Preservatives are antimicrobials put into products to kill or prevent the growth of microorganisms over a relatively long period of time.

challenge test is a microbiological evaluation of a product's ability to kill or prevent the growth of microorganisms over a relatively long period of time.

Preservatives are antimicrobial agents that can be found in nearly all consumer products, including foods. Since Microchem Laboratory specializes in the testing of consumer and medical antimicrobials, food preservatives will not be discussed at length (though fundamentally, they serve a similar purpose).

Without preservatives, many of the consumer products we take for granted (soaps, shampoos, cosmetics and cleaners) would literally spoil on the store shelf. Worse, they could support the growth of pathogenic (disease causing) microorganisms.

Many people are surprised to learn that virtually all consumer products contain preservatives. Truth be told, most consumer products are germ food without a preservative, meaning that they can serve as a growth medium for various microorganisms. It makes sense - most products on the market today are biodegradable, right? That means that microorganisms can break them down (feed on them) without trouble. That's a very good thing once a used product winds up in a wastewater treatment plant or landfill, but this means that they must be preserved prior to disposal.

Not all products must be preserved with chemical preservatives, though many are. One of the most common ways to prevent the growth of microorganisms in consumer products is to adjust the pH of the product to the point where it is either too acidic or too basic to support microbial life. Many cleaners, for example, have pH values of about 11, which assists with cleaning efficacy. Since very few microorganisms can grow in such a basic (high pH) environment, such products may not require an additional chemical preservative.

Preservatives are especially important in cosmetic products like makeup, because of the way the products are used. In contrast to a cleaner, which is simply sprayed from a trigger sprayer (a one-way street), cosmetics contact the body, which may harbor pathogenic microorganisms at low levels naturally. If the product were to pick up a harmful microorganism from the body, and then grow in the product, the next use of the product would essentially become a self-inoculation with pathogens.

The efficacy of a preservative (how well it kills or prevents germs from growing) is measured in a laboratory setting using what is known in the industry as a preservative challenge test. The common preservative challenge test is USP <51>. Preservative challenge tests consist of the following basic steps:

  • The product is evaluated for the presence of pathogens after manufacturing The test method used to evaluate a product for the presence of microbes is known as <USP> 61.
  • The product is separated out into 5 separate containers.
  • Each container is inoculated with a different microorganism at a fairly high concentration, and the initial "challenge" concentration is found.
  • The product is incubated at approximately room temperature for an extended period of time (usually at least 7 days).
  • The product is re-evaluated for the presence of the microorganisms that were inoculated into it.
  • OPTIONAL - the product is incubated further, sometimes analyzed at 14, 21, and 28 days.
  • OPTIONAL - the product is re-inoculated with even more microorganisms and allowed to incubate further (sometimes up to one year).

The typical microorganisms used for preservative challenge studies follow:

  • Pseudomonas aeruginosa : Gram-negative, very hardy bacterium
  • Escherichia coli : Gram-negative bacterium, similar to many pathogens
  • Staphylococcus aureus : Gram-positive bacterium, opportunistic pathogen
  • Candida albicans : Fungus (yeast), opportunistic pathogen
  • Aspergillus niger : Fungus (mold), opportunistic pathogen

Needless to say, the desired result in any preservative challenge study is a quick decline for all of the microorganisms. In practice, some microorganisms take longer, based on their natural resistance to the various preservatives on the market.

Preservative challenge tests for drugs (including cosmetic drugs such as sunscreens) do not have to be conducted under Good Laboratory Practice (GLP) test conditions, per this FDA guidance (see section 58.15).