Microbiology - Measuring Growth

Question 1

Total and viable counts

Answer 1

Total - counting total amount of bacteria from a sample (eg 1ml) using microscope dead and alive ( usually use haemocytometer)

Viable - counting the amount of colony (dot on Petri representing bacteria) using eyes form a sample (1ml) . Assumes each colony is caused by 1 bacteria. And avoid dead bacteria as they cannot grow a colony.

Question 2

Strengths + weakness of total count

Answer 2

+accurately count individual bacteria instead of colony
+give estimate of a larger sample form a small sample
-Does not differentiate dead or alive bacteria
-can over estimate true colony size of bacteria that can metabolise

Question 3

Strengths + weakness of viable count

Answer 3

+Accounts for living bacteria only. Avoid over estimation of bacteria that can metabolise
-can give under estimation if colony merge. 2 colony become 1

Question 4

Why serial dilution is needed

Answer 4

-When there are too much bacteria to be observed either total or viable.
-sample need to be diluted down to give estimation of a small sample form the original sample

Question 5

Serial dilution (10 fold - 10% of original sample)

Answer 5

1)Pour (eg 10 ml) of solution containing bacteria in tube 1

2)Pour 10% (1ml) of solution from tube 1 to tube 2. And use water to make volumn equal (9ml). Solution becomes 1/10 (fraction) or 10^-1 (standard form) of original amount.

3)Pour 10% (1ml) from tube 2 to 3. Fill up rest with water. 1/100 or 10^-2

4) 1/1000 or 10^-3

This allows for sample to be diluted
-if more dilution needed
-100 fold (1% of transfer)
-1000 fold (0.1%)

Question 6

How to deal with serial dilution test tube

Answer 6

-apply solution from each tube on petri to culture bacteria

-pick the Petri dish that has a countable range of bacteria or colony (50-300), the closer to 300 the better

Question 7

How to calculate number of total colonies or bacteria in the original sample using diluted samples

Answer 7

0.1cm3 of a 1/10000 or 10^-4 dilution of pond water gave a viable count of 54 bacteria. How many bacteria are there in 1cm3 pond water?

1)Times number of bacteria by the dilution factor
- 54x10000=540,000

2)times the answer by the adjusted volumn (0.1x10 =1cm3
- 540,000 x 10=5,400,000 or 5.4x10^6

Question 8

Haemocytometer

Answer 8

-a microscope slide to use on a microscope
-act similar to quadrant where you count the amount of bacteria in 1 large grid

-times answer by 10,000 as each grid is 1/10,000 of a ml

Question 9

Turbidimetry

Answer 9

Pass solution pass a colorimeter as particles can make solution cloudy

Compared data with a solution with known amount of particles