Describe antibiotics and antibiotic resistance

What Are Antibiotics?

How They Work: Selective Toxicity from First Principles

The Core Principle: Human cells and bacterial cells are both alive, but they're built differently. Antibiotics exploit those differences.

Key Structural Differences:

1. Cell walls: Bacteria have rigid peptidoglycan walls; human cells have flexible membranes only
2. Ribosomes: Bacterial ribosomes are 70S; human ribosomes are 80S (different protein-making machines)
3. DNA machinery: Bacterial DNA floats free; ours is in a nucleus with different enzymes
4. Folic acid synthesis: Bacteria make it from scratch; we get it from food

For a good antibiotic, this ratio should be very large (ideally ∞). The antibiotic fits the bacterial target like a key in a lock, but doesn't fit the human version of that machinery.

Example: Penicillin blocks the enzyme that builds peptidoglycan cross-links. Since humans have zero peptidoglycan (we don't have cell walls), penicillin's human toxicity is near zero. The "key" (penicillin) only fits the bacterial "lock" (transpeptidase enzyme).

Major Classes by Mechanism

1. Cell Wall Synthesis Inhibitors (β-lactams: Penicilin, Amoxicillin, Cephalosporins)

• Target: Transpeptidase enzymes (also called penicillin-binding proteins)
• Mechanism: Bacteria build cell walls by cross-linking peptidoglycan strands. Penicillin mimics the D-Ala-D-Ala end of peptidoglycan chains, permanently binds to the enzyme's active site
• Why this step? Without cross-links, the cell wall is weak. As the bacterium grows, osmotic pressure (water rushing in) bursts the cell like an overfilled water balloon
• Result: Bactericidal (kills bacteria) during active growth

2. Protein Synthesis Inhibitors (Tetracycline, Streptomycin, Chloramphenicol)

• Target: 70S bacterial ribosomes (specifically the 30S or 50S subunit)
• Mechanism:
  • Tetracycline binds the 30S subunit → blocks tRNA from delivering amino acids
  • Chloramphenicol blocks the 50S subunit → prevents peptide bond formation
• Why this step? No protein synthesis = no enzymes = no growth, no repair, eventual death
• Result: Usually bacteriostatic (stops growth, immune system finishes the job)

3. DNA/RNA Synthesis Inhibitors (Fluoroquinolones: Ciprofloxacin; Rifampin)

• Target: DNA gyrase (fluoroquinolones) or RNA polymerase (rifampin)
• Mechanism: DNA gyrase unwinds the double helix so it can be copied. Ciprofloxacin freezes this enzyme in a broken state → DNA stays tangled, can't replicate
• Why this step? Broken DNA triggers bacterial death pathways (like our apoptosis)
• Result: Bactericidal

4. Metabolic Pathway Inhibitors (Sulfonamides, Trimethoprim)

• Target: Folic acid synthesis pathway
• Mechanism:
  • Sulfonamides mic PABA (para-aminobenzoic acid), block the first enzyme
  • Trimethoprim blocks the second enzyme
  • Used together (co-trimoxazole) for synergy
• Why this step? Bacteria need folic acid to make DNA bases. No folic acid → no DNA → no replication
• Selectivity: Humans get folic acid from food (we can't make it anyway), so no human target exists
• Result: Bacteriostatic (but bactericidal when combined)

5. Cell Membrane Disruptors (Polymyxins)

• Target: Lipopolysaccharide in Gram-negative bacterial membranes
• Mechanism: Detergent-like molecules punch holes in the membrane
• Why this step? Membrane integrity lost → cell contents leak out → death
• Note: Rarely used (toxic to humans too) except for multi-drug resistant infections

Antibiotic Resistance: Evolution in Real Time

The Evolutionary Mechanism (Derived from First Principles)

Step 1: Variation Exists In any bacterial population of 10⁹ cells, random mutations create diversity. Maybe 1 in 10⁶ cells has a mutation that partially reduces antibiotic binding.

Step 2: Selection Pressure Applied When you take an antibiotic, it's a mass extinction event. Call the survival rate: $S = e^{-k \cdot [\text{Antibiotic}]}$ where $k$ is the kill rate constant. For sensitive bacteria, $k$ is large → $S \approx 0$. For the mutant with resistance, $k$ is smaller → $S > 0$.

Step 3: Differential Survival

• Sensitive bacteria:99.9999% die
• Resistant mutant: 50% survive
• Why this step? The resistant strain now has the whole environment (your body) to itself, with no competition

Step 4: Exponential Growth Bacteria divide every 20 minutes. One resistant cell becomes: $N(t) = N_0 \cdot 2^{t/20\text{ min}}$ After 10 hours (30 generations): $2^{30} \approx 10^9$ resistant cells. You're now infected with a resistant strain.

Step 5: Spread Horizontal gene transfer (plasmids, transposons) can copy resistance genes between unrelated bacteria—even different species. One resistant E. coli can donate resistance genes to Salmonella through a conjugation bridge in minutes.

Why so fast? Staph aureus generation time = 30 min. A single patient treated for 10 days experiences ~480 bacterial generations. That's equivalent to 10,000+ years of human evolution compressed into one infection.

Mechanisms of Resistance (The Four Strategies)

1. Enzymatic Destruction Bacteria produce enzymes that chemically destroy the antibiotic before it acts.

• Example: β-lactamase cuts the β-lactam ring in penicillin
• Chemical reaction: $\text{Penicillin (active)} \xrightarrow{\text{β-lactamase}} \text{Penicilloic acid (inactive)}$ The ring opens → structure collapses → can't bind target enzyme anymore

2. Target Modification Bacteria mutate the antibiotic's target so it no longer binds, but the target still functions.

• Example: MRSA has altered penicillin-binding proteins (PBP2a). Penicillin can't bind, but the protein still builds cell walls
• Analogy: Changing your door lock so the burglar's key doesn't fit, but your new key still works

3. Efflux Pumps Bacteria install molecular pumps in their membrane that actively spit out antibiotics.

• Mechanism: Transmembrane proteins use ATP energy to pump antibiotics out faster than they enter $\text{Rate}_{\text{out}} > \text{Rate}_{\text{in}} \implies[\text{Antibiotic}]_{\text{inside}} \text{ stays low}$
• Example: Tetracycline resistance (Tet pumps)
• Why this step? Antibiotic concentration inside never reaches lethal levels
• Broad resistance: One pump can export multiple different antibiotics (multi-drug resistance)

4. Reduced Permeability Bacteria alter porin channels in their outer membrane to block antibiotic entry.

• Mechanism: Mutate porin proteins to have smaller pores or different charge distribution
• Example: Carbapenem-resistant Enterobacteriaceae (CRE) close porins
• Analogy: Boarding up your windows so tear gas can't get in

Why it feels right: We see resistance appear after antibiotic use, so it looks like cause-and-effect.

Steel-man: The timing suggests causation. If you take an antibiotic and later find resistant bacteria, it's natural to think the drug created the resistance.

The fix: Mutations are random and constant—they happen whether antibiotics are present or not (cosmic rays, DNA copying errors, ~10⁻⁶ per base pair per generation). The antibiotic doesn't cause mutations; it selects for pre-existing resistant mutants by killing everything else. It's like a forest fire doesn't create fireproof trees; it just kills the flammable ones, leaving fireproof species to dominate.

Proof: The Luria-Delbrück experiment (1943) showed that resistant mutants exist before antibiotic exposure. They used statistical analysis of bacterial colonies—if antibiotics caused mutations, variance in resistant colonies would be low. Instead, variance was huge, proving mutations happened randomly before selection.

Why Resistance Is Inevitable (Thermodynamic Argument)

Given:

• Mutation rate: $\mu \approx 10^{-6}$ to $10^{-9}$ per base pair per generation
• Bacterial genome: ~5 million base pairs
• Generation time: 20-30 minutes
• Population size in an infection: $10^8$ to $10^{10}$ cells

Expected mutations per generation per population: $M = \mu \times \text{genome size} \times \text{population}$ $M \approx 10^{-7} \times 5 \times 10^6 \times 10^9 = 5000\text{ mutations per generation}$

Why this matters: With 5000 rolls of the dice per generation, bacteria will eventually hit a resistance mutation just by chance. It's not "if," it's "when." The more we use antibiotics, the faster we turn "when" into "now."

Combating Resistance: The Arms Race

1. Antibiotic Stewardship

• Use antibiotics only when needed (not for viral infections like colds)
• Use the right antibiotic for the specific bacteria
• Complete the full course even when feeling better
• Why? Partial treatment kills sensitive bacteria, leaves theoughest (partially resistant) to multiply

2. Combination Therapy Give2-3 antibiotics with different mechanisms simultaneously.

• Math: If resistance to drug A = $10^{-6}$ and resistance to drug B = $10^{-6}$, then resistance to both = $10^{-6} \times 10^{-6} = 10^{-12}$
• Why this step? Probability of random mutations confering dual resistance is vanishingly small
• Example: TB treatment uses4 drugs (rifampin + isoniazid + pyrazinamide + ethambutol) for 6 months

3. Cycling Antibiotics Hospitals rotate which antibiotics are used over time to reduce selection pressure on any one resistance gene.

4. New Drug Development

• Modify existing antibiotics (e.g., add side chains that block β-lactamase)
• Discover new targets (e.g., teixobactin targets lipid II, discovered2015)
• Problem: New antibiotic development is slow (10-15 years) and expensive ($1 billion), while resistance evolves in months

5. Preventing Spread

• Hygiene, sanitation, vaccination (reduce infections → reduce antibiotic use)
• Isolation of resistant infections hospitals

But here's the problem: bad guys have babies really fast—like, every 20 minutes. And sometimes, a baby bad guy is born with a random superpower, like armor that's shaped differently so the weapon bounces off. When you use the weapon (take antibiotics), you kill99.99% of the bad guys, but that one lucky mutant survives. Now he has the whole city to himself and makes a million copies of himself, all with the same superpower.

If you don't finish your medicine, you kill the weak bad guys but leave the tougher ones alive to breed. That's how we get "superbugs"—bad guys that no weapon can stop. Scientists are in a race to invent new weapons faster than the bad guys can evolve defenses.

Antibiotics are like mosquitoes trying to bite bacteria; DEET is how bacteria repel them.

Connections

• Natural Selection and Evolution – resistance is textbook Darwinian selection
• Bacterial Cell Structure – understanding targets requires knowing bacterial anatomy
• Horizontal Gene Transfer – plasmids spread resistance genes between species
• Minimum Inhibitory Concentration (MIC) – quantifying antibiotic effectiveness
• Gram Staining – Gram-positive vs Gram-negative affects which antibiotics work
• Public Health and Epidemiology – tracking resistant strains, outbreak control
• Enzyme Kinetics – how β-lactamase reaction rates determine resistance strength
• Selective Pressure in Ecosystems – same principle as pesticide resistance insects

#flashcards/biology

What is antibiotic? :: A substance that kills bacteria or stops their growth by targeting structures or processes unique to bacterial cells, leaving human cells unharmed.

Name the five major mechanisms by which antibiotics kill bacteria :: (1) Cell wall synthesis inhibition (β-lactams), (2) Protein synthesis inhibition (tetracycline), (3) DNA/RNA synthesis inhibition (fluoroquinolones), (4) Metabolic pathway inhibition (sulfonamides), (5) Cell membrane disruption (polymyxins).

Concept Map

Hinglish (regional understanding)