Quick questions on Antivirals, antibiotics, resistance and immunisation: HSC Biology Module 7

Antibiotics target structures or processes unique to bacteria, sparing host cells.

Antivirals target viral-specific enzymes and life-cycle steps.

Origin. Random mutations in bacterial DNA occasionally produce a resistance gene (e.g. beta-lactamase that degrades penicillin). In an antibiotic-free environment, resistance confers no advantage. Once antibiotics are applied, natural selection favours resistant individuals: susceptible bacteria die, resistant bacteria reproduce.

Vaccines provide active artificial immunity by exposing the immune system to a pathogen antigen without causing disease, generating memory B and T cells.

When a high enough proportion of a population is immune, transmission chains break and even unvaccinated individuals are protected.

- Cell wall synthesis inhibitors (penicillin, amoxicillin, cephalosporins) prevent peptidoglycan cross-linking, lysing actively growing bacteria. - Protein synthesis inhibitors (tetracycline, erythromycin) bind the bacterial 70S ribosome. - DNA replication inhibitors (fluoroquinolones such as ciprofloxacin) target bacterial DNA gyrase.

Antibiotics do not work against viruses, fungi, protozoa or prions. Many cause side effects by killing beneficial gut bacteria.

Random mutations in bacterial DNA occasionally produce a resistance gene (e.g. beta-lactamase that degrades penicillin). In an antibiotic-free environment, resistance confers no advantage.

Bacteria reproduce rapidly (every 20 minutes in good conditions) and share genes by horizontal gene transfer. - Conjugation. Plasmids carrying resistance genes are transferred between cells via a pilus. - Transformation. Bacteria take up free DNA from the environment.

Resistant infections cost lives and treatment dollars. Methicillin-resistant Staphylococcus aureus (MRSA), multi-drug resistant tuberculosis (MDR-TB) and carbapenem-resistant Enterobacteriaceae are major threats. The WHO ranks antibiotic resistance among the top ten threats to global health.

1. Prescribe only when bacterial infection is confirmed. 2.

- Live attenuated (MMR, oral polio, BCG, varicella). Weakened pathogen replicates briefly. Strong immunity.

The proportion of the population that must be immune is approximately 1 - 1/R0. - Measles (R0 = 12 to 18): 92 to 95 per cent. - COVID-19 ancestral strain (R0 = 2 to 3): 50 to 67 per cent.

- Protects those who cannot be vaccinated (newborns, immunocompromised, severely allergic). - Enables eradication (smallpox 1980; polio close). - Reduces selection pressure for new variants.

Vaccine hesitancy or supply gaps can drop coverage below the herd immunity threshold. Measles outbreaks resurged in 2019 in parts of Europe, North America and the Pacific where coverage had fallen.