In the pasteurisation of milk, the milk is heated to about

The correct answer is A, Lactobacillus. This bacterium ferments the lactose of milk into lactic acid, and the acid makes the milk protein coagulate, which is why a spoonful of old curd added to warm milk sets the whole vessel. Option B is wrong because Rhizobium lives in the root nodules of leguminous plants and fixes atmospheric nitrogen; it has nothing to do with milk. Option C is wrong because Penicillium is the mould from which penicillin was obtained and which also ripens certain cheeses, but it does not set curd. Option D is wrong because Plasmodium is the protozoan parasite of malaria, carried by the female Anopheles mosquito. The other food example worth remembering with this one is Acetobacter, which turns alcohol into vinegar.

The correct answer is C, Alexander Fleming. He noticed that a mould of the genus Penicillium growing by accident on a culture plate had stopped the growth of Staphylococcus around it, and Howard Florey and Ernst Chain later turned that observation into a usable drug, the three sharing the Nobel Prize. Option A is wrong because Louis Pasteur established the germ theory of disease, disproved spontaneous generation and developed pasteurisation and the rabies vaccine. Option B is wrong because Robert Koch identified the bacteria of anthrax and tuberculosis and framed the postulates for proving that a microbe causes a disease. Option D is wrong because Edward Jenner introduced vaccination against smallpox in 1796, long before antibiotics existed, using material from cowpox. Streptomycin, the next famous antibiotic, came from Streptomyces griseus, and remembering the pair Fleming with Penicillium and streptomycin with Streptomyces settles most questions on antibiotics.

The correct answer is B. Malaria is caused by the protozoan parasite Plasmodium, of which several species infect human beings, and it is transmitted by the bite of an infected female Anopheles mosquito, the male feeding on plant juices. Option A is wrong because Aedes aegypti carries the viral diseases dengue, chikungunya, yellow fever and Zika, and this pairing is the commonest confusion in the chapter. Option C is wrong because the rat flea carries plague, caused by the bacterium Yersinia pestis. Option D is wrong because no fungus causes malaria; waterborne diseases spread that way are cholera, typhoid and amoebic dysentery. Ronald Ross, working in India, showed the role of the mosquito in the spread of malaria. Prevention rests on destroying breeding places of mosquitoes, using nets and treating patients early, since the parasite completes part of its life cycle in the mosquito and part in the human liver and red blood cells.

The correct answer is C, peptidoglycan, also called murein, a mesh of sugar chains cross-linked by short peptides. It is the target of penicillin, which blocks its formation, and the amount of it in the wall is what makes a bacterium stain Gram-positive or Gram-negative. Option A is wrong because cellulose is the wall material of plant cells and of some algae. Option B is wrong because chitin forms the wall of fungi and also the exoskeleton of insects and other arthropods, and this is the pair most often interchanged in examinations. Option D is wrong because lignin strengthens the walls of woody plant tissue and is not a microbial wall material. The wall is absent in a virus altogether, since a virus is not a cell.

The correct answer is B, measles, a viral disease spread by droplets, which is why it is prevented by vaccination and not treated with antibiotics. Option A is wrong because tuberculosis is caused by the bacterium Mycobacterium tuberculosis, identified by Robert Koch, and is treated with a long course of antibiotics. Option C is wrong because typhoid is caused by the bacterium Salmonella typhi and spreads through contaminated food and water. Option D is wrong because cholera is caused by the bacterium Vibrio cholerae and also spreads through contaminated water. Other viral diseases worth listing together are influenza, the common cold, mumps, chickenpox, smallpox, poliomyelitis, rabies, hepatitis, dengue and the acquired immune deficiency syndrome. The simple test in such a question is whether a vaccine or an antibiotic is the remedy: a vaccine points to a virus and a course of antibiotics to a bacterium.