Invented initially for these unusual bacteria by department head (10)
I believe the answer is:
fabricated
'invented' is the definition.
('fabricate' can be a synonym of 'invent')
'initially for these unusual bacteria by department head' is the wordplay.
'initially' suggests taking the first letters.
'these unusual' indicates an anagram.
'by' means one lot of letters go next to another.
'head' suggests taking the first letters.
The first letter of 'for' is 'f'.
The initial letter of 'department' is 'd'.
'bacteria' anagrammed gives 'abricate'.
'f'+'abricate'+'d'='FABRICATED'
(Other definitions for fabricated that I've seen before include "Constructed; invented" , "cooked up?" , "Made" .)