In vivo

Examples of investigations in vivo include: the pathogenesis of disease by comparing the effects of bacterial infection with the effects of purified bacterial toxins; the development of non-antibiotics, antiviral drugs, and new drugs generally; and new surgical procedures. Consequently, animal testing and clinical trials are major elements of in vivo research. In vivo testing is often employed over in vitro because it is better suited for observing the overall effects of an experiment on a living subject. In drug discovery, for example, verification of efficacy in vivo is crucial, because in vitro assays can sometimes yield misleading results with drug candidate molecules that are irrelevant in vivo (e.g., because such molecules cannot reach their site of in vivo action, for example as a result of rapid catabolism in the liver).

The English microbiologist Professor Harry Smith and his colleagues in the mid-1950s found that sterile filtrates of serum from animals infected with Bacillus anthracis were lethal for other animals, whereas extracts of culture fluid from the same organism grown in vitro were not. This discovery of anthrax toxin through the use of in vivo experiments had a major impact on studies of the pathogenesis of infectious disease.

The maxim in vivo veritas ("in a living thing [there is] truth") is a play on in vino veritas, ("in wine [there is] truth"), a well-known proverb.

Levels of closeness to the natural state

Latin phrases used to describe the closeness of a wet lab experiment setup to the natural state include:

• In natura ("in nature"), the exact natural state
• In vivo ("in the living"), with a living being (usually the whole organism, in a controlled environment)
• Ex vivo ("out of the living"), with part of a living being (usually tissues, organs, or cells)
• In vitro ("in the glass"), usually either a cell culture or a mixture of sub-cellular components (disrupted cell, purified biomolecules)

Different subfields of biology have a tendency to use each word differently. Notable variations from the above include:

• Toxicologists lump ex vivo into in vitro: any data not obtained using a whole animal is in vitro.
• Molecular biologists working on single-celled organisms may refer to a living microbe culture as in vivo, reserving in vitro for cell-free systems.
• There are also cases of mammalian cell cultures being referred to as in vivo.