The following are useful effect of micro-organisms except

Microorganisms have many useful effects on human activities, but one of the options listed is not an example of such an effect. Specifically, "decay of meat" is not a useful effect of microorganisms, as it can lead to spoilage and contamination of the meat. In contrast, the other options are all examples of beneficial effects of microorganisms. For example, microorganisms can be used to produce vaccines that help prevent the spread of disease, and they can also be involved in the curing of tobacco and the tanning of leather.