Need help on Listeria

Nick Ferrala nferrala at
Fri May 7 09:05:05 EST 1999

A presumptive test in general means that there is a very strong possibility
that the target organism is present, but further confirmation/testing is
required.  A good example is a positive antibody test on a culture.  If the
antibody is specific for a certain bacteria (L. monocytogenes, for
example), then that culture is presumptive positive for that bacteria.  If
additional biochemical tests and microscopic observations are consistent
with L. monocytogenes, then that culture may be considered as
"confirmed-positive" for L. monocytogenes.

In your question, the non-hemolytic Listerias include L. innocua, L.
welshimerii and L. grayi.  There is no evidence to date that these bacteria
have been involved in a human illness or that they can cause illness such
as L. monocytogenes can.  However, it should be noted that when these
Listerias are detected in a food product, the manufacturer's process
sanitation should be questioned as it indicates a contamination route that
can ultimately lead to contamination by L. monocytogenes.

Hope this has been helpful.

Nick Ferrala
Binax/Northeast Laboratory

Lynn McCallum wrote:

> Can anyone please tell me the difference between Listeria monocytogenes
> and a presumptive Non haemolytica being detected in a sandwich.

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