1.1.6 Culturing Microorganisms - Required Practical

What is the purpose of culturing microorganisms?

Culturing microorganisms allows scientists to study them and test the effects of antibiotics and disinfectants on their growth.

What nutrients are contained in the culture medium?

The culture medium contains carbohydrates for energy, minerals, proteins, and vitamins.

What are two ways microorganisms can be grown in the lab?

Microorganisms can be grown in nutrient broth solution or on agar gel plates.

How is nutrient broth used to culture microorganisms?

Microorganisms are mixed with sterile nutrient broth in a flask, which is sealed with cotton wool to prevent contamination and shaken to provide oxygen.

How is agar gel used to culture microorganisms?

Hot sterilised agar jelly is poured into a sterilised Petri dish, left to set, and microorganisms are spread over the surface with a sterile inoculating loop.

Why must Petri dishes and culture media be sterilised before use?

They must be sterilised, often by autoclave or UV light, to kill unwanted microorganisms and prevent contamination.

Why must inoculating loops be sterilised?

Inoculating loops must be sterilised by passing through a flame to kill unwanted microorganisms.

Why is the lid of a Petri dish sealed with tape but not completely?

It prevents airborne microorganisms from contaminating the culture, but is not fully sealed so oxygen can enter and harmful anaerobic bacteria do not grow.

Why is the Petri dish stored upside down?

It prevents condensation from dripping onto the agar surface and disrupting bacterial growth.

Why are cultures incubated at 25 °C in school labs?

Incubating at 25 °C reduces the risk of harmful pathogens growing, which are more likely to grow near 37 °C (human body temperature).

What is binary fission?

Binary fission is the process by which bacteria reproduce by splitting into two.

How can you calculate the number of bacteria in a population?

Number of bacteria at end = Number of bacteria at beginning × 2^number of divisions.

How do you calculate the number of divisions?

Number of divisions = Total time ÷ Mean division time.

Why are bacterial population calculations often left in standard form?

Because the numbers at the end of growth periods are extremely large.

How can the effect of antibiotics on bacteria be tested?

Paper discs soaked in different antibiotics are placed on an agar plate spread with bacteria, and after incubation the clear area around each disc shows effectiveness.

What is the control in the antibiotic disc practical?

A control disc soaked in sterile water ensures that any inhibition is due to the antibiotic and not another factor.

What is the inhibition zone?

The inhibition zone is the clear area around an antibiotic disc where bacteria have been killed or cannot grow.

What does it mean if bacteria are resistant to an antibiotic?

If bacteria are resistant, they will continue to grow around the disc, leaving no inhibition zone.