Cherney Blog: Inside The Petridish

Your blog resource for education, experience, and a good read!

Cherney Blog: Inside The Petridish

Your blog resource for education, experience, and a good read!

Let's Talk About It: Listeria!

Posted by Lauren Ellie on Jan 12, 2021 9:22:24 AM
Lauren Ellie
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Why are bacteria so bad at math?

It’s because they multiply by dividing!

But honestly, bacterial growth is no joke! Under optimum conditions bacteria divide every twenty minutes. Just one cell can become sixteen million in one eight hour shift! It’s no wonder why cleaning and sanitation within production plants are vital to ensuring a quality product.

But if everyone is so focused on keeping areas clean, doesn’t it make you wonder just where does all this potential for bacterial contamination come from? Microbial contamination can come from sources you may not even consider, such as air flow within your plant, source water, soil contaminants, raw ingredients that are brought in, even animals and humans can be the source of contamination. And once bacterial contaminants are in your plant, they depend on certain growth factors to proliferate.

Bacterial growth depends on factors such as temperature, pH, moisture content, and water activity. It’s important to consider these factors both within your production environment as well as in your finished product, as the conditions could be just right to promote the growth of certain bacteria. As we go through some of the organisms most frequently tested here at Cherney keep in mind the optimal growth conditions and consider the potential for bacterial growth in both your product and environment.

The most frequently requested tests here at Cherney are Listeria and Salmonella pathogen testing, enumeration testing for Enterobacteriaceae, Staphylococcus, as well as Coliform and E coli.


About the Listeria Family: Listeria genus has a large family tree and there are still new members being discovered! Listeria genus consists of 17 species, and nine of those were recently described since 2009. The currently recognized species are: Listeria monocytogenes, Listeria seeligeri, Listeria ivanovii, Listeria welshimeri, Listeria marthii, Listeria innocua, Listeria grayi, Listeria fleischmannii, Listeria floridensis, Listeria aquatica, Listeria newyorkensis, Listeria cornellensis, Listeria rocourtiae, Listeria weihenstephanensis, Listeria grandensis, Listeria riparia, and Listeria booriae.

That’s quite the list!

The most commonly discussed members are L monocytogenes, L ivanovii, L innocua, L welshimeri, L grayi, and L seeligeri.

Although the main pathogen that causes the most concern is Listeria monocytogenes (L mono). And even the L mono species, on its own, is a very diverse group. It has three lineage divisions more than 14 different serotypes and all have the potential to cause illness and even death. L mono can cause the disease Listeriosis and human exposure to L mono bacteria through contaminated food has been linked to large outbreaks of Listeriosis.


L mono has the highest hospitalization rate and highest case fatality rate of any foodborne pathogen.

Growth Conditions: The conditions for Listeria growth can be as low as 30 degrees up to as high as about 120 degrees F (most optimal temperature is 86 degrees F). Listeria grows well in a pH range of 4.5-9.6 which is right around the neutral range, not too acidic but not too alkaline. Listeria can even grow/survive in the presence of salt, anywhere from 10-30%.

Listeria in food product: High risk foods have intrinsic characteristics that support growth of L mono (neutral pH, high water activity). Raw vegetables/fruits, fresh meat/fish, cheese products, as well as other food types, fall into this high risk category for Listeria.

Listeria in your environment: Frequent causes of outbreaks have resulted from contamination via the environment during manufacturing, processing or packing. Unlike most bacteria, Listeria is able to actually GROW in cold environments, within plants it’s often found in coolers, ice and condensation pans and lines. Listeria can survive in either wet or dry environments, although Listeria really thrives in moist conditions.

Did you knowthere is a second pathogenic species of Listeria? (Not only L mono) Listeria ivanovii has been linked to causing illness in ruminants, which includes cattle, sheep, deer, and their relatives.

Topics: Education, Continuing Education, Cherney Micro, Food Safety, Environmental Monitoring, Microbiology, Food Safety Modernization Act (FSMA), Test Results, Safe Quality Food