standard limestone, while a C4 plant will give δ13C values around −9‰. A δ13C of about −16‰ might represent either a mixture of the two sources or a plant that has grown in water (algae or a hydroponically grown plant). Animals will acquire the δ13C signature of their food sources. In the case of bacteria cultured in the laboratory, the signature of the growth medium will be reflected in that of the spores produced. Tests of this expectation for liquid growth medium bear this out (Kreuzer-Martin and Jarman, 2007).
In the case of δ2H and δ18O, cultured microorganisms record the isotopic signature of the water in the culture medium as well the nutrients in the medium. Since the anthrax attacks of 2001 there have been extensive studies testing these relationships using the nonpathogenic Bacillus subtilis. On average about 70 percent of the oxygen atoms in spores produced come from the water, while only about 30 percent of the hydrogen atoms come from the water (Kreuzer-Martin et al., 2003, 2005) Results show a strong positive correlation between the δ2H and δ18O content of the culture water and resultant spores for liquid cultures (Kreuzer-Martin and Jarman, 2007; Kreuzer-Martin et al., 2003). Samples grown on an agar medium are also subjected to isotopic fractionation from evaporation of water from the medium. It was found that the influence of evaporation was small for δ2H, but significant for δ18O (Kreuzer-Martin et al., 2005). Exchange of water vapor with the ambient atmosphere could also be important if the agar was prepared in one location and used in another, or anytime there is a different isotopic signature of the water used and that of the surrounding water vapor, as in the case of studies at Lawrence Livermore National Laboratory, where the source of the tap water is largely the Sierra Mountains and the ambient atmosphere can have a significant marine input (Kreuzer-Martin et al., 2005)
The forensic value of isotopic measurements on bacteria cultures depends on the individual circumstances. It has the potential to rule out certain combinations of water and growth media as well as to provide a distinguishing marker for discrimination between production batches of spores.