Antibiotic Sensitivity Tests

Sensitivity testing is often performed in a medical laboratory and uses culture procedures that introduce bacteria to antibiotics or genetic methods that screen to determine whether bacteria have resistance genes. Antibiotic sensitivity testing, also known as antibiotic susceptibility testing, determines bacteria’s responsiveness to antibiotics. It is used because certain antibiotics may be resistant to bacteria.

Antibiotics are often recommended on the basis of the patient’s symptoms and medical recommendations. This approach of antibiotic selection is known as an empiric treatment, and it is based on understanding which bacteria cause an infection and which antibiotics bacteria may be sensitive or resistant to.

Antibiotic sensitivity procedures and methods

Antibiotics are chosen for susceptibility testing when a bacteria has been discovered by microbiological culture. Methods of susceptibility testing include introducing bacteria to antibiotics and examining the effect on bacterial growth (phenotypic testing) or detecting particular genetic markers (genetic testing). Methods employed may be qualitative, in which a result reveals whether or not resistance exists or quantitative, in which a minimum inhibitory concentration (MIC) is used to define the concentration of antibiotic to which a bacteria is susceptible.

Many variables can influence the findings of antibiotic sensitivity tests and procedures, including equipment failure, temperature, moisture, and antimicrobial agent strength. Quality testing contributes to the accuracy of test results. Many international organisations give bacterium strains with established resistance characteristics that may be utilised for quality control.

Some of the most basic Antibiotic sensitivity test methods are explained below-

1. Broth Dilution Tests- The macro broth or Broth dilution method was one of the first antibiotic sensitivity testing procedures. Preparing two-fold dilutions of antibiotics in a liquid growth medium delivered in test tubes was required for this process. A uniform bacterial suspension was introduced into the antibiotic-containing tubes. The tubes were checked for evident bacterial growth as shown by turbidity after overnight incubation at the temperature of 35°C. The least inhibitory concentration was defined by the lowest concentration of antibiotics that halted growth (MIC). 
2. Due in significant part to the practice of physically producing repeated dilutions of the antibiotics, the accuracy of this procedure was judged to be plus or minus 1 two-fold concentration. The main disadvantages of the Broth dilution method of Antibody sensitivity tests were the time-consuming, manual task of preparing antibiotic solutions for each test, the risk of material misstatement in antibiotic solution preparation, and the relatively large quantity of reagents and space required for each test.
3. Antimicrobial gradient method- As a technique of measuring susceptibility, the antimicrobial gradient diffusion method employs the notion of establishing an antibiotic concentration gradient in an agar medium. The gradient diffusion method is intrinsically flexible in that it may test any medications that the laboratory chooses. If more than a few medications are examined, antimicrobial gradient strips can be a costly technique. This approach is most suited for cases where a MIC for only one or two medicines is required, or for testing an organism that requires an enriched medium or a particular incubation environment.
4. Disk Diffusion Antibiotic sensitivity test- The disk diffusion antibiotic sensitivity method is straightforward, practical Sensitivity te, and well-standardised. The advantages of the disk approach include test simplicity (no special equipment required), presentation of categorical data that are simply comprehended by all doctors, and freedom in the selection of discs for testing. It is the least expensive of all the other sensitivity approaches. Disk diffusion sensitivity test is one of the most effective antibiotic sensitivity test methods.

Uses and applications of Antibiotic Sensitivity tests

Antibiotic sensitivity testing is also used as a method of screening at the community level in various countries. This is to examine the baseline rates of antibiotic resistance, which may impact guidelines and public health initiatives. Antibiotic sensitivity testing determines which antibiotics are more likely to be successful and should thus be used to treat the infection, especially prior to clinical procedures. The findings of the antibiotic sensitivity tests are presented in the form of a table, which is frequently referred to as an antibiogram. 

• Antibiotic Sensitivity tests are important in treatment of Bacterial Infections, especially those caused by MultiDrug Resistant (MDR) bacteria.
• Antibiotic Sensitivity tests test the resistance of a microorganism to different antibiotic or antimicrobial drugs, this helps the chemists to produce more potent drugs and record the resistant types of beneficial microbes. 
• The information collected after antibiotic sensitivity tests  is essential to guide antimicrobial prescriptions and avoid unnecessary use of antibiotics. 
• Antibiotic Sensitivity tests also reveal the beneficial substances produced by the microbes after being subjected to an antibiotic or antimicrobial drug. Example – Restriction enzymes from the E. coli bacteria. 

Bacteria are classified as sensitive, resistant, or intermediately resistant to antibiotics based on the minimum inhibitory concentration (MIC), which is the lowest concentration of the antibiotic that inhibits bacterial growth.

Conclusion

Many microorganisms have developed resistance to conventional antibiotics. This indicates that the medicine will not be able to destroy the germs. The antibiotic sensitivity test is a valuable technique for determining if bacteria are resistant to specific medicines. After the bacterial cultures have been cultivated and antibiotic-tested, the findings are analysed in chemical laboratories. These findings can aid in determining the most effective medication to treat your illness.