Growth

Question 1

Media

Answer 1

: A culture medium provides nutrients for growth and multiplication of the microorganism - it is needed to grow microbes. It needs to provide the energy sources so they are able to generate ATP and NADPH and all building blocks they need

• they can be in liquid form or on a solid surface (agar- taken from the cell walls of a marine red algae and vary the concentration - firm = bacteria just sits on surface , soft= bacteria can swim through agar
• bacteria get the nutrients by the nutrients diffusing through the agar to where they are growing

We need to provide specific conditions in order to allow the bacteria grow:
- we need to look at the composition of the cell

• some organisms need a really wide range of exotic elements
• different microbes get their carbon in different ways (look at notes for diagram)
• The media must also supply other substances essential for bacterial growth e.g. (look at notes for diagram)
• And ‘trace elements’ in micrometers or nanometer amounts (e.g. Cu, Mn, Co, Zn)
  Proteins are the main source of nitrogen in humans

Question 2

Defined media

Answer 2

assembled from a specific list of chemicals. Allows very tight control of the factors affecting growth. Often important when investigating metabolism. - there are simple components and you know exactly what is in the media - for example nitrogen as ammonium etc.

Question 3

Undefined media

Answer 3

Sometimes it is difficult to grow bacteria in defined medium so we have:

Undefined media: include undefined things like meat broth, yeast extract, blood products. Sometimes it is much easier to grow bacteria on complex media, especially when you dont know all the growth factors required.

Question 4

Growth

Answer 4

An increase in biomass on a single cell level

Typically in microbiology growth rate refers to rates of population growth - i.e. Division rates

Most bacteria divide by binary fission

some microbes do the first three steps but do not end up splitting. Instead they form long chains of cells which are kind of stitched together

Each cell will keep splitting and you grow these filaments from the middle out

there are various types of asymmetric division of bacteria

Question 5

Exponential growth

Answer 5

Do practice questions

Question 6

Lag phase

Answer 6

inoculum adapting to new conditions. Synthesis of new cellular components (e.g. Enzymes for new nutrients in medium)

Question 7

Exponential phase

Answer 7

Maximal growth - population doubling at regular intervals (doubling time depends on organism; medium and conditions)

Question 8

Stationary phase

Answer 8

Growth rate slows, the plateaus as nutrients or oxygen are exhausted and waste products accumulate. Cell density may also stop further growth

Question 9

Death phase

Answer 9

Cells start to die - starvation and accumulation of toxic waste products. Survival of some ‘starvation resistant’ cells

Question 10

Counting cells

Answer 10

• Can count individual cells e.g. With a haemocytometer (used for cell counting - including red blood cells)
• small volumes by microscopy, using a counting chamber

Question 11

Disadvantages of the counting chamber

Answer 11

• Tedious (especially when counting small cells such as bacteria)
• counts both live and dead cells -by just looking at the cell you cannot tell whether the cells are dead or alive
• An alternative is serial dilution, followed by plating on agar

Concentration of original culture = [dilution factor x no. Of colonies]/vol plated

Do practice question from the notes

To get a reasonably accurate count, you should choose a plate with several hundred colonies

Question 12

Other approaches to count cells

Answer 12

• Biomass - harvest cells (centrifugation, filtration) and measure wet or dry weight
• Most Probable Number (MPN) - used, for example, in the water industry. Carry out a series of dilutions and then score each for ‘growth’ or ‘no growth’. Statistical tables used to estimate likely cell number
• Electrical impedance - measure the electrical properties of the sample. used for quality control in the food industry. Microbial metabolism usually results in decreased impedance due to production of ionic metabolites

Question 13

Another way is methods that do not rely on counting individual cells

Answer 13

• direct automated approaches : Coulter Counter or flow cytometer

Monitor changes in cell density by measuring light - scattering with spectrophotometer (OD600)

Question 14

Diauxic growth

Answer 14

meaning double growth, is cause by the presence of two carbon sources in a growth medium, one of which is easier for the microorganism to metabolise.

Question 15

Chemostat

Answer 15

Another way to grow bacteria - continuous culture (open-system) in a chemostat.
- culture maintained in a steady - state of “exponential growth” (careful here…)
- Conditions in the chemostat reman constant (c.f. Batch culture 0 changing conditions)
- For optimum cultivation in industry, all growth parameters carefully controlled