It may well be snowing where you live, but if you're in the Northern Hemisphere, spring isn't far off. Spring means flowers. And what is a flower to a microbiologist? It's a niche.
Yes, flowers are a microbial habitat like any other, and though they start out with few microbial members, these communities quickly acquire a broad range of microbial types and progress through a classical succession of different groups. In mBio this week, Shade et al. describe the unexpectedly great microbial diversity of apple blossoms, communities marked by the consistent presence of groups that are well-known for their abilities to withstand environmental stress.
Shade et al. used tag pyrosequencing of 16S genes to make a community-level assessment of blossoms from six different apple trees, half of which were sprayed with streptomycin after the blossoms emerged. (Alarmingly, streptomycin is often used to control fire blight in apples, a disease caused by Erwinia amylovora, a bacterium that infects the fruit at the flower stage.) The flower communities were rich and diverse, but exhibited a bump in diversity within the first couple days after opening. This makes sense, since the flower would be low on bacteria (and possibly low on resources for those bacteria) before opening to the world. And streptomycin didn't seem to impact diversity or dynamics in these communities either way: even flowers that had been sprayed with streptomycin were home to rich, ever-changing bacteria communities.
Among the OTUs that come and go over the course of the flower's life, Shade et al. identified six different successional groups with members that peaked in prevalence before the flowers opened, or on the day the flower opened, at 2 days after flower opening, etc. There were also OTUs they classified as "generalists" because they fluctuated but generally persisted over time.
The most abundant OTUs belonged to the groups TM7 and Deinococcus-Thermus - groups known to withstand environmental stresses. The remaining OTUs were relatively rare, but included sequences closely affiliated with Bacteroidetes, Firmicutes, and Proteobacteria.
What can we learn from who comes and goes on the surface of a flower? The authors point back to fire blight for one potential application. The bacterial disease seems to strike blossoms at a vulnerable point in time. Is the flower vulnerable because of holes in its bacterial defenses? Understanding the microbiome of apple blossoms could well reveal insights about the identities and dynamics of these commensals and how they interact with pathogens and the tree itself. Fire blight is a particular problem in apple orchards, and it will be interesting to see whether insights into the flower microbiome will eventually lead to a fire blight solution that doesn't require wholesale spraying of antibiotics all over a food product.
One thing this research cannot do is bring spring any closer. But sit tight. It's coming. Eventually.