Remove the drawer and give it a thorough was in hot soapy water - use a little interdental brush or pipe cleaner to get in the awkard bits. Dry thoroughly.
Wash the inside of the drawer as far back as you can - a bottle brush is excellent. It is important to clean the roof of the drawer as well. Use a pipe cleaner or cable tie to clean the jets in the roof - these do get clogged.
Reassemble the drawer and replace.
Wipe the inside of the door and the seal with hot soapy water.
Now use the cheapest biological powder you can find - it must be biological and it must be powder.
Put the correct amount in the drawer and run the machine on the hottest cycle without any washing in it. This will kill any bacteria lurking anywhere in the machine.
If you have very hard water use a product such as Oust 3 or 4 times a year.