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Antibiotics: Definition, Discovery, Production From Microbes, Classification, Examples and Mode of Action

Antibiotics: Definition, Discovery, Production From Microbes, Classification, Examples and Mode of Action

What do you generally do when you feel very sick? You might probably go to a doctor who will check you to diagnose the disease and its cause. After that he might prescribe some medicines which can help you to get better. Have you ever wondered what kind of medicines are prescribed by the doctors?

Does the term ‘antibiotics’ ring a bell? Generally for diseases caused due to bacterial infections, we are prescribed with antibiotics that help in eliminating the bacteria from our body. But did you know that antibiotics are actually produced by microorganisms and are capable of killing other microorganisms. Interesting, right? But how are these chemicals produced by microbes obtained and used as medicines? What are their source microbes? How did we find out about this technique? Let us find out the answers to all these questions in this article. 

Table of contents

  • Definition of antibiotics
  • Discovery of antibiotics 
  • Production of antibiotics from microbes
  • Classification of antibiotics
  • Examples of antibiotics
  • Practice Problems
  • FAQs

Definition of antibiotics

What is the first thing that comes to mind when you hear the term antibiotics? It will obviously be penicillin. Right? Penicillin was the first discovered antibiotic and it kills other microbes such as bacteria. So you can guess why it is called an antibiotic. In Greek anti means against and bio means life. So antibiotics are the chemicals that work against the life of microbes.

Hence antibiotics can be defined as the chemical substances(secondary metabolites), which are produced by some microbes and have the ability to kill or retard the growth of other microbes. Antibiotics can be used as drugs in appropriate concentrations and can help to eliminate the disease-causing microbes within a host body, without harming the host.

                                       Fig: Antibiotics

Characteristics of an ideal antibiotic

An ideal antibiotic is the one which does not harm the normal microflora of the host, does not have any side effects and is effective against a wide range of different pathogens, i.e, it should be broad spectrum in nature. It should also not take too long to start working against the pathogens.

Discovery of antibiotics

Did you know that Penicillin was the first antibiotic to be discovered, and it was an accidental discovery by Alexander Fleming? But how did it happen? Let us discuss more about the discovery of antibiotics now.

                                             Fig: Alexander Fleming and Penicillin

Penicillin was the first naturally produced antibiotic to be discovered by Alexander Fleming in 1928. Alexander Fleming observed a mould growing in one of his unwashed Staphylococcus culture plates around which Staphylococci could not grow. 

                                                  Fig: Discovery of penicillin

That mould was identified as Penicillium genus. Penicillium moulds generated a diffusible extract with antibacterial action against Staphylococci. Although Fleming conducted multiple in vitro trials, he did not test the extract against animal models. Thus, penicillin was only used as a local antiseptic for many years. Until the late 1930s, it was also difficult to isolate and purify penicillin.

                                               Fig: Penicillium and penicillin

Florey and Chain discovered the structure of penicillin in 1939. In 1940, they demonstrated that penicillin could be used to treat streptococcal infection in mice. Following this discovery, penicillin began to be used widely as an antibiotic, bringing in the antibiotic golden age. The Nobel Prize in Physiology or Medicine was awarded to Fleming, Chain, and Florey in 1945 for their discovery of penicillin.

                                          Fig: Chain and Florey

During World War II, this antibiotic was widely used to treat American soldiers who had been injured. Other antibiotics were isolated from other bacteria after Penicillin.

                                     Fig: Soldiers getting treated with penicillin

Antibiotics have significantly enhanced our ability to treat terrible diseases like plague, whooping cough, diphtheria, and leprosy, which used to kill millions of people around the world. Antibiotics are now so common that we can't envision a world without them.

Streptomyces is the greatest antibiotic-producing actinomycetes genus ever found in the microbial world. This group is notable for being the world's largest producer of antibiotics. The species are responsible for manufacturing the greatest amount of antibiotics. Antibiotics with a wide range of activities, such as antibacterial, antifungal, and antiparasitic, are synthesised by them. They are also known to create chemicals that can inhibit the immune system or are called immunosuppressants.

                                                 Fig: Streptomyces

Production of antibiotics from microbes

Antibiotics are industrially produced by culturing the source microorganism in big containers (100,000–150,000 litres or more) called bioreactors with a liquid growth medium.

                                             Fig: Production of antibiotics

Oxygen concentration, temperature, pH, and nutrient levels must all be at their optimum values, which are constantly checked and modified as needed. Because antibiotics are secondary metabolites, the population number must be carefully managed to achieve maximal output before the cells die. The antibiotic must next be removed and refined into a crystalline form once the process is complete. If the antibiotic is soluble in an organic solvent, this is easier to accomplish. Otherwise, ion exchange, adsorption, or chemical precipitation must be used to separate it from the culture medium. It is important to maintain aseptic conditions and avoid any sort of contamination during the entire process.

Sometimes the antibiotics obtained from microbes are modified to enhance their potency. These are known as semisynthetic antibiotics, e.g., ampicillin. Some antibiotics are completely designed and prepared in the laboratories and are known as synthetic antibiotics, e.g, norfloxacin.

For industrial production of antibiotics, Penicillium notatum has been replaced with Penicillium chrysogenum. The total number of known antibiotics is more than 7000. Bacillus subtilis produces around 60 varieties of antibiotics and Streptomyces griseus produces around 41 different antibiotics.

Classification of antibiotics

Antibiotics can be classified as bactericidal if they kill the bacteria or bacteriostatic if they just inhibit the growth. They can also be classified as broad spectrum or specific based on whether they are effective against a wide variety of pathogens or a single type of pathogen, respectively.

Based on their composition antibiotics are categorised into the following five classes -

1. Beta lactams, e.g., Penicillin
2. Aminoglycosides, e.g., Gentamicin
3. Quinolones, e.g., Ciprofloxacin
4. Sulfonamides, e.g., Sulfamethoxazole
5. Glycopeptides, e.g., Vancomycin

Examples of antibiotics

Now we will see some of the antibiotics and the microorganisms which are responsible for producing it and the diseases which they are used for treatment.



Mode of action

Diseases treated


Penicillium chrysogenum

Disrupt cell wall synthesis

Sore throat, Gonorrhoea, Rheumatic fever


Streptomyces griseus

Inhibit the activity of the 30S ribosomal subunit during protein synthesis

Meningitis, Pneumonia, Tuberculosis and Local Infection. 


Streptomyces aureofaciens

Inhibit protein synthesis by not allowing the binding of tRNAs charged with amino acids to the ribosomes.

Viral pneumonia, Osteomyelitis, Whooping Cough and Eye infections.


Streptomyces venezuelae

Inhibits protein synthesis

Typhoid, Typhus, Whooping cough, A typical Pneumonia, Bacterial Urinary Infections.


Micromonospora purpurea

Inhibit the activity of the 50S ribosomal subunit during protein synthesis

Effective against Gram (+) bacteria


Bacillus licheniformis

Inhibits cell wall synthesis

Syphilis, Lymphedema or Reticulosis.


Aspergillus fumigatus

Inhibits RNA synthesis

Was originally used against intestinal amoebiasis. Also used against microsporidian parasites in honeybees, effective against some myxozoan parasites in fishes, etc.

Practice Problems

Q 1. Assertion (A): Bacterial infections are treated using antibiotics.
Reason (R): Antibiotics are secondary metabolites released by microbes that can kill or inhibit the growth of other microbes.

a. Both assertion and reason are true and the reason correctly explains the assertion.
b. Both assertion and reason are true but the reason does not correctly explain the assertion.
c. Only the assertion is true.
d. Both assertion and reason are false.

Answer: Antibiotics are secondary metabolites produced by microbes that can either kill or stop the growth of other microbes. Due to this property, they are widely used in the field of medicine for the treatment of bacterial infections. Antibiotics can be used as drugs in appropriate concentrations and can help to eliminate the disease-causing microbes within a host body. Thus, both assertion and reason are true and the reason correctly explains the assertion. 

Hence the correct option is a.

Q 2. Match the following:



p. Penicillin

i. Streptomyces griseus

q. Streptomycin

ii. Streptomyces erythraeus

r. Erythromycin

iii. Penicillium notatum

a. p-i, q-iii, r-ii
b. p-ii, q-i, r-iii
c. p-iii, q-i, r-ii
d. p-iii, q-ii, r-i

Answer: Antibiotics are chemical compounds produced by microbes that can kill or inhibit the growth of other microbes. Penicillin is produced from Penicillium notatum. Streptomycin is produced from Streptomyces griseus and Erythromycin is produced from Streptomyces erythaeus.

Hence the correct option is c.

Q 3. Which of the following is not true for an ideal antibiotic?

a. It is effective only against specific strains of a bacterial species
b. It has no side effects on the host
c. It has no negative effects on the normal microflora of the host
d. It should quickly start acting against pathogens after administration

Answer:. An ideal antibiotic should be broad spectrum in action and should act against a wide variety of different pathogens. It should not only act against different strains of a bacterial species but should also work against both Gram positive and Gram negative bacteria. It should not have any side effects on the host and should not eliminate the normal microflora of the host. It should start working against pathogens within the body without losing much time.

Hence the correct option is a.

Q 4. A child was found to have been infected by Streptococcus pneumoniae. What would the treatment mainly consist of?

a. Antipyretics
b. Analgesics
c. Antibiotics
d. Laxatives

Answer: Streptococcus pneumoniae is a bacterium that causes a highly contagious respiratory tract infection called pneumonia. It is characterised by cough with phlegm, fever, chills and breathing difficulties. 

The treatment would mainly consist of antibiotics which are chemical agents obtained from microbes and work against other microbes by retarding their growth or killing them. Antibiotics are widely used for treating bacterial infections.

Antipyretics are drugs that are used to reduce fever. Analgesics are pain killers. Laxatives are used to reduce constipation and increase bowel movement.

Hence the correct option is c.

Q 5. How many of the following diseases can be treated with antibiotics?
Tuberculosis, common cold, polio, diphtheria, leprosy, flu

a. 3
b. 5
c. 4
d. 2

Answer: Antibiotics are chemical compounds released by some microbes that can retard the growth or kill other microbes. These are used as drugs to treat bacterial infections. Of the above mentioned diseases, tuberculosis, diphtheria and leprosy are caused by bacteria.

Antibiotics are not effective against viral diseases like common cold, flu, polio, etc as viruses are acellular in nature and live inside the cells of its host. Thus, antibiotics are not able to target them.

Hence the correct option is a.


Q 1. What are the side effects of antibiotics?
Answer: Antibiotics have a lot of adverse effects, despite the fact that they save a lot of lives. Allergic responses, digestive difficulties such as nausea, indigestion, vomiting, diarrhoea, and so on. Fungal infection, photosensitivity, staining on teeth, nails, bones etc are some of them other side effects. Antibiotics used to treat a urinary tract infection can sometimes result in a vaginal yeast infection.

Q 2. What is antibiotic resistance?
Answer: When bacteria gain the capacity to resist the antibiotic's ability to kill them, this is known as antibiotic resistance. Excessive use of antibiotics can often be the reason for the development of antibiotic resistance amongst pathogens. Antibiotic-resistant bacteria can cause infections that are resistant to all available antibiotics. Antibacterial-resistant illnesses can be extremely dangerous and even fatal. Antibiotic resistance can harm humans at any age, as well as the healthcare, veterinary, and agriculture industries. As a result, it is one of the world's most pressing public health issues. Antibiotic-resistant infections that need the use of second- and third-line medicines can harm patients by producing major side effects like organ failure and delaying therapy and recovery for months at a time.

Q 3. Why do antibiotics fail to treat viral infections?
Answer: Antibiotics are used to treat bacterial infections, but they are unable to kill viruses. Viruses differ from bacteria in terms of their structure and manner of replication. Antibiotics either destroy bacteria's cell walls or target their growth machinery. Viruses lack a cell wall, but the genetic component is contained within the protein coat. Viruses do not self-replicate; instead, they generate multiple copies of themselves using the host's machinery. They infect healthy cells and reprogram them to duplicate their genome numerous times. Antibiotics are ineffective against viruses because of this.

Q 4. In what forms antibiotics can be consumed?
Answer: Antibiotics aren't only available in pill form. You can use them in a variety of ways. Tablets, capsules, liquid (to drink), creams, sprays, drips, lotions, injections, and a drip that goes straight into the blood can all be used to administer them. Antibiotics come in a variety of forms. This is fortunate since we may employ different ones to address various health issues. Furthermore, different antibiotics can perform the same function. This is important when someone is allergic to one antibiotic and needs to switch to another.

YOUTUBE LINK: https://youtu.be/HeTWhOdv0mA (34:45 - 37:14)

Related Topics

Microbes in chemicals, enzymes and other bioactive molecules

Microbes in household products 

Microbes in fermented beverages

Microbes as Biofertilisers 


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