Listeria monocytogenes is a serious food pathogen. If you are looking for this pathogen, you want to detect this bacteria down to the last cell. The quality of the ALOA medium used is a crucial part of the analysis.
Research shows that there is a difference in electivity between suppliers. The color of the colony can vary from dark green to light green. The opaque halo around the colony is very difficult to see for at least one supplier and can easily be overlooked. If this is the case at your laboratory, it would be best to try another medium supplier for better electivity.
Below are the ALOA plates from the different suppliers with the spread and pouring plate technique.
The used strain is Listeria monocytogenes from reference material CRM-LM.