An antibiotic susceptibility test is mandatory when a doctor suspects that a patient's disease is bacterial in nature. This is due to the fact that doctors are trying to control the prescription of these drugs so as not to stimulate mutations and not cause resistance in microorganisms.
Definition
Antibiotic susceptibility testing is a laboratory method for identifying a drug that will have the greatest effect on the pathogenic flora in this particular case of the disease.
At the moment, antibiotic therapy is used quite widely where it is needed, as well as in cases where it is not necessary at all, to reinsure against possible complications. For example, after caesarean section, laparoscopic surgery, removal of stones from the kidneys or ureters, etc.
The pharmaceutical industry can offer a wide range of drugs in terms of both price and potency. In order not to "poke a finger in the sky" and appoint an effectiveantibiotic, need culture for sensitivity.
Indications
Before the doctor selects the therapy, the patient needs to pass some tests. Antibiotic susceptibility culture is indicated if it is necessary to determine the drug that is most appropriate in this case. Most often, this test is prescribed for the treatment of sexually transmitted diseases, or STDs. For children, the need to determine the antibiotic is a prerequisite.
In addition, susceptibility testing is needed to avoid bacterial resistance to treatment. If the patient was recently treated with antibiotics, and now a second course is needed again, then a replacement drug is required. This will allow the use of smaller doses of the drug and not cause mutations in the pathogen. In purulent surgical departments, antibiotics are changed every two to three months.
This analysis is necessary even if the patient has an allergic reaction to the main group of antibiotics.
Diffusion methods
An analysis of urine for sensitivity to antibiotics, and not only it, can be done in several ways. The first one is the disk method. It is carried out as follows. Agar is poured into the Petri dish, and when it hardens, the test material is applied with a special tool. Then paper discs impregnated with antibiotics are laid out on the surface of the agar. After the cup is closed and placed in a thermostat. Gradually, the disk is immersed in gelatin, and the antibiotic diffuses into the surrounding space. A “growth inhibition” zone forms around the paper. The cups are held in the thermostat for twelve hours, then they are removed and the diameter of the above zone is measured.
The second way is the E-test method. It is similar to the previous one, but instead of paper discs, a strip is used, which is impregnated with an antibiotic to varying degrees along its length. After twelve hours of exposure in a thermostat, the Petri dish is taken out and it is observed where the zone of growth suppression is in contact with the strip of paper. This will be the lowest concentration of the drug needed to treat the disease.
The advantage of these tests is the speed and ease of their implementation.
Breeding Methods
An analysis of flora and sensitivity to antibiotics can be done in another way. This method is based on the sequential decrease in the concentration of the antibiotic (from maximum to minimum) in order to determine which of the tubes will stop inhibiting the growth of bacteria.
First prepare solutions of the drug. Then they are introduced into a liquid medium with bacteria (broth or agar). All test tubes for the night (that is, 12 hours) are placed in a thermostat at a temperature of 37 degrees, and in the morning the results are analyzed. If the contents of the tube or Petri dish are cloudy, this indicates the growth of bacteria and, therefore, the ineffectiveness of the antibiotic at this concentration. The first tube that will not be visually determinedthe growth of colonies of microorganisms, will be considered a sufficient concentration for treatment.
This dilution of the drug is called the minimum inhibitory concentration (MIC). It is measured in milligrams per liter or micrograms per milliliter.
Interpretation of results
Analysis for sensitivity to antibiotics must be able not only to do it right, but also to correctly decipher it. Based on the results obtained, all microorganisms are divided into sensitive, moderately resistant and resistant. In order to distinguish between them, conditional borderline drug concentrations are used.
These values are not constant and may change depending on the adaptability of microorganisms. The development and revision of these criteria is entrusted to chemotherapists and microbiologists. One of the official structures of this kind is the US National Committee on Clinical Laboratory Standards. The standards they have developed are recognized worldwide for use in evaluating antibiotic potency, including for randomized multicentre trials.
There are two approaches to evaluating antibiotic susceptibility testing: clinical and microbiological. Microbiological evaluation focuses on the distribution of effective antibiotic concentrations, while clinical evaluation focuses on the quality of antibiotic therapy.
Resistant and susceptible microorganisms
Analysis - determination of sensitivity to antibiotics - is prescribed to identify sensitive and resistant microorganisms.
Sensitive are pathogens that can be treated with antibiotics in an average therapeutic concentration. If there is no reliable information on the sensitivity category of the microorganism, then the data obtained in the laboratory are taken into account. They are combined with knowledge about the pharmacokinetics of the drug used, and after the synthesis of this information, a conclusion is made about the susceptibility of bacteria to the drug.
Resistant, that is, resistant, microorganisms are those bacteria that continue to cause disease even when using maximum concentrations of drugs.
Intermediate resistance is established in the event that the disease in the course of treatment can have several outcomes. The patient's recovery is possible if high doses of antibiotics are used or if the drug is targeted at the site of infection.
Minimum bactericidal concentration
An analysis of the microflora and sensitivity to antibiotics determines such an indicator as the minimum bactericidal concentration, or MBC. This is the lowest concentration of the drug, which in laboratory conditions causes the elimination of almost all microorganisms within twelve hours.
Knowledge of this indicator doctors use when prescribing therapy not bactericidal, but bacteriostaticmedicines. Or in cases where standard antibiotic therapy is ineffective. Most often, this test is ordered for patients with bacterial endocarditis, osteomyelitis, as well as opportunistic infections.
What can be a sample?
Antibiotic susceptibility testing can be done using body fluids:
- saliva;
- blood;
- urine;
- cum;
- breast milk.
In addition, swabs are taken from the urethra, cervical canal and upper respiratory tract to determine local sensitivity.
Preparing for tests
Buck. Antibiotic susceptibility testing does not require significant preparation from patients, but there are still some limitations.
- For research, an average portion of morning urine is used, which is collected in a sterile dish. Before this, the patient must necessarily toilet the external genital organs and hands.
- Breast milk is collected before the baby is fed. The first portion is drained, and then a few milliliters from each breast are expressed into a sterile container.
- Before taking a smear from the nasopharynx, you should refrain from eating for five to six hours.
- In the case of taking a swab from the genital tract, it is recommended to refrain from sexual intercourse for a couple of days.
Today, there are no clinical or laboratory methods that could predict the effect of antibacteri altherapy. But at the same time, determining the sensitivity of bacteria to drugs can be a guide for doctors in choosing and correcting treatment.
