Sensitivity tests are carried out in vitro and not many in vivo factors into account (eg. As pharmacodynamic and pharmacokinetic factors, location based substance concentrations, host immune status, local host defense) that influence the success of therapy. For these reasons, the results of susceptibility testing do not always allow a statement about the success of therapy.

Sensitivity tests determine the sensitivity of an infectious agent to antimicrobial substances by standardized pathogen concentrations specified concentrations of antimicrobials are exposed. Sensitivity tests can be performed on bacteria, fungi and viruses. In some organisms, test results can be transferred from one antibiotic to another, which is why not all potentially useful substances are tested. Sensitivity tests are carried out in vitro and not many in vivo factors into account (eg. As pharmacodynamic and pharmacokinetic factors, location based substance concentrations, host immune status, local host defense) that influence the success of therapy. For these reasons, the results of susceptibility testing do not always allow a statement about the success of therapy. Susceptibility testing can qualitatively, semi-quantitatively or be performed by using molecular biology methods. The effect of antibiotic combinations can be determined by means of synergy testing. Qualitative methods Qualitative methods are not as accurate as semiquantitative. The results are typically reported as one of the following: Susceptible (S) Zwischenstuflich (I) Resistant (R) Some strains in which no resistance criteria are to be found, can only be listed as vulnerable or sensitive. The division which specific drug concentration with S, I and R is known, depends on several factors, including pharmacokinetic, pharmacodynamic, clinical and microbiological data. The widespread agar diffusion method (also known as Kirby-Bauer test known) can be used for fast-growing microorganisms. Here antibiotic-impregnated wafers are placed on agar media inoculated with the tested pathogens. After incubation (typically 16-18 h) the diameter of the zone of inhibition is measured for each wafer. Each pathogen-antibiotic combination has different diameters for S, I or R. Other methods that require a less stringent observance of the test parameters can be applied to the rapid screening of resistance of a pathogen in terms of a single substance, substance class or a specific combination of antibiotics to allow (z. B. oxacillin resistance of methicillin-resistant Staphylococcus aureus, ?-lactamase-formers). Semi-quantitative methods Semi-quantitative methods to determine the minimum concentration of a substance that is required to inhibit the growth of a particular pathogen in vitro. This minimal inhibitory concentration (MIC) is given as a numerical value, which may then be in one of four categories S (sensitive), I (intermediate), R (resistant) or sometimes not sensitive stock. The determination of the MIC is used in particular in bacterial isolates (including mycobacteria and anaerobic) and sometimes for fungi, particularly Candida sp. The minimum bactericidal concentration (MBC) can also be determined, but this is technically demanding and the interpretation of standards has not yet been approved. The benefits of the MBC tests is that it indicates whether a drug may be bacteriostatic or bactericidal. The antibiotic can be dissolved in agar or broth, which are then inoculated with the pathogen. The Bouillondilutionsverfahren is the gold standard, but labor intensive, because only one antibiotic concentration per tube can be tested. A more efficient method uses a polyester strip which is impregnated over the whole length with a concentration gradient. The strip is placed on an agar plate containing the inoculum, and the MIC is after the incubation period determined by the location on the strip, from which the inhibition begins; more antibiotics can be tested on a plate. The MIC allows the correlation between the antibiotic sensitivity of the pathogen and the achievable tissue concentration of the free substance (i. E. Not protein bound substance). Successful therapy is likely if the tissue concentration of free substance is higher than the MIC. The designations S, I and R from the MIC study normally correlate with the achievable concentrations of the free substance in the serum, plasma or urine. Nucleic acid-based methods These methods include molecular biological methods similar to those for pathogen identification, but are modified to detect known resistance genes or mutations. An example is mecA, a gene for Oxacillinresistenz S. aureus; if the gene is present, the pathogen is considered independent of the phenotypic resistance results to be resistant to most beta-lactam antibiotics. Although a number of such genes is known, their presence does not automatically mean in vivo resistance. Also, because new mutations or other resistance genes may be present, their absence does not guarantee antibiotic sensitivity. For these reasons, remain routine, phenotypic susceptibility testing methods, the standard approach to evaluate the susceptibility of bacteria and fungi for antimicrobial drugs. However, DNA purification for rapid diagnosis of multidrug-resistant TB is preferred in high-risk groups and for the rapid detection of possible resistance in organisms that are obtained directly from positive blood cultures.

Health Life Media Team

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