Splitting microbial hairs Defining antibiotics

Microbes are a diverse collection of organisms, many of which have almost nothing in common except that they are extremely little. They fall into three major categories — bacteria, viruses, and eukaryotes, which include fungi (which have some important disease organisms) — as well as a bunch of other critters, like Giardia and other protozoan parasites, that cause diseases like diarrhea and malaria. Humans tend to lump them all together because, as a group, they're responsible for infectious diseases.

Antimicrobials, as you may cleverly guess, are compounds that are active against microbes. Each antimicrobial has a name that indicates which microbe it's active against:

1 Antivirals: Antimicrobial compounds that target viruses. 1 Antifungals: Compounds active against fungi.

1 Antimalarials and other compounds: These are active against specific kinds of eukaryotic diseases (see the nearby sidebar "Eukaryotes for you and me"). (Note: Antimicrobials that are active against these types of infections tend to be named more specifically.)

1 Antibacterials: Compounds that are active against bacteria.

Here's where matters get confusing: Antibacterials are often referred to as antibiotics, a term that is also occasionally used interchangeably with the term antimicrobial.


Antibiotic can be used generally to refer to all disease-fighting compounds and specifically to mean only those compounds that are effective against bacterial agents. This dual-purpose use of the term leads to one of the biggest misunderstandings about what antibiotics can and can't do. You may have

heard that antibiotics aren't effective against viral infections. That's true, when the term antibiotics is used specifically to mean antibacterial, which by definition means active only against bacteria, not against other microbes.

Technically, antibiotics are chemicals that kill or inhibit the growth of biotic (biological, or living) things, as opposed to abiotic things (like rocks). Rat poison is an antibiotic; so is Round Up. But when scientists use the term antibiotic, they're almost always referring to antibacterial agents — things that kill or inhibit the growth of bacteria — because the original antibiotic compounds, such as penicillin, were active against bacteria.

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