The antibiogram is the final report that results after carrying out a study of bacterial sensitivity to antibiotics. When a body fluid or tissue is "cultivated" in search of bacteria and one can be isolated, it is subjected to a sensitivity study to determine the antibiotic that would best act against it.
This study is frequently done in medical practice. The information provided by it is of vital importance when treating infectious diseases. The expert staff in carrying out sensitivity tests, cultures and antibiograms is made up of microbiological bioanalysts and the results are interpreted by infectious disease doctors.
The antibiogram report reports the sensitivity or resistance of a bacterium to one or more antibiotics. In case several germs are isolated, an antibiogram is carried out for each one. The final decision regarding the use of one or the other antimicrobial rests solely with the treating physician and should not be based solely on that result.
What is it for?
The antibiogram is a guiding element for doctors when indicating a treatment with antibiotics. The information provided by this study is very useful to initially decide whether or not to order antimicrobial therapy and, if it is decided to do so, it helps to choose the best treatment option.
It is also critical to determine whether or not the antibiotic rotation is prudent. When antibiotic therapy is started empirically, without being certain which bacteria is causing the infection, once the result of the antibiogram is available, it should be considered whether to continue with it or change to a more specific or appropriate one.
Another utility of the antibiogram is quality control and validation of susceptibility. It is frequently used in clinical research work, epidemiological evaluations, and job safety.
Outside the strict medical sphere, cultures and antibiograms of surfaces and inanimate objects reveal the possibilities of local contamination.
Types
The antibiogram is the final report of the result of a culture. As such, it does not have different types, beyond the particular differences in the way of offering the information that each laboratory has.
All will report the type of bacteria isolated, the number of colony-forming units, and the susceptibility to different antibiotics.
The report of antibiotic sensitivity is expressed in three terms: sensitive, intermediate or resistant. It seems like a no-brainer, but based on the response of the antibiotic against the isolated germ, its condition will be assigned:
- Sensitive, when bacterial growth is inhibited in vitro by an amount of the antibiotic that would correspond to a normal dose in humans.
- Intermediate, when bacterial growth is partially inhibited by a concentration of the antibiotic corresponding to a usual dose in humans; or when very high doses are needed to achieve an effective result with a risk of toxicity.
- Resistant, when bacterial growth is not inhibited by a usual concentration of the antibiotic. It is associated with a high percentage of treatment failure.
Some literature available in the microbiological world raises a possible classification of the antibiogram. It is very simple and divides the antibiogram into two large classes: qualitative and quantitative.
Qualitative
It is obtained through diffusion techniques. The qualitative report of the antibiogram provides information on the presence of the isolated germ and information on sensitivity.
Sometimes you can have a preliminary report whose task is only to tell the doctor what bacteria was found in order to start a treatment.
Quantitative
It is obtained through dilution techniques. This type of report not only informs which bacteria was isolated, but also provides the number of colony-forming units; This data is important to determine the aggressiveness of the germ, the concentration of antibiotic to attack it or the possible presence of other germs.
Process
Bacterial cultures are carried out through any method established for it at the request of the specialist doctor. There are many types of cultures and the choice of which one to use will depend on the intended purpose, the type of suspected infection, the characteristics of the sample and the capabilities of the laboratory and the personnel working there.
However, there are basic characteristics that every growing medium must have, among which we have:
- Presence of oxygen for aerobic bacteria.
- Absence of oxygen for anaerobic bacteria.
- Adequate supply of nutrients.
- Sterile medium.
- Ideal temperature.
- Consistency according to the germ sought.
- adequate pH.
- Artificial light.
- Availability of laminar flow hood.
Once the ideal culture medium is available, the sample is sown in it. These samples can be blood, urine, feces, cerebrospinal fluid, exudates or transudates, other body secretions, pus, or pieces of solid tissue.
Reading and analysis
Once the bacteria begin to grow and are identified, they are added to the antibiotic discs to study their action.
The size of the circle formed around the inoculation point is related to the degree of sensitivity of the microorganism: small circles, resistant bacteria; large circles, sensitive bacteria.
Then, specialized teams or trained personnel analyze each halo and report it. This information should be interpreted as part of a whole and not as isolated information.
The clinical picture of the patient, the phenotypic characteristics of the bacterium, the known resistance and the response to treatment are key data when selecting an antibiotic.
The final antibiogram report must be printed or written on paper with all the data obtained. Each antibiotic studied (they are not always the same) must be reported with the above-mentioned classification as sensitive, intermediate or resistant. The minimum inhibitory concentration and the number of colony forming units should be added.
Other antibiograms
Although until now only antibiograms obtained by bacterial cultures have been mentioned, they also exist for fungi. These pathogens require special culture media, but if they can be isolated, sensitivity or resistance to their typical treatments can be determined.
Viruses cannot be incubated in traditional culture media, so embryonated bird eggs, cell cultures or live experimental animals are used. Therefore, it is not possible to perform antibiograms.
References
- Cantón, R. (2010). Interpretive reading of the antibiogram: a clinical necessity. Infectious Diseases and Clinical Microbiology, 28 (6), 375-385.
- Joshi, S. (2010). Hospital antibiogram: a necessity. Indian Journal of Medical Microbiology, 28 (4), 277-280.
- Najafpour, Ghasem (2007). Production of Antibiotics. Biochemical Engineering and Biotechnology, Chapter 11, 263-279.
- Cercenado, Emilia and Saavedra-Lozano, Jesús (2009). The antibiogram. Interpretation of the antibiogram, general concepts. Anales de Pediatría Continuada, 2009; 7: 214-217.
- Tascini, Carlo; Viaggi, Bruno; Sozio, Emanuela and Meini, Simone. Reading and understanding an antibiogram. Italian Journal of Medicine, 10 (4), 289-300.