bacterial culture and serial dilution

Question 1

explain three methods that reduce food spoilage

Answer 1

-freezing= decreases rate of enzyme activity
-pickling (put in acid which increase pH) denatures enzymes
-irradiation= destroys microbial DNA

Question 2

what are two growth medium

Answer 2

Broth
Agar in petri dish

Question 3

explain the four ways of inoculation

Answer 3

-streaking= dilutes sample whilst spreading it around the plate thinly = single colonies
-seeding = drop of solution on agar
-spreading= spreading inoculum over surface evenly
-swabs= spreading with cotton swab

Question 4

what are the three main ways of growing microorganisms

Answer 4

1.sterilisation
2.inoculation
3.incubation

Question 5

what are the steps for producing an agar plate

Answer 5

sterilise:
-heat inoculating loop in blue flame and allow to cool
-remove cap from broth culture and flame mouth of bottle, Dip cool sterile inoculating loop in broth. flame and recap bottle

inoculate:
-spread a streak of culture over surface of agar and cover with a lid. Reheat inoculating loop in blue flame

incubate:
-use adhesive tape to hold tape lid on peter dish= not completely sealed to not allow anaerobic respiration
-flip plate upside down so condensation on the lid doesn’t drip onto agar
-incubate at 25 degrees= no hitter so harmful pathogens don’t grow

Question 6

explain the steps of the serial dilution to reduce the population density of microorganism

Answer 6

1) add 1cm3 of broth solution to test tube 1 and add 9cm3 of water = 0.1 dilution
2) from test tube 1, take 1cm3 of the solution and add to beaker 2 with 9cm3 of water
3)repeat for two more test tubes= creates 0.001 and 0.0001 dilutions
4)for each test tube, spread the solution over the agar plate, count colonies on 0.0001 dilution and multiply by dilution factor = 10,000 e.g 18x10,000

Question 7

explain the curve of microorganisms in a closed culture

Answer 7

1) lag phase= slow initial growth as they adapt to their new micro ment and produce essential enzymes, low reproduction

2)log phase= rapid growth occurs under ideal conditions, plentiful resources

3)stationary = plateaus, carrying capacity reached, reproduction equal to death, small variation in environment

4) population may decline, resources start to decline, competition, death > birth

Question 8

why do we flame the neck of the bottle and also hold the lid over the petri dish

Answer 8

-kill bacteria on neck of bottle
-avoids infection with bacteria in the air

Question 9

a medical technician cultures blood on the agar plate. What measures should the technician take to keep the agar plate culture sterile

Answer 9

-work in an inoculating cabinet/ maintain minimum plate opening time
-flame inoculating loop

Question 10

student studied an agar plate, it was completely covered by a film of bacteria and impossible to count colonies
describe a modification to the procedure that would enable the student to estimate the population size

Answer 10

-dilute the sample taken from the colony
-multiply results from agar
by dilution factor

Question 11

give and explain the formula for the number of individual organisms

Answer 11

N = N0 x 2n

N0= number of bacteria at the start
2 = bacteria divide in 2 each time
n= number of generations/ how many divisions

Question 12

solve this questions
2 bacteria are dividing every 20 minutes how many will there be after 4 hours

Answer 12

4 hours= 240 mins
240/20 = 12

2 x 2^12 = 8192