1.1.6a Culturing microorganisms

Bacteria Multiply Quickly by Binary Fission

1. Bacteria reproduce by binary fission, simple cell division that splits one cell into two identical cells.
2. With enough nutrients and a suitable temperature, they can divide as often as once every 20 minutes.
3. Dividing this fast, a single bacterium can build a huge population in only a few hours.

Microorganisms Are Grown on a Culture Medium

1. To study microorganisms, they are grown, or cultured, on a medium that supplies their nutrients.
2. The medium can be a liquid nutrient broth in a flask, or nutrient agar jelly poured into a Petri dish.
3. The nutrients include carbohydrates for energy and sources of nitrogen and minerals for growth.

Aseptic Technique Prevents Contamination

1. Petri dishes and culture media are sterilised before use to kill any microbes already present, so only the microbe being studied grows.
2. The inoculating loop is passed through a flame until it glows, then cooled, to kill microbes on it.
3. The dish lid is lifted only slightly and briefly, and work is done near a flame, to stop airborne microbes landing on the agar.
4. The lid is held on with adhesive tape to keep airborne microbes out.
5. The dish is stored upside down so condensation collects in the lid instead of dripping onto the colonies.

Incubating Cultures and Testing Antibacterials

1. In school laboratories, cultures are incubated at no more than 25 degrees Celsius.
2. This lets the culture grow but makes harmful pathogens, which often grow best at body temperature, less likely to develop.
3. Antibiotics or antiseptics can be tested by placing paper discs soaked in them onto a lawn of bacteria.
4. A clear inhibition zone forms where the bacteria have been killed, and a larger zone shows a more effective substance.

Counting Colonies and Measuring Their Area

1. When a single bacterium is spread onto agar, it divides many times to form a visible colony.
2. Each colony grows from one original cell, so counting the colonies estimates how many cells were spread on the plate.
3. A colony and an inhibition zone are both roughly circular, so their cross-sectional area is found with the area of a circle.
4. Measure the diameter, halve it to get the radius r, then use the formula.
5. $A=\pi r^2$
6. For example, a zone 20 mm across has a radius of 10 mm, so its area is $\pi \times 10^2=314{mm}^2$.
7. Count only well-separated colonies, and take a mean from several plates for a more reliable estimate.

1.1.6b Culturing microorganisms (Higher tier)

Bacterial Populations Grow by Repeated Doubling

1. Each time a bacterium divides by binary fission, one cell becomes two, so the whole population doubles.
2. After each division the number doubles: 1, then 2, 4, 8, 16, and so on.
3. The number of divisions in a given time is the total time divided by the mean division time.
4. The population is the starting number multiplied by 2 raised to the number of divisions.

$number\; of\; bacteria=starting\; number\times 2^n$, where $n$ is the number of divisions

Working Out the Population and Writing It in Standard Form

1. First find the number of divisions by dividing the total time by the mean division time.
2. Then raise 2 to that number of divisions and multiply by the starting number.
3. The answer is often very large, so it is written in standard form, a number between 1 and 10 multiplied by a power of ten.
4. For example, one bacterium dividing every 20 minutes for 5 hours has $300÷20=15$ divisions.
5. The population is $1\times 2^{15}=32768$, which in standard form is $3.2768\times 10^4$.
6. In standard form the first part is always between 1 and 10, and a large number gives a positive power of ten.