Uncontaminated cultures of microorganisms are required for investigating…

The idea

Investigating disinfectants and antibiotics requires a culture containing only the microorganism being studied. If other microbes are present, results become unreliable because the disinfectant or antibiotic may affect the contaminants as well as the intended species, and contaminant colonies may be mistaken for growth of the test organism. An uncontaminated culture is achieved by sterilising equipment and media, using aseptic technique, and transferring only the chosen microorganism. For example, when testing an antibiotic disc on a lawn of a known bacterium, any stray fungal or bacterial colony would confuse measurement of the inhibition zone. Contamination also risks the safety of the worker. Therefore, valid conclusions about the action of disinfectants and antibiotics depend on starting with a pure culture and keeping it pure throughout the investigation.

What earns the marks

• An uncontaminated culture contains only the microorganism under investigation, with no unwanted bacteria or fungi.
• Contaminants can grow on the medium and produce colonies that are not caused by the test organism, making results invalid.
• Disinfectants and antibiotics may affect contaminating microorganisms as well as the intended species, so the observed effect would not be attributable to the test organism alone.
• Aseptic technique, sterilised media and sterilised equipment are used to obtain and maintain an uncontaminated culture.
• Valid comparison of disinfectants or antibiotics requires that all plates start with the same pure culture and are treated identically apart from the variable being tested.
• Contamination can also pose a health risk to the person carrying out the investigation.

Common mistakes and how to put them right

• Assuming any visible growth is the test organism.

  Put it right: Contaminant colonies can appear and must be avoided by aseptic technique.

• Thinking sterilisation kills only harmful bacteria.

  Put it right: Sterilisation removes or kills all microorganisms, including those that could contaminate the culture.

• Believing results are still valid if a plate is contaminated.

  Put it right: Contamination invalidates conclusions about the action of the disinfectant or antibiotic.

Exam tips

• Explain validity by referring to the need for only the test organism to be present.
• Link aseptic technique directly to preventing contamination, not just to safety.
• When describing an investigation, state that the same pure culture and conditions are used so that the disinfectant or antibiotic is the only variable affecting growth.