John Humphrys was vilified by listeners to the Today programme on Radio 4 for using bacteria to refer to a singular bacterium. This morning he invited Dr Cressida Ryan, a classicist from Oxford University, to clarify the grammar (listen to the interview here for a few more days).
Dr Ryan sat on the fence a bit; she said there were three points to consider: a) what is correct Latin? b) what is correct English? and c) what is the relationship between the two? People make choices depending on whether they consider the word a Latin word, or a fully assimilated English word. She implied that scientists and specialists might prefer one version, and the general public another. People are also inclined to make false assumptions eg by making the plural of syllabus syllabi, even though syllabus, to quote the OED, "appears to be founded on a corrupt reading syllabos in some early printed editions—the Medicean MS. has sillabos —of Cicero Epp. ad Atticum iv. iv, where the reading indicated as correct by comparison with the MS. readings in iv. v. and viii. is sittybas or Greek σιττύβας, accusative plural of sittyba, σιττύβα parchment label or title-slip on a book", in other words we make things up!