Consider a nutritious mixture containing abundant soluble sugars, soluble lipids and soluble protein etc. Would heterotrophic microbes preferentially use sugars first > lipids > proteins > insoluble polymer such as cellulose (please propose your own order if this is wrong)? If appropriate, you can further differentiate these compounds into e.g. sugars into monosaccharides and oligosaccharides. Is there a general trend or is it highly variable depending on the particular microbe concerned?