Antimicrobial Assay media

While this approach has limitations related to the peptidic nature of the linker such as stability to harsh reaction conditions and requirement of specific functional groups to be coupled with the linker arms, its application to particular libraries and chemistries may be useful. Its biological utility was assessed in a bead-based antimicrobial assay on bacterial cells [54], which produced good correlations between the MIC (minimal inhibitory concentration) values for the compounds released in situ in the culture medium from the beads or tested as standard solutions. 

For antimicrobial assays, there are several common methods employed. Due to its ease of operation, the most common method used is the disk diffusion method, which involves the application of a material onto a filter paper disk, and then the disk is placed onto solid medium previously seeded with the test microorganism of interest. Sometimes, the sample is dissolved in an appropriate solvent before application onto the paper disk. This method is very common in the evaluation of antibiotics and is the method adopted by the National Committee for Clinical Laboratory Standards (NCCLS). The method depends on the aqueous solubility of the antibiotics in order to facilitate diffusion through the solid medium. Essentials oils, however, are generally hydrophobic, do not readily diffuse through an aqueous medium and, therefore, the prevalence of false negatives or reduced activity might then be anticipated. 

In this form of assay, the extract or pure compound is run on a TLC plate, which IS then covered by a medium seeded with the appropriate microorganism. As with direct bioautographic assays, both fungi and bacteria may be investigated. Rahalison et al. (54) have applied this technique for the evaluation of antimicrobial extracts against the yeast Candida albicans and the bacterium Bacillus subtilis. 

From the beginning, laboratory studies on the antibacterial properties of cefaclor pointed to an inherent chemical instability in solution and in neutral to alkaline bacteriological media. Test medium composition, pH, and inoculum density can significantly affect in vitro susceptibility assay results. For example, in both Mueller-Hinton broth and agar, chemical degradation of cefaclor is observed as a rapid loss of antimicrobial activity (50%) after 6 hr at 37°C (Preston, 1979). Special care is, therefore, recommended in the interpretation of in vitro test results. 

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