One day last week I decided to make myself a sandwich for lunch. I grabbed the mustard, turkey, cheese and lettuce from the fridge, and then proceeded to the cupboard to get my hearty, whole-wheat, pre-sliced bread. What I found next has likely been experienced by nearly everyone: Bread mold!
We’ve all seen it — that purple-grey, almost greenish fuzz that slowly takes over our various bread products just short of their expiration date. That fuzzy mold is Rhizopus stolonifer, from the fungal order Mucorales. Rhizopus can also grow on and steal nutrients from other foods in your house, including an array of fruits and vegetables (see our strawberries horror movie). As it turns dark, its spores are released into the air where they float to their next prey, making this mold a bit of a household menace.
Time lapse video of a tasty bagel inoculated with the evil mold Rhizopus stolonifer by Kent Loeffler. If your bagel looks like this, please don’t eat it…
Although in our daily lives we usually only think about the Rhizopus eating our foods, other species of Rhizopus can cause disease in various commercial crops, while still others are used for making tempeh and certain alcoholic beverages. Rhizopus species are very common molds in human environments, and can contribute to sick building syndrome. We are exposed to their spores all the time, and normally Rhizopus species do not infect people, but those whose immune systems are compromised are more vulnerable to zygomycosis as a result of exposure to the mold. Zygomycosis is a very grave fungal infection that causes rapid tissue death around the point of infection –- whether your immune system is hale or not, avoid breathing the spores of molds, including Rhizopus.
The coolest thing about Rhizopus microsporus lies buried inside its cells -– bacteria from the genus Burkholderia live inside the fungal hyphae and spores. Many fungi harbor bacteria on their surface, and some others inside their cells, often in mutually beneficial symbiotic (living together) relationships, but little is known about why these relationships exist. In the case of Rhizopus microsporus and its Burkholderia, however, a research group in Germany has recently made a fascinating discovery: the endobacteria actually make the toxins that the fungus uses to attack plants!
The toxins rhizoxin and rhizonin are most commonly associated with a disease called Rice Seedling Blight. Blight is a generic name for a disease that leads to rapid browning and shriveling of plant tissues, and the eventual death of the plant. In this case it’s caused by Rhizopus microsporus invading rice seedlings. For years plant pathologists (scientists who study plant diseases) have been trying to figure out just how Rhizopus produces its toxins. Their hope has been that by identifying the source of the toxins they will be able to develop a targeted method for stopping their production.
The researchers expected that these toxins were mycotoxins –- produced by the fungus (myco) –- but were surprised to find that if they removed the bacteria from the Rhizopus, then poof! No more toxin production! What’s even more fascinating is that these bacteria control the reproduction of the Rhizopus. Stripped of its endobacteria, Rhizopus microsporus struggled to produce asexual spores, which account for the greenish black color of the mold, and serve as the fungus’s primary method of propagation.
So let’s just eliminate all of those pesky Burkholderia to stop the Rhizopus from eating our rice! This sounds all fine and good except for two major caveats: at this point targeting the endobacteria is not much simpler than targeting the fungus, and not all Rhizopus strains have these Burkholderia bacteria inside of them. This whole story has just emerged over the past few years, and we clearly have more to discover about the partnership between Rhizopus microsporus and its bacterial pal.
Rice Seedling Blight is of tremendous concern in countries that rely on rice as a primary cash crop–and for the rest of us, since rice accounts for about 20% of calories consumed by people around the world. Understanding how these toxins are produced is an important step towards combating plant disease, economic loss, and hunger. For me, however, when I see my bread mold, I’m no longer grossed out but instead am reminded of the intricate networks connecting all the kingdoms of life, and of the coolness that is Rhizopus.
- G. Lackner, N. Mobius, et al. 2009. Global Distribution and Evolution of a Toxinogenic Burkholderia-Rhizopus Symbiosis. Appl. Environ. Microbiol. 75(9): 2982-2986. doi:10.1128/AEM.01765-08
- L.P. Partida-Martinez, S. Monajembashi, K.-O. Greulich, C. Hertweck. 2007. Endosymbiont-Dependent Host Reproduction Maintains Bacterial-Fungal Mutualism. Curr. Biology 17(9): 773-777. doi:10.1016/j.cub.2007.03.039
- J.E. Gee, M.B. Glass, et al. 2011. Characterization of Burkholderia rhizoxinica and B. endofungorum Isolated from Clinical Specimens. PLoS ONE 6(1): e15731. doi:10.1371/journal.pone.0015731
- B. Rohm, K. Scherlach, N. Mobius, L.P. Partida-Martinez, C. Hertweck. 2010. Toxin production by bacterial endosymbionts of a Rhizopus microsporus strain used for tempe/sufu processing. Int. J. Food Microbiol. 136(3): 368-371. doi:10.1016/j.ijfoodmicro.2009.10.010 [not to scare you, but if you eat much tempeh, you might want to ask your favorite tempeh producer about the toxin issue identified in this paper. --Ed.]