Which microorganism is responsible for the setting of milk into curd?

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.

The correct answer is A, Rhizobium. It lives in a symbiotic relationship inside the nodules on the roots of gram, pea, groundnut and other legumes, converting atmospheric nitrogen into compounds the plant can absorb while drawing its food from the plant. Option B is wrong because Nitrobacter does not fix nitrogen but carries out the second step of nitrification, oxidising nitrite to nitrate after Nitrosomonas has turned ammonia into nitrite. Option C is wrong because Pseudomonas performs denitrification, returning nitrogen from the soil to the atmosphere, the opposite of fixation. Option D is wrong because Saccharomyces is the yeast used in baking and brewing. Azotobacter fixes nitrogen while living free in the soil, and the cyanobacteria Nostoc and Anabaena do so in water. This is why pulses are sown in rotation with cereals, and why Rhizobium is sold to farmers as a biofertiliser in place of costly nitrogenous fertiliser.