There are three types of patents: Utility patents, Design patents, and Plant patents. (http://www.uspto.gov/patents/).
Utility patents are granted to those who have invented or discovered a “process, machine, *article of manufacture, or composition of matter” that is NEW and USEFUL.
Design patents must also be new but “original” with respect to designing “an *article of manufacture.”
Plant patents are granted when a distinct and new type of plant is invented/discovered and asexually reproduced.
Essentially there are four categories of discoveries or inventions that are patentable as a “utility” patent:
2) article of manufacture
4) composition of matter
The U.S. Court of Appeals defines an *article of manufacture as “the production of articles for use from raw or prepared materials by giving to these materials new forms, qualities, properties, or combinations, whether by hand-labor or by machinery.” http://en.wikipedia.org/wiki/Article_of_manufacture
A more in depth analysis applying these categorical terms to discoveries in microbiology will come later in the discussion.