The most common and most likely answer to this crossword clue is the 9 letter word BACTERIAL
I've seen this crossword clue in The Guardian
B
A
C
T
E
R
I
A
L
More clues leading to the result BACTERIAL
Buddy explains!

The straight part
We know that "regarding micro-organisms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case regarding micro-organisms = bacterial.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BACTERIAL mean?
BACTERIAL adjective- Of, relating to, or caused by bacteria.