spirillum vs spirilla

spirillum and spirilla both are nouns.

Word	Noun	Adjective	Verb	Adverb
spirillum	Yes	No	No	No
spirilla	Yes	No	No	No

As nouns, spirillum and spirilla are synonyms defined as:

spirillum and spirilla: any flagellated aerobic bacteria having a spirally twisted rodlike form

spirillum (noun)	spirilla (noun)
any flagellated aerobic bacteria having a spirally twisted rodlike form	any flagellated aerobic bacteria having a spirally twisted rodlike form
spirally twisted elongate rodlike bacteria usually living in stagnant water