Environmental liberal, one causing food poisoning, perhaps (5)

I believe the answer is:

ecoli

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'food poisoning perhaps' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'environmental liberal one' is the wordplay.
'environmental liberal' becomes 'ecol' (I can't justify this - if you can you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'ecol'+'i'='ECOLI'

'causing' is the link.

Can you help me to learn more?

(Another definition for ecoli that I've seen is " Bacterium".)

I've seen this clue in The Irish Times.

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