NATURE OF MICROBES

What are microbes and are there different types?

How are they seen?How can they be grown?

How were their effects first proved?

Learning outcomes

• Know that microbes include bacteria, viruses and fungi.

• know that viruses are smaller than bacteria.

• know that yeasts are fungi.

Bacteria

• A bacterial cell consists of cytoplasm, cell membrane and a cell wall.

• There is no distinct nucleus. • Bacteria reproduce by dividing into

two.

Viruses

• Viruses are smaller than bacteria• they consist of a protein coat

surrounding a small number of genes and can only reproduce inside a host cell.

• The production of new viruses results in the destruction of the host cell and the release of new viruses which can then attack new cells.

Fungi

• Yeast are a type of fungi• Yeast cells are larger than bacteria• They have a nucleus, cytoplasm and

a membrane surrounded by a cell wall.

• Yeasts reproduce by budding.

Pupil Activity

• Read through the hand out and make a labelled diagram of– A bacterial cell– A virus

Learning outcomes

• understand the safe use of basic aseptic techniques

• investigate the presence of bacteria in milk using agar plates.

Aseptic Techniques

• Aseptic techniques are used to safely inoculate, plate and incubate microbes.

• Micro organisms can be grown in a culture medium containing carbohydrates as an energy source, mineral ions, and in some cases supplementary protein and vitamins.

• These nutrients are often contained in an agar medium which can be poured into a petri dish.

Culturing micro organisms

Aseptic Techniques

• For this:– petri dishes and culture media must be

sterilised before use to kill unwanted micro organisms;

– inoculating loops used to transfer micro organisms to the media must be sterilised by passing them through a flame;

– the lid of the petri dish should be sealed with adhesive tape to prevent micro organisms from the air contaminating the culture.

Risk assessment

• cultures should be incubated at a maximum temperature of 25°C to prevent the growth of pathogens that might be harmful to humans.

• In industrial conditions higher temperatures can produce more rapid growth.

Learning outcomes

• understand how Pasteur used scientific method to devise experiments and make deductions.

Spontaneous Generation

• Read through the worksheet– Spallanzani’ experiment

• What did Spallanzani's experiment show? • Was his hypothesis correct or incorrect?

– Proponents of spontaneous generation argued that Spallanzani had only proven that spontaneous generation could not occur without air.

Louis Pasteur

• Pasteur's Experiment Results– What did Pasteur's experiment show?– Was his hypothesis correct or incorrect?

• The theory of spontaneous generation was finally laid to rest in 1859 by Louis Pasteur.

• Pasteur's experiment was a variation of earlier methods

• He boiled meat broth in a flask, heated the neck of the flask in a flame until it became pliable, and bent it into the shape of an S. – Air could enter the flask, but airborne

microorganisms could not - they would settle by gravity in the neck.

• As Pasteur had expected, no microorganisms grew.

• When Pasteur tilted the flask so that the broth reached the lowest point in the neck, where any airborne particles would have settled, the broth rapidly became cloudy with life.

More Louis Pasteur

Learning outcomes

• explore information about the effect of temperature on the growth of bacteria and understand its application in food storage

Stages of Growth

• In micro-organisms, such as bacteria and yeast, growth leads to cell division and consequently an increase in population size.

• The growth of a population of single-celled microorganisms grown in a closed environment typically shows four stages: – lag phase; – exponential phase; – stationary phase; – death phase.

Growth Curve

• The lengths and characteristics of these phases will depend upon factors such as the nature of the growth medium and temperature of incubation.

• In industry, it is important to understand the factors which affect the growth rate of a given micro-organism in order to generate maximum product by the most economic means.

Example

• If the product is produced when the organism has stopped growing the manufacturer will want to provide optimum conditions for the culture to:– reach maximum numbers in stationary

phase – in the shortest time possible

Food Storage

• food preservation – by canning, – freezing, – drying, – ultra heat treatment, – high solute concentrations)

Temperature and food storage

• Freezing

• UHT

Micro-organisms in the food industry

• Micro-organisms are heterotrophic – they take in ready made foods molecules from their environment.

• Examples of Microbes useful in food production– Acetobacter – vinegar production– Lactobacillus - yoghurt and cheese production– “ripening” bacteria in cheese production– Fusarium and mycoprotein

Use of micro-organism

process Use of microorganism

Baking

Yeast is mixed with flour, sugar and water to make dough. The yeast respire the sugar and produce pockets of carbon dioxide that makes the dough rise.When the bread is baked the yeast is killed.

BrewingYeast respires sugar to form ethanol and carbon dioxide. The ethanol makes the drink alcoholic and the carbon dioxide gives the drink its fizz. The process is called fermentation.

Yoghurt making

Bacteria are used to ferment milk at 46oC. As a result lactic acid is produced which makes the milk lumpy (coagulate). The temperature is then reduced to 5oC to prevent further bacterial action. The whole process is carried out in sterile conditions

Single-cell

protein

This is a protein produced by microorgansism such as bacteria. Fungi or unicellular algae in a fermentation vessel. While the product contains 72% protein it does not taste very nice and is expensive to produce. An alternative is quorn – this is a mycoprotein made from a filamentous FUNGUS, which has 12.2% protein, low fat content and 6% fibre.

Food Additives

• Food Additives are substances with no nutritional value

• They are added to improve the– Appearance– Flavour– Texture– Storage properties of food

antioxidants

• Stop food reacting with oxygen, which may spoil the taste of the food or change it’s colour

Colourings

• Improve the appearance of food• Some colourings can be hazardous

– Sunset yellow • Hyperactivity• Allergic reactions and asthma

Flavourings

• Enhance the taste of the food– MSG – monosodium glutamate in

processed food– Sugar– Artificial sweetners e.g. aspartame

Preservatives

• Give processed foods a longer shelf life by preventing the growth of micro-organisms

• Disadvantages of preservatives– Sulphur dioxide - destroys vitamin B1

– Sodium nitrite - carcinogen