Here's why:
* It's not a proper noun: "Fecal coliform bacteria" is a common noun referring to a type of bacteria. Proper nouns are names of specific people, places, or things (e.g., John, Paris, the Eiffel Tower).
* It's not a scientific name: Even though "coliform" might sound like a scientific name, it's not written in the scientific naming convention (genus species). The scientific name of the most common fecal coliform bacteria is *Escherichia coli*.
* Capitalization rules: In general, only the first word of a sentence and proper nouns are capitalized.
Examples:
* Incorrect: Fecal coliform bacteria are indicators of water contamination.
* Correct: Fecal coliform bacteria are indicators of water contamination.
* Correct: Fecal coliform bacteria are important in the study of environmental health.
* Correct: The *Escherichia coli* bacteria is a common fecal coliform.