Glossary of Antimicrobial Terms

From A-Z: discover the meanings behind commonly utilized antimicrobial terms.

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A.

Antibacterial agent – any chemical which kills bacteria (bactericide) or interferes with the multiplication and growth of bacteria (bacteriostat).

Antifungal agent – any chemical which kills (fungicide) or inhibits the growth of fungi.

Antimicrobial agent – any chemical which kills or inhibits the growth of microorganisms including bacteria, mold, mildew and fungi. 

Antimicrobial efficacy – the efficacy of antimicrobial products judged from the value of antimicrobial activity. The value of antimicrobial activity is obtained by utilizing standard test methods.

Autoclave – a pressure chamber capable of steam sterilizing articles at 121°C and 15 psi.


B.

Bacteria – microscopic living organisms, usually one-celled, that can be found everywhere. They can be dangerous, such as when they cause infection, or beneficial, as in the process of fermentation and that of decomposition.

Bacteriostatic -  when an antimicrobial agent inhibits the growth and reproduction of bacteria but does not kill them. Quantitatively, the bacterial population does not grow or increase in number on the treated sample throughout the challenge period, but the control sample allows the bacteria to reproduce. 


C.

Challenge testing – generally, test methods specify environmental testing conditions: the length of time bacteria should be in contact with the test piece, nutrient concentration, and temperature.  The challenge conditions for most antibacterial efficacy testing are 5% nutrient concentration with a contact time of 18-24 hours at 37°C.

Colony forming units (CFU) unit to estimate the number of bacteria on a sample. This estimate is made by growing the bacteria on a microbiological medium (nutrient source) at a temperature that supports the growth of the organisms so they can be seen by the naked eye and counted.

Control sample – untreated (non-antimicrobial) sample used in testing as a comparison for the treated sample. The number of organisms on the control sample after the challenge period can be compared to the number of organisms surviving on the treated (antimicrobial) sample. The antimicrobial efficacy can be calculated as percent or log reductions to indicate the level of antimicrobial activity.


E.

Enumerate – to determine the population of bacteria on a sample. 


G.

Gram staining -  a common technique used to differentiate two large groups of bacteria based on their different cell wall constituents. The Gram stain procedure distinguishes between Gram-positive and Gram-negative groups by coloring these cells red or violet.

Gram-positive Bacteria -  bacteria that give a positive result (violet staining) in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their cell wall. This group includes some of the most common soil bacteria and some pathogens, such as Mycobacterium and Corynebacterium. Antimicrobial efficacy tests generally use Staphylococcus aureus as the surrogate Gram-positive bacteria. 

Gram-negative Bacteria -  bacteria that give a negative result (red staining) in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their cell wall. Bacteria in this group are a particularly common cause of nosocomial bloodstream infections, pneumonia, and meningitis. Escherichia coli is the most common gram-negative pathogen. Antimicrobial efficacy tests generally use Escherichia coli and Klebsiella pneumoniae as the surrogate Gram-negative bacteria.


I.

Incubation – the process of maintaining controlled environmental conditions that favor the growth or development of microorganisms. 

Incubation period – the length of time that microorganisms are kept in contact with test samples under controlled environmental conditions.

Inoculum – the starting population of test organisms (colony forming units/CFU) applied to a test piece.  The inoculum value can be compared to the number of organisms remaining at the end of testing. 


L.

Log reduction – a measure of change in the microbial population on a treated sample after a challenge period compared to the initial inoculum applied or compared to the untreated sample.


M.

Microorganisms – unicellular or cell-cluster microscopic organisms that are too small to see with the naked eye. Include eukaryotes such as fungi and protists, prokaryotes such as bacteria and certain algae, and some viruses.


N.

Neutralize – steps taken to end the challenge period of a test.  The challenged test sample is exposed to a neutralizer (e.g. letheen broth, lecithin) that stops the antimicrobial activity and ends the challenge.

Neutralizer – chemical agents used to inactivate, neutralize, or quench the antibacterial properties of antibacterial agents.

Neutralization – the act of immersing a test piece in a neutralizer to stop the antimicrobial from acting on the bacteria. The bacteria still living at the end of the challenge period can then be determined. 

Nutrient level/Nutrient load – level of nourishment made available to test organisms during the challenge (testing) period. Some test methods specify nutrient level, however some do not. If a test is conducted without nutrient (e.g. saline) the change in surviving organisms may be due to the lack of nutrient as well as the antimicrobial effect. The standard growth nutrient conditions (nutrient broth: water) for most testing of non-absorbent material (e.g. polymers) is 1:500 (0.2%) and for most absorbent testing (e.g. textiles) the nutrient is diluted to 1:20 (5%).


P.

Percent reduction – a measure of change in the microbial population on a treated sample after a challenge period compared to the initial inoculum applied or compared to the untreated sample. 

Plate count method – method by which the number of bacteria present after incubation is calculated by counting the number of colony forming units (CFU) on an agar surface.


Q.

Qualitative testing – testing that gives an indicator of the effect of an antimicrobial but does not indicate the level of the effect. It gives a binary “Yes or No” reading, but not a numerical determination of level of antimicrobial activity. Examples of qualitative tests include AATCC 147, AATCC 90, Kirby Bauer, AATCC 30-III.

Quantitative Measure of Efficacy (QMR) – a measure of percent reduction or log10 reduction to determine the change in the microbial population.  1-Log Reduction is equivalent to a 90% reduction. 2-Log Reduction is equivalent to a 99% reduction.

Quantitative testing –  testing that gives a numerical determination of the effect of an antimicrobial (percent or log reduction).  Quantitative tests include AATCC 100, JIS L 1902, ISO 22196.


R.

Recover – steps taken to stop the antimicrobial activity, end the challenge period, and determine the bacterial population still living after the challenge period.  


T.

Treated sample – sample of a product being tested for antimicrobial properties.   


V.

Value of antimicrobial activity -  showing the difference in logarithmic value of viable counts between antimicrobial products and untreated products after the inoculation and incubation of bacteria.


Z.

Zone of inhibition – a clear zone of no growth of a microorganism around a test sample placed in direct contact on an agar surface.