'bacterious' originates from New Latin 'bacterius', ultimately from Greek 'bakterion', where 'bakter-' meant 'small staff' or 'rod' (the shape of many bacteria).

'bacterious' was formed in English by combining the stem from New Latin/Greek ('bacter- / bacteri-') with the adjective-forming suffix '-ous'; the sequence is Greek 'bakterion' → New Latin 'bacterium'/'bacterius' → English 'bacterious'.

Initially related to the notion of 'rod-like (organisms)' coming from 'bakterion'; over time it has been used to mean 'relating to or characterized by bacteria' in modern usage.