Bacteria

• Bacteria — unicellular prokaryotes (no true nucleus).
• Useful — curd formation (Lactobacillus), nitrogen fixation, decomposition.
• Harmful — diseases (cholera, tuberculosis), food spoilage.
• Preservation — refrigeration, pasteurisation, pickles (salt/oil).

1. Structure — single cell, cell wall, no organised nucleus; reproduce by fission.
2. Useful bacteria — Lactobacillus converts milk to curd; Rhizobium fixes nitrogen in legume root nodules.
3. Harmful bacteria — Salmonella (food poisoning), pathogenic types cause cholera, typhoid.
4. Food preservation — low temperature slows bacterial growth; salt and sugar reduce moisture.
5. Diagram (text) — rod-shaped bacterium with flagellum (optional).
6. Real world — curd set overnight; antibiotics from some bacteria (Streptomyces).

Making curd from milk (NCERT activity)

Step 1 — Warm milk (~40°C), add spoon of curd (starter with Lactobacillus).
Step 2 — Keep undisturbed 6–8 hours.
Step 3 — Bacteria multiply and convert lactose to lactic acid.
Step 4 — Milk proteins coagulate → curd formed.

• Thinking all bacteria harmful (many essential for ecosystem and health).
• Misconception: antibiotics work on viruses (only bacteria; misuse causes resistance).
• Boiling milk then adding curd when too hot kills starter bacteria.
• Confusing bacteria with virus size and structure.

• Name one useful and one harmful bacterium.
• How does Lactobacillus help in curd formation?
• Two food preservation methods against bacteria.

Open the Practice tab for graded questions on Bacteria.