15. ANTIBIOTICS

ANTIBIOTICS: A TEACHER’S COMPREHENSIVE GUIDE

Welcome, future pharmacists and healthcare professionals!

The word antibiotic was derived from the word antibiosis which means “against life.” Historically, antibiotics were believed to be organic compounds produced by a microorganism toxic to other microorganisms. Due to this belief, an antibiotic was initially defined as a substance produced by a microorganism which can prevent the growth of, or are fatal to, other microorganisms even at low concentrations. However, this definition has been modified at the present time to include antimicrobials produced by synthetic means either partially or wholly.

Antibiotics can either kill other bacteria or inhibit their growth. Those antibiotics which kill bacteria are termed bactericidal and those which inhibit bacterial growth are termed bacteriostatic. Even though antibiotics are referred to as antibacterial agents, they are differentiated as antibacterials, antifungals, and antivirals to indicate the type of microorganisms against which they act.

As a pharmaceutical chemistry educator with years of experience teaching antimicrobial pharmacology, I have observed that students often find this topic challenging due to the variety of drug classes and their complex mechanisms of action. Let me tell you: Understanding antibiotics is essential for every pharmacy professional.

In this comprehensive guide, I will walk you through the major classes of antibiotics—β-lactam antibiotics, aminoglycosides, tetracyclines, macrolides, and miscellaneous agents. I will explain their classifications, mechanisms of action, uses, stability, and formulations. By the end of this article, you will have a thorough understanding of these essential antimicrobial agents. Let us begin our journey!

Dpharmguru’s exam insights:

Antibiotics are frequently tested in pharmacy exams. Remember: Bactericidal antibiotics kill bacteria, while bacteriostatic antibiotics inhibit bacterial growth. β-lactam antibiotics (penicillins, cephalosporins) inhibit cell wall synthesis. Aminoglycosides inhibit protein synthesis. Tetracyclines and macrolides also inhibit protein synthesis but at different ribosomal subunits. Pay special attention to the mechanisms of action and classifications—these are almost always asked in exams!

12.1. CLASSIFICATION OF ANTIBIOTICS

• β-Lactam Antibiotics: Penicillins, Cephalosporins, Carbapenems, Monobactams
• Tetracyclines: Doxycycline, Minocycline, Oxytetracycline
• Macrolides: Erythromycin, Azithromycin, Clarithromycin
• Aminoglycosides: Streptomycin, Gentamicin, Tobramycin, Amikacin
• Miscellaneous Agents: Chloramphenicol, Clindamycin, Rifampicin, Fusidic acid, Griseofulvin

12.2. β-LACTAM ANTIBIOTICS

β-Lactam antibiotics are the most widely used antibiotics among all antibiotics. They possess a highly reactive 3-carbon, 1-nitrogen ring in their molecular structure, i.e., the β-lactam ring. All members of this category, including penicillins (penams) and cephalosporins (cephems), act by inhibiting the cell wall synthesis of the bacterial organism.

The β-lactam antibiotics are generally given with β-lactamase inhibitors (e.g., clavulanic acid) because bacteria obtain resistance to β-lactam antibiotics by producing β-lactamase enzyme which attacks the β-lactam ring.

12.2.1. Classification of β-Lactam Antibiotics

• Penicillins: Penicillin G, Penicillin V, Methicillin, Nafcillin, Oxacillin, Cloxacillin, Dicloxacillin, Amoxicillin, Carbenicillin, Ticarcillin
• Cephalosporins:
  • 1st Generation: Cefazolin, Cefadroxil, Cephalexin, Cephradine
  • 2nd Generation: Cefaclor, Cefamandole, Cefonicid, Cefmetazole, Cefoxitin, Cefotetan, Cefuroxime
  • 3rd Generation: Cefoperazone, Cefotaxime, Ceftazidime, Ceftriaxone, Cefixime, Moxalactam
  • 4th and 5th Generation: Cefepime, Cefpirome, Cefquinome, Ceftobiprole, Ceftaroline Fosamil
• Carbapenems: Biapenem, Ertapenem, Doripenem, Imipenem, Panipenem
• Monobactams: Aztreonam, Tigemonam, Carumonam

12.2.2. Examples of β-Lactam Antibiotics

12.2.3. Penicillin G (Benzyl Penicillin)

Benzylpenicillin is a narrow spectrum natural penicillin antibiotic. It shows poor oral absorption, thus is administered intravenously or intramuscularly.

Chemical Name: (2S,5R,6R)-3,3-dimethyl-7-oxo-6-[(2-phenylacetyl)amino]-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid

Structure: The nucleus of penicillin consists of fused rings of thiazolidine and β-lactam, with side chains attached through an amide linkage. Penicillin G with a benzyl side chain is the original penicillin used clinically.

Mechanism of Action: Penicillin G attaches to penicillin binding proteins (PBPs) found inside the bacterial cell wall and inactivates them. PBP inactivation disrupts the cross-linking of peptidoglycan chains, which is required for bacterial cell wall strength and stiffness.

Uses: Benzylpenicillin is used in septicaemia, meningitis, pericarditis, endocarditis, and severe pneumonia caused by penicillin G-susceptible microorganisms.

Stability & Storage: Stable at pH range of 5.0-8.0. Refrigerate between 2-8°C (36-46°F). Prevent from freezing.

Formulations: Injection powders, Solutions

Brand Names: Pfizerpen, PenG or BENPEN, Bicillin L-A

12.2.4. Amoxicillin

Amoxicillin is a penicillin derivative used to treat infections produced by gram-positive bacteria, particularly in upper respiratory tract infections caused by streptococcal bacteria.

Chemical Name: (2S,5R,6R)-6-{[(2R)-2-Amino-2-(4-hydroxyphenyl)acetyl]amino}-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid

Mechanism of Action: Penicillin-binding proteins are competitively inhibited by amoxicillin. This results in the crosslinking of D-alanine and D-aspartic acid. Without the action of penicillin binding proteins, bacteria are unable to build and repair the cell wall, leading to bactericidal action.

Uses:

• Bacterial infections including tonsillitis, bronchitis, pneumonia, and infections of the ear, nose, throat, skin, and urinary tract
• Stomach ulcers caused by Helicobacter pylori infection (with clarithromycin and lansoprazole)

Stability & Storage: Capsules and pills should be kept at room temperature, away from heat and moisture. Liquid medication should be refrigerated. After 14 days, throw away any unused liquid medication.

Formulations: Suspensions, Tablets

Brand Names: Amoxil, Trimox, Moxatag

12.2.5. Cloxacillin

Cloxacillin is an antibiotic used to treat infections caused by beta-hemolytic streptococcal, pneumococcal, and staphylococcal infections.

Chemical Name: (2S,5R,6R)-6-[[3-(2-chlorophenyl)-5-methyl-1,2-oxazole-4-carbonyl]amino]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylate

Mechanism of Action: Cloxacillin inhibits the third and final stage of bacterial cell wall formation by attaching to penicillin-binding proteins (PBPs) found inside the bacterial cell wall.

Stability & Storage: Capsules should be kept away from light and moisture at room temperature (59-86°F / 15-30°C). Liquid solution should be refrigerated at 36-46°F (2-8°C).

Formulations: Powder, Liquid

Brand Names: Amclo-AIIPL, Cloxapen, Cloxacap, Tegopen

Dpharmguru’s exam insights:

β-Lactam antibiotics are frequently tested. Remember: Penicillin G is the original natural penicillin. Amoxicillin is a broad-spectrum penicillin. Cloxacillin is penicillinase-resistant. Cephalosporins are classified into generations based on their antibacterial spectrum. A common exam question is: “What is the mechanism of action of penicillins?” (Answer: Inhibition of bacterial cell wall synthesis by binding to PBPs).

12.3. AMINOGLYCOSIDES

Aminoglycosides are molecules composed of amino-modified sugars. They are water-soluble, stable in solution, and more active at alkaline pH. They are widely used against gram-negative enteric bacteria in bacteremia and sepsis. Along with vancomycin or penicillin, they are used for treatment of endocarditis and tuberculosis.

12.3.1. Classification of Aminoglycosides

• According to Side Effects:
  • Cochlear Nerve Damage: Amikacin, Netilmicin, Kanamycin
  • Vestibular Nerve Damage: Streptomycin, Tobramycin, Gentamicin
• According to Route of Administration:
  • Local Use: Tobramycin, Kanamycin
  • Systemic Use: Gentamicin, Amikacin, Streptomycin
• According to Antibacterial Spectrum:
  • Both Gram-negative and Gram-positive: Gentamicin, Tobramycin, Kanamycin, Neomycin
  • Primarily Gram-negative: Amikacin, Streptomycin
• According to Type of Ring:
  • Containing Streptidine: Streptomycin
  • Containing 2-Deoxy Streptamine: Gentamicin, Tobramycin, Neomycin, Amikacin, Kanamycin

12.3.2. Example – Streptomycin

Streptomycin is an antibiotic produced by Streptomyces griseus (a soil actinomycete). It is an aminoglycoside antibacterial and anti-mycobacterial.

Mechanism of Action: Streptomycin irreversibly binds to specific 30S-subunit proteins and 16S rRNA. This interferes with the decoding site, causing misreading of mRNA; incorrect amino acids are inserted into the polypeptide to produce non-functional or toxic peptides.

Uses:

• Treating tuberculosis
• Tularemia (caused by Francisella tularensis)
• Plague (caused by Yersinia pestis)
• Brucellosis
• Enterococcal endocarditis

Stability & Storage: Can be refrigerated for up to 24 hours after reconstitution. Can be stored at room temperature before being reconstituted.

Formulation: Powder for injection

Brand Names: Brucella, Agrimycin-17, Agrept, Rimosidin

Dpharmguru’s exam insights:

Aminoglycosides are frequently tested. Remember: They bind to the 30S ribosomal subunit and cause misreading of mRNA. They are primarily used against gram-negative bacteria. Streptomycin is used for tuberculosis. A common exam question is: “What is the mechanism of action of aminoglycosides?” (Answer: Inhibition of protein synthesis by binding to 30S ribosomal subunit).

12.4. TETRACYCLINES

Tetracyclines are potent, broad-spectrum antibacterial agents with activity against gram-positive and gram-negative aerobic and anaerobic bacteria. Most of the marketed tetracyclines occur naturally and are obtained by fermentation of Streptomyces spp. broths.

12.4.1. Classification of Tetracyclines

• Short Acting (Half-Life 6 Hours): Chlortetracycline, Oxytetracycline
• Intermediate Acting (Half-Life 16 Hours): Demeclocycline, Methacycline
• Long Acting (Half-Life 18-24 Hours): Doxycycline, Minocycline

12.4.2. Examples of Tetracyclines

12.4.3. Doxycycline

Doxycycline is a broad-spectrum antibiotic and a synthetic derivative of oxytetracycline. It is a second-generation tetracycline with lesser toxicity than first-generation tetracyclines.

Chemical Name: (4S,4aR,5S,5aR,6R,12aR)-4-(dimethylamino)-1,5,10,11,12a-pentahydroxy-6-methyl-3,12-dioxo-4a,5,5a,6-tetrahydro-4H-tetracene-2-carboxamide

Mechanism of Action: Doxycycline binds to the 16S rRNA portion of the bacterial ribosome and inhibits translation, thus prevents tRNA from binding to the RNA-30S bacterial ribosomal subunit. Therefore, initiation of protein synthesis is obstructed, producing a bacteriostatic effect.

Uses:

• Rocky Mountain spotted fever, typhus fever, Q fever, rickettsial pox
• Respiratory tract infections caused by Mycoplasma pneumoniae
• Lymphogranuloma venereum (LGV), trachoma, inclusion conjunctivitis
• Psittacosis (ornithosis) caused by Chlamydia psittaci
• Non-gonococcal urethritis caused by Ureaplasma urealyticum
• Relapsing fever due to Borrelia recurrentis

Stability & Storage: Tablets, capsules, and syrup should be stored at room temperature (59-86°F / 15-30°C) in airtight, light-resistant containers.

Formulations: Powder for Suspension, Capsule, Tablet

Brand Names: AdoxaTT, Doryx, Monodox, Oracea, Periostat, Vibramycin Calcium, Vibra-Tabs

12.4.4. Minocycline

Minocycline is a tetracycline analogue effective against tetracycline-resistant Staphylococcus infections.

Chemical Name: (4S,4aS,5aR,12aR)-4,7-bis(dimethylamino)-1,10,11,12a-tetrahydroxy-3,12-dioxo-4a,5,5a,6-tetrahydro-4H-tetracene-2-carboxamide;hydrochloride

Mechanism of Action: Minocycline crosses the lipid bilayer or undergoes passive diffusion through porin channels. It binds to the 30S ribosomal subunit, prevents the binding of tRNA to the mRNA-ribosome complex, and interferes with protein synthesis.

Formulations: Tablets, Capsules, Injection

Brand Names: Dynacin, Minocin, Minocin Kit, Solodyn, Ximino

Dpharmguru’s exam insights:

Tetracyclines are frequently tested. Remember: They bind to the 30S ribosomal subunit and inhibit protein synthesis. Doxycycline and minocycline are long-acting tetracyclines. Tetracyclines are the drugs of choice for rickettsial infections, chlamydia, and mycoplasma infections. A common exam question is: “What is the mechanism of action of tetracyclines?” (Answer: Inhibition of protein synthesis by binding to 30S ribosomal subunit).

12.5. MACROLIDES

Macrolides are compounds with a macrocyclic lactone ring (containing 14 or 16 atoms) with attached deoxy sugars. Erythromycin is the prototype drug which was obtained in 1952 from Streptomyces erythreus.

12.5.1. Classification of Macrolides

• 13-membered Ring: Semi-synthetic (e.g., Telithromycin)
• 14-membered Ring: Natural (Erythromycin, Oleandomycin), Semi-synthetic (Clarithromycin, Roxithromycin, Dirithromycin)
• 15-membered Ring: Semi-synthetic (Azithromycin)
• 16-membered Ring: Natural (Tylosin, Josamycin, Spiramycin, Midecamycin), Semi-synthetic (Miokamycin, Rokitamycin)

12.5.2. Examples of Macrolides

12.5.3. Erythromycin

Erythromycin is a bacteriostatic macrolide antibiotic produced by a strain of Saccharopolyspora erythraea in 1952. It is widely used in various infections caused by gram-positive and gram-negative bacteria.

Mechanism of Action: Erythromycin binds to the 23S ribosomal RNA molecule in the 50S subunit of ribosomes and inhibits protein synthesis by inhibiting the transpeptidation/translocation step.

Uses:

• Respiratory tract infections (bronchitis, pneumonia, Legionnaires’ disease, pertussis, diphtheria)
• STDs (syphilis)
• Ear, intestine, gynaecologic, urinary tract, and skin infections
• Prevention of recurrent rheumatic fever

Stability & Storage: Keep at temperatures below 86°F (30°C). Protect tablets from moisture and extreme heat.

Formulations: Tablet, Capsules, Powder, Solution, Eye drops

Brand Names: Erythrocin, E-Mycin, Tlosone, E.E.S. Granules

12.5.4. Azithromycin

Azithromycin is a broad-spectrum macrolide antibiotic with a long half-life and a high degree of tissue penetration. It is part of the azalide sub-class of macrolides, having a 15-membered ring.

Chemical Name: (2R,3S,4R,5R,8R,10R,11R,12S,13S,14R)-11-[(2S,3R,4S,6R)-4-(dimethylamino)-3-hydroxy-6-methyloxan-2-yl]oxy-2-ethyl-3,4,10-trihydroxy-13-[(2R,4R,5S,6S)-5-hydroxy-4-methoxy-4,6-dimethyloxan-2-yl]oxy-3,5,6,8,10,12,14-heptamethyl-1-oxa-6-azacyclopentadecan-15-one

Mechanism of Action: Azithromycin binds to the 23S rRNA of the bacterial 50S ribosomal subunit, blocking the transpeptidation/translocation step of protein synthesis.

Uses:

• Sinus infections (Moraxella catarrhalis, Streptococcus pneumoniae)
• Community-acquired pneumonia (Chlamydia pneumoniae, Haemophilus influenzae, S. pneumoniae)
• Skin infections (Staphylococcus aureus, Streptococcus pyogenes)
• Tonsillitis (S. pyogenes)
• Urethritis and cervicitis (Chlamydia trachomatis)
• Ear infections in children

Stability & Storage: Keep at room temperature, away from heat and moisture. Extended-release suspension should not be refrigerated or frozen. Discard after 10 days.

Formulations: Tablet, Solution, Powder

Brand Names: Azasite, Zithromax, Zmax

Dpharmguru’s exam insights:

Macrolides are frequently tested. Remember: They bind to the 50S ribosomal subunit and inhibit protein synthesis. Erythromycin is the prototype macrolide. Azithromycin has a longer half-life and better tissue penetration. Macrolides are alternatives for patients allergic to penicillin. A common exam question is: “What is the mechanism of action of macrolides?” (Answer: Inhibition of protein synthesis by binding to 50S ribosomal subunit).

12.6. MISCELLANEOUS ANTIBIOTICS

12.6.1. Chloramphenicol

Chloramphenicol is bacteriostatic in nature and is a prototypical broad-spectrum antibiotic along with the tetracyclines. It is active against many gram-positive and gram-negative organisms.

Chemical Name: 2,2-dichloro-N-[(1R,2R)-1,3-dihydroxy-1-(4-nitrophenyl)propan-2-yl]acetamide

Mechanism of Action: Chloramphenicol inhibits protein synthesis at the peptidyl transferase reaction by binding to the bacterial 50S ribosomal subunit.

Uses:

• Enteric Fever: Reserve drug (safer alternatives available)
• Meningitis: Activity against H. influenzae meningitis; used in brain abscess
• Topical Uses: Eye and ear infections as drops and ointments
• Other Uses: Severe rickettsial infections, anaerobic sepsis, pneumonia caused by Haemophilus

Stability & Storage: Eye drops are most stable when kept in the refrigerator at 3-7°C.

Formulations: Eye drops, Eye Ointment, Powder for Solution

Brand Names: Chloromycetin, Econochlor, Ocu-Chlor

Note: Chloramphenicol is not in use any longer because of its adverse drug reactions, bacterial resistance, and the availability of better alternatives.

12.6.2. Clindamycin

Clindamycin is a semi-synthetic lincosamide antibiotic that has replaced lincomycin due to its improved side effect profile. It binds to bacterial 50S ribosomal subunit and inhibits bacterial protein synthesis.

Chemical Name: (2S,4R)-N-[(1S,2S)-2-chloro-1-[(2R,3R,4S,5R,6R)-3,4,5-trihydroxy-6-methylsulfanyloxan-2-yl]propyl]-1-methyl-4-propylpyrrolidine-2-carboxamide

Mechanism of Action: Clindamycin binds to 50S bacterial ribosomal subunits and inhibits bacterial protein synthesis. It mainly binds to the 23S RNA subunit.

Uses:

• Infections caused by susceptible anaerobic bacteria (Bacteroides spp., Peptostreptococcus, Clostridium spp.)
• Polymicrobial infections (intra-abdominal or pelvic infections, osteomyelitis, diabetic foot ulcers, aspiration pneumonia, dental infections)
• MSSA and respiratory infections (S. pneumoniae, S. pyogenes)
• Bacterial vaginosis (vaginal use)
• Necrotizing fasciitis (reduces toxin-producing effects of S. aureus and S. pyogenes)
• Topical treatment of acne

Stability & Storage: Keep tightly wrapped in its container. Store at room temperature, free from heat and moisture. Liquid preparation should not be refrigerated (may thicken).

Formulations: Powder for Solution, Capsule, Foam, Solution, Lotion, Pad, Gel

Brand Names: Cleocin, Cleocin Phosphate, Cleocin Pediatric, Cleocin HCl

Dpharmguru’s exam insights:

Miscellaneous antibiotics are frequently tested. Remember: Chloramphenicol binds to the 50S ribosomal subunit and is a broad-spectrum antibiotic but has serious side effects (bone marrow toxicity). Clindamycin also binds to the 50S ribosomal subunit and is used for anaerobic infections and acne. A common exam question is: “What is the mechanism of action of clindamycin?” (Answer: Inhibition of protein synthesis by binding to 50S ribosomal subunit).

COMPARISON: ANTIBIOTIC CLASSES

FREQUENTLY ASKED QUESTIONS (FAQs)

1. What is the difference between bactericidal and bacteriostatic antibiotics?

Bactericidal antibiotics kill bacteria directly (e.g., penicillins, aminoglycosides). Bacteriostatic antibiotics inhibit bacterial growth and replication, allowing the immune system to eliminate the bacteria (e.g., tetracyclines, macrolides, chloramphenicol).

2. What is the mechanism of action of β-lactam antibiotics?

β-lactam antibiotics (penicillins, cephalosporins) inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), which disrupts the cross-linking of peptidoglycan chains.

3. What is the difference between 1st and 3rd generation cephalosporins?

1st generation cephalosporins have good activity against gram-positive bacteria. 3rd generation cephalosporins have enhanced activity against gram-negative bacteria and can cross the blood-brain barrier.

4. What is the mechanism of action of aminoglycosides?

Aminoglycosides (streptomycin, gentamicin) bind to the 30S ribosomal subunit and cause misreading of mRNA, leading to the production of non-functional proteins.

5. What is the mechanism of action of tetracyclines?

Tetracyclines bind to the 16S rRNA portion of the bacterial 30S ribosomal subunit, preventing tRNA from binding, thus inhibiting protein synthesis.

6. What is the mechanism of action of macrolides?

Macrolides (erythromycin, azithromycin) bind to the 23S rRNA of the bacterial 50S ribosomal subunit, blocking the transpeptidation/translocation step of protein synthesis.

7. Why is chloramphenicol no longer widely used?

Chloramphenicol is no longer widely used due to its serious adverse effects, including bone marrow toxicity (aplastic anaemia) and bacterial resistance, and the availability of safer alternatives.

SUMMARY

Antibiotics are essential antimicrobial agents that have revolutionised the treatment of infectious diseases. This guide covered the major classes:

• β-Lactam Antibiotics (Penicillins, Cephalosporins) inhibit cell wall synthesis
• Aminoglycosides (Streptomycin, Gentamicin) inhibit protein synthesis (30S)
• Tetracyclines (Doxycycline, Minocycline) inhibit protein synthesis (30S)
• Macrolides (Erythromycin, Azithromycin) inhibit protein synthesis (50S)
• Miscellaneous Antibiotics (Chloramphenicol, Clindamycin) have various mechanisms and uses

As I always tell my students: “Understanding antibiotics is essential for treating infectious diseases. Each class has a unique mechanism, spectrum, and clinical application. Choose wisely to ensure optimal patient outcomes and minimise resistance.”

REFERENCES AND FURTHER READING

• Pharmacy Council of India (PCI). (2022). Pharmacology Syllabus. New Delhi: PCI.
• Rang, H. P., & Dale, M. M. (2021). Rang & Dale’s Pharmacology (9th ed.). Elsevier.
• Goodman, L. S., & Gilman, A. (2018). Goodman & Gilman’s The Pharmacological Basis of Therapeutics (13th ed.). McGraw-Hill.
• Katzung, B. G. (2021). Basic and Clinical Pharmacology (15th ed.). McGraw-Hill.
• Williams, D. A., & Lemke, T. L. (2019). Foye’s Principles of Medicinal Chemistry (8th ed.). Wolters Kluwer.
• World Health Organization (WHO). (2023). WHO Model List of Essential Medicines. Retrieved from https://www.who.int.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns. Pharmaceutical regulations and guidelines may vary by region—always refer to your local regulatory authorities for specific requirements.

written by:
Dr. N. Sujith Kumar

Dr. N. Sujith Kumar is a pharmacy educator, Pharm.D graduate, and academic content creator from Jawaharlal Nehru Technological University Kakinada (JNTUK), where he completed his Doctor of Pharmacy (Pharm.D) degree between 2015 and 2021. He has more than 7 years of experience creating pharmacy educational content, writing study materials, and reviewing academic articles for pharmacy students. He has also contributed guest articles to pharmacy education platforms, including PharmD Guru. At D.PharmGuru, his work focuses on simplifying complex Diploma in Pharmacy (D.Pharmacy) subjects into easy-to-understand notes, practical explanations, and exam-oriented educational resources for students across India. His areas of focus include Human Anatomy and Physiology, Pharmaceutics, Pharmacology, Pharmaceutical Chemistry, Hospital and Clinical Pharmacy, and other core D.Pharmacy subjects.

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