Antibiotics Explained: Prokaryotic vs Eukaryotic Targets | Microbiology Basics

This comprehensive lecture covers all major antibiotic classes and their clinical applications. Antibiotics target four mechanisms: cell wall synthesis (beta-lactams, glycopeptides), DNA replication (fluoroquinolones, sulfonamides, rifampicin), RNA synthesis (rifampicin), and protein synthesis (aminoglycosides, macrolides, tetracyclines). Beta-lactams include penicillins (amoxicillin, amoxicillin-clavulanate, methicillin, penicillin G/V), extended-spectrum penicillins (piperacillin-tazobactam), and carbapenems (imipenem, meropenem). Cephalosporins span generations from first (cefazolin) to fourth (cefepime). Gram-positive coverage includes vancomycin for MRSA and C. difficile, daptomycin with CK monitoring. Fluoroquinolones cause tendinopathy and QT prolongation. Cotrimoxazole and metronidazole target anaerobes and protozoa. Macrolides treat intracellular pathogens. Aminoglycosides require combination therapy. Tetracyclines cause dental staining. Pregnancy contraindications follow QR Coca mnemonic.

Antibiotics are medications targeting bacterial infections only, not viral or fungal infections. Major classes include: glycopeptides (vancomycin) for hospital MRSA infections; sulfonamides (Bactrim) for urinary and pulmonary infections; quinolones (ciprofloxacin) for renal, vesicular, and genitourinary infections; macrolides (erythromycin, clarithromycin) for respiratory infections; tetracyclines (doxycycline) for intracellular organisms and STIs; aminoglycosides (gentamicin) for severe Gram-negative infections; and beta-lactams (amoxicillin) as the main class with approximately 100 molecules. Beta-lactams divide into penicillins, cephalosporins, and carbapenems, with varying hospital/outpatient use.

Antibiotics may be ineffective due to poor penetration to infection sites, insufficient dosing, resistant bacterial mutants, or the infection being caused by organisms not susceptible to the antibiotic. Major antibiotic families include beta-lactamines (penicillins, cephalosporins), aminoglycosides, macrolides, quinolones, sulfonamides, and others. Each family has distinct mechanisms of action, spectra of activity, and clinical applications. Understanding this classification helps in selecting appropriate antibiotics for different infections and optimizing treatment outcomes.

Antibiotics are classified into families based on their chemical structure and mechanism of action: Beta-lactams (penicillins, cephalosporins) destroy bacterial cell walls; Macrolides (erythromycin, azithromycin) inhibit protein synthesis by binding to 50S ribosomal subunits; Tetracyclines (doxycycline, minocycline) bind to 30S subunits and are contraindicated in children under 8 and pregnant women due to tooth discoloration and bone effects; Aminoglycosides (gentamicin, streptomycin) bind to 30S subunits and carry risks of ototoxicity and nephrotoxicity; Fluoroquinolones (ciprofloxacin, levofloxacin) inhibit bacterial DNA gyrase and topoisomerase IV.

Major antibiotic classes include: (1) Beta-lactams (penicillins, cephalosporins) - bactericidal agents that inhibit cell wall synthesis, effective against gram-positive and gram-negative bacteria; (2) Tetracyclines - broad-spectrum bacteriostatic agents that inhibit protein synthesis, historically used in agriculture; (3) Aminoglycosides (neomycin, gentamicin) - bactericidal agents effective against gram-negative bacteria; (4) Macrolides (azithromycin) - bacteriostatic/bactericidal agents effective against gram-positive bacteria, gram-negatives, and atypical pathogens; (5) Fluoroquinolones - bactericidal agents that inhibit DNA synthesis, particularly effective for urinary tract infections. Each class has distinct mechanisms, spectra, and clinical applications.

Mitochondrial ribosomes (55-60S) resemble prokaryotic ribosomes in structure and antibiotic sensitivity, reflecting their evolutionary origin from bacteria. They contain 16S and 4S rRNAs in the large subunit and 12S rRNA in the small subunit. This similarity explains why some antibiotics that target bacterial ribosomes can also affect mitochondrial ribosomes, potentially causing side effects.

Antibiotics target different bacterial structures: cell wall synthesis (penicillins, cephalosporins, vancomycin), DNA/RNA synthesis (quinolones), protein synthesis (tetracyclines, aminoglycosides), folic acid synthesis (sulfonamides), and cell membranes (polymyxins). Cell wall inhibitors have the best therapeutic indices because humans lack cell walls. The more different the target is from human structures, the more likely the drug will be selectively toxic with fewer side effects. Bacterial ribosomes are 70S, while human ribosomes are 80S, but mitochondrial ribosomes are also 70S, so drugs targeting bacterial ribosomes may affect human cells, causing side effects.

Different antibiotics target bacterial machinery through specific mechanisms. Tetracyclines bind to the 30S subunit of bacterial ribosomes, blocking amino acid entry and halting protein synthesis. Macrolides like erythromycin bind to the 50S subunit, preventing ribosome advancement along mRNA. Aminoglycosides like gentamicin cause ribosomes to misread genetic codes, producing defective proteins that damage bacterial membranes. These mechanisms exploit geometric differences between bacterial (70S) and human (80S) ribosomes. However, human mitochondria contain ribosomes similar to bacterial ribosomes (approximately 55S) because mitochondria originated from free-living bacteria engulfed by ancestral eukaryotic cells 2,000 million years ago. This evolutionary history explains why some antibiotics cause side effects: aminoglycosides can damage hearing by affecting inner ear mitochondria, and some tetracyclines cause liver toxicity partly through mitochondrial effects. These side effects reflect our evolutionary history rather than random errors.

This section addresses the medical consequences of mitochondrial ribosome similarities to bacterial ribosomes. Since many antibiotics target bacterial ribosomes, they can also affect mitochondrial function, causing toxicity. The video explains how aminoglycoside antibiotics (streptomycin, kanamycin) bind to the 30S subunit and disrupt quality control. Certain mitochondrial rRNA mutations (m.1555A>G, m.12494T>C) alter RNA secondary structure, creating binding sites similar to bacterial ribosomes. This increases susceptibility to aminoglycoside-induced deafness (ototoxicity), explaining why streptomycin is avoided as first-line treatment despite effectiveness against infections like TB.

Antibiotics exploit ribosome differences between bacteria and human cells. Chloramphenicol blocks peptide bond formation during protein synthesis. Since bacteria have 70S ribosomes (30S and 50S subunits) while humans have 80S ribosomes (40S and 60S subunits), antibiotics can selectively target bacterial ribosomes without harming human cells. This selective toxicity is fundamental to antibiotic therapy.

Protozoans and fungi are eukaryotic organisms that share more similarities with human cells than bacteria do. Because they lack peptidoglycan cell walls and use 80S ribosomes like human cells, there are fewer differences to exploit for selective toxicity. This makes developing effective antifungal and antiprotozoal drugs more challenging compared to antibacterial drugs.

Antifungal drugs face challenges due to fungal eukaryotic similarity to human cells. Ergosterol biosynthesis inhibitors (amphotericin B, nystatin, azoles) disrupt fungal membranes by targeting ergosterol (fungal equivalent of cholesterol). Griseofulvin disrupts nucleic acids and cell division; flucytosine disrupts RNA function. Antiviral development is difficult because viruses use host machinery. Nucleoside analogs (acyclovir, ganciclovir, foscarnet) cause chain termination during viral DNA replication. Neuraminidase inhibitors (oseltamivir, zanamivir) block influenza virus release. Direct-acting antivirals (sofosbuvir, ledipasvir, velpatasvir) target specific HCV enzymes. HIV treatment requires combination therapy: NRTIs (zidovudine, lamivudine), NNRTIs (efavirenz), integrase inhibitors, and protease inhibitors attack multiple viral lifecycle stages simultaneously. Antiprotozoals like artemisinin produce reactive oxygen species against malaria parasites, while metronidazole targets anaerobes through nitrogen oxide damage.

Fungal and protozoan infections are often harder to treat than bacterial infections because fungi and protozoa are eukaryotic cells, similar to human cells. This makes it difficult to develop drugs that kill the microbe without harming the host. Bacterial infections are easier to treat because bacteria are prokaryotic and very different from human cells.

This section explains drug spectrum: narrow-spectrum drugs target specific organisms (e.g., penicillins for gram-positive bacteria), while broad-spectrum drugs target multiple groups including gram-positive, gram-negative, mycobacteria, and intracellular pathogens. Different drug generations have varying spectra. Treating eukaryotic infections (fungi, protozoans) is challenging due to cellular similarity to human cells. Viruses are particularly difficult because they use host machinery and have high mutation rates, creating multiple strains within single infections.

Fungal infections present significant treatment challenges because fungi are eukaryotic organisms biologically similar to human cells. The antifungal drug fluconazole works by inhibiting enzymes involved in ergosterol synthesis, a molecule in fungal cell membranes similar to human cholesterol. However, because ergosterol is structurally similar to human cholesterol, the drug can also affect human cells, limiting its effectiveness. Fungi can develop both resistance (where the drug no longer works) and tolerance (where the drug becomes less effective over time). Protozoan infections like toxoplasmosis and amebiasis are equally challenging because protozoa are single-celled eukaryotic organisms that closely resemble human cells. Some protozoan infections have extremely high mortality rates with very few patients surviving. The lack of effective treatments for these diseases highlights the challenges of developing medications that can kill eukaryotic pathogens without harming human cells.

The Minimum Inhibitory Concentration (MIC) is the lowest concentration that prevents visible bacterial growth, while the Minimum Bactericidal Concentration (MBC) is the lowest concentration that kills 99.9% of bacteria. The ratio of MBC to MIC determines whether an antibiotic is bactericidal (>4) or bacteriostatic (<4).

Bacteriostatic antibiotics inhibit bacterial growth without killing cells, while bactericidal antibiotics kill bacteria. This distinction is measured by Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC). If MIC and MBC are close, the antibiotic is bactericidal; if they differ significantly, it's bacteriostatic. Time-dependent antibiotics (beta-lactams, glycopeptides) achieve maximum killing once threshold concentrations are reached, benefiting from prolonged exposure. Concentration-dependent antibiotics (aminoglycosides) show killing proportional to concentration regardless of duration. Bactericidal antibiotics are preferred for severe infections and immunocompromised patients who cannot mount adequate immune responses.

This section distinguishes bactericidal from bacteriostatic drugs. Among anti-tubercular drugs, ethambutol is bacteriostatic while rifampicin, isoniazid, pyrazinamide, and streptomycin are bactericidal (rip apart mnemonic). Triclabendazole is the drug of choice for Fasciola hepatica (flag mnemonic). Most protein synthesis inhibitors are bacteriostatic, except aminoglycosides and streptogramins which are bactericidal. Cell wall synthesis inhibitors (beta-lactams, cycloserine, fosfomycin, vancomycin, bacitracin) are bactericidal. Linezolid inhibits 28S subunit and is bacteriostatic. Aminoglycosides have no activity against anaerobes due to oxygen-dependent activation.

Bactericidal antibiotics kill bacteria directly, while bacteriostatic antibiotics inhibit bacterial growth. Bactericidal agents are preferred for serious infections, immunocompromised patients, and endocarditis. Bacteriostatic agents rely on the host immune system to clear the remaining bacteria. The Minimal Inhibitory Concentration (MIC) is the lowest concentration of an antibiotic that prevents visible bacterial growth in vitro. This value is determined through laboratory testing and helps guide antibiotic selection and dosing. Understanding the difference between these mechanisms is essential for appropriate antibiotic selection.

MIC (Minimum Inhibitory Concentration) is the lowest concentration of an antibiotic that prevents visible growth of bacteria. MBC (Minimum Bactericidal Concentration) is the lowest concentration that kills bacteria. MIC is always less than or equal to MBC, as killing requires higher concentrations than merely inhibiting growth.