Antibiogram

  An antibiogram is the result of a laboratory testing for the sensitivity of an isolated bacterial strain to different antibiotics. It is by definition an in vitro-sensitivity.

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In clinical practice, antibiotics are most frequently prescribed on the basis of general guidelines and knowledge about sensitivity: e.g. uncomplicated urinary tract infections can be treated with a first generation quinolone, etc. This is because Escherichia coli is the most likely causative pathogen, and it is known to be sensitive to quinolone treatment. Infections that are not acquired in the hospital, are called "community acquired" infections.

However, many bacteria are known to be resistant to several classes of antibiotics, and treatment is not so straight-forward. This is especially the case in vulnerable patients, such as patients in the intensive care unit. When these patients develop a "hospital-acquired" (or "nosocomial") pneumonia, more hardy bacteria like Pseudomonas aeruginosa are potentially involved. Treatment is then generally started on the basis of surveillance data about the local pathogens probably involved. This first treatment, based on statistical information about former patients, and aimed at a large group of potentially involved microbes, is called "empirical treatment".

Before starting this treatment, the physician will collect a sample from a suspected contaminated compartment: a blood sample when bacteria possibly have invaded the bloodstream, a sputum sample, a urine sample,... This samples are transferred to the microbiology lab, which looks at the sample under the microscope, and tries to culture the bacteria. This can help in the diagnosis.

Once a culture is established, there are two possible ways to get an antibiogram:

• a semi-quantitative way based on diffusion (Kirby-Bauer method); small discs containing different antibiotics, or impregnated paper discs, are dropped in different zones of the culture in the petri dish. The antibiotic will diffuse in the area surrounding each tablet, and a disc of bacterial lysis will become visible. Since the concentration of the antibiotic was the highest at the centre, and the lowest at the edge of this zone, the diameter is suggestive for the Minimum Inhibitory Concentration (conversion of the diameter in millimeter to the MIC, in µg/ml, is based on known linear regression curves).
• a quantitative way based on dilution: a dilution series of antibiotics is established (this is a series of reaction vials with progressively lower concentrations of antibiotic substance). The last vial in which no bacteria grow contains the antibiotic at the Minimal Inhibiting Concentration.

Once the MIC is calculated, it can be compared to know values for a given bacterium and antibiotic: e.g. a MIC > 0,06µg/ml may be interpreted as a penicillin-resistant Streptococcus pneumoniae. Such information may be useful to the clinician, who can change the empirical treatment, to a more custom-tailored treatment that is directed only at the causative bacterium.

• Antibiogram technique video (diffusion method)