Component 1.4 - Conditions For Culturing Bacteria

Question 1

What is supplied in nutrient media?

Answer 1

1) carbon compounds
2) usually organic compounds such as glucose
3) nitrogen, organic or inorganic
4) growth factors such as vitamins and mineral salts.

• Also require water

Question 2

What are obligate aerobes?

Answer 2

Micro-organisms which require oxygen for metabolism

Question 3

What are facultative anaerobes?

Answer 3

Micro-organisms which grow best in the presence of oxygen but can survive in its absence

Question 4

What are obligate anaerobes?

Answer 4

Micro-organisms which cannot grow in the presence of oxygen

Question 5

What temperature and pH are required to grow micro-organisms?

Answer 5

Different species vary in their requirements and usually grow over a range of temperatures and pH values, with an optimum within the range.

E.g mammalian pathogens grow best at 37°C

Question 6

What is an aseptic technique used to prevent?

Answer 6

1) Contamination of the environment by the microbes being handled
2) Contamination of microbial cultures by unwanted microbes from the environment

Question 7

Give examples of how equipment is sterilised before use?

Answer 7

1) Use of an autoclave at a suitable temperature (121°C) for 15 minutes
2) heating an inoculating loop in a Bunsen flame
3) Exposing plastics to radiation

Question 8

What is the difference between a total cell count and a viable cell count?

Answer 8

Total cell count - A count of all the dead and living cells in a known volume
Viable cell count - A count of the living cells only in a known volume

Question 9

For a viable cell count how do you estimate the number of bacteria in a given volume?

What assumption is made?

Answer 9

• Aknown volume of organisms is added to agar plates
• It is incubated
• The colonies are counted

Assumption: It is assumed that one cell gives rise to one colony.

Question 10

What is the problem with the assumption made when estimating the numbers of the original plated cells?

Answer 10

This makes no allowance for clumping of cells (colony could have arisen from more than one bacteria) and also you do not count dead cells so this may lead to an underestimate of the numbers.

Question 11

How do you provide a final number of colonies with a countable range?

Answer 11

Culture usually requires dilution by ten fold steps.

(Questions could do two ten fold steps in one so be careful.

Question 12

Problem with having few colonies to calculate number of bacteria in original sample

Answer 12

There are not enough to produce a reliable estimate