Antibiogram
An antibiogram is the result of a laboratory test to determine the sensitivity or resistance of microbial pathogens to antibiotics . The test procedure checks whether a bacterium is inhibited in growth by an antibiotic in a certain concentration , then it is sensitive ( sensitive , sometimes also referred to as sensitive ) to this or whether there is no growth inhibition, then it is resistant (resistant) to the active ingredient. In medical jargon , this microbiological test procedure is also referred to as sensitivity testing or resistance testing. The term Antimicrobial Susceptibility Testing (AST) is common in English-speaking countries .
Medical importance
An antibiogram is recommended before any antibiotic therapy. In the case of infections that require therapy, treatment should be carried out in parallel with the creation of an antibiogram in order not to delay the necessary antibiotic therapy . As long as the pathogen has not been identified, a calculated therapy based on empirical values is started. After identification and the result of the antibiogram, targeted therapy can then be initiated if necessary. The antibiogram is helpful because no antibiotic is effective against all possible pathogens and the risk of multi- resistance in clinically relevant bacteria is a serious problem.
Agar diffusion test
In the agar diffusion test - often using the Kirby Bauer method - the microorganism to be examined is distributed in small quantities on the surface of a special, flat nutrient medium and the nutrient medium is then covered with circular plates made of absorbent material that contain various antibiotics in a defined amount. The antibiotics dissolve in the gel culture medium and diffuse radially outwards. The further the antibiotic diffuses away from the platelet, the lower its concentration becomes. At a certain distance from the platelet, the concentration of the antibiotic is so low that the microorganism can grow despite the presence of the antibiotic. After an incubation time that is sufficient for the microorganisms to multiply until the nutrient medium is visibly fouled (usually 16 to 20 hours for pathogenic bacteria), the radii of the zones of inhibition are measured and evaluated with regard to the resistance or sensitivity of the microorganism.
This test procedure is also known as the plate diffusion test, since it is carried out in a plate, more precisely in a Petri dish . For antibiograms, which are produced on a large scale in laboratories of medical microbiology, technical aids are available, for example stamps with which several different antibiotic discs can be applied to the nutrient medium at the same time.
Dilution method for determining the MIC or MBK value
The minimum inhibitory concentration (MIC) is the lowest concentration of an inhibitor at which the reproduction of the microorganism is still prevented. In contrast to the minimum inhibitory concentration (MIC), the minimum bactericidal concentration (MBK) is the concentration that is sufficient to kill 99.9% of the individuals of the microorganism tested. These sizes cannot be determined so precisely by the agar diffusion test described; a dilution test is used for this .
Microdilution method
With the microdilution method (Latin: diluere , "to dilute"), the microorganism to be examined is suspended in a liquid nutrient medium ( broth ). A defined amount of an antibiotic is added to this. If this is done with different concentrations of the antibiotic, e.g. B. with a dilution series (1, 2, 4, 8, 16, 32 mg / L), an MIC determination can be carried out, which is more precise than the determination of the sensitivity with the agar diffusion test. The multiplication of the microorganism can be recognized by the turbidity of the nutrient medium: If it remains clear, the examined microorganism is sensitive to this antibiotic in this concentration. The lowest concentration (highest dilution) at which inhibition occurs provides the MIC value. The microdilution method is used when the process is carried out with small volumes, e.g. B. is carried out in a microtiter plate . If larger volumes are used, such as in culture tubes, one speaks more generally of the bouillon dilution test.
Automated resistance determinations
There are various devices that carry out an antibiotic resistance determination. These devices work according to the microdilution method with microtiter plates. The reading of the measurement results is also taken over by the devices (plate readers) with the help of photometric measurement methods. The software belonging to the devices evaluates the results and checks them for plausibility . Examples of such automated laboratory devices are:
- VITEK from bioMérieux
- Phoenix by Becton Dickinson
evaluation
Evaluation is based on selected standards, for example
Further test procedures
A special case of an antibiogram is the epsilometer test (E-test), which can also be used to determine the minimum inhibitory concentration, although the method is based on the agar diffusion test.
When researching antibiotic active ingredients, a test can also be carried out to determine their spectrum of effects on microorganisms. For this purpose, the antibiotic is applied to the middle of the surface of the nutrient medium, for example on a filter plate, and various indicator organisms are spread radially to it. After incubation , an inhibition can be recognized by the fact that the vaccination line is not completely covered by the antibiotic. The antibiotic therefore acts on the associated indicator organism, which is sensitive.
Web links
- Entry on antibiogram in the Flexikon , a Wiki of the DocCheck company
- Antibiogram yeasts. In: Laborlexikon . Retrieved March 12, 2018 .
Individual evidence
- ↑ a b c Michael T. Madigan, John M. Martinko, Jack Parker: Brock Mikrobiologie. German translation edited by Werner Goebel, 1st edition. Spektrum Akademischer Verlag GmbH, Heidelberg / Berlin 2000, ISBN 3-8274-0566-1 , pp. 966-968.
- ^ A b c Herbert Hof, Rüdiger Dörries: Dual series: Medical microbiology . 3. Edition. Thieme Verlag, Stuttgart 2005, ISBN 978-3-13-125313-2 , p. 11-12, 289-296 .
- ^ Marianne Abele-Horn: Antimicrobial Therapy. Decision support for the treatment and prophylaxis of infectious diseases. With the collaboration of Werner Heinz, Hartwig Klinker, Johann Schurz and August Stich, 2nd, revised and expanded edition. Peter Wiehl, Marburg 2009, ISBN 978-3-927219-14-4 , pp. 260-267 ( antimicrobial therapy of specific pathogens according to an antibiogram ).
- ^ A b Hans G. Schlegel, Christiane Zaborosch: General microbiology . 7th edition. Thieme Verlag, Stuttgart / New York 1992, ISBN 3-13-444607-3 , p. 364-367 .