I'm working analyzing new treatments to eliminate pathogen flora in food such as Listeria, but we have a problem: We use a strain control of Listeria innocua but we found other bacterial species can grow in Palcam and Oxford agar (supplemented with antibiotics), possibly other species of Bacillus Catalase + , Gram + and oxidase - , giving us a false positive. Somebody have some idea ? The matrix analyzed are complex ( dairy products).
Some staphylococci and enterococci may grow also. If you are willing to work with L. monocytogenes you may use ALOA. The differentiation from other interfering bacteria is very simple (green with white halo), and only L. ivanovii has a similar growth.
Thank you for your answer. We performed some gram Stain and effectively we found some bacillus, coccus and streptpcoccus, all of them Gram positive, with this information we can perform some additional test.
Certainly ALOA agar it's one of the best ways to isolate L.monocytogenes, but we don't work with L. monocytogenes, just with L. innocua.
Best regards and thank you again.
Enterococci can grow in presence of the salt concentrations used in PALCAM formulation and they are also able to hydrolise esculin. Although antibiotics are added to PALCAM formulation, some enterococci strains, together with Bacillus spp., are able to grow.
See attached paper. It appears that many more species can grow on PALCAM and ALOA
As it has been corectly pointed by all the colleagues, enterococci and staphylococci can indeed grow on PALCAM agar but these bacteria can easily be differentiated since they ferment mannitol contained in the medium, changing the color of the medium from red to yellow.
L. innocua is mannitol-negative like most of the Bacillus spp. that could possibly interfere and produce false positive results. In this case, further testing (biochemical or molecular) is needed for complete differentiation. Great work from Dr. Angelidis and his colleagues on this subject!
