Petri and his dishes (and then some)

I was inspired by today's Google doodle, honoring Julius Richard Petri's birthday, to discuss the Petri dish.

A Petri dish culture of a Legionella sp. The genus includes Legionella pneumophila which causes Legionnaires' disease. This is a streak plate, a method used to dilute down a sample to the point where individual colonies can be seen and then counted. 

In modern science, Petri dishes have become a vital tool. Of course, much has changed since the days of Julius Richard Petri. Though the concept of the Petri dish is rather simple--a small isolated environment used for growing whatever small organism the scientist desires--these dishes are used quite widely. Most important, perhaps, are the uses in microbiology, especially in relation to the study of disease.

Bacteria, fungi, and other life cannot simply be grown in a plastic dish. They need a source of food and water. This is provided through some sort of growth medium that is added prior to the addition of the bacteria or other lifeforms. This medium is usually agar-based, agar being an extract from red algae that gels in a similar manner to gelatin. Nutrients are added to the mix, the types of nutrients depending on what is being grown on the plate. Trypticase soy agar, for example, is a very basic medium that a lot of bacteria will readily grow on. After the addition of the growth medium, Petri dishes can referred to as Petri plates.

There are a variety of specialized nutrient media that are used when more information is needed beyond a basic "yep, it's a bacterium." There are selective media, which will prevent the growth of certain lifeforms. This can be helpful when, for example, a scientists wants to know what bacteria are present in a soil sample. Fungi in the soil can overwhelm a plate unless the sample is grown on a medium that inhibits fungal growth.

A plate showing Alpha, Beta, and Gamma hemolysis

Differential media are especially helpful when trying to identify a specific species. Bacteria can show widely different characteristics when grown in contact with various nutrients. A blood plate, one type of differential media, is often used to identify whether bacteria can digest blood cells. This is especially useful when diagnosing infection diseases. Streptococcus pneumoniae, a species that causes one type of pneumonia, can be identified partly thanks to its hemolytic activity, hemolysis being the breakdown of red blood cells. There are three different characteristic types of hemolysis that can be seen by growing bacteria on blood agar: alpha, beta, and gamma. Alpha hemolytic bacteria will only affect the medium that the colony is touching, turning the agar from red to green. Beta hemolytic bacteria cause a rather more dramatic change, removing the color from the media for some distance around each colony. Alpha hemolysis is only a partial digestion of the blood cells, while beta is a complete breakdown of the blood; thus the difference in the results. Gamma hemolytic bacteria, in contrast, cannot digest blood at all. They will feed on the other nutrients in the medium, but the blood will be completely unaffected. The results of growing a pathogen on blood agar can help provide a diagnosis for a sick patient.

One of the most important parts of the process of growing something in a Petri plate is cleanliness. Aseptic technique is vital to prevent contamination of samples, which would skew results. This includes everything from flame sterilization to autoclaving materials; anything that can possibly be done to keep unwanted contaminants away from the sample. I'll never forget the hours I spent in the laboratory during my days learning Microbiology, going through the procedures for aseptic technique over and over again.

Interestingly enough, after an inoculum is added dishes are actually incubated upside down, which may surprise many of you. One of the main reasons for this is to prevent contamination, as spores from various living things then cannot fall down onto the growth medium. Also, condensation cannot drip down from the lid, which would interfere with the otherwise rather stable conditions the Petri dishes provide.

Source is Microbiology with Diseases by Taxonomy, 2nd edition. Images are from Wikimedia Commons and are under Creative Commons licensing or are copyright free: one, two.