Microorganisms: Bacteria, Viruses, Fungi and Diseases

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.

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.

The correct answer is B, chitin. Fungi are eukaryotes with a true nucleus, and their wall is built of chitin, the same tough polysaccharide that forms the exoskeleton of insects and crustaceans. Option A is wrong because peptidoglycan is the bacterial wall material, and keeping the two apart, chitin for fungi and peptidoglycan for bacteria, answers a whole family of questions. Option C is wrong because cellulose is characteristic of plant cell walls, though some fungal relatives among the water moulds do contain it. Option D is wrong because protein alone forms no cell wall; proteins are present in the wall but it is the polysaccharide that gives it strength. Fungi are heterotrophic, living as saprophytes, parasites or symbionts, never making their own food. Yeast is the exception among them, a single-celled fungus, and mushrooms, moulds and the mildews are the forms a student meets most often.

The correct answer is B, yeast producing carbon dioxide. The yeast Saccharomyces cerevisiae ferments the sugars of the dough into ethanol and carbon dioxide, and the gas, trapped in the gluten, makes the dough rise and leaves the bread spongy; the alcohol evaporates during baking. Option A is wrong because lactic acid bacteria sour milk into curd and are what make idli and dosa batter tangy, though their gas plays a smaller part than yeast's. Option C is wrong because a virus cannot multiply except inside a living host cell and has no role in dough. Option D is wrong because protozoa are not used in baking at all. The same fermentation by yeast, carried further, yields alcoholic drinks and industrial ethanol. The gluten of wheat flour is what traps the gas, which is why bread made from wheat rises better than bread of flours poor in gluten.

The correct answer is B, the bacterium Mycobacterium tuberculosis, identified by Robert Koch in 1882, which is why the organism is also called Koch's bacillus. It spreads through droplets in the air, chiefly attacks the lungs, and is treated with a long and uninterrupted course of several antibiotics. Option A is wrong because tuberculosis is bacterial; treating it as viral is the error that leads candidates to rule out antibiotics. Option C is wrong because Aspergillus causes other lung conditions in people with weak immunity and is used industrially for citric acid, but it does not cause tuberculosis. Option D is wrong because Entamoeba histolytica causes amoebic dysentery, a disease of the intestine spread by contaminated food and water. Leprosy is caused by a related species, Mycobacterium leprae.

The correct answer is B. A virus has heredity and mutates like a living thing, yet it has no cell, no metabolism and no means of reproduction of its own; outside a host it is an inert particle that can even be crystallised, as W. M. Stanley showed with the tobacco mosaic virus in 1935, while inside a host cell it directs that cell to make copies of itself. Option A is wrong because small size alone says nothing about being alive; many bacteria are tiny and are fully living. Option C is wrong because a virus carries either DNA or RNA and never both, a favourite point of examiners. Option D is wrong because antibiotics act on bacterial structures and are useless against viruses, which are controlled by vaccines and antiviral drugs.

The correct answer is B, 70 degrees Celsius for 15 to 30 seconds, followed by rapid chilling and storage. The short, sharp heating kills the disease-causing and most spoilage microbes while leaving the taste and most of the food value of the milk unchanged, which is exactly what boiling does not do. Option A is wrong because forty degrees is close to the temperature at which microbes grow best and would preserve nothing. Option C is wrong because a hundred degrees for ten minutes is ordinary boiling, which alters taste and destroys some vitamins. Option D is wrong because a hundred and twenty-one degrees under pressure for twenty minutes describes autoclaving, used to sterilise instruments and culture media, not milk. The method is named after Louis Pasteur.

The correct answer is A, Aspergillus niger, a fungus grown on a sugar medium to make most of the world's citric acid, which is used in soft drinks, food and medicines. Option B is wrong because Lactobacillus species produce lactic acid and are used in curd and other fermented milk products, not citric acid. Option C is wrong because Clostridium tetani is the bacterium of tetanus, an anaerobe that enters through wounds; it is a pathogen, not an industrial organism, although a related species yields butyric acid. Option D is wrong because Anabaena is a nitrogen-fixing cyanobacterium, used with the water fern Azolla as a biofertiliser in paddy fields. Other industrial pairs to remember are Saccharomyces for ethanol and Monascus purpureus for statins. Remembering a product with its organism, acid with Aspergillus, alcohol with yeast and antibiotic with Streptomyces, answers a whole set of questions on industrial microbiology.