Antibiotics

Definition and Overview

Antibiotics are pharmacological agents used to treat or prevent bacterial infections by either killing bacteria (bactericidal) or inhibiting their growth and reproduction (bacteriostatic)

They are ineffective against viral, fungal, or parasitic infections

Antibiotics vary in spectrum (narrow or broad), mechanism of action, resistance profile, and pharmacokinetics

Proper selection, dosing, and duration of antibiotic therapy are critical to optimize outcomes, reduce toxicity, and prevent resistance

Main Classes of Antibiotics and Their Mechanisms

1. Beta-Lactam Antibiotics

Inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs)

Bactericidal

Penicillins

Amoxicillin

Ampicillin

Flucloxacillin

Benzylpenicillin

Piperacillin

Frequently combined with beta-lactamase inhibitors such as clavulanic acid or tazobactam

Cephalosporins

First Generation: Cefalexin

Second Generation: Cefuroxime

Third Generation: Ceftriaxone, Ceftazidime

Fourth Generation: Cefepime

Fifth Generation: Ceftaroline

Carbapenems

Meropenem

Imipenem

Ertapenem

Broadest spectrum among beta-lactams

Monobactams

Aztreonam

Only active against gram-negative aerobes

2. Macrolides

Inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit

Bacteriostatic or bactericidal depending on concentration and organism

Examples

Erythromycin

Azithromycin

Clarithromycin

3. Tetracyclines

Inhibit protein synthesis by binding to the 30S ribosomal subunit

Broad-spectrum and bacteriostatic

Examples

Tetracycline

Doxycycline

Minocycline

4. Aminoglycosides

Bind to the 30S ribosomal subunit and disrupt protein synthesis

Bactericidal and used mainly for gram-negative infections

Require monitoring for toxicity

Examples

Gentamicin

Amikacin

Tobramycin

5. Fluoroquinolones

Inhibit bacterial DNA gyrase and topoisomerase IV

Bactericidal with broad-spectrum coverage

Examples

Ciprofloxacin

Levofloxacin

Moxifloxacin

6. Sulfonamides and Trimethoprim

Inhibit folate synthesis, disrupting bacterial DNA and RNA synthesis

Examples

Trimethoprim

Sulfamethoxazole

Combined as cotrimoxazole (TMP-SMX)

7. Glycopeptides

Inhibit bacterial cell wall synthesis by binding to D-Ala-D-Ala terminus of peptidoglycan precursors

Primarily used for resistant gram-positive infections

Examples

Vancomycin

Teicoplanin

8. Lincosamides

Inhibit protein synthesis by binding to the 50S ribosomal subunit

Example

Clindamycin

9. Oxazolidinones

Inhibit protein synthesis by preventing the formation of the initiation complex

Effective against multidrug-resistant gram-positive organisms

Example

Linezolid

10. Nitroimidazoles

Cause DNA strand breakage in anaerobic bacteria and certain protozoa

Example

Metronidazole

11. Others

Fosfomycin: inhibits cell wall synthesis

Nitrofurantoin: damages bacterial DNA, used in UTIs

Rifampin: inhibits bacterial RNA polymerase

Chloramphenicol: inhibits protein synthesis but limited due to bone marrow toxicity

Spectrum of Activity

Narrow-spectrum antibiotics target specific bacteria (e.g. penicillin G for Streptococcus pyogenes)

Broad-spectrum antibiotics cover a wide range of bacteria (e.g. amoxicillin-clavulanic acid)

Selection depends on infection site, suspected organism, resistance patterns, and patient factors

Common Clinical Uses by Infection Site

Respiratory tract infections

Amoxicillin for otitis media, sinusitis

Macrolides or doxycycline for atypical pneumonia

Levofloxacin or amoxicillin-clavulanic acid for community-acquired pneumonia

Urinary tract infections

Nitrofurantoin for uncomplicated UTI

Ciprofloxacin for complicated UTI

Trimethoprim-sulfamethoxazole for cystitis

Skin and soft tissue infections

Flucloxacillin for cellulitis

Clindamycin or doxycycline for MRSA

Amoxicillin-clavulanic acid for bite wounds

Gastrointestinal infections

Metronidazole for anaerobic and Clostridioides difficile infections

Ciprofloxacin for traveler’s diarrhea

Rifaximin for hepatic encephalopathy or IBS

Sexually transmitted infections

Azithromycin for chlamydia

Ceftriaxone for gonorrhea

Doxycycline for syphilis (alternative)

Bone and joint infections

Flucloxacillin or cefazolin for osteomyelitis

Vancomycin for MRSA

Rifampin in prosthetic joint infections

Central nervous system infections

Ceftriaxone or cefotaxime plus vancomycin for bacterial meningitis

Ampicillin added if Listeria suspected

Sepsis and bacteremia

Empiric therapy with piperacillin-tazobactam or meropenem

Tailored after culture and sensitivity results

Dosage and Administration

Dosage varies by age, renal function, severity of infection, and drug class

Beta-lactams: typically 2 to 4 times daily

Macrolides: azithromycin once daily; erythromycin multiple doses

Aminoglycosides: once-daily dosing or traditional divided dosing with drug level monitoring

Fluoroquinolones: once or twice daily

IV, IM, or oral routes depending on severity and agent bioavailability

Side Effects and Adverse Reactions

Common side effects

Nausea

Diarrhea

Skin rashes

Headache

Serious or class-specific adverse effects

Penicillins: anaphylaxis, serum sickness, interstitial nephritis

Cephalosporins: cross-allergy with penicillins

Macrolides: QT prolongation, hepatotoxicity

Tetracyclines: photosensitivity, esophagitis, teeth staining in children

Aminoglycosides: nephrotoxicity, ototoxicity

Fluoroquinolones: tendon rupture, CNS effects, QT prolongation

Vancomycin: red man syndrome, nephrotoxicity

Chloramphenicol: aplastic anemia

Rifampin: orange body fluids, hepatotoxicity

Nitrofurantoin: pulmonary fibrosis with long-term use

Contraindications

Penicillins: severe hypersensitivity

Tetracyclines: pregnancy, children under 8

Fluoroquinolones: pregnancy, children (cartilage toxicity)

Aminoglycosides: caution in renal impairment

Chloramphenicol: bone marrow suppression

Rifampin: caution in liver disease

Precautions

Monitor renal and hepatic function in long-term or high-dose therapy

Use probiotics or antifungals to prevent superinfection

Adjust dosage in renal impairment for drugs like aminoglycosides and vancomycin

Avoid sunlight with tetracyclines

Check interactions in patients on anticoagulants or anticonvulsants

Drug Interactions

Warfarin: potentiated by macrolides, metronidazole, fluoroquinolones

Anticonvulsants: altered levels with rifampin and erythromycin

Statins: increased toxicity with macrolides

Digoxin: increased levels with erythromycin

Oral contraceptives: effectiveness reduced with rifampin

Diuretics: increased nephrotoxicity with aminoglycosides

Antibiotic Resistance

Mechanisms

Enzymatic degradation (e.g. beta-lactamases)

Target modification (e.g. altered PBPs)

Efflux pumps

Reduced permeability

Contributing factors

Overuse in viral infections

Incomplete courses

Agricultural misuse

Poor infection control

Strategies to combat resistance

Antibiotic stewardship programs

Culture-based therapy

Education on appropriate use

Development of new agents

Use of combination therapy in multidrug-resistant infections

Special Populations

Pregnancy

Penicillins, cephalosporins, and macrolides generally safe

Tetracyclines and fluoroquinolones contraindicated

Pediatrics

Avoid tetracyclines and fluoroquinolones

Dosing often weight-based

Elderly

Monitor renal function closely

Avoid nephrotoxic agents when possible

Renal impairment

Dose adjustments for aminoglycosides, vancomycin, nitrofurantoin

Hepatic impairment

Caution with macrolides and rifampin

Prophylactic Uses

Surgical prophylaxis

Cefazolin or cefuroxime 30–60 minutes before incision

Endocarditis prophylaxis

Amoxicillin before dental procedures in high-risk patients

Traveler’s diarrhea

Ciprofloxacin or azithromycin in select cases

Malaria prophylaxis

Doxycycline used in endemic areas

Key Clinical Principles

Always confirm indication and suspected pathogen

Use narrow-spectrum agent when possible

Avoid unnecessary antibiotics for viral infections

Base choice on local antibiogram and resistance trends

De-escalate therapy once cultures are available

Ensure full course completion

Monitor for side effects, especially in long-term therapy

Educate patients on antibiotic purpose and adherence

Avoid empirical reuse of previous antibiotics without reassessment

Prevent cross-infection by good hygiene and hospital infection control practices