According to the ISI Web of Knowledge, since Spring 2009 ~300 scientific publications have been published containing the term "unculturable" as an adjective describing microorganisms. Furthermore, "unculturable" appeared with highest frequency in 2008; thus, use of this term is increasing. As a researcher and educator, I oppose the proliferation of conceptually erroneous terminology because, once accepted, it influences how individuals ask and answer scientific questions. I argue here that the term "unculturable" should not be used to label microorganisms in a taxonomic sense. Instead, "uncultured" is the accurate term. "Unculturable" should be considered synonymous with "moribund" and be applied only as a descriptor of a temporary physiological state of microbial cells (in laboratory culture or in nature) that have lost their ability to grow or be resuscitated.

It is widely recognized that naturally occurring microbial communities in soils, sediments, water, and other habitats are highly diverse and are dominated by Bacteria and Archaea (and Eukarya) that have not yet been cultivated. What separates this vast zoo of undomesticated taxa from the ones microbiologists can use to generate physiological, genetic or other laboratory experimental data is the discovery of media that allow these life forms to grow. Efforts to meet the physiological needs of the "uncultured majority" are expanding; thus, what may appear "wild and uncultured" today will surely become "tame and domesticated" (cultured, or cultivated) tomorrow. Implicit in the term "unculturable" to classify microorganisms is a prediction that no microbiologist will ever devise a medium that can meet the physiological needs of such cells. Because of microbiologists' propensity to devise new media in the future, I argue that taxonomically, there is no such thing as "unculturable cells"—the correct terms are "uncultured" or "not-yet-cultivated" cells.

To drive home the important distinction between the uses of "unculturable" in physiological versus taxonomic contexts, consider a neglected petri dish inoculated with Escherichia coli that may have been allowed to age and desiccate on a laboratory bench. These cells may not resuscitate when streaked on fresh LB media because certain essential cellular components have lost function, preventing growth. Such cells are indeed physiologically "unculturable" (moribund or nonviable), but we would never use our observation of a temporary inability to proliferate to taxonomically classify E. coli as an "unculturable" bacterium. Because clear scientific thinking can be occluded by misconceptions, I urge microbiologists everywhere to use the term "unculturable" with care.

Eugene L. Madsen
Ithaca, N.Y.

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