For the concentration determination on the Certificate of Analysis for CRM-LM (Listeria monocytogenes), we use ALOA agar from three different suppliers.
It appears that there is a difference in electivity between suppliers. The colour of the colony can vary from dark green to light green.
The opaque halo around the colony is by one supplier very difficult to see and can easily be overlooked.
If this is the case at your laboratory, it is advisable to try another medium supplier for better electivity.