The most common and most likely answer to this crossword clue is the 12 letter word STREPTOCOCCI
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "Bacteria" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bacteria = streptococci.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 STREPTOCOCCI mean?
STREPTOCOCCI noun- A spherical, gram-positive bacterium of the genus Streptococcus. Although commonly found benignly in the human mouth and gut, and though many species are non-pathogenic, other species can cause diseases including strep throat and more serious conditions.