Антибиотиктерге сезімталдық тестілеу және бактериялардың төзімділігі

Microbiology test used in medicine

Antibiotic sensitivity testing or antibiotic susceptibility testing is the measurement of the susceptibility of bacteria to antibiotics. It is used because bacteria may have resistance to some antibiotics. Sensitivity testing results can allow a clinician to change the choice of antibiotics from empiric therapy, which is when an antibiotic is selected based on clinical suspicion about the site of an infection and common causative bacteria, to directed therapy, in which the choice of antibiotic is based on knowledge of the organism and its sensitivities. Sensitivity testing usually occurs in a medical laboratory, and uses culture methods that expose bacteria to antibiotics, or genetic methods that test to see if bacteria have genes that confer resistance. Culture methods often involve measuring the diameter of areas without bacterial growth, called zones of inhibition, around paper discs containing antibiotics on agar culture dishes that have been evenly inoculated with bacteria. The minimum inhibitory concentration, which is the lowest concentration of the antibiotic that stops the growth of bacteria, can be estimated from the size of the zone of inhibition. Antibiotic susceptibility testing has been needed since the discovery of the beta lactam antibiotic penicillin. Initial methods were phenotypic, and involved culture or dilution. The Etest, an antibiotic impregnated strip, has been available since the 1980s, and genetic methods such as polymerase chain reaction (PCR) testing have been available since the early 2000s. Research is ongoing into improving current methods by making them faster or more accurate, as well as developing new methods for testing, such as microfluidics.

In clinical medicine, antibiotics are most frequently prescribed on the basis of a person's symptoms and medical guidelines. This method of antibiotic selection is called empiric therapy, This is because Escherichia coli is the most likely causative bacterium, and may be sensitive to that combination antibiotic. Antibiotic sensitivity testing provides information about which antibiotics are more likely to be successful and should therefore be used to treat the infection. This is to assess the background rates of resistance to antibiotics (for example with methicillin resistant Staphylococcus aureus), and may influence guidelines and public health measures.

The disc diffusion method involves selecting a strain of bacteria, placing it on an agar plate, and observing bacterial growth near antibiotic impregnated discs. This is also called the Kirby Bauer method, although modified methods are also used. This type of test is considered a diffusion test. In agar and broth dilution methods, bacteria are placed in multiple small tubes with different concentrations of antibiotics. The lowest concentration of antibiotics that inhibits growth is considered the MIC. An example is the use of PCR to detect the mecA gene for beta lactam resistant Staphylococcus aureus. However, whether resistance genes are detected does not always match the resistance profile seen with phenotypic method. The ionised particles are then accelerated, and spectral peaks recorded, producing an expression profile, which is capable of differentiating specific bacterial strains after being compared to known profiles.

Bacteria are marked as sensitive, resistant, or having intermediate resistance to an antibiotic based on the minimum inhibitory concentration (MIC), which is the lowest concentration of the antibiotic that stops the growth of bacteria. The MIC is compared to standard threshold values (called "breakpoints") for a given bacterium and antibiotic. Breakpoints for the same organism and antibiotic may differ based on the site of infection: for example, the CLSI generally defines Streptococcus pneumoniae as sensitive to intravenous penicillin if MICs are ≤0.06 μg/ml, intermediate if MICs are 0.12 to 1 μg/ml, and resistant if MICs are ≥2 μg/ml, but for cases of meningitis, the breakpoints are considerably lower. Sometimes, whether an antibiotic is marked as resistant is also based on bacterial characteristics that are associated with known methods of resistance such as the potential for beta lactamase production. Specific patterns of drug resistance or multidrug resistance may be noted, such as the presence of an extended spectrum beta lactamase. The results of antimicrobial susceptibility tests performed during a given time period can be compiled, usually in the form of a table, to form an antibiogram. Antibiograms help the clinician to select the best empiric antimicrobial therapy based on the local resistance patterns until the laboratory test results are available.|alt=See caption.]] Ideal antibiotic therapy is based on determining the causal agent and its antibiotic sensitivity. Empiric treatment is often started before laboratory microbiological reports are available. This might be for common or relatively minor infections based on clinical guidelines (such as community acquired pneumonia), or for serious infections, such as sepsis or bacterial meningitis, in which delayed treatment carries substantial risks. and other opportunistic infections. Other considerations may influence the choice of antibiotics, including the need to penetrate through to an infected site (such as an abscess), or the suspicion that one or more causes of an infection were not detected in a sample. Traditional techniques typically take between 12 and 48 hours, although it can take up to five days. Additional research is focused at the shortcomings of current testing methods. As well as the duration it takes to report phenotypic methods, they are laborious, have difficult portability and are difficult to use in resource limited settings, and have a chance of cross contamination. Quantitative PCR, with the view of determining the percent of a detected bacteria that possesses a resistance gene, is being explored. Whole genome sequencing of isolated bacteria is also being explored, and likely to become more available as costs decrease and speed increases over time. Additional methods explored include microfluidics, which uses a small amount of fluid and a variety of testing methods, such as optical, electrochemical, and magnetic. Such assays do not require much fluid to be tested, are rapid and portable. The use of fluorescent dyes has been explored. These involve labelled proteins targeted at biomarkers, nucleic acid sequences present within cells that are found when the bacterium is resistant to an antibiotic. An isolate of bacteria is fixed in position and then dissolved. The isolate is then exposed to fluorescent dye, which will be luminescent when viewed. Improvements to existing platforms are also being explored, including improvements in imaging systems that are able to more rapidly identify the MIC in phenotypic samples; or the use of bioluminescent enzymes that reveal bacterial growth to make changes more easily visible.