Culturing Microorganisms and investigating the effect of antibiotics and disinfectants

when culturing microorganisms why should all work be carried out in front of a bunsen brurner?

the flame creates a convection current around the bench preventing any contamination of microorganisms in the air

how is the petri dish and agar jelly sterilised and why?

they are heated to a high temperature to kill any potential microorganisms that could contaminate the experiment

why should the petri dish only be opened as little as possible?

it decreased the risk of microorganisms entering the petri dish and contaminating it, and if you open it only open it around the open flame

why should the petri dish be stored upside town and secured with tape?

prevents drops of condensation which is another form of contamination, from dropping onto the surface of the agar ( don’t put tape all around though, as air needs to enter to prevent dangerous anaerobic bacteria from growing)

why should the cultures not be incubated above 25’C in a school laboratory?

to restrict the growth of harmful pathogens which are more likely to grow at higher temperatures

what is the inoculating loop used for?

to transfer the bacteria to the culture medium in zig zag streaks

how do you sterilise the inoculating loop?

pass it through a hot flame

how do you calculate the zone of inhibition?

use the pi r squared equation and work out the area of the entire circle that’s clear

how do you calculate the number of divisions for a bacterial cell?

time spent dividing/ the mean division time

how do you calculate the number of cells produced and why?

it’s two to the power of the number of divisions as every time a bacterial cell divides it produces two cells

how would you carry out an experiment for investigating the effect of different antiseptics on bacterial growth?

1. make sure your workspace and hands are clean so wash your hands and spray your desk

2. use a permanent marker to divide the plate into sections for each of your different antiseptics and draw dots to indicate where you’ll place your antiseptics

3. soak a filter paper disk in your first antiseptic

4. carefully place paper disk on one of the dots ensuring the lid is lifted away from your face

5. soak the paper disks in the remaining antiseptics including the sterile water as a control and add them to the agar plate

6. secure the lid of the plate with two pieces of tape and incubate the plate at 25’C for 48 hours

7. conclude which type of antibiotic was most effective against bacterial growth comparing the zones of inhibition of each one

why is sterile water used as a control?

to ensure that any differences in bacterial growth observed can be attributed to the presence of the antiseptic or antibiotic used and not some other factor (such as the paper discs for example)

what are the four factors affecting the speed of bacterial growth?

- temperature - most bacteria grow fast in warm environments

- nutrients availability - bacteria need a good supply of nutrients in order to grow rapidly

- oxygen - different types of bacteria either need the presence or absence of oxygen for growth

- moisture - most bacteria grow fastest in moist conditions

list the aseptic techniques used when culturing microorganisms

disinfect hands / work surface

• sterilise Petri dish or culture medium (before use)

• pass inoculating loop / forceps through a flame (before use)

• work near a flame or work in a fume cupboard

• tilt lid (of Petri dish) when placing discs on agar (to minimise contact with air / breath)

• secure lid of Petri dish with adhesive tape