The Latin (and Greek) plurals will get you every time: bacteria, data, criteria, phenomena. All of them are plural and commonly used, but, if they are plural, it means that they have a singular, and it’s the singular that is commonly ignored. So in the above example, a single bacteria can’t exist; a single bacterium can (and does. Similarly, datum (though very uncommon; data has become a common enough word that it functions as both singular and plural), criterion, phenomenon. The latter two are the Greek singulars (-on), while the former two are Latin singulars (-um).