Merriam-Webster Collegiate Dictionary - bacteria

 noun  (plural ~; also -rias)  Etymology: plural of bacterium  Date: 1881 bacterium — not usually used technically Usage:  Bacteria is regularly a plural in scientific and pedagogical use; in speech and in journalism it is also used as a singular, and it is sometimes pluralized as ~s caused by a ~ borne by certain tiny ticks — Wall Street Journal more resistant to chlorine and elevated water temperatures than other ~s — Allan Bruckheim, M.D., Chicago Tribune. These journalistic uses are found in British as well as American sources.