My Science School

The source of the smell is most probably caused by bacteria and fungi feeding on organic matter in the rubbish. It will be more noticeable if the bin is in a warm and damp place.

The smell will not always be exactly the same, but it will be more characteristic of the different organisms than on the type of food they consume.
The smell one gets from penicillin mould growing on an orange will be exactly the same as that from penicillin mould grown in a laboratory culture. It is pungent, characteristic and very common.
Analyses of household rubbish have detected pathogenic bacteria, including Pasteurella pestis, the bacterium responsible for causing bubonic plague. So do not sniff too hard.