- How the antibiogram is made
- Uroculture with antibiogram
- How to interpret the result
- Why is it necessary to identify the correct antibiotic?
The antibiogram, also known as Antimicrobial Sensitivity Test (TSA), is an exam that aims to determine the sensitivity and resistance profile of bacteria to antibiotics. Through the result of the antibiogram, the doctor can indicate which antibiotic is most suitable to treat the patient's infection, thus avoiding the use of unnecessary antibiotics that do not fight the infection, in addition to the emergence of resistance.
Normally the antibiogram is performed after the identification of microorganisms in large quantities in the blood, urine, feces and tissues. Thus, according to the identified microorganism and sensitivity profile, the doctor can indicate the most appropriate treatment.
How the antibiogram is made
To perform the antibiogram, the doctor will request the collection of biological material such as blood, urine, saliva, phlegm, feces or cells from the organ contaminated by the bacteria. These samples are then sent to a microbiology laboratory for analysis and cultivation in a culture medium that favors bacterial growth.
After growth, the microorganism is isolated and subjected to identification tests in order to reach the conclusion of the bacteria responsible for the infection. After isolation, the antibiogram is also performed to know the sensitivity and resistance profile of the bacteria, which can be done in two ways:
- Antibiogram by agar diffusion: in this procedure small paper discs containing different antibiotics are placed on a plate with appropriate culture medium for the growth of the bacteria. After 1 to 2 days in the greenhouse, it is possible to observe whether or not you hear bacterial growth around the disc. In the absence of bacterial growth, it is said that the bacteria is sensitive to that antibiotic, being considered the most suitable for the treatment of infection; Antibiogram based on dilution: in this procedure there is a container with several dilutions of antibiotic with different doses, where the bacteria to be analyzed are placed, and the Minimum Inhibitory Concentration (CMI) of the antibiotic is determined. In the container where no bacterial growth was observed, the correct dose of the antibiotic is present.
Currently in the laboratories, the antibiogram is performed by equipment that tests the resistance and sensitivity of bacteria. The report released by the equipment informs which antibiotics the bacterium was resistant to and which were effective in combating the microorganism and in what concentration.
Uroculture with antibiogram
Urinary tract infection is one of the most common infections in women, mainly, and in men. Therefore, it is common for physicians to request in addition to the type 1 urine test, the EAS, and the urine culture accompanied by an antibiogram. In this way, the doctor is able to check if there is any change in the urine that is indicative of kidney problems, through EAS, and the presence of bacteria in the urinary tract that may indicate infection, through urine culture.
If the presence of bacteria in the urine is verified, the antibiogram is performed next so that the doctor can know which antibiotic is most suitable for treatment. However, in the case of urinary infections, antibiotic treatment is only recommended when the person has symptoms to prevent the development of bacterial resistance.
Understand how urine culture is made.
How to interpret the result
The result of the antibiogram can take up to 3 to 5 days and is obtained by analyzing the effect of antibiotics on the growth of bacteria. The antibiotic that inhibits the growth of bacteria is indicated to treat the infection, but if the bacteria grow and the antibiotics have no effect, it indicates that the bacteria is not sensitive to that antibiotic, that is, resistant.
A very common example is the antibiogram performed for urinary infections. The bacterium Escherichia coli (E. coli) is one of the main causes of urinary tract infection, and in most cases the antibiogram identifies that this bacterium is sensitive to antibiotics such as Fosfomycin, Nitrofurantoin, Amoxicillin with Clavulonate, Norfloxacino or Ciprofloxacino, for example. example. However, there are already reports of E. coli resistant to the antibiotics normally used. Thus, it is important that the doctor knows what the result of the antibiogram is so that treatment can begin. Learn how to recognize the signs and symptoms of E. coli infection .
The result of the antibiogram must be interpreted by the doctor, who observes the values of the Minimum Inhibitory Concentration, also called CMI or MIC, and / or the diameter of the inhibition halo, depending on the test that was performed. The IMC corresponds to the minimum concentration of antibiotic that is able to inhibit bacterial growth and complies with the standards of the Clinical and Laboratory Standards Institute , the CLSI, and may vary according to the antibiotic to be tested and the microorganism that has been identified.
According to the CMI values, it is possible to say whether the microorganism is non-susceptible, susceptible, intermediate or resistant to the tested antimicrobial. For example, in the case of E. coli , CLSI determines that CMI for Ampicillin less than or equal to 8 µg / mL is indicative of susceptibility to the antibiotic, its use being recommended for treatment, while values equal to or greater than 32 µg / mL indicates that the bacterium is resistant. Thus, from the antibiogram data it is possible for the doctor to identify which is the best antibiotic for the person.
In the case of the agar diffusion antibiogram, where papers containing certain concentrations of antibiotics are placed in the culture medium with the bacterium, after incubation for about 18 hours it is possible to perceive the presence or not of inhibition halos. From the size of the diameter of the halos, it is possible to verify whether the bacteria is non-susceptible, susceptible, intermediate or resistant to the antibiotic. The result should also be interpreted based on the determination of the CLSI, which determines that for the E.coli susceptibility test to Ampicillin, for example, the inhibition zone less than or equal to 13 mm is indicative that the bacterium is resistant antibiotic and a halo equal to or greater than 17 mm indicates that the bacteria is sensitive.
Why is it necessary to identify the correct antibiotic?
The use of antibiotics that are not suitable and effective for a microorganism delays the person's recovery, partially treats the infection and favors the development of resistance mechanisms of the bacteria, making the infection more difficult to treat.
For this same reason, it is very important not to use antibiotics without the doctor's guidance and unnecessarily, as this may end up selecting bacteria that are more resistant to antibiotics, reducing the options for medicines to fight infections.
