In a case of microbial cross-species regulation, the bacterium Streptococcus mutans has been found to produce a natural product called mutanobactin A that inhibits the growth of another microorganism—Candida albicans, a pathogenic yeast. What makes this finding unusual is that the cross-species regulation occurs inside yet a third species—humans. S. mutans is a regular denizen of tooth surfaces in the human mouth. C. albicans, which is also found in the mouth, forms threadlike mycelium structures that can lead to an oral yeast infection called thrush. Robert H. Cichewicz and coworkers at the University of Oklahoma now find that mutanobactin A produced by the bacterium stops yeast from forming mycelia. The bacterium thus apparently earns its keep by protecting the host from thrush (Org. Biomol. Chem., DOI: 10.1039/c0ob00579g). When genes for mutanobactin A are knocked out, the inhibitory effect disappears. The natural product also blocks C. albicans mycelial growth directly. The researchers are currently evaluating the use of mutanobactin A as a treatment for thrush and related Candida infections.