antibiotics

Microbiology

The study of microorganisms (microbes) concerning the structure, function, and classification of such organisms and ways to exploit and control their activities.

Medical microbiology

The study of microbes that causes human illness and their role in disease.

Pathogen

A biological agent that causes disease or illness to its host.

Infectious diseases

Disorders caused by organisms, such as bacteria, viruses, fungi, or parasites.

Joseph Lister

“Germ theory of disease” — introduced the use of carbolic acid as an antiseptic for operating theatres and wards —> post-surgery survival rates significantly improved.

Robert Koch

• Identified Koch’s postulates of disease: microorganisms cause disease

  • Suspected causative agent is present in diseased organism but not present in all healthy disease

  • Causative agent must be isolated from the diseased organism and grown in culture

  • Cultured agent must cause the same disease when inoculated in healthy, susceptible organism

  • Same agent must be reisolated from inoculated, diseased organism

Alexander Fleming

• Discovered penicillin: 1928 (anecdotal)

• Isolating penicillin was not a problem problem, purifying penicillin in larger quantities was a problem

• this was not available until 1944 (WW2)

Paul Ehrlich

• Father of chemotherapy

Chemotherapy

Chemical that can directly interfere with the proliferation of microorganisms at concentrations tolerated by the host. Chemicals are selectively toxic to microbial cells.

Salvarsan

• First purely synthetic antimicrobial drug discovered in 1910 by Paul Ehrlich

• Effective against trypanosomes (parasites) and syphilis (microbial)

• Cause damage to essential proteins by disrupting thiol groups due to arsenic’s high affinity to thiol

• Arsenic in its chemical structure can be harmful to the human body (low selectivity)

Bacteriostatic agent

Inhibit cell growth, rely on immune system to kill pathogens. Usually target processes that can be quickly recovered —> decrease in bacterial numbers

Bactericidal agent

Actively kills bacterial cells — cell death. Targets activities vital for life

Broad-spectrum

Target a broad range of bacteria (Gram-positive and Gram-negative)

Narrow-spectrum

Target specific types or species of bacteria (Gram-negative vs Gram-positive)

Antibiotic targets in bacteria

• Cell wall synthesis machinery (beta lactams)

• Nucleic acid synthesis machinery (DNA gyrase, RNA polymerase, folate synthesis)

• Protein synthesis machinery (50s ribosome subunit, 30s ribosome subunit)

Gram-negative vs Gram-positive bacteria:

Gram-negative bacteria (stain pink) has a thin peptidoglycan cell wall sandwiched between an outer membrane (contains O antigen) and an inner membrane.

Gram-positive bacteria (stain purple) has the cell wall accessible with multiple layers of peptidoglycan (thick).

Mechanism of action of beta-lactams

• Disrupt cell wall biosynthesis by mimicking terminal D-ala-ala substrate

  • Binds to serine hydroxyl and covalently binds to transpeptidase

  • Missing components for adjacent peptide chain to join —> blocks synthesis of peptide chain

• Inhibits transpeptidase irreversibly

• Total depletion of transpeptidase available for cell wall synthesis —> unstable cell wall

Bacterial cell wall structure

• Made up of parallel series of sugar backbones alternating between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG)

• Off of NAM are peptide chains (think window light that drapes down)

• Peptide chains end in D-Ala-D-Ala

• These peptide chains are crosslinked between adjacent rows by transpeptidases (penicillin-binding proteins — PBPs) via pentaglycine link

Describe the biosynthesis of peptidoglycan cell wall

• Transpeptidase binds to D-ala-D-ala and fosters crosslinking between D-ala-D-ala and Glycine on an adjacent strand

  • Attacks from serine hydroxyl group onto 4th D-alanine

  • 5th D-alanine is kicked off

  • Covalent bond between peptide chain and transpeptidase

  • Adjacent glycine joins peptide chain and form peptide bond

• Leads to covalent bond between glycine and D-ala-D-ala

• Leads to reinforcement of cell wall

Structure similarity between beta-lactam antibiotics and

• Beta-lactam ring!

Mechanisms of beta-lactam antibiotic resistance

• Liposaccharide layer on outermembrane of Gram-negative bacteria acts as a physical barrier

  • LPS outer layer is negatively charged (negatively charged / neutral particles cannot cross easily)

• Resistance genes coding for beta-lactamases

  • Break beta-lactam ring and prevents drug from binding to active site of transpeptidase

• Mutations in transpeptidase which makes them “blind” to the drug

When administering antibiotics, physicians (used to) usually go for:

Bactericidal antibiotics that are broad-spectrum —> reliant overuse causing resistance

Classes of beta-lactam antibiotics

• Cephalosporins

• Penicillins

• Carbapenems

• Monobactams

General trend in earlier and later generations of beta-lactam antibiotics

• Earlier generations are more effective against Gram-positive bacteria

• Later generations are more effective against Gram-negative bacteria (they are better at crossing the outer membrane layer)

Classes of beta-lactamases

• All are found in nature

• A, C, D all have serine and lysine in the active site

• Class B (NMD-1) has zinc metals (metalloproteases) which makes them resistant to many beta-lactams

Sulphonamide mechanism of action

• Target tetrahydrofolate biosynthesis (pyrimidine) involved in cell metabolism

• Mimics PABA (para-aminobenzoic acid)

  • Interacts via intermolecular forces with enzyme involved

  • Competitive reversible inhibitors (bacteriostatic agents)

Examples of sulphonamides

• Trimethoprim

• Sulphones

• Sulphamethoxazole

• Trimethoprim and sulphamethoxazole are usually used as combination drugs to treat UTI, intestinal infections

Sulphonamides drawbacks

• Can cause allergic reactions

• Ineffective against certain types of bacteria

Protonsil

• A red dye found to have antibacterial properties only when ingested (in vivo)

• Gut bacteria chemically alter the drug, sulphanilamide (so it’s a sulphonamide)

Pro-drug

• Compound which is inactive until converted by the body into an active drug

Inhibition of protein translation in bacteria

• Tetracyclines blocks tRNA binding

• Chloramphenicol blocks peptide chain transfer

• Aminoglycosides block translocation

Aminoglycosides properties

• Carbohydrate