What is an antibiotic?

We’ve all heard about antibiotics, and chances are good that we’ve all taken them at some time or another. But what are they, exactly?

The word “antibiotic” is commonly used to describe medications that kill bacteria, fungi, or protozoa. Technically, this is incorrect. “Antimicrobials” are drugs that kill bacteria, fungi, protozoa, or viruses. Antibiotics only kill bacteria. (Antifungals kill fungi; antiprotozoals kill protozoa; antivirals kill viruses.) Doctors and veterinarians frequently use “antibiotics” and “antimicrobials” interchangeably. For the purposes of this article, I’m mostly going to use the word antibiotic to talk about all these kinds of medications.

So now that the definitions are as clear as mud, I’m going to give you a few more. There are two ways that antibiotics can work. They can directly kill the bacteria that cause disease (these are bacteriocidal), or they can just stop the bacteria from growing and reproducing without actually killing them (these are bacteriostatic). Needless to say, if the bacteria can’t grow and reproduce they will eventually die, but curing an infection with a bacteriostatic medication can take longer than using a bacteriocidal medication.

Antibiotics can be produced by one microorganism as protection against another microorganism (like penicillin produced by a fungus that kills bacteria). They can also start as a natural substance, but be modified to have slightly different properties (like Keflex, which started from a fungus but was modified in a laboratory to be the functional medication we use today).

Antibiotics are commonly used in human and veterinary medicine to treat a variety of diseases. As you know, there are lots of diseases that are caused by bacteria (and fungi, protozoa, and viruses). Some antibiotics can be used in both people and animals; some are only used in animals and some are only used in people.

The Federal Drug Administration has stringent testing requirements for any antibiotic that is approved for use in people and animals. When testing a new drug for use in people, the safety and efficacy of the drug must be proven in people.

When testing a new drug for use in food animals (like cattle, poultry, or pigs), the drug must be tested for safety and efficacy in the particular animal species and it must be tested for safety in people. In addition, a withdrawal time must be established for each drug. This is the length of time between the date when the medication was last administered and the date that animal can be sold for food. These dates are based on the half-life of the medication (how long it takes to be eliminated from the body). Farmers must keep accurate records of which animals they administer medications to, and what dose of medication they give. If a company wants to gain approval for one drug in more than one animal species, they must undergo all the same testing in each species.