That is categorically incorrect honey and your GP needs to get up to date. It is not the dressing that causes listeriosis (although it could as well)...

Did you see on the news today that NORCO custard is being recalled as some batches have tested positive to listeriosis???

Have a look on the Australia New Zealand food standards website. It is very very clear about the risk in salads. Thus they are advised against serving them in maternity units!