Bacteria can be grown in a nutrient broth solution or as colonies on an…

The idea

Bacteria are cultured in the laboratory to study their growth and behaviour. Two common methods exist. In a nutrient broth solution, bacteria are suspended in a liquid containing nutrients, so they multiply throughout the liquid and the culture becomes turbid; this suits measuring population growth over time. On an agar gel plate, a solid medium containing nutrients is used, and each bacterium divides in place to form a visible colony, a cluster of genetically identical cells. A colony arises from one original cell, so colonies allow separation and counting of viable cells. For example, a serial dilution of a culture spread on an agar plate can produce countable colonies, whereas the same culture in broth gives an overall cloudiness. Both methods supply nutrients, water and suitable conditions for growth.

What earns the marks

• Nutrient broth is a liquid medium containing nutrients in which bacteria multiply throughout the solution, producing a cloudy suspension.
• Agar gel plates are solid media; bacteria grow on the surface or within the gel, each original cell giving rise to a visible colony.
• A colony is a group of genetically identical bacteria descended from one parent cell, so colonies can be counted to estimate numbers of viable bacteria.
• Broth cultures are useful for growing large numbers of bacteria or measuring growth over time; agar plates are useful for isolating and counting colonies.
• Aseptic technique is needed with both methods to prevent contamination by unwanted microorganisms.

Common mistakes and how to put them right

• Thinking bacteria grow only on solid agar.

  Put it right: They also grow throughout a liquid nutrient broth, which becomes turbid.

• Believing a colony contains many different species.

  Put it right: A colony usually arises from one cell and contains genetically identical cells.

• Confusing the nutrient source with the physical state of the medium.

  Put it right: Both broth and agar supply nutrients, but broth is liquid and agar gel is solid.

Exam tips

• When comparing the two methods, state the physical state of each medium and one advantage of each.
• Use the term colony precisely: a visible cluster of genetically identical cells derived from one parent cell.
• Link the method chosen to the purpose, for example counting colonies on agar or measuring turbidity in broth.