available.