"Rosettes" of Caulobacter.
Caulobacter is a neat bacterium, found living in fresh water--it likes really, really, really dilute nutrients, so it's good at living in tap water or even bottled water. Students in the Microbial Diversity lab are given the task of isolating pure cultures of organisms from mixed cultures--such as soil or horse poop. The way you isolate Caulobacter from the zillions of other organisms in a soil sample are by providing all the organisms with the thinnest possible gruel to eat--no other organisms can survive, but Caulobacter is happy and thrives while everything else starves. After a few weeks you have a culture that's almost all Caulobacter.
Fortunately for my students, Caulobacter is pretty easy to recognize--it forms these rosettes, bunches of cells all joined at their tails. This is a result of another unique feature of Caulobacter--its stalk. This picture shows a different species of Caulobacter, with an unusually long stalk.
I'll highlight the stalks here:
Usually Caulobacter uses the stalk to hold on to a surface. In fact, the "glue" that Caulobacter uses is one of the strongest natural glues we know of--a square millimeter of this has been calculated to be sufficient to hold about 7 kilograms of weight. That's stronger than dental adhesive. However, if you grow up lots of Caulobacter in a culture, the cells end up sticking to each other, tail to tail--hence, a rosette, and an indicator of a success in Mic105L.
All photos 100 X, oil immersion, phase contrast, snapped through the eyepiece with my handy Canon point-n-shoot.
Peter Tsang, Guanglai Li, Yves Brun, L. Ben Freund, and Jay X. Tang (2006). Adhesion of Single Bacterial Cells in the Micronewton Range. Proceedings of the National Academy of Science USA 103(15):5764-8.