Measure bacterial composition (9)
I believe the answer is:
calibrate
'measure' is the definition.
(I know that calibrate is a more specific form of the action measure)
'bacterial composition' is the wordplay.
'composition' indicates an anagram.
'bacterial' is an anagram of 'CALIBRATE'.
(Other definitions for calibrate that I've seen before include "Adjust and align" , "Adjust (gauge for accuracy)" , "Mark the scale of a measuring instrument" , "Mark (a gauge) with a scale of readings" , "Mark a scale to use in taking readings" .)