3. Useful Micro-organisms
●yeast
○carry out alcoholic fermentation in the presence of
carbohydrates & the absence of O2
○baking bread
■produces carbon dioxide to raise the dough
○brewing beer / making wine
■fermentation to produce alcohol
●nitrogen-fixing bacteria in leguminous plant [root nodules]
○carry out nitrogen fixation which changes N2 gas from the air to
nitrogenous compounds for plants to produce protein
●decomposers & nitrifying bacteria
○decomposers decompose organic waste & dead body into
ammonia
○nitrifying bacteria change ammonia into nitrate for plants to
make protein
4. Harmful Micro-organisms
●disease causing (pathogens)
○bacterial disease : e.g. cholera, sore throat, tuberculosis
○viral disease : e.g. AIDS, bird flu, common cold, influenza
○fungal disease : e.g. athlete’s foot , ringworm
○protozoa disease : e.g. malaria, sleeping sickness
●bacteria
○prokaryotic cells (without nuclear membrane)
○reproduce by binary fission
○some are pathogenic, some are useful
●viruses
○with only DNA/RNA + protein coat
○all are parasitic (they are considered living only when they
are inside living cells)
5. ●decay food & other useful
materials
○microorganisms are saprophytic
○take in decaying food: cause food
poisoning
Harmful Micro-organisms
6. Antibiotics
●chemicals produced by microorganisms which
can kill or stop the growth of bacteria & fungi
○e.g. penicillin
●importance: effective in treating many
diseases such as meningitis, syphilis, etc
●Broad-spectrum antibiotics:
○can attack many different strains of bacteria
●Narrow-spectrum antibiotics
○can attack only one or a few strains of bacteria
7. Clear zone
- indicates the
absence of
bacteria which had
been killed by the
antibiotic
Antibiotics
Clear zone with larger diameter
indicates a more powerful antibiotic
against the bacteria
8. Food Preservation
1. Heating
●kill most microorganisms & their spores by high
temperature
2. Canning & Bottling
●sealing in cans to avoid bacterial entry
●should work together with sterilization
3. Pasteurization
●kill most microorganisms without changing the
flavour of milk
●75 ℃ for 15s and then quickly cooled & bottled
4. UHT (Ultra High Temperature) treatment
●common in killing bacteria in milk
●superheat to ~150℃ for a few seconds and then
sealed
5. Refrigeration
●to decrease or stop the activities & growth of
microorganisms (NOT killing them)
6. Freezing
7. Drying (Dehydration)
●micro-organism dies due to water
deficiency
8. Smoking
●dry, coat with smoke which has
chemical to stop bacterial growth
9. Pickling
●add vinegar (usually together with
salt)
●kill bacteria by acid
10. Osmotic preservation
●by adding salt or sugar
●to remove water from micro-
organisms by osmosis
11. Irradiation
●expose food to radiation
●kill bacteria and moulds & prevent
