The Singular Forms of Criteria and Bacteria
English has borrowed numerous words from Classical Latin and Ancient Greek while conserving the original plurals, which are known as neoclassical plurals. Examples are vertebrae and syllabi. Some Greek and Latin borrowings, such as formula and dogma, take Anglicized - s plurals (formulas and dogmas), which may compete with the neoclassical plurals (formulae and dogmata) in some or all senses. When a regular Anglicized plural becomes available, the neoclassical plurals can start to feel affected or overly formal. Criteria and bacteria are neoclassical plurals in English whose singular forms merit some discussion due to conflicting popular usages.
Criteria and bacteria are more commonly used in the plural than in the singular. This is for pragmatic reasons: one often refers to a set of criteria or a population of bacteria rather than a single criterion or bacterium in isolation. This has meant that English speakers rarely hear the singular inflections, which has led to doubt over their proper form.
However, because the singular is infrequently used, some English speakers instead use the innovative singular criteria. It is especially common with this and that.
This criteria is specifically geared to the requirements.
This use is considered non-standard. Criterion is the only standard singular. However, some speakers may avoid it in neutral or informal contexts because it can sound overly formal. In Google Ngram Viewer, that criteria is nearly as common as that criterion, although criterion is still more common and is overwhelmingly preferred in formal texts. This criterion is still much more common than this criteria, and a criteria is rarely attested. These patterns of usage show that the innovative singular criteria is heavily conditioned by grammatical context and is by no means replacing criterion.
As a side note, the word criterium also exists, meaning “a bicycle race taking place in one day on a circuit road course.” The plural is criteriums. Any use of this word in place of criterion in non-standard.
However, there is a singular use of bacteria found in informal and even neutral-register texts.
This appears to be an innovative use of bacteria as an uncountable noun, similar to milk, sugar and water, meaning “a strain or species of bacteria”. The plural of this use is bacteria. However, this use may be considered non-standard, despite its growing presence (Google Ngram Viewer shows that the frequency of this bacteria and a bacteria has exploded since around 1980).
Perhaps it is this singular use of bacteria that has led to the innovative, non-standard singular bacteria with reference to a singular bacterium. Some people who say this pluralize it as bacterias.
Antibiotics and alcohol can kill good bacterias.
Natural bacterias are feeding on the sugars.
This use is highly non-standard and is still virtually non-existent in formal writing. Some of these uses arguably have a “types of” reading as well. Sequences like even a few bacterias, which focus on the countability of the organisms, are very rare.