Antibiotics: Mode of Action and Types

Antibiotics are common agents used in modern healthcare that are used against certain microbial infections. Antibiotics are compounds that fight infections caused by bacteria in humans and animals. They have undoubtedly transformed the world and have certainly contributed to increasing human life expectancy by treating several life-threatening infectious diseases by the administration of antibiotics.

Antibiotics are common agents used in modern healthcare that are used against certain microbial infections. Antibiotics are compounds that fight infections caused by bacteria in humans and animals. They have undoubtedly transformed the world and have certainly contributed to increasing human life expectancy by treating several life-threatening infectious diseases by the administration of antibiotics.

Contents: Mode of action, Types of Antibiotics

In 1928, Alexander Fleming made an unintended discovery that eventually led to the development of an antibiotic. He observed a substance (penicillin) made by the fungus Penicillium notatum selectively killed a variety of bacteria without harming the host cells. The discovery of penicillin by Fleming transformed contemporary medicine.

Scientists experimented with creating semi-synthetic penicillin molecules in the late 1950s by adding various chemical groups to the core of the penicillin molecule. Thus, a wide variety of penicillins became accessible to treat a number of illnesses brought on by various bacteria.

Penicillin seemed to have no effect on the tubercle bacillus (Mycobacterium tuberculosis), but streptomycin, an antibiotic discovered in 1943 from Streptomyces griseus, caused this organism to become extremely sensitive. Cephalosporins, which resemble penicillins but are made by a mold called Cephalosporium acrimonium were discovered by scientists in the 1950s. In the same decade, the quinolone antibiotic class was also discovered.

Antibiotics may also differ in their spectrum of action because certain antibiotics are relatively specific and some, like tetracyclines, work efficiently against a wide range of various bacteria. They are notably helpful in treating infections when there is not enough time to conduct susceptibility tests and in battling mixed infections. While some antibiotics, like quinolones and semi-synthetic penicillins, must be administered intravenously or intramuscularly, others can be taken orally. 

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Mode of action

Antimicrobial agents are classically grouped into two main categories based on their In vitro effect on bacteria: bactericidal and bacteriostatic. The bactericidal antibiotics “kill” bacteria while bacteriostatic antibiotics “prevent the growth” of bacteria.

Antibiotics: Mode of Action and Types;
Figure: Vancomycin mode of action

The antibiotics are classified on the basis of their mode of action which includes inhibition of cell wall synthesizing. Antibiotics including penicillin, cephalosporins, and vancomycin suppress the synthesis of peptidoglycan membrane structures as part of their antimicrobial effect. Peptidoglycan is a series of polymers made of N-acetylglucosamine and N-acetylmuramic acid repeating units that are cross-linked with short peptides to provide cell wall stiffness.

There are many processes involved in the biosynthesis of the peptidoglycan. Any disruption in the process might stop the production of the cell wall, which would make it impossible for the bacteria to live. Aside from aminoglycosides and chloramphenicol, other antibiotics that hinder bacteria from synthesizing protein include aminoglycosides, chloramphenicol, erythromycin and clindamycin.

Antibiotics like polymyxin B and polymyxin E adhere to phospholipids in the bacterial cell membrane and prevent it from serving as a selective barrier, allowing essential macromolecules to flow out and attack the cell.

The synthesis of nucleic acid requires a certain class of antibiotics called sulfonamides as a required precursor. Because they mimic the intermediary component (para-aminobenzoic acid), which is converted by an enzyme to folic acid, sulfonamides have the capacity to block the synthesis of folic acid.

The rivalry for the enzyme that converts the intermediate into folic acid between para-aminobenzoic acid and sulfonamide is triggered by their identical chemical structures. By eradicating the chemicals from the process, the reaction can be reversed, which inhibits the germs.

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Types of Antibiotics

1. Penicillins

Penicillin notatum, also known as Penicillium chrysogenum, is the source of the first modern antibiotics. Both gram-positive and gram-negative bacteria can be killed by penicillin. The beta-lactam ring, a distinctive feature of penicillins, controls their antibacterial effects. The final stage in the cell wall is accomplished by the beta-lactam ring, which interacts with proteins in bacterial cells.

Penicillins are divided into semi-synthetic penicillins and naturally occurring penicillins (penicillin g, penicillin v, and benzathine penicillin). By cultivating penicillium mold under conditions that only allow it to produce the primary molecule (6 amino penicillanic acid), semi-synthetic penicillins can be developed.  

Despite the fact that penicillins are less hazardous to humans, they nonetheless have the potential to produce allergic reactions such as rashes, swelling, and anaphylaxis. Penicillins come in three different types: penicillinase-resistant, antipseudomonal, and beta-lactamase inhibitors. Meningitis, syphilis, urinary tract infections, and throat infections are all treated using penicillin.

2. Cephalosporins

A class of antibiotics called cephalosporins are produced by the bacterium Cephalosporium acremonium and have similarities to the penicillium species. They only attack gram-positive and gram-negative bacteria.  Cefazolin, Cephalothin, and Cephalexin are first-generation cephalosporins that exhibit a spectrum of antibacterial activity similar to that of broad-spectrum penicillins. 

Cefamandol, cefaclor, cefoxitin, and cefuroxime are second-generation cephalosporins with significant efficacy against additional gram-negative bacilli and a broad spectrum of activity. Cephalosporins are used to treat lung infections, strep throat, skin infections, urinary tract infections, and ear infections.

3. Tetracyclines

Tetracyclines are an anti-inflammatory class of antibiotics that can be used to treat a number of viral infections. They are used to treat inflammatory skin disorders like acne, rosacea, and perioral dermatitis as well as chest, urethral, and pelvic infections. Tetracyclines are risky for oral health and should not be used by anyone under the age of 12 or those who are pregnant or nursing.

Oesophageal discomfort is another side effect of tetracycline. It’s recommended to stay out of the sun as well because doxycycline might cause photosensitivity and sunburn. The most popular tetracyclines are sarecycline, doxycycline, and minocycline.

4. Macrolides

A class of antibiotics known as macrolides have anti-inflammatory effects. They may deal with microorganisms resistant to penicillin. Additionally, it is a good option for those who are allergic to cephalosporin or penicillin. These antibiotics are frequently used to treat STIs, respiratory infections, and skin problems like rosacea, acne, and rosacea. 

Among the common macrolides are erythromycin, clarithromycin, azithromycin, and roxithromycin. Elderly people and people with compromised kidney or liver function are more likely to experience toxicity.

5. Fluoroquinolones

Fluoroquinolones, sometimes referred to as quinolones, can combat bacterial infections that are dangerous or challenging to treat. Prostatitis, acute salmonellosis, and shigellosis are all treated with them as the first line of defense. Additionally, it is used to treat some forms of gonorrhoea and TB.

Due to their interactions with antimicrobial resistance, they should only be taken when absolutely necessary. The most widely used fluoroquinolones include ciprofloxacin, ofloxacin, and moxifloxacin. Aortic regurgitation, QT prolongation, and other heart problems are some of the side effects.

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6. Sulfonamides

Synthetic antibiotics called sulfonamides also referred to as sulfa medicines, are recommended by doctors when first-line therapy is inadequate. The most popular kind is co-trimoxazole, which is sulfamethoxazole combined with trimethoprim. It can treat illnesses like pneumocystis pneumonia and nocardiosis in adults with compromised immune systems and lessen the prevalence of various diseases in children.

The most prevalent sulfonamide kinds are sulfasalazine, sulfacetamide, and silver sulfadiazine. Due to the potential for increased miscarriage risk, sulfonamides shouldn’t be used during pregnancy.

7. Glycopeptides

Drug-resistant bacteria, such as methicillin-resistant Staphylococcus aureus and multidrug-resistant Streptococcus pneumonia, can be treated with glycopeptide antibiotics. Vancomycin, dalbavancin, and oritavancin are some of the most prevalent glycopeptides.

8. Lincomycins

This class is effective against some gram-negative bacteria as well as gram-positive aerobic and anaerobic bacteria. Derivatives of lincomycin are useful for treating serious illnesses like gastric ulcers, infections of the lower respiratory tract, and infections of the bones and joints. Some of them can even be used to cure acne on the skin.

Clindamycin is an example of a typical lincomycin. Clindamycin comes in three salt forms, including clindamycin phosphate, clindamycin hydrochloride, and clindamycin nicotinamide, all of which are equally effective.

9. Aminoglycosides

Aminoglycosides swiftly work as antibiotics by attaching to the 30S ribosome, which prevents bacteria from synthesizing new cells. In addition to being accessible in inhalation and eye dosage forms, these drugs are typically administered intravenously. Aminoglycosides that are frequently used include gentamicin, tobramycin, and amikacin.

Author: Athulya B S

Edited: Ajmal Aseem, Achuth B S

Published: 30 September 2023


Athulya B S

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