Serratia marcescens

Serratia marcescens is a Gram-negative bacteria which belongs to a family of bacteria called the Enterobacteriaceae.

Now, the genus consists of at least 20 species, of which eight are known to have caused infections in humans, with Serratia marcescens being the main human pathogen.

Serratia marcescens is widely distributed in water, soil and plants and it causes a variety of hospital-acquired infections.

Now, Serratia marcescens has a thin peptidoglycan layer, so it doesn’t retain the crystal violet dye during Gram staining.

Instead, like any other Gram-negative bacteria, it stains pink with safranin dye.

And since it’s a Gram-negative bacillus, it looks like a little pink rod under the microscope.

Ok, now Serratia marcescens is motile and also facultative anaerobic which means it can survive in both aerobic and anaerobic environments.

It’s urease positive which means it can produce an enzyme called urease that dissociates urea into carbon dioxide and ammonia.

This can be tested by transferring a pure sample of bacteria from the culture to a sterile tube containing a mixture of “urea agar” broth and phenol red.

Then, the mixture is incubated. So, with Serratia marcescens, urease makes urea dissociate into carbon dioxide and ammonia.

Ammonia then makes the mixture change color from orange-yellow to bright pink.

Also, it’s catalase positive which means it can produce an enzyme called catalase.

To test for this, a few drops of hydrogen peroxide are added to the colony of the suspected bacteria.

So, if catalase is present, it makes the hydrogen peroxide dissociate into water and oxygen, causing the mixture to foam.

Furthermore, it produces another three enzymes, DNase, lipase, and gelatinase, which are unique to Serratia and this can help easily differentiate it from other Enterobacteriaceae.

Finally, Serratia marcescens grows well on MacConkey agar which is a medium that contains a pH sensitive dye and lactose.

This medium helps identify whether Gram-negative bacteria are lactose fermenters or not.

Some bacteria, like Klebsiella, Enterobacter and Escherichia coli can ferment lactose, which results in the production of acid, that makes the pH sensitive dye turn pink - so their colonies will be pink.

Others, like Salmonella, Yersinia and Shigella can’t ferment lactose so their colonies will be colorless.