How Do Antibiotics Work: Mode & Mechanism Of Action Of Antibiotics

How do Antibiotics Work? Mechanism Explained !

Dear Reader, do you have any idea that, “How do antibiotics work” . You might be having a fair idea about it, but the same thing has been brought out in detail in this article below with Mode of action of antibiotics & Mechanism Of Action Of Antibiotics. So, lets start with the introduction to antibiotics.

What Are Antibiotics ?

Antibiotics are the group of drugs which are either produced by microorganisms or may be produced synthetically. How do antibiotics work can be answered as these are the substances which can either kill the bacteria or slow down the bacterial growth.

Antibiotics belong to a broader class of drugs called the “Antimicrobials. Antibiotics are specifically targeted against Bacteria. Whereas other microorganisms like the Viruses, Fungi or Parasites have other specific targeted antimicrobials which are Anti-viral, Anti-fungal and Anti-parasitic drugs.

Antibiotics are the most frequently used drugs in medicine. But all antibiotics will not have same working technique. Antibiotics or antimicrobials will either kill the bacteria (Bactericidal) or it will inhibit the growth of bacteria (Bacteriostatic).

Other than these two types of mechanisms of action antibiotics affect the bacterial cell structure as well as the bacterial physiology. Antibiotics are also either Target specific (Narrow Spectrum ) or these may act on a wider gamut of bacteria (Broad Spectrum).

Another important feature of antibiotics is that they are capable of differentiating the human cells from bacterial cells. For example human cells do not have any cell wall but many bacterial cells do have a cell wall.

Some antibiotics are specific for acting on the cell wall and therefore can easily identify and differentiate between the bacterial and host cells.

How Do Antibiotics Work Without Harming the Surrounding Human Cells?

Antibiotics/Antimicrobials have a specific chemical structures which have the capability to differentiate between Bacterial and Human cells.

Also Structurally, a human is very different from that of bacterial cell. A human cell is devoid of any cell wall whereas many bacterial cells contain a cell wall.

So Antibiotics will identify this structural and chemical compositional difference between the two and hence destroy only the bacterial cell.

A human cell has different structures when compared to a bacterial cell

A bacterial cell has presence of cell wall which is absent in a human cell

What are Different Types of Antibiotics and How they work in Different Manner?

Antibiotics or the Antimicrobials are a very vast group of drugs and they have been classified on different grounds –

Based on the Mechanism Antibiotics Destroy Bacterial Cell

Inhibition of Cell Wall Synthesis – Penicillin, Vancomycin, Bacitracin, Cephalosporins.
Protein Synthesis Inhibition – Tetracyclin, Clindamycin, Erythromycin, Linezolid.
Leakage from the Cell Membrane – Polypeptides – Colistin, Bacitracin. ;Polyenes –Amphotericin B, Hamycin
Misreading of m-RNA code – Aminoglycosides: Streptomycin, Gentamicin
DNA-gyrase Inihibiton- Ciprofloxacin
DNA synthesis interference – Zidovudine
Metabolism Disturbance – Ethambutol, Sulphonamides, Trimethoprim.

Based on the Type of Micro organisms those are Active Against

Antibacterial – Penicillins, Aminoglycosides, Erythromycin
Antifungal – Amphotericin B, Ketoconazole etc
Antiviral – Acyclovir, Amantidine
Antiparasitic – Metronidazole, Pyrimethamine, Mebendazole.

Based on the Type of Spectrum of Acitivity

Narrow Spectrum
Penicillin
Streptomycin
Erythromycin
Broad Spectrum
Tetracyclines
Chloremphenicol

All these antibiotics do not work in a similar fashion and all of them have a different action on the bacterial cell. It depends on the type of infection and the causative organism, what kind of antibiotics will be effective.

Main types of Antibiotic are –

Penicillins –

First discovered by Sir Alexander Fleming in the year 1929.
Effectively used against skin, ear, respiratory, dental and urinary tract infections etc.
Mechanism of Action – These drugs block the construction of bacterial cell wall and thus cause the breakage of cell wall finally killing the bacteria.

Penicillin bind to the Penicillin Binding Protein present on the bacterial cell wall and thereby destroy the bacteria.
This group of antibiotics is safe but is known to cause serious allergic reactions.
Most commonly used penicillins are –
Ampicillin
Amoxicillin
Oxacillin

Tetracyclins

This group of antibiotics was discovered in the 1940s.
Tetracyclins are used as broad spectrum antibiotics to treat many infections.
Mechanism of Action – Inhibition of protein synthesis and the drug binds to the bacterial ribosome thereby preventing binding of RNA and hence protein synthesis.

The most commonly used Tetracyclins are –
Minocycline
Doxycycline

Fluoroquinolones

These antibiotics are known as “Broad-spectrum antibiotics”.
Frequently used to treat respiratory (Sinusitis, Bronchitis) skin, urinary infections.
Mechanism of Action – These antibiotics interfere with the formation of DNA. This affects the ability for the bacteria to divide.

Side effects from fluoroquinolones affect the digestive system. This antibiotic is prohibited for Pregnant females.
Most commonly fluoroquinolones are –
Norfloxacin
Levofloxacin
Ofloxacin
Gatifloxacin

Cephalosporins

Cephalosporins have been divided into generations as these have different modes of action.
Till now three generations have been identified and currently are prescribed.
The fourth generation is still under research and yet to hit the medical practice.
Mechanism of Action – Cephalosporins alike the Penicillins inhibit the cell wall synthesis and therefore have a Bactericidal effect on the microbe.

The effectiveness of Cephalosporins towards the Gram Negative bacteria increases with each subsequent generation. That means the third generation Cephalosporins have the highest effectiveness.
Cephalosporins are closely related to Penicillins.
These are used to treat Throat (Staphylococcal, Streptococcal) infections. Skin infections and Gonorrhea.
They are also used for antibiotic coverage in the pre-operative period.
The most commonly prescribed Cephalosporins are:
First Generation Cephalosporins
Cephradin
Cephadroxil
Cephalexin
Second Generation Cephalosporins
Cefaclor
Cefuroxime
Cefprozil
Third Generation Cephalosporins
Cefixime
Cefotaxime
Ceftazidime
Fourth Generation Cephalosporins
Cefepime
Cefpirome

Aminoglycosides –

These are the drugs commonly used with either Penicillins or Cephalosporins as an adjunct in cases of suspected Antibiotic Resistance. Also can be used to get a better response to antibiotic therapy
Aminoglycosides are usually not give orally as they will breakdown in the stomach, destroying the effect of Antibiotic.
It is given in injectable form.
Mechanism of Action – Aminoglycosides prevent the formation of Protein in the bacterial cell thereby having a Bactericidal effect.

When injected, Amioglycosides may have major side effects on the ears and kidney.
To reduce this effect aminoglycoside doses are given in small quantity and it also becomes important to keep checking the amount of drug in blood.
Moreover, Aminoglyosides are given for a short period of time.

Macrolides

These are Erythromycin like antibiotics and have fewer side effects.
Macrolides belong to Polyketide class of natural products.
Mechanism of Action- Macrolides work on the basis of amount of drug given. In lower concentrations, these bind to the ribosomes and prevent the formation of Protein and hence have a Bacteriostatic role.
If given at higher concentrations, they have Bactericidal role.
- The biggest side effect of these antibiotics is that they can irritate the stomach.
- They may have allergic reactions too but very few as compared to Penicllins and cephalosporins.

There are many routes by which an antibiotic can be administered in an individual. Those routes are :

Oral Route
Injectable (Intramuscular, Intravenous, Subcutaneous)
Dermal Patches

Antibiotics being different in chemical structures are always liable to different environments of the human body. This difference in chemical structures also helps in deciding the choice of drug route.

It is always debatable as to which route of drug administration can give maximum effect and is also least painful for the patient. So amongst all the routes, oral route is most preferable. But it is also important to consider the time which the drug will take to reach the site of infection.

Considering the oral route as the safest option it should be answered that in how much time does the oral antibiotic will show its effect.

How Long Do Oral Antibiotics Take To Work?

Advantages of Oral Antibiotics

It is easy for the patient
Non invasive so no fear of needles
It is inexpensive as compared to other routes
Can be self administered

Oral route has a few Disadvantages too, which are-

Inefficient in case of few antibiotics and most part of the drug goes unabsorbed
Maximum part of the drug goes into the liver which has its own blood supply
High chances of Stomach upset as a side effect
Slow effect in cases of emergencies
Unpleasant taste
Cannot be given in unconscious patients.

Considering all these pros and cons of an Oral Antibiotic, if we still consume the drug, it has to pass through many stages to reach the site of infection.

Once an antibiotic pill is consumed through oral route it will travel through the gastrointestinal tract which stretches from the oral cavity till the anus.
During this course the amount of acidity keeps on changing which can sometimes destroy the drug making it ineffective.
Other than this there is an Effect which is called the First Pass Metabolism, which means the drug gets absorbed in the blood circulation of liver. So more this first pass metabolism effect lesser amount of drug will reach the site of infection.

So Oral route should be considered in cases where such a slow release of antibiotics can also be beneficial for the patient.

How Fast Do Antibiotics Work?

The effect of any antibiotic or any other drug depends on the fact that how will the body respond to the drug. This question “What the body does to any drug or Antibiotic” is called as Pharamacokinetics.

Pharmaokinetics is that brach of Pharamcology which describes:

The time duration of action of antibiotic drug
How will the body act towards the drug
The exact relationship between the amount of drug administered in the body and the amount used in the body.
Pharamacokinetics of Antibiotics is affected by two important facts:
1. The chemical nature of Antibiotic drug
2. Amount of drug concentrated at infection site in body.

The basic action plan of any drug is –

Administration of drug in the body through any route which can be oral, injection, inhalation, through the skin.
The second step is Absorption of th drug in the blood stream and Distribution
Elimination of the remaining drug

Pharmacokinetics of some Important Antibiotics

Pharmacokinetics of Penicillins
Penicillins are Acid Labile which means they will be destroyed by the acid of the stomach and only 1/3^rd of the administered drug is available to show the effect.
Since New borns and older individuals have low stomach acidity they will have a higher fraction of dug available.
If an injection of Penicllin is given intramuscularly then it is the most rapid and complete route of drug distribution in the body.
About 60% of the drug will be bound the Plamsa proteins and there is rapid excretion.(The longer the binding capacity of drug with the plasma proteins the longer will be the effect)
Duration of working of Drugs depends on the time they are present in the Plasma which has been called the t^1/2 of the drug. This duration in a healthy adult is about 30 minutes.

Pharmacokinetics of Fluoroquinolones (Ciprofloxacin)

When given by oral route Ciprofloxacin is rapidly absorbed.
But if food is present in the stomach it will delay the absorption.
Ciprofloxacin and other Fluoroquinolones have high penetration capacity through the tissues so concentration is higher in the lungs, bone muscle etc.
On oral administration around 20-30% of drug binds to the plasma proteins.
Plasma Half life t^1/2 is 4-6 hrs.
Bioavailability of drug after taking through oral route is around 90%.

Pharmacokinetics of Tetracyclines

Tetracyclines are administered most commonly through oral route in the form of capsules other types of preparations were available in the past like dry syrups and liquid oral preparations.
These are not advised to take as injections intramuscularly because injections are exceptionally painful and absorption form the injection site is also poor.
Tetracycline absorption from the stomach is better when it is taken empty stomach.
Another important property of Tetracyclines is that they have a tendancy to bind with Calcium. So if taken in combination with milk it can reduce the absorption as well as bioavailability of the drug.
Plasma Half Life t^1/2 is around 6-10 hrs.

Pharmacokinetics of Aminoglycosides

This class of drugs is highly resistant to the stomach acids nor it gets absorbed neither gets absorbed in the stomach.
When administered from through intra muscular route, it gets rapidly absorbed.
Plasma protein binding is insignificant.
Plasma t^1/2– 2-4 hours.

What Antibiotics Can Do To Your Body?

After consumption of antibiotic by any mode of administration, the body will react towards the drug so as to obtain the best effect and to flush out the remaining unused drug out of the body.

The Antibiotic so consumed will have its effects on the body which are the therapeutic effects. The branch of Pharmacology which studies the effect of any drug or an Antimicrobial is Pharmacodynamics.

Pharamacodynamics comprises of three aspects:

Spectrum of Activity
Bactericidal or Bacteriostatic effect

Spectrum of Activity

This aspect helps in determining the drug of choice as some antibiotics are Broad Spectrum which means they are active against both the Gram negative as well as Gram positive bacteria.

Example – Fluoroquinolones, Tetracyclines, “Third and Fourth Generation” Cephalosporins.

Whereas Narrow spectrum Antibiotics are those which are primarily useful against a particular organism and /or a particular species of organism.

Example – Glycopeptides and Aminoglycosides are effective only against Gram Positive Bacteria. While, Polymyxins are effective only for Gram negative bacteria.

Bactericidal or Bacteriostatic Effect

To understand this aspect of Pharmacodynamics it is important to get introduced to a few terms like–

Minimum Inhibitory Concentration(MIC) – It is that lowest concentration of the Antimicrobial or Antibiotic drug that will prevent the growth of the targeted organism within 24 hrs of antibiotic consumption.
Minimum Bactericidal Concentration (MBC) – It is the minimum concentration of the Antibiotic which kills 99.9% bacteria.
Post-Antibiotic Effect (PEA) – It refers to a persistent antibacterial effect even during the lowest levels of antibiotic in body.

The activity of the Antimicrobial or Antibiotic agent can be either Bacteriostatic or Bactericidal which depends on few factors. These factors are the following:

Concentration of the drug at the site of Infection
The Infection causing Microorganism

Bacteriostatic Drugs

These drugs inhibit the growth of bacteria and also few of them can act as Bactericidal when given at high concentration. Moreover, these drugs once ceased may develop the condition again.

Example – Macrolides, Sulphonamides, Chloremphenicol.

Bactericidal Drugs

These drugs act by killing the microorganism. There are various mechanisms by which antibiotics show bactericidal action like Inhibition of Cell Wall Synthesis, Destruction of Bacterial Cell Protein and Prevention of cell DNA to replicate.

Common examples of such bactericidal drugs are – Penicillins, Cephalosporins, Aminoglycosides, Fluoroquinolones.

Bactericidal drugs work in a simple manner of killing microbes but when it comes to Bacteriostatic dugs the working mechanism is quite different.

How Do Bacteriostatic Antibiotics Work?

The Bacteriostatic drugs temporarily inhibit the growth of Bacteria and this effect can be reversed when the drug is removed. To have a clinically visible effect form this group of drugs it is important to achieve the concentration above MIC.

So, how will a Bacteriostatic antibiotic will clear up the infection if it is not killing the bacteria?

In such a case of Bacteriostatic antibiotic we can see the role of Human immune system, as once the growth of bacteria is inhibited, the immune system will tackle with the remaining Bacteria.

A very common example of Bacteriostatic agents is the Tetracycline drug, this antibiotic works by binding to the Bacterial cell Ribosome so that no new proteins are made.

This inhibition of protein synthesis will not kill the bacterium as to survive the bacterium can use the existing proteins, so it will not divide further in such a deficiency of Proteins.

Another example of Bacteriostatic agents are the Sulfonamides or the Sulfa Drugs, these drugs inhibit the production of important metabolites needed for the formation of DNA and RNA which help in further bacterial cell division.

But what will happen if these antibiotics do not respond and nor inhibit the growth neither kill the microbes. In such a scenario the bacteria are said to develop Antibiotic/Antimicrobial Resistance.

How Do Antibiotic Develop a Resistance?

Antibiotic resistance develops when a bacterium stops responding to a given antibiotic, Major causes of Antibiotic Resistance are :

Indiscriminate use of Antibiotics in routine practice.
Insufficient research and development in the field of Antibiotic Resistance
Unnecessary consumption of Antibiotics

Indiscriminate use of Antibiotics in Routine Practice –

Inappropriate consumption of Antibiotics can either be done by the individual himself or may be because of health care professionals.

A few situations where such indiscriminate use of antibiotics can be seen are:

Consumption of Antibiotics during the course of a common cold which is a viral infection have a duration of 5-7 days and will heal on its own.
Persistent prescription of Broad-Spectrum antibiotics rather specific drug in primary course of treatment.

Insufficient Research and Development in the field of Antibiotic Resistance

The bacteria keeps on changing according to the misused antibiotic, hence research and development in this field is least. Therefore, there is a need for such antibacterial agents which can act against such highly evolved bacteria.

Unnecessary Consumption of Antibiotics

Antibiotics do kill or inhibit microorganisms but they affect host immunity as well. It has become a common practice to take antibiotics even for slightest of Infection.
All the bacterial infections have a tendency to go through phases and human body acts accordingly to cure the infection on its own called host immune response.
Antibiotics work after lowering the host immunity. Unnecessary antibiotic consumption not only bring resistance in bacteria to the drug. But it also reduces the host immune response on second encounter with same infection.

Does all Antibiotics work in a similar fashion for all age groups or is there some different mechanism. Another question are there any limitations or few facts to consider while treating children with Antibiotic Therapy.

How Do Antibiotics Work for Kids?

The mechanism of action of any antibiotic will remain similar in a child as in an adult patient. The only thing which varies in treating a child with antibiotic therapy is the concentration of drug in the child.

What are the causes for which a child should be treated with antibiotic therapy?

In routine practice it is seen that children need to be given antibiotics in situation like:

Throat Infections
Diarrhea
Ear Infections
Fever with Unknown Cause

But before treating a child with Antibiotic drug few things considered are:

Does the child needs an Antimicrobial?
Which antibiotic is appropriate?
What route of Antibiotic administration should be given?
Monitoring of the prescribed antibiotic effectiveness

There are also a few more factors which are considered while antibiotic therapy. Those factors are as such:

Age of Child
Weight of Child
Identification of the Causative microorganism to avoid giving broad spectrum antibiotics
Avoid giving drugs with known toxicity like Tetracycline is one such drug which is not given to new borns or infants.
The Drug Pharmacodynamics in child varies from an adult individual.

After knowing about the indications, metabolism, effects, routes of administration, Antibiotic resistance and the effectiveness of antibiotics for kids, the question arise that is it really good to have antibiotics.

Is Antibiotic Good For You?

Everything has a correct time and place. Consuming antibiotics is not bad at all, after all they have been the life saviors in cases of severe infections as rightly called the “Magic Bullets”

But with this arising problem of Antibiotic Resistance, it is very difficult to comment whether these drugs are good or bad. How do antibiotics work also determine whether should be consumed or not.

The odds of changing weather are usually seen as catching a cold or flu. These infections usually turn as upper respiratory tract infection or commonly known as sore throat. So you end up in a jam packed room full of people coughing around you.

The doctor will prescribe you an antibiotic to treat the infection as a last resort. But many times the moment you feel fine within two days of antibiotic consumption.

You will leave the drug course in between giving time to the bacteria to develop a resistance to the old drug. By the next season when you again catch a sore throat the bacteria would have become resistant to lower dose of the earlier antibiotic.

So bottom line for this statement is that antibiotics when taken under proper supervision, complete course, preferably narrow spectrum and calculated doses, they are definitely good for you.