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The Need for Sterilisation

You can wash your hands with soap, but that is not enough when growing bacteria in a lab.
Before starting an experiment, all equipment, Petri dishes, and culture media (nutrient broth or solid agar gel) must undergo strict sterilisation.
An autoclave is typically used to sterilise dishes and media by applying high-pressure steam at 121°C.
This process is necessary to kill any pre-existing, unwanted microorganisms.
Killing these microbes ensures you grow a pure culture and prevents environmental bacteria from competing with your desired bacteria for nutrients (like glucose) and oxygen.

Preparing the Uncontaminated Culture

Understanding how heat changes airflow explains why we use fire to keep a workspace clean.
Preparing an uncontaminated culture requires aseptic technique, a strict set of step-by-step procedures to prevent contamination.
All work must be performed directly near a lit Bunsen burner. The heat creates a convection current of warm air that constantly draws airborne microorganisms upwards and away from the open workspace.
The metal inoculating loop used to transfer bacteria is sterilised by passing it through a roaring blue Bunsen burner flame until it glows red hot.
The loop must be allowed to cool in the air for a few seconds; if used while too hot, it will instantly kill the desired bacteria you are trying to transfer.
The neck of the culture bottle should also be passed through the flame immediately after opening and before closing to prevent airborne microbes from entering.

Inoculation and Sealing the Petri Dish

Why do we tape a Petri dish shut, but specifically avoid sealing it completely?
During inoculation, the Petri dish lid should be opened as little as possible (held at an angle) and for the shortest time possible to reduce exposure to airborne microbes.

Students often mistakenly write that the Bunsen burner 'kills bacteria in the air'. Examiners will not accept this; you must state that the flame creates a 'convection current' to move airborne microbes away.

When explaining the 25°C incubation limit, never write 'to kill the bacteria' — state precisely that it is 'to prevent the growth of harmful pathogens'.

In exam questions asking why the inoculating loop is flamed, always use the specific phrasing 'to kill unwanted microorganisms' or 'prevent contamination'. Avoid vague words like 'germs' or 'dirt'.

If asked why Petri dish lids are only partially taped, you must state both halves of the answer: 'to allow oxygen in AND prevent the growth of harmful anaerobic bacteria'.

In binary fission math questions, always check the units of time first! You must convert hours to minutes before dividing to find the number of divisions ( $n$ ).

Nutrient-rich substances, such as liquid broth or solid agar jelly, used to support the growth of microorganisms.

A solid, jelly-like substance used as a culture medium to grow microorganisms in Petri dishes.

The process of making something entirely free from bacteria or other living microorganisms.

A machine that uses high-pressure steam at around 121°C to sterilise equipment and culture media.

Microbes from the air, skin, or environment that can ruin a culture by competing for nutrients and space.

A laboratory culture containing only one single, intended species of microorganism.

A culture that has been prepared carefully so that no unwanted environmental microbes are present.

A set of strict laboratory procedures used to prepare an uncontaminated culture and prevent microbial contamination.

The accidental introduction of unwanted microorganisms into a culture.

A flow of warm air created by a Bunsen burner that draws airborne microorganisms upwards and away from the workspace.

A metal wire tool used to transfer a small sample of a microorganism culture from one place to another.

The process of intentionally introducing microorganisms into a culture medium for growth.

Harmful, disease-causing microorganisms that thrive and grow in the absence of oxygen.

Microorganisms, such as certain bacteria or viruses, that cause disease.

Storing a Petri dish upside down (with the agar base on top) to prevent condensation from ruining the culture.

A method of asexual reproduction in which a single bacterial cell divides into two identical daughter cells.

The clear area around an antibiotic or antiseptic disc on an agar plate where bacterial growth has been prevented.