Microorganisms and Disease Questions and Answers

Answer: C

The correct match is P-3, Q-4, R-1, S-2.

• Cholera toxin acts on the intestine, causing fluid loss: an enterotoxin.
• Diphtheria toxin kills cells by blocking protein synthesis (it inactivates elongation factor 2): a cytotoxin.
• Lipopolysaccharide is part of the Gram-negative outer membrane and is the classic endotoxin.
• Tetanus toxin blocks inhibitory neurotransmitter release: a neurotoxin.

P-3, Q-2, R-1, S-4 swaps diphtheria and tetanus toxins. The other two options match lipopolysaccharide wrongly; it is the endotoxin.

Answer: C

The correct matching is a-4, b-3, c-1, d-2.

• Inhalation: mumps spreads by respiratory droplets.
• Ingestion: food poisoning comes from eating contaminated food.
• Inoculation: arboviruses such as dengue are inoculated into the skin by the bite of an arthropod vector.
• Congenital: toxoplasmosis passes from mother to fetus across the placenta.

The other codes fail on at least one of these: a-1 makes arboviruses airborne, and b-3 with c-2 sends toxoplasmosis in through the skin.

Answer: B

The role of bacteria in causing disease was demonstrated by Robert Koch.

In 1876 Koch showed that Bacillus anthracis caused anthrax, the first proof that a specific bacterium causes a specific disease. He went on to discover the tubercle bacillus (1882) and the cholera vibrio (1883), and his postulates set the criteria for proving that a microbe causes a disease.

Joseph Lister introduced antiseptic surgery with carbolic acid. Edward Jenner developed the smallpox vaccine using cowpox. Alexander Fleming discovered penicillin in 1928. Each worked on prevention or treatment rather than proving that bacteria cause disease.

Answer: C

It can cause a variant of Creutzfeldt-Jakob disease.

Bovine spongiform encephalopathy (printed 'encephalitis'), 'mad cow disease', is a prion disease of cattle. Eating beef products contaminated with the BSE prion led, from the mid-1990s in Britain, to variant CJD (vCJD), a fatal neurodegenerative disease of younger people, distinct from classic sporadic CJD.

Kuru was spread by ritual cannibalism in Papua New Guinea, and fatal familial insomnia (printed 'familiar') and Gerstmann-Straussler-Scheinker syndrome are inherited prion diseases. None is linked to eating BSE-infected meat.

Answer: D

Lyme disease is spread by ticks.

It is caused by the spirochaete Borrelia burgdorferi, transmitted by the bite of hard ticks of the genus Ixodes (deer ticks). The first sign is often a spreading 'bull's-eye' rash (erythema migrans), followed if untreated by arthritis, heart and nerve problems. It is named after the town of Lyme, Connecticut.

Bed bugs and leeches do not transmit it, and mosquitoes carry other diseases such as malaria, dengue and filariasis.

Answer: A

The resident flora are commensals.

The normal microbiota of the skin, mouth, gut and other surfaces live on the host and gain food and shelter without, as a rule, harming it, which is the definition of commensalism. They also protect the host by competing with pathogens, so some textbooks describe parts of the relationship as mutualistic, but 'commensals' is the standard term for the normal flora.

Parasites and pathogens harm the host, which normal flora do not under ordinary conditions. Mutualism implies a clear benefit to both partners, which is true only of certain members of the flora.

Answer: D

Prions are classically known as the cause of Madcow disease, bovine spongiform encephalopathy (BSE).

Prions are misfolded forms of the normal prion protein PrP. The misfolded form (PrP^Sc), rich in beta sheet, converts normal PrP into more of itself, and the aggregates destroy brain tissue, leaving it sponge-like. BSE in cattle, scrapie in sheep, and Creutzfeldt-Jakob disease and kuru in humans are the classic transmissible spongiform encephalopathies.

Schizophrenia is a psychiatric disorder with no infectious agent, and a stroke is caused by blocked or ruptured blood vessels. Alzheimer's disease does involve misfolded amyloid-beta and tau, and these are sometimes described as prion-like, but it is not transmissible and is not the classic prion disease.

Answer: A

Polio spreads mainly by Ingestion (Oral-fecal) route.

Poliovirus is an enterovirus. It is shed in large amounts in the faeces of infected people and spreads through contaminated water, food and hands. It multiplies first in the throat and intestine and, in a small fraction of infections, invades the nervous system to cause paralysis. The oral polio vaccine works by imitating this route, producing gut immunity.

Respiratory droplets play a minor role early in infection, when the virus is in the throat, but they are not the primary route. Polio is not sexually transmitted, and blood transfusion is not a recognised route of spread.

Answer: D

In prion disease the brain becomes sponge like.

Prion diseases such as Creutzfeldt-Jakob disease, kuru, bovine spongiform encephalopathy and scrapie are caused by misfolded prion protein (PrP^Sc) that converts normal PrP into the abnormal form. Neurons die and vacuoles form in the grey matter, giving the brain a spongy appearance, hence the name transmissible spongiform encephalopathies.

The damage is in the central nervous system, not the liver, so the two liver options are wrong. Squeezed ventricles are not the characteristic change; brain atrophy, if anything, enlarges them.

Answer: D

Prions are reported to kill humans and cattle.

Prion diseases are fatal neurodegenerative disorders caused by misfolded prion protein. In cattle the disease is bovine spongiform encephalopathy (mad cow disease); in humans it includes Creutzfeldt-Jakob disease, its variant form linked to eating BSE-infected beef, and kuru.

Since both are affected, 'humans only' and 'cattle only' are incomplete. Prion diseases also occur in sheep (scrapie) and deer, but not as a recognised disease of monkeys and poultry.