Economic Importance of Bacteria and Fungi — ICSE Class 9 Biology Important Questions

Answer: (b) Lactobacillus.

Lactobacillus ferments the milk sugar lactose into lactic acid, which curdles the milk protein and turns milk into curd.

Answer: (c) Penicillium.

Alexander Fleming discovered that the mould Penicillium notatum produces penicillin, which kills or inhibits many disease-causing bacteria.

Answer: (a) Rhizobium.

Rhizobium lives symbiotically in the root nodules of legumes and fixes atmospheric nitrogen into compounds the plant can use, enriching the soil.

Answer: (b) Enrich the soil with nitrogen through Rhizobium in their root nodules.

Legumes harbour nitrogen-fixing Rhizobium in their root nodules; ploughing the crop back (green manuring) returns this fixed nitrogen to the soil, improving fertility naturally.

Answer: (a) Saprophytic bacteria and fungi are decomposers; by breaking down dead organic matter they recycle nutrients such as nitrogen and carbon back into the soil, which is exactly why they are important. R correctly explains A.

Useful activities of bacteria:

1. Nitrogen fixation by Rhizobium to enrich soil fertility.
2. Formation of curd from milk by Lactobacillus.

Harmful activities of bacteria:

1. Spoilage of food by decomposing it.
2. Causing diseases such as cholera, typhoid and tuberculosis in humans.

(i) Bakery: Yeast (Saccharomyces) ferments the sugar in the dough, producing carbon dioxide gas. The trapped $CO_2$CO_2 makes the dough rise and gives bread its soft, porous, spongy texture.

(ii) Brewing: Yeast ferments the sugars in malted grains or fruit juice into ethyl alcohol and carbon dioxide, and is thus used in making beer, wine and other alcoholic beverages.

1. Nitrogen fixation and soil fertility: Bacteria such as Rhizobium (in legume root nodules) and free-living Azotobacter fix atmospheric nitrogen into usable compounds, while nitrifying bacteria convert ammonia to nitrates, keeping the soil fertile.

2. Dairy and food products: Lactobacillus turns milk into curd, and other bacteria are used in making cheese and vinegar.

3. Decomposition (recycling) and industry: Saprophytic bacteria decompose dead plants and animals, returning nutrients to the soil; bacteria are also used in the retting of fibres (jute, flax) and in the curing of tea, coffee and tobacco leaves.

Other examples include bread mould (Rhizopus) spoiling bread, and rust and smut of wheat affecting the grains and leaves. Fungal diseases usually spread in warm, moist conditions.

Food spoils because bacteria and fungi grow on it, digest it and release wastes that make it unfit to eat. These micro-organisms and their enzymes need warmth and moisture to multiply.

In a refrigerator the low temperature greatly slows down the growth and enzyme activity of bacteria and fungi (it does not kill them), so decay is delayed and the food stays fresh much longer than at warm room temperature.

Two other methods of preservation:

1. Drying / dehydration: removing moisture (e.g. sun-drying grains, fish, or making milk powder) so that microbes cannot grow without water.
2. Adding salt or sugar / preservatives: high concentrations of salt (pickles, salted fish) or sugar (jams) draw water out of microbes by osmosis, preventing their growth; canning and pasteurisation are also used.

(i) In food and baking

• Yeast (Saccharomyces) is used in baking bread (its $CO_2$CO_2 makes dough rise) and in brewing alcohol.
• Mushrooms such as Agaricus are grown and eaten as a protein-rich food.
• Certain fungi are used to ripen and flavour cheese (e.g. blue cheese).

(ii) In medicine

• The mould Penicillium yields the antibiotic penicillin, which cures many bacterial infections.
• Other fungi provide antibiotics and useful chemicals (e.g. sources of statins and organic acids like citric acid).

(iii) In decomposition (recycling)

• Saprophytic fungi break down dead leaves, wood and animal remains, returning carbon, nitrogen and minerals to the soil and keeping the environment clean.

(iv) Harmful effects

• Fungi cause food spoilage — bread mould (Rhizopus), rotting of fruits and vegetables.
• They cause diseases in plants (potato blight, wheat rust, smut) leading to crop losses.
• They cause diseases in humans and animals such as ringworm and athlete's foot.

Thus fungi are both greatly beneficial (food, medicine, recycling) and harmful (spoilage and disease).

Different groups of bacteria act at different stages to keep the soil supplied with usable nitrogen:

1. Nitrogen-fixing bacteria: Rhizobium (in the root nodules of legumes) and free-living Azotobacter and Clostridium convert inert atmospheric nitrogen ($N_2$N_2) into ammonia / nitrogenous compounds that plants can absorb.

2. Decomposers (putrefying/ammonifying bacteria): Saprophytic bacteria break down the proteins of dead plants, animals and their wastes, releasing ammonia back into the soil (ammonification).

3. Nitrifying bacteria: Nitrosomonas converts ammonia to nitrites, and Nitrobacter converts nitrites to nitrates, which are the form of nitrogen most easily taken up by plant roots.

(Denitrifying bacteria such as Pseudomonas return some nitrogen to the air, completing the cycle.)

If all soil bacteria were destroyed:

• Atmospheric nitrogen could no longer be fixed, and dead matter would not be decomposed, so nitrates and other nutrients would not be replenished.
• Dead plants and animals would pile up undecomposed, locking up nutrients.
• Plants would suffer severe nitrogen deficiency, growth would fail, and soil fertility would collapse, breaking down the whole food chain.

Thus soil bacteria are essential for continuously recycling nitrogen and maintaining fertile soil.

(i) The organism is the bread mould Rhizopus (a fungus); the tiny black dots are its sporangia containing spores.

(ii) Its mode of nutrition is saprophytic (saprotrophic) — it secretes enzymes onto the bread, digests the organic matter externally and absorbs the soluble food.

(iii) Two favourable conditions in the cupboard were:

1. Warmth (a warm temperature suitable for spore germination and growth).
2. Moisture / dampness (the moist air and bread provided the water the mould needs).
   (Presence of food and darkness also helped.)

(iv) The refrigerated loaf stayed fresh because the low temperature greatly slows the germination of spores and the growth and enzyme activity of the mould, so the bread spoils much more slowly than in the warm cupboard.