Glossary of Terms Related to Antibiotic Resistance

CDC works with federal, state, and local partners to reduce antibiotic resistance and protect the public’s health. This work includes investigating outbreaks caused by resistant bacteria, identifying new antibiotic resistance threats, and recommending ways to help prevent illnesses and outbreaks caused by resistant bacteria.

The National Antimicrobial Resistance Monitoring System (NARMS) assists in outbreak investigations by testing bacteria from sick people, animals, and retail meats for antibiotic resistance. When paired with information collected during interviews with sick people about what they ate or were exposed to before becoming ill, resistance data can provide investigators with clues about the source of outbreaks.

This glossary defines some key terms related to antibiotic resistance that CDC uses to describe multistate outbreak investigations.

Antibiotic: A drug that kills or stops the growth of bacteria. Antibiotics are a type of antimicrobial. Penicillin and ciprofloxacin are examples of antibiotics.

Antibiotic resistance: Antimicrobial resistance of bacteria.

Antibiotic resistance testing (also known as antimicrobial susceptibility testing): Laboratory testing performed on bacteria to find out if they are resistant to one or more antibiotics.

Antimicrobial: A substance, such as an antibiotic, that kills or stops the growth of microbes, including bacteria, fungi, or viruses. Antimicrobials are grouped according to the microbes they act against (antibiotics, antifungals, and antivirals). Also referred to as drugs.

Antimicrobial resistance: The ability of a microbe (germ) to resist the effects of a drug. Antimicrobial-resistant germs are not killed by the drugs that are typically used against them and may continue to multiply. Antimicrobial resistance includes antibacterial, antifungal, and antiviral resistance. Learn more about antimicrobial resistance.

Antimicrobial susceptibility testing (AST): Laboratory testing performed on microbes to find out if they are susceptible or resistant to one or more drugs. Results of antimicrobial susceptibility testing show if bacteria are susceptible (can be treated with the drug), intermediate (may be treatable with the drug, but may require adjusted dosage), or resistant (cannot be treated with drug). AST is also referred to as antibiotic susceptibility testing when tests are being performed on bacteria.

Bacteria: Single-celled organisms that live in and around us with a distinct structure from other microbes. Bacteria can be helpful, but can also cause illnesses such as strep throat, ear infections, and pneumonia.

Drug class: Antibiotics are categorized into classes of drugs. Each class is defined by the way it kills or stops the growth of bacteria. NARMS uses the Clinical and Laboratory Standards Institute’s drug classes  to categorize antibiotics.

Genes: Genes, which are made up of DNA, are the basic units that define the characteristics of every organism. Genes carry information that determine traits, such as eye color in humans and resistance to antibiotics in bacteria.

Genome: A genome is an organism’s complete set of genes that carry the genetic instructions for building and maintaining that organism.

Isolates: Bacteria isolated from a specimen (e.g., stool, blood, food).

Microbes: Living organisms, like bacteria, fungi, or viruses, which can cause infections or disease. Also referred to as germs.

Multidrug-resistant (MDR) isolate: CDC typically uses this term to refer to an isolate that is resistant to at least one antibiotic in three or more drug classes.

Non-susceptible isolate: An isolate that is either resistant or not completely susceptible to one or more antibiotics.

Organism: any living thing, including humans, animals, plants, and microbes.

Panel: A set of antibiotics used to determine if an isolate is resistant to antibiotics and, if so, which ones. The set of antibiotics used differs by type of bacteria being tested.

Predicted resistance pattern: The antimicrobial resistance expected based on analysis of an organism’s genome. This analysis identifies resistance genes and mutations.

Resistant isolate: An isolate that is resistant to one or more antibiotics.

Resistance determinant (or resistance mechanism): A catch-all term that includes both resistance genes and resistance mutations that give a microbe the ability to resist the effects of one or more drugs.

Resistance gene: A gene that gives microbes the ability to resist the effects of one or more drugs. The gene may be naturally present in the microbe, or it may be transferred from other microbes.

Resistance mutation: A change in the normal genetic code that gives a microbe the ability to resist the effects of one or more drugs.

Resistance pattern: A description of the antibiotic resistance testing results for an isolate.

Resistance profile: A description of the resistance patterns for all isolates in an investigation. A resistance profile differs from a resistance pattern, which refers to the characteristics of a single isolate.

Specimen: A sample collected for laboratory testing. During outbreak investigations, samples may be collected from the blood, stool, or another location of a human or animal, and from food and the environment.

Susceptible isolate: An isolate that is not resistant to any of the antibiotics tested.

Whole genome sequencing: A technology that determines the genetic code (genome) of an organism (for example, people, bacteria, and viruses). Learn more about whole genome sequencing.