Growing bacteria: Petri dishes between technique and art

If they tell you « biological laboratory » what are the first images that come to your mind?
Very often when we think of a healthcare or research profession, whether it is a doctor, a biologist or a biotechnologist, in addition to the inevitable white coat and latex gloves, we associate it with the « classic » images of the most popular laboratory instruments: the optical microscope , the liquid aspirator pipette (when it comes to biomedical discoveries on the news, the image of the researcher who uses it working in another iconic equipment, the biological hood that ensures a sterile work environment), the pyrex glass test tubes and not least, transparent saucers such as those in the following figure, the Petri dishes.

What are Petri dishes for?

 Bacteria are single-celled organisms with a fairly simple structure and lifestyle that are the cause of many diseases that afflict other living beings, including us humans.
They are also responsible for managing the most important passages of the so-called « bio-geo-chemical cycles », the cyclical paths made by the elements essential for life, especially Carbon and Nitrogen, in the various compartments of the environment (soil, atmosphere, waters, beings living).
They deserve credit, for example, to extract nitrogen from the atmosphere and transform it into forms that can be used by plants to feed on it.

Then the herbivores eat the plants, the carnivores eat the herbivores and so on, a carousel that could not turn for long without the bacteria.

Bacteria are responsible for many other chemical processes useful to humans such as fermentations, which allow us to produce both yogurt and biogas.

In short, studying bacteria is of enormous importance both to exploit the positive characteristics of some species and to defend against the negative effects of others, and the first step to studying bacteria is … to have bacteria!

Petri dishes are just that, small-scale « bacteria farms ».

As with any other living being, in order to breed bacteria, it is necessary to give them food, water, shelter and an adequate temperature.
Inside the Petri dish there is precisely this, a jelly made of Agar-Agar, a complex sugar extracted from an alga, containing different food substances depending on the bacterium that you want to grow but which essentially must supply it with Carbon, Nitrogen, Oxygen, Hydrogen, Sulfur and Phosphorus plus some metal ions present in traces.

Working in a sterile environment to avoid contamination, a drop of the sample containing the bacteria we want to study is placed on the surface of this jelly, suitably diluted to decrease the number of bacteria sampled, and it is spread as much as possible to cover the entire plate.

Then you close the plate and put it in an incubator that provides the optimal temperature for bacterial growth, usually around 37 degrees (not by chance the internal temperature of our body).
The single bacteria will adhere to the Agar-Agar jelly and begin to feed on it, grow and multiply.

Each bacterium undergoes a cell division that causes two « daughter » cells to originate from a « mother » cell, exactly the same as the mother, ie clones.
After an appropriate period of time has passed, the first bacterium that has settled at a certain point on the surface of the Petri dish will therefore originate many descendants, who will organize themselves to form a structure visible to the naked eye called a colony.

Some examples of bacterial colonies

Colonies can have very different shapes, colors, edges and surface appearance and these characteristics allow researchers or technicians to identify the bacteria from which they were formed. In this way, a colony, made up exclusively of bacteria of a given species or of a given strain, can be entirely sampled to make it grow in another culture medium (there are also liquids) and have sufficient quantities of bacteria for the study or the intended use.

Different types of culture media

Depending on how they are prepared, they are first divided into synthetics and complexes.
A complex culture medium contains as an nutrient source an extract of a plant (usually soy) or an animal (for example cattle) with all the nutrients it contains, in quantities and in unknown numbers. It allows the growth of a large number of different species that will be identified and isolated later.

On the other hand, in a synthetic medium, the composition of nutrients is planned on a table and possibly allows to exclude species that have particular nutritional needs, by making a first selection.

Based on the use that is made of it, the land is divided into:

Electives – particularly rich in nutrients, allow the growth of strains with demanding nutritional needs
Selective – they contain inhibiting substances (for example antibiotics) that block the growth of certain species and not others
 Differentials – allow the growth of different species but contain substances that help to visually identify some of these thanks to colors that distinguish them from the others
Enrichment – based on their nutrient content, they allow more species to grow than others

In addition to the technical and research uses, the Petri dishes have recently become canvases on which to paint both ephemeral and fascinating works.
The Spanish artist Maria Peñil Cobo cleverly uses different bacterial strains on differential soils to create drawings with a biological background such as this tree with dragonfly.

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