Antibiotics Classification: Examples, Chart, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 05:24 PM IST

Since their discovery, antibiotics have undoubtedly transformed modern medicine into the contemporary successful treatment of bacterial infections that claimed so many lives before. From penicillin in the 1920s to the wide array of antibiotic classes in use today, these drugs rewrote how infectious diseases are treated. All of this may now be lost due to the growing problem of antibiotic resistance, which has made a huge dent in their efficacy.

This Story also Contains

Understanding Antibiotics
Types of Antibiotics
Real-life relevance and applications
Some Solved Examples
Summary

Antibiotics

Equally important to the meaning of antibiotics, their mechanism of action and their place in history is understanding such. We will explain each of the classes of antibiotics, detailing the mechanism of action, spectrum of activity, and ability for resistance. We will use real examples and case studies to detail why antibiotics play a paramount role in clinical and public health settings.

Understanding Antibiotics

Antibiotics represent very potent medicines designed to combat different kinds of bacterial infections. In principle, they act in two ways: bactericidal antibiotics kill bacteria directly, while the action of bacteriostatic antibiotics consists in slowing down the growth of bacteria until the immune system is able to fight off the infection. The first antibiotic, penicillin, was discovered by Alexander Fleming in 1928 and totally revolutionized medical science. Since then, millions have been saved from various diseases by being treated with antibiotics for small infections to potentially fatal ones.

The importance of antibiotics extends beyond the treatment of individual patients. Without them, surgical operations, cancer treatments, and control of chronic conditions like diabetes and lung disease would be radically different because of the occurrence and severity of infections. Inappropriate and excessive uses of the drugs, however, have paved the way for emerging resistant strains of bacteria against them. This growing crisis engulfs immediate attention from healthcare providers, researchers, and policymakers alike.

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The term antibiotics has been derived from the word 'Antibiosis' which means survival of fittest, i.e., a process in which one organism may destroy another to preserve itself. It is a chemical substance produced by or derived from living cells that is capable of inhibiting life processes or even destroying microorganisms.

The first antibiotic, discovered by Alexander Fleming in 1929 from the mold Penicillium notatum, was penicillin. In 1938, Ernst Chain and Howard Florey isolated penicillin in pure form and proved its effectiveness as an antibiotic. It was against the large number of infections caused by various cocci, gram-positive bacteria, etc. It is an effective drug for pneumonia, bronchitis, sore throat, and abscesses.

In penicillin, a four-membered ring is fused to another five-membered ring. Structures of individual penicillins are given in the table. Penicillin-G is the most commonly used. The penicillins are only sparingly soluble in water. However, their sodium or potassium salts are soluble in water. Penicillins are found to be active against gram-positive stains. However, these are ineffective against gram-negative organisms. Organisms sometimes develop resistance to penicillins. Penicillins generally have low toxicity in comparison to sulpha drugs. However, in some cases, allergic reactions may result. Penicillin is thus, given after a test prick.

The antibiotics can either be bactericidal or bacteriostatic. Examples are:

Bactericidal: The drugs that kill the organism in the body. For example, penicillin, ofloxacin, and aminoglycosides.
Bacteriostatic: The drugs that inhibit or check the growth of the organism in the body. For example, tetracycline, chloramphenicol, and erythromycin.

Types of Antibiotics

There are several classes of antibiotics according to their chemical structure and mechanism of action. The most frequently prescribed include:

Penicillins: This class of antibiotics includes many widely used antibiotics, such as amoxicillin and ampicillin, that are indicated for treatment against a wide variety of infections.
Cephalosporins: Similar to penicillins, they also have a broad spectrum of activity against microbes and are often used preoperatively.
Aminoglycosides: These are very potent antibiotics reserved mainly for serious infections caused by Gram-negative bacteria.
Macrolides: Against infections of the respiratory tract, macrolides—of which azithromycin is a noted example—are prescribed for those who show allergies to penicillin.
Tetracyclines: They are used against a wide array of infections, from acne to respiratory infections.

Most class activities display a characteristic spectrum of activity, side effects, and potential for resistance. Understanding these sections of information from each drug class is valuable in detailing appropriate prescribing and effective treatment outcomes.

Real-life relevance and applications

The role played by antibiotics in real life is simply not possible to overstate. They are useful in treating bacterial infections, preventing infections after surgeries, and in the management of chronic diseases. However, antibiotic resistance has serious concerns for public health. It is estimated that, according to the CDC, each year in the U.S., at least 2 million people get infected with antibiotic-resistant bacteria. This causes more than 23,000 deaths.

As an example, resistant bacteria with infections can complicate once-routine surgical procedures, turning minor operations into matters of life and death. Moreover, the financial cost due to antibiotic resistance is a very heavy one: it involves an increased healthcare cost due to prolonged hospital stays.

It is therefore important to have research in antibiotics and resistance within academia to help develop new treatments and strategies against the crisis. Alternative therapies for conditions where the arsenal of existing antibiotics is limited, such as bacteriophage therapy and novel antimicrobial agents, are being explored by investigators searching for remedies.

Public health campaigns for promoting the rational use of antibiotics, improving sanitation, and increasing infection control are critical to reducing the threat of antibiotic resistance. On this basis, education and awareness campaigns will enable patients and healthcare providers to make a proper decision about the use of antibiotics with a view to preserving these lifesaving drugs.

Some Solved Examples

Example 1: Matching Items

Question:
Match the items in column (I) with the items in column (II):
- (A) Norethindrone
- (B) Ofloxacin
- (C) Equanil

Column (II):
- (P) Antibiotic
- (Q) Anti-fertility
- (R) Hypertension
- (S) Analgesics

Solution:
- Norethindrone is an anti-fertility drug.
- Ofloxacin is an antibiotic.
- Equanil is used for treating hypertension.

So, the correct match is:
- (A) → (Q)
- (B) → (P)
- (C) → (R)

Hence, the answer is option (2).

Example 2: Streptomycin's Effectiveness

Question:
Streptomycin is effective for infections related to:
1. Throat, lungs, fever, and ears.
2. Throat, fever, ears, and lungs.
3. Throat, lungs, ears, and kidneys.
4. Fever, lungs, and ears.

Solution:
Streptomycin is highly effective against tuberculosis and common infections in the throat, lungs, ears, and kidneys.

Therefore, the correct answer is option (3).

Example 3: Drug Interaction with Oral Contraceptives

Question:
The drug which may not interfere with the efficiency of oral contraceptives is:
1. Barbiturates
2. Rifampicin
3. Ampicillin
4. Amoxicillin

Solution:
Rifampicin and barbiturates can lower the effectiveness of oral contraceptives. Ampicillin can also make birth control pills less effective. However, amoxicillin does not interfere with the effectiveness of oral contraceptives.

Hence, the correct answer is option (4).

Example 4: Aminoglycosides Use

Question:
Aminoglycosides are usually used as:
1. Antibiotic
2. Analgesic
3. Hypnotic
4. Anti-fertility

Solution:
Aminoglycosides are commonly used as antibiotics.

Therefore, the correct answer is option (1).

Example 5: Non-Bactericidal Antibiotic

Question:
Which of the following is NOT a bactericidal antibiotic?
1. Penicillin
2. Aminoglycosides
3. 0.03 ppm Cl2
4. Ofloxacin

Solution:
Penicillin, aminoglycosides, and ofloxacin are all bactericidal antibiotics. However, 0.03 ppm Cl2 is a disinfectant, not a bactericidal antibiotic.

Hence, the correct answer is option (3).

Summary

Shortly, antibiotics were gigantic victories of medicine in the treatment of bacterial infections and brought immense improvements in health outcomes in all fields of medicine. Their discovery changed the tide of medical history because it opened an avenue to cure, with great ease, what was considered hitherto to be fatal infections. From penicillin to a variety of classes of antibiotics in current clinical applications, they allowed surgeries and treatment of cancers alongside chronic disease management since they prevented infections.

Frequently Asked Questions (FAQs)

1. What are antibiotics?

Antibiotics were originally substances produced by one microorganism that selectively inhibited the growth of another microorganism. Since then, synthetic antibiotics have been developed that perform similar functions and are usually chemically similar to natural antibiotics. They have zero effect on viral infections.

Antibiotics are antibiotics that help stop infections caused by bacteria. They do this by killing bacteria or preventing them from multiplying or multiplying.

2. On which basis antibiotics are classified?

Although there are many schemes for classifying antibiotics based on bacterial chain (broad vs. narrow) or route of administration (injectable vs. oral vs. topical) or mode of action (bactericidal vs. bacteriostatic), chemical structure is the most useful. Antibiotics generally show similar trends in potency, toxicity, and allergenic potential across systemic classes.

3. Who discovered penicillin?

In 1926, Alexander Fleming discovered penicillin, a compound that could inhibit the growth of bacteria produced by fungi. Edward Chin and Howard Florey studied more penicillin in 1939 and later on penicillin in humans (thought to be a fatal bacterial infection). Fleming, Florey and Chain received the Nobel Prize in 1945 for their work in the beginning of the antibiotic era.

4. Write about the Classification list of antibiotics.

The classification list of antibiotics are-

1. Penicillin:

Penicillin V is an antibiotic from the penicillin group that helps fight bacteria in the body. Penicillin is used to treat many types of infections caused by bacteria, including ear infections. During active replication, penicillin G is bactericidal against penicillin-susceptible organisms.

2. Cephalosporins:

In wealthy countries, cephalosporins have dominated hospital antibiotic prescribing. Recommended daily for a wide range of infections. Their undoubted success is focused on reducing the risk of allergens and toxicity in various sports.

3. Fluoroquinolones:

Fluoroquinolones are antibiotics approved for dangerous and serious bacterial infections. As with all antibiotic products, official guidelines regarding the correct use of antimicrobial agents should be considered.

4. Tetracycline:

Tetracyclines are commonly used to treat acne, many skin infections, respiratory infections, and have been shown to be effective in treating urinary tract infections.

5. Macrolides:

Macrolides are often indicated for the treatment of community-acquired bacterial pneumonia because they act against multiple causative organisms. However, microbial resistance is becoming increasingly common.

5. What is the work of Antibiotics?

It kills bacteria in the body. This is the work of antibiotics.

6. Why is it important to classify antibiotics?

Classifying antibiotics is crucial because it helps healthcare providers choose the most appropriate treatment for specific infections. Classification is based on factors like chemical structure, mechanism of action, and spectrum of activity, which determines which bacteria they can target.

7. What is antibiotic resistance and why is it a concern?

Antibiotic resistance occurs when bacteria evolve to survive exposure to antibiotics, making infections harder to treat. It's a major concern because it can lead to longer illnesses, increased healthcare costs, and potentially untreatable infections, threatening modern medicine's ability to treat common diseases.

8. How do aminoglycoside antibiotics work, and why are they often used in combination with other antibiotics?

Aminoglycosides (like gentamicin) work by binding to the 30S subunit of the bacterial ribosome, causing misreading of the genetic code and inhibiting protein synthesis. They're often used in combination with other antibiotics to enhance effectiveness and reduce the risk of resistance, especially for serious gram-negative infections.

9. What is the significance of the "gram" classification in antibiotics?

The gram classification (gram-positive or gram-negative) refers to how bacteria react to a specific staining technique, which reflects differences in their cell wall structure. This classification is crucial for antibiotic selection because some antibiotics are more effective against one type than the other due to differences in cell wall permeability.

10. Why are some antibiotics classified as "last resort" treatments?

"Last resort" antibiotics, like colistin, are used when other antibiotics have failed, often due to antibiotic resistance. They're reserved for severe infections to prevent the development of resistance to these crucial drugs. Often, these antibiotics have more severe side effects or are less well-tolerated than first-line treatments.

11. What is the difference between broad-spectrum and narrow-spectrum antibiotics?

Broad-spectrum antibiotics are effective against a wide range of bacteria, both gram-positive and gram-negative. Narrow-spectrum antibiotics target specific types of bacteria. While broad-spectrum antibiotics may seem more versatile, using narrow-spectrum antibiotics when possible helps reduce antibiotic resistance.

12. How do macrolide antibiotics differ from beta-lactams in their mechanism of action?

While beta-lactams target cell wall synthesis, macrolide antibiotics (like erythromycin) inhibit protein synthesis in bacteria. They bind to the 50S subunit of the bacterial ribosome, preventing the formation of peptide bonds and thus stopping protein production essential for bacterial growth.

13. What are fluoroquinolones, and how do they work?

Fluoroquinolones are a class of broad-spectrum antibiotics that work by inhibiting bacterial DNA gyrase and topoisomerase IV enzymes. This prevents DNA replication and transcription in bacteria, leading to cell death. Examples include ciprofloxacin and levofloxacin.

14. Why are some antibiotics classified as bacteriostatic while others are bactericidal?

Bacteriostatic antibiotics (like tetracyclines) inhibit bacterial growth and reproduction without necessarily killing them, allowing the immune system to clear the infection. Bactericidal antibiotics (like penicillins) directly kill bacteria. The distinction is important for treating different types of infections and in patients with compromised immune systems.

15. How do beta-lactam antibiotics work?

Beta-lactam antibiotics, which include penicillins and cephalosporins, work by interfering with bacterial cell wall synthesis. They contain a beta-lactam ring that binds to penicillin-binding proteins (PBPs), preventing the formation of peptidoglycan cross-links in the bacterial cell wall, leading to cell lysis.

16. What are antibiotics and how do they work?

Antibiotics are medications that fight bacterial infections. They work by either killing bacteria directly or preventing them from reproducing, allowing the body's immune system to eliminate the infection. Antibiotics are not effective against viruses.

17. How do tetracyclines work, and why are they considered broad-spectrum antibiotics?

Tetracyclines work by binding to the 30S ribosomal subunit, inhibiting protein synthesis in bacteria. They're considered broad-spectrum because they're effective against a wide range of both gram-positive and gram-negative bacteria, as well as some atypical organisms like Chlamydia and Mycoplasma.

18. How do glycopeptide antibiotics like vancomycin differ from beta-lactams in their mechanism of action?

While both target cell wall synthesis, glycopeptides like vancomycin work differently from beta-lactams. Vancomycin binds to the D-Ala-D-Ala terminus of peptidoglycan precursors, preventing cross-linking of the peptidoglycan layers. This makes it effective against some beta-lactam-resistant bacteria.

19. How do sulfonamides work, and why are they often combined with trimethoprim?

Sulfonamides inhibit bacterial folate synthesis by competing with para-aminobenzoic acid (PABA). They're often combined with trimethoprim, which inhibits a different step in the folate synthesis pathway. This combination (e.g., co-trimoxazole) provides synergistic action, enhancing effectiveness and reducing the risk of resistance.

20. What is the mechanism of action of polymyxins, and why are they considered a "last resort" antibiotic?

Polymyxins (like colistin) are cationic polypeptides that disrupt the bacterial cell membrane, leading to cell death. They're considered a last resort because they can have significant side effects, particularly nephrotoxicity. However, they're crucial for treating some multidrug-resistant gram-negative infections.

21. What is the role of efflux pump inhibitors in overcoming antibiotic resistance?

Efflux pumps are bacterial mechanisms that expel antibiotics from the cell, contributing to resistance. Efflux pump inhibitors are compounds that block these pumps, potentially restoring the effectiveness of antibiotics. While not yet widely used clinically, they represent a promising strategy for combating antibiotic resistance.

22. What is the role of prodrugs in antibiotic therapy?

Prodrugs are inactive compounds that are metabolized in the body to produce the active drug. In antibiotic therapy, prodrugs can improve absorption, distribution, or reduce side effects. For example, the antibiotic cefuroxime axetil is a prodrug that improves oral absorption of cefuroxime.

23. What is the significance of the "generations" in cephalosporin classification?

Cephalosporins are classified into generations (1st to 5th) based on their spectrum of activity and resistance to beta-lactamases. Generally, later generations have broader spectrum activity, particularly against gram-negative bacteria, and improved stability against beta-lactamases. This classification helps in selecting the most appropriate cephalosporin for specific infections.

24. How do carbapenem antibiotics differ from other beta-lactams?

Carbapenems are a class of beta-lactam antibiotics with the broadest spectrum of activity. They have a unique chemical structure that makes them highly resistant to most beta-lactamases. This makes them effective against many multidrug-resistant bacteria, but they're often reserved as last-line treatments to prevent resistance development.

25. How do oxazolidinones like linezolid work, and why are they important in treating resistant infections?

Oxazolidinones work by binding to the 50S ribosomal subunit, inhibiting protein synthesis in a unique way that doesn't overlap with other antibiotics. This makes them effective against some multidrug-resistant gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA).

26. What is the significance of time-dependent versus concentration-dependent killing in antibiotic therapy?

Time-dependent killing antibiotics (like beta-lactams) are most effective when bacteria are exposed to them for a prolonged time, even at lower concentrations. Concentration-dependent killing antibiotics (like aminoglycosides) are most effective when they reach high peak concentrations. This distinction is crucial for determining dosing strategies to maximize efficacy and minimize resistance development.

27. How do nitrofurans like nitrofurantoin work, and why are they primarily used for urinary tract infections?

Nitrofurans are reduced by bacterial enzymes to reactive intermediates that damage bacterial DNA, RNA, and other cellular components. They're primarily used for urinary tract infections because they achieve high concentrations in urine while having low systemic absorption, minimizing side effects.

28. What is the role of beta-lactamase inhibitors in antibiotic therapy?

Beta-lactamase inhibitors, like clavulanic acid, are combined with beta-lactam antibiotics to overcome bacterial resistance. They work by irreversibly binding to beta-lactamase enzymes, preventing them from inactivating the antibiotic. This combination strategy extends the spectrum of activity of the antibiotic.

29. How do streptogramins work, and why are they often used in combination?

Streptogramins (like quinupristin/dalfopristin) work by binding to the 50S ribosomal subunit, inhibiting protein synthesis. They're often used in combination because different types of streptogramins (A and B) act synergistically, enhancing their antibacterial effect and reducing the risk of resistance development.

30. What is the significance of the minimum inhibitory concentration (MIC) in antibiotic classification and use?

The MIC is the lowest concentration of an antibiotic that prevents visible growth of a bacterium. It's crucial for determining the susceptibility of bacteria to antibiotics and guiding appropriate dosing. Lower MICs generally indicate greater antibiotic effectiveness against a particular bacterium.

31. How do lipopeptide antibiotics like daptomycin work?

Lipopeptides work by inserting into the bacterial cell membrane, causing rapid depolarization and cell death. Daptomycin, for example, is effective against many gram-positive bacteria, including some resistant strains. Its unique mechanism of action makes it less likely to induce cross-resistance with other antibiotic classes.

32. What is the importance of pharmacokinetics and pharmacodynamics in antibiotic classification and use?

Pharmacokinetics (how the body processes the drug) and pharmacodynamics (how the drug affects the body) are crucial for determining optimal dosing regimens. They help in classifying antibiotics based on their time- or concentration-dependent killing, tissue penetration, and elimination, which guides appropriate use and dosing strategies.

33. How do pleuromutilin antibiotics work, and why are they important in veterinary medicine?

Pleuromutilins (like retapamulin) work by binding to the peptidyl transferase center of the 50S ribosomal subunit, inhibiting protein synthesis. They're important in veterinary medicine due to their effectiveness against certain resistant bacteria and their limited use in humans, which helps preserve their efficacy for animal health.

34. How do fidaxomicin and rifaximin differ from systemic antibiotics in their use and mechanism of action?

Fidaxomicin and rifaximin are antibiotics that work primarily in the gastrointestinal tract and have minimal systemic absorption. Fidaxomicin inhibits bacterial RNA synthesis and is used to treat Clostridioides difficile infections. Rifaximin inhibits bacterial RNA synthesis and is used for traveler's diarrhea and hepatic encephalopathy. Their localized action reduces systemic side effects and disruption of the normal gut microbiome.

35. What is the significance of the mutant prevention concentration (MPC) in antibiotic therapy?

The MPC is the antibiotic concentration that prevents the growth of the least susceptible single-step mutant. It's important in preventing the selection of resistant mutants during therapy. Dosing strategies that maintain antibiotic concentrations above the MPC can help reduce the development of resistance.

36. How do bacteriophage therapies differ from traditional antibiotics, and why are they gaining interest?

Bacteriophage therapies use viruses that specifically infect and kill bacteria. Unlike traditional antibiotics, phages are highly specific to particular bacterial strains and can evolve with bacteria. They're gaining interest as a potential alternative to antibiotics, especially for treating multidrug-resistant infections.

37. What is the role of combination antibiotic therapy, and when is it typically used?

Combination antibiotic therapy involves using two or more antibiotics simultaneously. It's used to broaden the spectrum of activity, achieve synergistic effects, prevent resistance development, and treat polymicrobial infections. It's particularly important in severe infections, immunocompromised patients, and when dealing with suspected resistant organisms.

38. How do antiseptics differ from antibiotics in their classification and use?

Antiseptics are chemical agents applied to living tissue to prevent or stop the growth of microorganisms. Unlike antibiotics, which are typically used systemically, antiseptics are used topically and have a broader spectrum of activity, often affecting multiple cellular targets. They're classified based on their chemical structure and are crucial in preventing infections in wounds and during surgical procedures.

39. What is the significance of the post-antibiotic effect in antibiotic classification and dosing?

The post-antibiotic effect refers to the continued suppression of bacterial growth after antibiotic concentrations have fallen below the MIC. It's important in determining dosing intervals and can influence whether an antibiotic is classified as time- or concentration-dependent. Antibiotics with a longer post-antibiotic effect may allow for less frequent dosing.

40. How do topical antibiotics differ from systemic antibiotics in their classification and use?

Topical antibiotics are applied directly to the skin or mucous membranes and are classified based on their ability to penetrate tissues and their spectrum of activity. They're used for localized infections and to prevent wound infections. Unlike systemic antibiotics, they achieve high local concentrations with minimal systemic absorption, reducing the risk of systemic side effects and resistance development.

41. What is the role of antibiotic stewardship programs in the context of antibiotic classification and use?

Antibiotic stewardship programs aim to optimize antibiotic use to improve patient outcomes, reduce adverse effects, and combat antibiotic resistance. They involve classifying antibiotics into categories (e.g., unrestricted, restricted, reserved) based on their importance and potential for resistance development. These programs guide appropriate antibiotic selection, dosing, and duration of therapy.

42. How do antifungal antibiotics differ from antibacterial antibiotics in their classification and mechanisms of action?

Antifungal antibiotics target fungi rather than bacteria and are classified based on their chemical structure and mechanism of action. Major classes include azoles (inhibit ergosterol synthesis), polyenes (bind to ergosterol in cell membranes), and echinocandins (inhibit cell wall synthesis). Unlike most antibacterial antibiotics, many antifungals target eukaryotic cells, which can increase the risk of toxicity to human cells.

43. What is the significance of the minimum bactericidal concentration (MBC) in antibiotic classification and use?

The MBC is the lowest concentration of an antibiotic required to kill 99.9% of bacteria. It's important in distinguishing between bacteriostatic and bactericidal antibiotics and can guide dosing strategies. Antibiotics with MBCs close to their MICs are generally considered more bactericidal, which can be crucial in treating severe infections or in immunocompromised patients.

44. How do antibiotic-antibiotic interactions influence their classification and use in combination therapy?

Antibiotic-antibiotic interactions can be synergistic, additive, indifferent, or antagonistic. These interactions influence how antibiotics are classified for combination therapy. Synergistic combinations (like beta-lactams with aminoglycosides) are often used for severe infections, while antagonistic combinations (like bactericidal with bacteriostatic antibiotics) are generally avoided.

45. What is the role of pharmacogenomics in antibiotic classification and personalized antibiotic therapy?

Pharmacogenomics studies how genetic variations affect drug response. In antibiotic therapy, it can help classify patients based on their likelihood of responding to certain antibiotics or experiencing side effects. This allows for more personalized antibiotic selection and dosing, potentially improving efficacy and reducing adverse reactions.

46. How do antibiotics used for mycobacterial infections (like tuberculosis) differ from those used for typical bacterial infections?

Antibiotics for mycobacterial infections often have unique mechanisms of action due to the unusual cell wall structure of mycobacteria. They include drugs like isoniazid (inhibits mycolic acid synthesis), rifampin (inhibits RNA synthesis), and ethambutol (inhibits arabinogalactan synthesis). These antibiotics often require longer treatment durations and are used in combination to prevent resistance development.

47. What is the significance of the mutant selection window in antibiotic therapy and resistance prevention?

The mutant selection window is the range of antibiotic concentrations between the MIC and the MPC where resistant