Gram-positive bacteria: Refers to the ability of a microorganism to retain a certain stain. This ability is related to the cell wall composition of the microorganism and has been useful in classifying bacteria.

Hemolytic uremic syndrome (HUS): A rare disease that is marked by the formation of thrombi in the capillaries and arterioles especially of the kidney that is characterized clinically by hemolytic anemia, thrombocytopenia, and varying degrees of kidney failure and is precipitated by a variety of etiologic factors (such as infection with Escherichia coli or Shigella dysenteriae) and that primarily affects infants and young children.

Heterotropic: An organism that cannot manufacture its own food and instead obtains its food and energy by taking in organic substances, usually plant or animal matter (American Heritage Science Dictionary. 2011. Houghton Mifflin Harcourt Publishing Company).

Homolog: One of two or more genes that are similar in sequence as a result of derivation from the same ancestral gene. The term covers both orthologs and paralogs.

Hospital-acquired infections: Infections not present and without evidence of incubation at the time of admission to a health care setting. As a better reflection of the diverse health care settings currently available to patients, the term health care–associated infections replaced old ones such as nosocomial, hospital-acquired, or hospital-onset infections.

Host: Animal or plant that harbors or nourishes another organism.

Infection: The invasion of the body or a part of the body by a pathogenic agent, such as a microorganism or virus. Under favorable conditions the agent develops or multiplies, the results of which may produce injurious effects. Infection should not be confused with disease.

Inflammatory bowel disease (IBD): A term covering a group of disorders in which the intestines become inflamed (red and swollen), probably as a result of an immune reaction of the body against its own intestinal tissue. IBD includes Crohn’s disease and ulcerative colitis.

Keystone species: A species whose presence and role within an ecosystem has a disproportionate effect on other organisms within the system. A keystone species is often a dominant predator whose removal allows a prey population to explode and often decreases overall diversity. Other kinds of keystone organisms are those—such as coral—that significantly alter the habitat around them and thus

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