All jars of food, be it Marmite [food of the Gods] Jam [comfit food] or Pickles [food of the Wilfreds] should be kept in the fridge once opened. The top shelf of our larder fridge at this moment contains at least 20 various jams jellies and condiments in the process of being consumed.
We do not suffer from blue mould on the top of our opened jars of food in this house.
We also almost always are able to ignore use by dates because this extends their keeping properties.
Incidentally it does say on the label of a Marmite jar "Store in a cool dark place"
Giok