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Types of Micro-organisms


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Bacteria
Viruses
Fungi
Protozoa
•
•
•
•
fungi ( eg. Mucor , Rhizopus )
protozoa ( eg. Amoeba , Paramecium )
algae (unicellular / colonial form)
( eg. Spirogyra )
Algae - Diatoms
Zooplankton
(some multicellular)
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
Useful Micro-organisms

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
Useful Micro-organisms

decomposers & nitrifying bacteria
– decomposers decompose organic waste &
dead body into ammonia
– nitrifying bacteria change ammonia into
nitrate for plants to make protein
Harmful Micro-organisms

disease causing (pathogens)
–
bacterial disease : e.g. cholera, sore
throat, tuberculosis
Dust Mite - multicellular
–
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
athlete's foot
• 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)
Harmful Micro-organisms

decay food & other useful materials
– microorganisms are saprophytic
– take in decaying food: cause food
poisoning
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

Antibiotics

Broad-spectrum antibiotics:
can attack many different strains of
bacteria

Narrow-spectrum antibiotics
can attack only one or a few strains of
bacteria
Antibiotics
Clear zone
- indicates the
absence of bacteria
which had been
killed by the
antibiotic
Clear zone with larger diameter
indicates a more powerful antibiotic
against the bacteria
Food Preservation

Heating
– kill most microorganisms & their spores
by high temperature

Canning & Bottling
– sealing in cans to avoid bacterial entry
– should work together with sterilization
Food Preservation

Pasteurization
– kill most microorganisms without changing the
flavour of milk
– 75  for 15s and then quickly cooled & bottled

UHT (Ultra High Temperature) treatment
– common in killing bacteria in milk
– superheat to ~150 for a few seconds and then
sealed
Food Preservation

Refrigeration
– to decrease or stop the activities & growth of
microorganisms (NOT killing them)

Freezing
– stop the activities of micro-organisms
– NOT killing them
– they can resume activity when ice melts
Food Preservation

Drying (Dehydration)
– micro-organism dies due to water deficiency

Smoking
– dry, coat with smoke which has chemical to
stop bacterial growth
Food Preservation

Pickling
– add vinegar (usually together with salt)
– kill bacteria by acid

Osmotic preservation
– by adding salt or sugar
– to remove water from micro-organisms by
osmosis
Food Preservation

Irradiation
– expose food to -radiation
– kill bacteria and moulds & prevent food
spoilage

Preservatives
– add to food to stop bacterial growth or kill
them
~END~