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Listeria Identification Test

In my Parsley Patch blog I was talking about squash seedlings infected by soil-borne fungi and bacteria. To make sure I am not dealing with human pathogenic species growing on my squash plants, I decided to perform a few identification tests. To begin the tests, I started with a Gram stain and found the microorganism to be a Gram positive rod. I also noticed that the bacteria had tumbling motility which is characteristic of Listeria species and is often used as a conventional marker for Listeria identification. Bergey's Manual and other sources identify Listeria habitat as soil, decaying vegetable matter, silage, sewage, stream water, animal feed, food, and dust. This is consistent with the source of my isolate. Listeria group contains six species, only two of which, L. monocytogenes and L. ivanovii are pathogenic. L. monocytogenes causes listeriosis, a potentially deadly disease, especially in newborns and those with weakened immune system. In these people L. monocytogenes may cause meningitis and blood poisoning. Listeria is not transmissible from animal to animal, animal to human, or human to human; rather it is acquired from the external environment or food (Source: Infectious diseases of wild mammals By Elizabeth S. Williams, Ian K. Barker). According to the Encyclopedia of Medical Genomics and Proteomics, all Listeria species are potential food contaminants and coexistence of several species on the same substrate is not unusual. Therefore the presence of any of these species can be considered as an indicator of their contamination by the pathogenic species. To rule out the pathogenic species I decided to do further tests. The nextt step was soluble starch test which I used to separate the two pathogenic species (negative) from non-pathogenic L. grayi (positive). The picture below shows that the tested organism is able to hydrolyze starch.

To be sure, I also performed catalase (+), citrate (+), esculin hydrolysis (+), Vogues-Proskauer (+), mannitol (+) and glucose fermentation (+) tests, as well as H2S production in TSI agar (-). The tests results confirmed that the organism is L. grayi.


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