Antibiotic sensitivity

Antibiotic sensitivity

Antibiotics, also called as antibacterial, are the drugs that kill microbes, and fight infections caused by microbes, hence are used in the treatment of bacterial infections or diseases. Antibiotics prevent diseases by either killing bacteria or keeping them away from reproducing.
Antibiotics generally do not fight infections that are caused by viruses like cold, flu, sore throats, unless the infection is caused by Streptococcus. Using antibiotics when we don’t need them may cause antibiotic resistance. If antibiotics are being taken, it should be on prescription only and the directions should be followed carefully. It is very important that the medicine is completed even if feeling better in the middle of the prescription schedule, if done so; some bacteria may survive and re-infect the patient or may get even stronger.

An antibiotic should have the following characteristics:
• It should be toxic to the infecting organism while harmless to the host cells and the microbiota of the host.
• It should stay in toxic form for a sufficient amount of time to affect the infecting microorganism. If it changes to another form or is broken down in the body, it may not be useful.
• Sufficient amounts of it should reach the site of infection to kill the infecting agent.
• The infecting agent should be sensitive to it.

Antibiotic sensitivity refers to sensitivity of bacteria to an antibiotic i.e. the ability of the antibiotic to kill those bacteria. If it is said that a bacteria is sensitive to an antibiotic it means that the antibiotic can kill that bacteria and if it is said that the bacteria is resistant to an antibiotic it means that the antibiotic is no longer capable of killing that bacteria.
Sensitivity analysis, also known as susceptibility testing, is the test to check if an antibiotic can kill the disease causing bacteria or the pathogen. Doctors use sensitivity testing to choose the appropriate antibiotic and to determine the right antibiotic treatment for an infection. Antimicrobial susceptibility tests are used to determine which specific antibiotics a particular bacteria or fungus is sensitive to. Antimicrobial susceptibility tests can guide the physician in drug choice and dosage for difficult-to-treat infections.
Sensitivity analysis is recommended if the infection of the disease does not respond to the particular treatment, which will help to find out if the bacterium is resistant to the antibiotic and to find out the appropriate drug for that disease.

Many bacteria are resistant to common antibiotics. This means that the drug can’t kill the bacteria. Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic.
It is a specific type of drug resistance.
Antibiotic resistance evolves naturally via natural selection through random mutation, but it could also be engineered by applying an evolutionary stress on a population.
Once such a gene is generated, bacteria can then transfer the genetic information in a horizontal fashion (between individuals) by plasmid exchange.
If a bacterium carries several resistance genes, it is called multiresistant or, informally, a superbug.
Causes Antibiotic resistance can also be introduced artificially into a microorganism through transformation protocols.
This can be a useful way of implanting artificial genes into the microorganism.
Antibiotic resistance is a consequence of evolution via natural selection.
The antibiotic action is an environmental pressure; those bacteria which have a mutation allowing them to survive will live on to reproduce.
They will then pass this trait to their offspring, which will be a fully resistant generation.
Several studies have demonstrated that patterns of antibiotic usage greatly affect the number of resistant organisms which develop.
Overuse of broad-spectrum antibiotics, such as second- and third-generation cephalosporins, greatly hastens the development of methicillin resistance.
Other factors contributing towards resistance include incorrect diagnosis, unnecessary prescriptions, improper use of antibiotics by patients, and the use of antibiotics as livestock food additives for growth promotion.

One of the most efficient methods for testing antibiotic sensitivity is Disc Diffusion method.
The disc diffusion method uses the antibiotic discs on a bacterial culture plate to check the extent to which the bacteria are affected by that antibiotic.


The agar diffusion method uses commercially available filter paper disks, each containing a defined concentration of a specific antibiotic. The relative effectiveness of different antibiotics provides the basis for a sensitivity spectrum of the organism. This information, together with various pharmacological considerations, is used in the selection of an antibiotic for treatment.
It should be emphasized that chemotherapeutic agents are not chosen simply on the basis of the drug producing the widest ZOI (Zone Of Inhibition). This is because of the nature of the growth-inhibition substances. The size of the zone may be affected by the density or viscosity of the culture medium, the rate of diffusion of the antibiotic, the concentration of the antibiotic on the filter disc, the sensitivity of the organism to the antibiotic, and the interaction between the antibiotic and the medium. In addition, an agent that has been found to have a significant antibiotic effect may not be therapeutically useful because it may also have significant adverse effects in the system for which it is intended. The disk diffusion method represents a simple procedure for screening substances to determine if they have significant antibiotic activity.
An important task of the clinical microbiology laboratory is the performance of antimicrobial susceptibility testing of significant bacterial isolates. The goals of testing are to detect possible drug resistance in common pathogens and to assure susceptibility to drugs of choice for particular infections.
The disk diffusion method is simple and practical and has been well-standardized. The test is performed by applying a bacterial inoculum of approximately 1–2×108CFU/mL to the surface of a large (150 mm diameter) Mueller-Hinton agar plate. Up to 12 paper antibiotic disks can be placed on the inoculated agar surface . Plates are incubated for 16–24 h at 35°C prior to determination of results. The zones of growth inhibition around each of the antibiotic disks are measured to the nearest millimeter. The diameter of the zone is related to the susceptibility of the isolate and to the diffusion rate of the drug through the agar medium. The results of the disk diffusion test are “qualitative,” in that a category of susceptibility i.e., susceptible, intermediate, or resistant.

By | 2018-02-27T06:30:27+00:00 February 21st, 2018|Fermentation, Food Microbiology, Molecular Biology, Plant Tissue Culture|Comments Off on Antibiotic sensitivity

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