CDS (I) 2021 General Knowledge · Question 109 of 120

Correct — D, A virus does not use biochemical pathways which can be blocked by an antibiotic. But a vaccine can boost an immune system to fight the virus. An antibiotic works by jamming a piece of machinery that a bacterium runs and a human cell does not — the building of a peptidoglycan cell wall, protein assembly on the bacterial 70S ribosome, the making of folic acid, the unwinding of bacterial DNA. A virus owns none of that. Outside a cell it is little more than genetic material in a protein coat, and inside a cell it borrows the host's own machinery, so there is no separate viral pathway for the drug to attack without wrecking the patient's cells too. A vaccine takes an entirely different route: it presents the immune system with a harmless form of the antigen, the body makes antibodies and memory cells, and on real exposure the response comes fast enough to prevent disease. The drug attacks the microbe; the vaccine prepares the patient.

• (a)An antibiotic can break RNA only, whereas virus has DNA. — Wrong on both halves. Antibiotics do not work by breaking RNA, and viruses come with either DNA or RNA — influenza, polio and the coronaviruses are all RNA viruses.
• (b)An antibiotic is a carbohydrate in its chemical nature, whereas a vaccine is a protein which works well to kill a virus. — Antibiotics are a chemically varied group and are not carbohydrates. A vaccine does not kill anything either — it trains the immune system in advance.
• (c)Only a vaccine can break the genetic material of a virus. — No vaccine breaks viral genetic material. It supplies an antigen so that the body's own antibodies and memory cells can deal with the virus when it arrives.

Antibiotics and vaccines sit on opposite sides of the treatment and prevention divide. An antibiotic is a chemical that kills or stops bacteria, and its usefulness rests on selective toxicity — it must hit something the bacterium has and we do not. A vaccine contains a killed or weakened pathogen, or a piece of one, and works by teaching the immune system to recognise it, so protection comes from the patient's own antibodies rather than from the medicine.

Three of the four options can be discarded on a single point of fact each, so this rewards a reader who checks every clause. Option (a) collapses on the existence of RNA viruses, option (b) on the chemistry of antibiotics, option (c) on what a vaccine actually does. The remaining option states the real reason and states it in two parts, which is why it is the longest — and length is not the reason to choose it. The consequence worth carrying beyond the exam is that taking antibiotics for a viral fever or a common cold does nothing for the illness and does add to antimicrobial resistance.

• Antibiotics act on bacterial structures and pathways — cell-wall synthesis, the 70S ribosome, folate synthesis, DNA gyrase.
• Viruses have no metabolic machinery of their own and use the host cell's, so those drug targets do not exist in them.
• Viral genetic material may be DNA or RNA; influenza, polio and the coronaviruses are RNA viruses.
• A vaccine supplies an antigen that prompts antibody production and immunological memory before the real infection.
• Antiviral drugs, which do exist, work on virus-specific steps such as entry, replication or the release of new particles — they are not antibiotics.

• Assuming all viruses carry DNA — many of the ones that matter carry RNA.
• Thinking a vaccine kills the pathogen; it prepares the immune system, which does the killing.
• Treating antivirals and antibiotics as the same class of medicine.

As a reasoning item on why antibiotics fail against viral illness, or as a statement set on what vaccines and antibiotics each do.

• practice — not a real PYQ

  Penicillin is effective against many bacteria because it interferes with

  • (a)the synthesis of the bacterial cell wall
  • (b)the host's immune response
  • (c)viral replication
  • (d)the fungal cell membrane

  Answer(a) the synthesis of the bacterial cell wall — human cells have no cell wall, so the drug damages the bacterium and leaves the patient's cells alone.

• practice — not a real PYQ

  Which one of the following statements about vaccines is correct?

  • (a)A vaccine dissolves the genetic material of the pathogen
  • (b)A vaccine primes the immune system to respond quickly on later exposure
  • (c)A vaccine is a type of antibiotic
  • (d)A vaccine works only after the disease has developed

  Answer(b) A vaccine primes the immune system to respond quickly on later exposure — it supplies an antigen so that antibodies and memory cells are ready in advance.