Bad smell in fridge ZUD 9154?

Hi, I have an integrated fridge model ZUD 9154 that has started to smell very badly. I have cleaned the fridge out and had the kick board off and cleaned under the fridge as the smell seems to be coming from where the vent is and the drip tray hole in the fridge. I have put bicarbonate of soda both under the fridge and in the fridge. I did put a couple of drops of zoflora in the drip hole in the fridge but the smell is still awful.
Can anyone help/offer advice.
Many thanks in advance