'bacterioidal' originates from Greek + New Latin elements: Greek 'bakterion' meaning 'small staff, rod' (via New Latin 'bacterium') combined with the suffix '-oid' from Greek '-oeidēs' meaning 'resembling'.

'bakterion' (Greek) → New Latin 'bacterium' (meaning 'rod-shaped microorganism') → adjective forms such as 'bacterioid' (meaning 'resembling a bacterium') → modern English adjectival formation 'bacterioidal'.

Initially related to the idea of a 'small rod' (the root for bacterial organisms); over time it came to refer to organisms (bacteria) themselves and later to the adjectival sense 'resembling or characteristic of bacteria', which is the modern usage.