'bacteroidal' originates from New Latin/Neo-Latin and Greek, specifically the word 'Bacteroides' and Greek 'bakterion', where 'bakterion' meant 'small rod' and the element '-oid' meant 'resembling'.

'bacteroidal' changed from Neo-Latin 'Bacteroides' into the English formation 'bacteroid' (meaning 'resembling Bacteroides' or 'rod-like') and eventually took the adjectival suffix '-al' to become 'bacteroidal'.

Initially it carried the general sense of 'rod-like' or 'resembling a rod/bacteria'; over time it became specialized to mean 'relating to the genus Bacteroides or organisms similar to it'.