Growth of Microbes-why should we know this?

• Control of growth is important for • infection control

• growth of industrial and biotech organisms

• Controlling food infections

Why Bacteria?

• Reproduce rapidly

• Can produce complex molecules (e.g.enzymes)

• Genetic code similar to us

• Presence of plasmids

• Less ethical concerns

Shapes of bacteria

Binary Fission

• Bacteria divide by Binary Fission.• This is a form of asexual reproduction.• Under ideal conditions it can take place every 20 minutes!• In this way huge numbers of bacteria can be produced very rapidly.

Generation Time• Time required for cell to divide/for population to double• Average for bacteria is 1-3 hours• E. coli generation time = 20 min

• 20 generations (7 hours), 1 cell becomes 1 million cells!

Bacterial Doubling Time

• Escherichia coli 20 minutes

• Mycobacterium tuberculosis 18 hours

• Mycobacterium leprae 14 days

Fig. 7.14a

Plotting growth on graphs

Factors Regulating Growth

• Nutrients

• Environmental conditions: temperature, pH, osmotic pressure

• Generation time

Environmental Factors Influencing Growth• Temperature

• O2

• pH

• Osmotic Pressure

• Others: radiation, atmospheric pressure

How to see their growth?Turbidity

Standard Growth Curve

Growth curve for Bacteria

Log

arit

hm

ic S

cale

Time

Lag phase Log Phase Stationary

Phase

Death Phase

Nu

mb

er o

f B

acte

ria

The Lag Phase

•After inoculation there is normally a brief

period of adaptation by the cells to the

new conditions.

•Bacteria are producing the enzymes

necessary to digest the nutrients.

•The rate of growth begins to increase

towards the end of this phase.

Return to graph

The Log (Logarithmic / Exponential)

phase

• There is a rapid period of growth during this

phase due to the fact that:

• Bacteria have developed the necessary

enzymes and there are plenty of nutrients.

• There are few waste products being

produced.

• The rate of cell division is currently at its

maximum with the number of bacteria

doubling as often as every 20 minutes.

Return to graph

The Stationary Phase

• The rate of growth levels off during this

period.

• This is because:

• The nutrients are becoming used up.

• The amount of waste produced by the bacteria themselves is increasing.

• The rate at which new cells are produced is equal to the rate at which other cells

are dying.

Return to graph

The Death (Decline) Phase

During this phase more bacteria are

dying than are being produced. This is

because:

• Very few nutrients are left.

• Many bacteria are poisoned by the

waste produced by such large numbers

•Thus the rate of growth is falling.

Return to graph

In unfavourable conditions many bacteria can

form endospores

These are highly resistant to drought high

temperature and other environmental

hazards.

Endospore Formation

Batch and Continuous Flow

food processing

A bioreactor is a vessel in which

biological reactions take place

Bio-processing (Food processing and Antibiotic

production)

• Modern bio-processing methods involve the use of bacteria and fungi to produce a wide range of products.

• These include dairy products e.g. yoghurts and cheeses, artificial sweeteners, flavourings, vitamins and alcohol products such as wines and beers, and antibiotics

There are two main methods of food processing:

• Batch food processing

• Continuous flow food processing

Batch processing

• In batch food processing a fixed amount of sterile nutrient is added to the micro-organisms in the bioreactor.

• The micro-organisms go through the stages of a typical growth curve

• i.e The Lag, Log, Stationary and Death stages

• The reaction may be stopped before the death stage as very little product will be formed at this stage.

• At the end of production the bioreactor is cleared out. The product is separated from the rest of the solution and is purified.

• The bioreactor is cleaned and re-sterilised.

• The process can then be repeated.

Time (days)

Lag phaseLog Phase Stationary

Phase

Death Phase

Nu

mb

er o

f B

acte

ria

In Batch Processing most of the product is formed during the

stages highlighted below

Nutrients and

micro-

organisms

added here

Gas out here

Oxygen (if required)

in here

Stirrer

Sparger

Bioreactor for Batch Food Processing

Product is released

at the end of the

process

E.g: Penicillin produced using batch fermentation in a bioreactor

(Penicillin is a secondary metabolite i.e it is produced when growth slows down)

Continuous flow food processing

• In continuous flow food processing nutrients are continuously fed into the bioreactor.

• At the same time the culture medium (containing some micro-organisms) is continually withdrawn.

• In this method of food processing micro-organisms are maintained in the Log phase of growth and the process can continue uninterrupted for weeks, even months.

• In continuous flow bioreactors factors such as temperature, pH, rate of stirring, concentration of nutrients, oxygen and waste products are constantly monitored in order to maintain growth in the Log phase and therefore produce the maximum yield.

Time (months)

Lag phase Log Phase

Nu

mb

er o

f B

acte

ria

In Continuous Flow Processing most of the product is formed

during the stage highlighted below

Nutrients fed in

continuously Gas out here

Oxygen

in here

Stirrer

Sparger

Product (and

culture medium)

flows out

continuously

Bioreactor for Continuous Flow Food

Processing

Product

Enzymes in food production

• Protease from bacteria softens gluten, making rolling of biscuit easier

• Pectinase breaks down clumps of plant cells, makes fruit juices from cloudy to clear

• Lipase and Proteases in biological washing powders can remove stain from fatty food/grease and from blood (working at low temperature, and clothes do not shrink)

• Lipase from fungi helps make chocolate flow to cover biscuits and sweets

• Lactose free milk using Lactase enzyme

Haber process of producing ammonia without using enzymes requires - 750 degrees C Temperature- - 30 times of atmospheric pressure

- Nitrogen fixing bacteria can perform this process at 25 degrees C and at atmospheric pressure because of enzymes it contains

- Majority of enzymes used commercially are obtained from microbes (usually fungi and bacteria). These microbes can be grown under carefully controlled conditions in a bioreactor (=fermentor)

Video on Enzymes

Enzymes – speed up biological reactions