(see Antibacterial agents)

(see About bacteria and antibiotics)

Antibiotic resistance
(see About antibiotic resistance)

Antimicrobial is a broad term used to describe any agent which interferes with the normal function of microscopic organisms, including bacteria, fungi, yeast, viruses and protozoa. Antibiotics, which affect only bacteria, are one type of antimicrobial.


(see About bacteria and antibiotics)

Biological diversity (Biodiversity) 
Refers to the number of living organisms and variability among them and their environments.

Broad-spectrum antibiotics
(see Narrow-spectrum vs. broad-spectrum antibiotics)


Colonization occurs when a new species of bacteria develops a colony (a group of the same type of bacteria) in a new location, such as the human intestinal tract. Bacteria can colonize a host without causing infection or disease.

Usually refers to a microorganism that lives in close contact with a host organism (human, animal or plant) without causing disease in the host. Commensal organisms can be beneficial to the host. Some microorganisms can be a commensal for one host species but cause disease in a different species. 


Ecological impact
The changes induced by natural or human activity on the ecology and living organisms.

The study of the relationships and interactions between organisms and the environment.

A place with living (animals, plants, microorganisms, and other organisms) and nonliving (soil, water, rocks) elements that form a complex web of interdependency.

Wastewater (treated or untreated) that leaves a water treatment plant, sewer, or industrial operation; generally, waste that is discharged into surface water.

Enteric bacteria
Bacteria that live in the intestines of humans or animals.

Physical elements that form one's surroundings.


(Plural: fungi) A multicellular organism with cell walls and nuclei, but lacking chlorophyll. The fungi include many unrelated or only distantly related organisms, such as mushrooms, yeast (such as that used in making bread or beer), and the molds (for example, those that are used in making cheese or that cause rotting of food). Fungi can cause many plant and animal diseases. However, they are also the source of a number of useful antibiotics (for example, penicillin, which comes from the Penicillium mold).


Segment of a DNA molecule carrying instructions for the construction of a protein; a unit of heredity. 

Generic vs. trade name (non-generic) antibiotics
Commercially available antibiotics may be referred to by two different names. The generic name is the common family identification provided by chemists, for example "Amoxicillin." The trade name is given to it by the manufacturer and is often used by doctors and pharmacists when prescribing and dispensing the drugs. One trade name for Amoxicillin is Augmentin.

Gram-positive vs. gram-negative bacteria
When gram-positive bacteria are stained with a dye, the cell wall holds the dye inside and the bacteria are stained dark purple. Cell walls of gram-negative bacteria are more permeable - they do not retain much of the dye, and so their cell walls do not show much stain.

Growth promoters
A class of substances, usually antibiotics, used at low doses to promote growth in food animals.


Horizontal gene transfer
Exchange of genetic material between two microorganisms; no new microorganism is created. 

A multicellular organism (such as a tree, dog, or human) colonized by either commensal or pathogenic microorganisms.


Living organisms that are microscopic or submicroscopic: they cannot be seen with the human eye. They include bacteria, some fungi, and protozoa. Viruses are sometimes included in this category, although some scientists do not include viruses as microorganisms because they do not think that viruses should be classified as living organisms.

Multiple drug resistance
The ability of an organism to resist several different drugs.


Narrow-spectrum vs. broad-spectrum antibiotics
An antibiotic may be classified as "narrow-spectrum" or "broad-spectrum" depending on the range of bacterial types that it affects. Narrow-spectrum antibiotics are active against a select group of bacterial types. Broad-spectrum antibiotics are active against a wider number of bacterial types and, thus, may be used to treat a variety of infectious diseases. Broad-spectrum antibiotics are particularly useful when the infecting agent (bacteria) is unknown. Examples of narrow-spectrum antibiotics are the older penicillins (penG), the macrolides and vancomycin. Examples of broad-spectrum antibiotics are the aminoglycosides, the 2nd and 3rd generation cephalosporins, the quinolones and some synthetic penicillins.

Natural selection
A process by which organisms that are better adapted to their environment thrive and multiply, while organisms that are less well adapted to their environment fail to thrive and do not reproduce successfully. 

Non-public health antimicrobial agents
Agents that control or inhibit odor-causing bacteria. (See public health antimicrobial agents.)

Nosocomial infections
Infections that are acquired in a hospital while undergoing treatment for a different condition.


A microorganism, virus, or other substance that causes disease in another organism, the host. 

A small loop of genetic material, not part of the chromosomes, that can be easily transferred between bacteria.

Drugs used to prevent disease, before any symptoms of the disease have been observed. 

Public health antimicrobial agents
Agents that are intended to control infectious microorganisms that may be a hazard to human health. To obtain the designation of "public health" antimicrobial for an agent, a manufacturer must present data to the EPA demonstrating that the agent is effective against specific infectious microorganisms and meets standards of safety and toxicity. An agent is considered effective if it controls the specified microorganisms, not necessarily the diseases caused by the microorganisms. The manufacturer cannot claim that the agent prevents diseases. 


Reservoir of resistance
A phrase used to describe commensal bacteria that are resistant to antimicrobials. These commensal bacteria will not cause disease in their hosts; however, the resistance may eventually be transferred to an organism that will cause an antimicrobial-resistant disease in another host.

(see About antibiotic resistance)


Selective pressure
The influence exerted by some factor (such as an antibiotic) on natural selection to promote one group of organisms over another. In the case of antibiotic resistance, antibiotics cause a selective pressure by killing susceptible bacteria, allowing antibiotic-resistant bacteria to survive and multiply.

Drugs used at levels that are too low to be effective in controlling disease; antibiotics are commonly used in subtherapeutic doses for growth promotion for food animals.

Some bacterial strains have become resistant to so many antibiotics that they are sometimes referred to as "superbugs" or "supergerms." Examples of superbugs are Staphylococcus aureus that are resistant to methicillin and vancomycin, Pseudomonas aeruginosa (a cause of many lung and burn infections), vancomycin-resistant Enterococcus faecalis (VRE) (can cause an infection in the digestive system), and multi-resistant Mycobacterium tuberculosis (the bacteria that causes TB). Some of these strains resist all known antibiotics - more than 100 different drugs.

A measure of how well antimicrobials affect bacteria. Susceptible bacteria can be killed or inhibited by an antimicrobial.


Drugs used to treat disease. 

A small amount of DNA that can easily move between genetic elements such as chromosomes and plasmids. Transposons often carry genes specifying antimicrobial resistance.


An extremely small infective agent, visible only with an electron microscope. Viruses can cause disease in humans, animals and plants. Viruses consist of a protein coat and either a DNA or RNA strand of genetic material. Viruses can reproduce only inside a host cell by using the cell's machinery to reproduce. Thus, viruses are considered either the simplest type of organism or a very complex, non-living, molecule. Viruses are not killed by antibiotics.