Salmonella enterica Time-lapse

By Michael Beavitt

Salmonella enterica subsp. enterica is one of the best-known food-poisoning culprits. While many associate it with undercooked chicken meat, in fact a large number of cases stem from the consumption of contaminated meat in general, as well as eggs and egg-containing products, and even sometimes fresh vegetables and fruit.

While a number of pathogenic varieties of Salmonella enterica exist, the most common are the Typhimurium and Enteritidis serovars. They account for the vast majority of food poisoning cases in the UK and can cause some pretty nasty symptoms.

Once they get into the body, they can trick the macrophage immune cells into “eating” them, before escaping the deadly digestive enzymes and hitching a ride to a multitude of organs and infectable sites. They can also trigger debilitating immune over-reactions, as well as producing large quantities of toxins.

Shown is a 48-hour time lapse of Salmonella enterica growing on XLD agar, which is used in our lab in the early confirmation stages of salmonella detection. One change that’s obvious is that the agar turns from orange/red to pink– this is due to the fermentation of xylose sugar and the decarboxylation of lysine (the “X” and “L” in XLD), turning the previously pH neutral surroundings alkaline. The pink colour comes from the indicator phenol red, which changes colour depending on the pH of the surrounding media. 

It’s important to note that not all Salmonella species look like this – some ferment other sugars to produce acid, turning the agar a bright yellow colour.

The other change of note is the formation of black spots in the centres of the colonies – Salmonella enterica species commonly produce H2S, a black precipitate, from sources of iron and sulphur. This allows you to quickly tell typical Salmonella species apart from other unwanted background flora that might still grow on the media.

Due to the various ways different Salmonella serovars grow on the XLD (and other selective agars used) it requires experienced microbiologists to interpret the plates as having or not having Salmonella present, and thus verifying if a food product is safe or not safe to eat.