Viruses and Bacteria
The Microbial World of Pathogens
Viruses
Viruses are obligate intracellular parasites — they are not cells, cannot reproduce alone, and debate exists about whether they are 'alive'. Structure:
- Capsid: protein coat enclosing the genome
- Genome: DNA or RNA (single or double-stranded)
- Envelope (in some): lipid bilayer from host cell
Lytic cycle: virus injects genome → hijacks host replication → new virions assemble → cell lyses → virions released.<br>Lysogenic cycle (in bacteriophages): viral DNA integrates into host chromosome (prophage) → replicates silently → can switch to lytic on stress.
Bacteria and Antibiotic Resistance
Bacteria are diverse prokaryotes. Key groups:
- Cocci (spherical), bacilli (rod), spirilla (spiral)
- Gram-positive (thick peptidoglycan, stains purple) vs Gram-negative (thin peptidoglycan + outer membrane, stains pink)
Antibiotics exploit prokaryote-specific targets: cell wall synthesis (penicillin → blocks peptidoglycan cross-linking), protein synthesis (streptomycin → 70S ribosome), DNA replication (fluoroquinolones → DNA gyrase). Antibiotic resistance arises by natural selection on resistant mutants — a major global health crisis.
Antibiotic Misuse
Completing antibiotic courses is critical. Stopping early leaves the most resistant bacteria surviving and reproducing, accelerating the evolution of resistant strains. WHO lists antimicrobial resistance as one of the top 10 global public health threats.
What is the difference between the lytic and lysogenic cycles of a bacteriophage?
Lytic cycle: immediate viral replication → cell death. Lysogenic cycle: viral DNA (prophage) integrates and replicates with host — can activate to lytic later.
Correct answer: Lytic: immediate replication and cell lysis; Lysogenic: viral DNA integrates into host chromosome and lies dormant
Penicillin is effective against bacteria because it:
Penicillin inhibits the enzymes that cross-link peptidoglycan chains, weakening the bacterial cell wall and causing lysis — it has no effect on eukaryotic cells (which lack peptidoglycan).
Correct answer: Inhibits peptidoglycan cross-linking in the cell wall
Why are antibiotics ineffective against viral infections such as influenza?
Antibiotics are selectively toxic to prokaryotic bacteria, not viruses or eukaryotic cells.
Correct answer: Antibiotics target prokaryotic-specific structures (peptidoglycan cell walls, 70S ribosomes, prokaryotic enzyme targets). Viruses use host cell machinery to replicate and have none of these targets. Antivirals are needed instead, exerting different mechanisms (e.g., neuraminidase inhibitors for influenza).