Antibiotics mechanism and uses

Antibiotics: Cell Wall Synthesis Inhibitors

• The ability of an antibacterial agent to permeate bacterial cell walls affects its effectiveness, especially through the peptidoglycan layer.
• Drugs must get through barriers to reach targets, like penicillin-binding proteins (PBPs), which are the enzymes that synthesize and maintain bacterial cell walls.

Gram-Positive vs Gram-Negative Bacteria

• Gram-positive bacteria feature a thick peptidoglycan layer and no outer membrane
• Gram-negative bacteria feature a thin peptidoglycan layer and an outer membrane comprised of lipopolysaccharide
• Penicillin is highly effective against gram-positive bacteria, easily reaching penicillin-binding proteins.
• Penicillin is less effective against gram-negative bacteria, needing porin channels to reach penicillin-binding proteins.

Key Physiochemical Properties Influencing Drug Penetration

• Smaller-sized drugs penetrate more easily.
• Size effect on Gram + bacteria: Easy penetration of thick peptidoglycan layer
• Size effect on Gram – bacteria: Harder to penetrate due to outer membrane
• Charge has minimal impact on Gram + bacteria
• Charge effect on Gram – bacteria: Charged drugs might struggle with the LPS barrier
• Hydrophobic drugs cross lipid membranes, while hydrophilic drugs need porins in Gram + bacteria
• Hydrophobicity effect on Gram + bacteria: Easy penetration (no outer membrane)
• Hydrophilic drugs require porin channels for penetration in Gram - bacteria

Penicillin's Mechanism of Action

• Penicillins are a type of β-lactam antibiotic, targeting PBPs in the periplasmic space.
• Penicillins inhibit PBPs, preventing peptidoglycan cross-linking, and disrupt the bacterial cell wall, leading to cell lysis.
• In Gram-positive bacteria, the thick but porous peptidoglycan layer assists penicillins in easily reaching PBPs and disrupting cell wall synthesis.
• In Gram-negative bacteria, penicillins must cross the LPS outer membrane through porin channels to reach PBPs, which makes them less effective.
• Alterations in porins or efflux pumps can lower drug uptake.
• Penicillins are hydrophilic, making it harder to cross the lipid-rich outer membranes of Gram-negative bacteria without porin channels.
• The LPS membrane in Gram-negative bacteria prevents the easy entry of hydrophilic molecules.
• Porin channels allow small, hydrophilic drugs like penicillin to pass into the periplasmic space, where PBPs are located.
• Bacteria can modify porins or develop efflux pumps to expel the drug, reducing how effective the drugs are.

Antibacterial Spectrum of Penicillins

• The antimicrobial spectrum defines the range of bacteria an antibiotic can effectively target.
• Penicillins can be categorized based on activity spectrum and origin.
• Broad-spectrum antibiotics are effective against both gram-positive and gram-negative bacteria
• Examples of Broad-spectrum antibiotics: Tetracyclines, Carbapenems, Amoxicillin-Clavulanate and Fluoroquinolones
• Advantage of Broad-spectrum antibiotics: Treats unknown/mixed infections
• Disadvantage of Broad-spectrum antibiotics: Risk of resistance, disrupts normal flora
• Narrow-spectrum antibiotics target a specific group of bacteria, either gram-positive or gram-negative
• Examples of Narrow-spectrum antibiotics: Penicillin G, Vancomycin, and Isoniazid
• Advantage of Narrow-spectrum antibiotics: Lower resistance risk, preserves normal flora
• Disadvantage of Narrow-spectrum antibiotics: Ineffective for mixed infections, requires pathogen ID

Natural vs Synthetic Antibiotics

• Natural Antibiotics are derived from microorganisms (bacteria, fungi).
• Examples of Natural Antibiotics: Penicillin, Streptomycin, and Tetracycline
• Advantage of Natural Antibiotics: Potent against certain bacteria evolved to combat microbes
• Disadvantage of Natural Antibiotics: Complex structures, susceptible to natural resistance mechanisms
• Synthetic Antibiotics are manufactured entirely in laboratories using chemical processes
• Examples of Synthetic Antibiotics: Sulphonamides, Fluoroquinolones, and Linezolid
• Advantage of Synthetic Antibiotics: Tailored efficacy, stable, mass-producible
• Disadvantage of Synthetic Antibiotics: High development costs, potential for rapid resistance development
• Semi-synthetic Antibiotics are a hybrid of natural and synthetic antibiotics, derived from natural sources with chemical modifications.
• Examples of Semi-synthetic Antibiotics: Amoxicillin, Cefuroxime
• Advantage of Semi-synthetic Antibiotics: Enhanced activity or improved bioavailability or resistance
• Disadvantage of Semi-synthetic Antibiotics: May still face resistance issues similar to natural antibiotics

Pharmacokinetics of Penicillins

• Penicillins' effectiveness hinges on their stability in stomach acid and susceptibility to β-lactamases.
• Stability to stomach acid is important for oral administration because a drug must survive the acidic conditions to be absorbed into the bloodstream and exert its therapeutic effect.
• ẞ-lactamases are bacterial enzymes that hydrolyze the ẞ-lactam ring of antibiotics, which inactivates the antibiotic, allowing bacteria to resist treatment.

Stability of Stomach Acid

• Acid-Stable Penicillins are resistant to stomach acid degradation
• Structural modifications protect the β-lactam ring in Acid-Stable Penicillins
• Examples of Acid-Stable Penicillins: Amoxicillin, Penicillin V
• An advantage of Acid-Stable Penicillins is convenient oral administration.
• None is listed as a disadvantage for Acid-Stable Penicillins.
• Acid-Stable Penicillins are suitable for outpatient treatment where oral intake is preferred.
• Acid-Labile Penicillins are rapidly degraded by stomach acid.
• An example of Acid-Labile Penicillins is Penicillin G
• Acid-Labile Penicillins are effective when administered via non-oral routes.
• Acid-Labile Penicillins cannot be taken orally.
• Acid-Labile Penicillins are used in severe infections when oral administration is not feasible (IV or IM required).

Susceptibility to B-Lactamases

• Many natural penicillins (penicillin G & V) are highly susceptible to β-lactamases produced by resistant bacteria.
• These enzymes cleave the β-lactam ring, making the antibiotic ineffective against the bacteria.
• Bacteria produce various types of β-lactamases, some of which are highly specific to certain antibiotics (penicillinases), while others (such as extended-spectrum β-lactamases, or ESBLs) can degrade a wide range of β-lactam antibiotics.
• Penicillinase-Resistant Penicillins feature Structural modifications (at R-side chain) that prevent β-lactamase access to the β-lactam ring.
• Examples of Penicillinase-Resistant Penicillins: Methicillin, Oxacillin, Nafcillin, and Dicloxacillin
• Advantage of Penicillinase-Resistant Penicillins: Effective against β-lactamase-producing bacteria.
• Penicillinase-Resistant Penicillins are used in infections caused by β-lactamase-producing bacteria (e.g., Staphylococcus aureus, excluding MRSA).
• β-lactamase Inhibitors inhibit β-lactamase enzymes, protecting antibiotics from degradation.
• Examples of β-lactamase Inhibitors: Clavulanic acid, Sulbactam, and Tazobactam
• β-lactamase Inhibitors extends the spectrum of penicillins.
• β-lactamase Inhibitors are combined with penicillins (e.g., Amoxicillin-Clavulanate) to combat β-lactamase-producing bacteria.

Pharmacokinetics & Safety of Penicillins

• Penicillin is widely used because of its broad activity and relatively low toxicity.
• Penicillin can cause various side effects, with hypersensitivity being the most significant concern.
• Cross-reactivity occurs when patients with a known penicillin allergy also show cross-reactivity to other β-lactam antibiotics, such as cephalosporins, because they share a similar β-lactam structure.
• Cross-reactivity is less common with newer-generation cephalosporins.

Adverse Effects

• Hypersensitivity (Allergy) presents as immune-mediated reactions ranging from mild to severe (anaphylaxis).
• Cross-reactivity with other ẞ-lactams may occur during Hypersensitivity (Allergy)
• Examples/Symptoms of Hypersensitivity (Allergy):
  • Mild: Rash, itching
  • Moderate: urticaria (hives)
  • Severe: anaphylactic shock (difficulty breathing, swelling, drop in BP, rapid pulse, dizziness)
  • Treatment: immediate medical intervention, often with epinephrine, antihistamines, and corticosteroids.
• GI Effects involves nausea, vomiting, diarrhea, C. Difficile colitis
• Neurological Effects involves central nervous system disturbances
• Examples/Symptoms of Neurological Effects: Seizures, confusion (in high doses or patients with kidney impairment)
• Haematological Effects involves blood cell abnormalities.
• Examples/Symptoms of Haematological Effects: Eosinophilia, hemolytic anemia, leukopenia
• Renal Effects involves kidney inflammation
• Examples/Symptoms of Renal Effects: Interstitial nephritis (rare)
• Hepatic Effects involves liver dysfunction
• Examples/Symptoms of Hepatic Effects: Elevated liver enzymes, jaundice, hepatitis (rare)

Antibiotics – Protein Synthesis Inhibitors

• mRNA template, ribosomes, tRNAs, and enzymatic factors are NB
• The small ribosomal subunit forms on the mRNA template at the Shine-Dalgarno sequence (prokaryotes) or the 5' cap (eukaryotes).
• Translation begins at the initiating AUG on the mRNA, specifying methionine.
• The formation of peptide bonds occurs between sequential amino acids specified by the mRNA template according to the genetic code.
• Charged tRNAs enter the ribosomal A site, and their amino acid bonds with the amino acid at the P site.
• The entire mRNA is translated in three-nucleotide "steps" of the ribosome.
• When a nonsense codon is encountered, a release factor binds and dissociates the components and frees the new protein.
• Folding of the protein occurs during and after translation.

Tetracyclines

• Mechanism of Action: Binds to 30S ribosomal subunit, preventing aminoacyl-tRNA from attaching to mRNA-ribosome complex (bacteriostatic).
• Uses: Respiratory infections, STIs (chlamydia, syphilis), Rickettsial infections, Acne vulgaris, Malaria prophylaxis (Doxycycline), Lyme disease.
• Pharmacokinetics:
  • Reduced absorption with food, dairy, antacids (chelation).
  • Well-distributed, except in CSF.
  • Excreted in urine (except Doxycycline & Minocycline, excreted via liver).
  • Half-life: 6-20 hrs (Doxycycline has longer half-life).
• Effects & Action: Bacteriostatic
• Side Effects: GI distress, photosensitivity, teeth discoloration (in children), hepatotoxicity, nephrotoxicity.
• Antimicrobial Spectrum: Broad-spectrum, effective against Gram-positive and Gram-negative bacteria, atypical bacteria (Mycoplasma, Chlamydia), rickettsiae, spirochetes, protozoa.

Glycylcyclines

• Mechanism of Action: Binds to 30S ribosomal subunit, overcoming resistance in tetracycline-resistant bacteria (bacteriostatic)
• Uses: Complicated skin/soft tissue infections, intra-abdominal infections, community-acquired pneumonia, drug-resistant pathogens (MRSA, VRE).
• Pharmacokinetics:
  • IV administration (poor oral bioavailability).
  • Extensive tissue distribution; low blood levels.
  • Liver excretion; biliary and renal routes.
  • Half-life: 36 hours.
• Effects & Action: Bacteriostatic
• Side Effects: Nausea, vomiting, increased death risk in severe infections (sepsis).
• Antimicrobial Spectrum: Broad-spectrum, effective against Gram-positive (MRSA, VRE), Gram-negative bacteria, and anaerobes, while lacking activity against Pseudomonas and Proteus.

Aminoglycosides

• Mechanism of Action: Binds to 30S ribosomal subunit, causing misreading of mRNA (bactericidal).
• Uses: Serious Gram-negative infections (e.g. Pseudomonas), sepsis, endocarditis (combined with β-lactams), and TB (second-line).
• Pharmacokinetics:
  • IV or IM (poor oral absorption).
  • Extracellular fluid, low CSF penetration.
  • Not metabolized, excreted by kidneys.
  • Half-life: 2-3 hours (adjust dosing based on renal function).
• Effects & Action: Bactericidal.
• Side Effects: Ototoxicity, nephrotoxicity, and neuromuscular blockade (high doses).
• Antimicrobial Spectrum: Primarily Gram-negative aerobes (e.g. Pseudomonas, E. coli, Klebsiella), limited Gram-positive (synergy with β-lactams).

Macrolides

• Mechanism of Action: Bind to 50S ribosomal subunit, inhibiting nascent peptide exit tunnel (bacteriostatic, bactericidal at higher doses).
• Uses: Respiratory infections, atypical infections (Mycoplasma, Chlamydia, Legionella), STIs (Chlamydia, Gonorrhoea), H. pylori eradication (peptic ulcer disease).
• Pharmacokinetics:
  • Well absorbed orally.
  • Good tissue penetration, poor CSF.
  • Liver metabolism (CYP450 enzymes).
  • Azithromycin (40-68 hrs), Clarithromycin (3-7 hrs).
• Effects & Action: Bacteriostatic/Bactericidal.
• Side Effects: GI upset, QT prolongation, cholestatic hepatitis.
• Antimicrobial Spectrum: Effective against Gram-+ cocci, atypical bacteria (Mycoplasma, Chlamydia, Legionella), and some Gram-negative bacteria (Haemophilus influenzae).

Clindamycin

• Mechanism of Action: Binds to 50S ribosomal subunit, inhibiting protein synthesis (bacteriostatic).
• Uses: Anaerobic infections (Bacteroides), skin/soft tissue infections (MRSA), osteomyelitis, and dental infections.
• Pharmacokinetics:
  • Well absorbed orally.
  • Penetrates tissues, including bone, but not CSF.
  • Liver metabolism; excreted in bile and urine.
  • Half-life: 2-3 hours.
• Effects & Action: Bacteriostatic
• Side Effects: Diarrhea (risk of C. difficile infection), rash, and hepatotoxicity.
• Antimicrobial Spectrum: Effective against Gram-+ cocci (MRSA), anaerobes (Bacteroides), Ineffective against Gram-neg aerobes.

Chloramphenicol

• Mechanism of Action: Binds to 50S ribosomal subunit, inhibiting peptide bond formation (bacteriostatic, bactericidal in high doses).
• Uses: Typhoid fever, bacterial meningitis (limited resource settings), and Rickettsial infections (Rocky Mountain spotted fever if tetracyclines are contraindicated).
• Pharmacokinetics:
  • Well absorbed orally.
  • Wide distribution, including CSF.
  • Liver metabolism; excreted in urine.
  • Half-life: 1.5-3.5 hours.
• Effects & Action: Bacteriostatic/Bactericidal at high doses SE: Bone marrow suppression, aplastic anemia, grey baby syndrome (neonates), GI upset.
• Antimicrobial Spectrum: Broad-spectrum: Gram-positive, Gram-negative, anaerobic bacteria, Rickettsia, Chlamydia.

Antibiotics – Folate Synthesis Inhibitors

• Folate is an essential vitamin
• People likely to suffer from folic acid deficiency:
  • Pregnant women → advised to takke folic acid supplements as deficiency can cause Spina bifida (congenital abnormality – neural tube defect)
  • Alcoholics
• 2 important forms of folic acid include methyltetrahydrofolate & methylene tetrahydrofolate

Folate Antagonists

• Mechanism of Action: Inhibit folate synthesis, essential for bacterial DNA synthesis; Sulfamethoxazole inhibits dihydropteroate synthase, trimethoprim inhibits dihydrofolate reductase, blocking folate synthesis (bactericidal).
• Uses: Urinary tract infections, respiratory infections, e.g., pneumonia in immunocompromised patients.
• Pharmacokinetics:
  • Well absorbed orally, good tissue penetration, including CSF.
  • Metabolized in the liver; excreted in urine.
  • Half life: 8-10 hours.
• Effects & Action: Bactericidal.
• Side Effects: Allergic reactions, skin rashes, Gl upset, bone marrow suppression (with long-term use).
• Antimicrobial Spectrum: Effective against many Gram-positive (e.g. Streptococcus pneumoniae) and Gram-negative bacteria (e.g. E. coli, Haemophilus influenzae).

Fluoroquinolones

• Inhibit bacterial DNA synthesis by targeting DNA gyrase and topoisomerase IV, leading to bacterial death (bactericidal).
• Uses respiratory infections, GI infections (e.g., Salmonella), bone/joint infections, skin/soft tissue infections, anthrax prophylaxis (Ciprofloxacin).
• Pharmacokinetics:
  • Well absorbed orally; impaired by divalent/trivalent cations.
  • Widely distributed (lungs, prostate, bone).
  • Eliminated in urine; some hepatic metabolism.
  • Half life: 4-12 hours.
• Effects & Action: Bactericidal - -Side Effects: GI disturbances, dizziness, tendon rupture, QT prolongation, risk of $C$. difficile. Antimicrobial Spectrum: Broad-spectrum: Effective against Gram-negative (e.g. $E$. coli, Pseudomonas aeruginosa) and some Gram-positive bacteria (e.g. Staphylococcus aureus).

Sulphonamides

• Mechanism of Action: Inhibit dihydropteroate synthase, blocking folate and DNA synthesis (bacteriostatic).
• Uses: UTIs and burns (topical).
• Pharmacokinetics:
  • Well absorbed orally.
  • Widely distributed, including CNS - Hepatically metabolised; renal excretion - HL: 6-12 hours.
• Effects & Action: Bacteriostatic
• Side Effects: Allergic reactions, Stevens-Johnson syndrome, GI disturbances, renal toxicity
• Antimicrobial Spectrum: Effective against Gram-positive e.g. $Staphylococcus aureus and some gram-negative bacteria e.g. $E$. coli

Trimethoprim

• Selectively inhibits dihydrofolate reductase - blocking folate synthesis.
• Pharmacokinetics:
  • Well absorbed orally
  • Widely distributed
  • Liver metabolism and excreted in urine
  • Half life 8-11 hours
• Effects & Actions: Bactericidal
• Antimicrobial Spectrum: Effective against Gram-positive Streptococcus pneumoniae and gram-negative e.g. E. coli and Klebsiella.

Urinary Tract Antiseptics

• Uses: Uncomplicated UTIs
• Action: Inhibit baterical enzymes and disrupts cell wall synthesis.

Nitrofurantoin

• Well absorbed orally-
• Concentrates in urine. Minimal hepatic metabolism and excreted in urine.

Effects & Actions:

• Bactericidal in urine-
• Side Effects: nausea and vomiting
• Antimicrobial Spectrum: Effective against Gram_positive
• e.g. $Staphylococcus saprophyticus and gram-negative bacteria in urine

Methenamine

• Hydrolyses in acidic urine to form formaldehyde.
• Well absorbed orally
• Excreted in urine
• Requires acidic urine for activation.

Effects & Actions

• Bactericidal in acidic urine Effective against Gram and Gram-positive bacteria in urine ,Requires acidic urine for activation

Questions cover the mechanisms of action, spectrum of activity, excretion routes, and side effects of antibiotics like Clindamycin, Chloramphenicol, and Clarithromycin. Also covers the differences between natural, semi-synthetic, and synthetic antibiotics.