Culturing Microorganisms Flashcards

All 6 cards in this deck

How do bacteria multiply?

By simple cell division (binary fission), as often as once every 20 minutes given enough nutrients and a suitable temperature.

In what two ways can bacteria be grown in the laboratory?

In a nutrient broth solution, or as colonies on an agar gel plate.

What are the steps to calculate the number of bacteria after a given time from the mean division time?
e.g. 1 bacterium, mean division time 20 minutes, 60 minutes of growth

1. Number of divisions = time $\div$ division time: $60 \div 20 = 3$
2. Multiply the starting number by $2^{\text{number of divisions}}$: $1 \times 2^3 = 8$ bacteria

What are the steps to prepare an uncontaminated culture using aseptic technique?
e.g. inoculating an agar plate with bacteria

1. Sterilise the Petri dish and agar before use
2. Pass the inoculating loop through a flame, then spread the bacteria on the agar
3. Tape the lid on loosely, store the plate upside down and incubate at 25 °C

Why are cultures of microorganisms incubated at 25 °C in school laboratories?

At higher temperatures harmful pathogens are more likely to grow.

Why is the lid of a Petri dish secured with tape but not sealed all the way round?

The tape stops microorganisms in the air contaminating the culture, while air can still get in so harmful anaerobic bacteria do not grow.