Filamentous bacteria under the microscope

Filamentous bacteria from soil, seen with phase contrast microscopy.

It's pretty easy to isolate soil bacteria: take a scoop of soil, mix it with some water, then plate the liquid on a Petri dish and incubate it overnight at 25-30 °C. Voilà. 

The isolated bacterial species will vary with the conditions (type of medium, temperature); here I found many filamentous bacteria on the plate. They look a little bit like filamentous fungi (since they also form a mycelium), but usually you can easily tell them apart (with a microscope) because of their smaller diameter. 

Filamentous bacteria from soil, seen with phase contrast microscopy

What are they? Most likely they are actinomycetes, a large group of Gram-positive bacteria that play an important ecological role as decomposers of organic matter. Via the production of geosmin, actinomycetes are also responsible for the earthy smell of soil! And what is more, most of the antibiotics that we use were first discovered in species of actinomycetes (in particular Streptomyces species). [Edit November 2012: After talking with a colleague, I realized that another solid possibility is that these filamentous bacteria could be Paenibacillus, another group of Gram-positive bacteria.]

Segmentation of the filaments after one day in the fridge

These filamentous bacteria can also form spores to get dispersed. Hence, after some time I observed the segmentation of the filaments and their transformation into spores called conidia.

As seen with phase contrast microscopy, spores are bright and refractile objects that are easily recognizable, for instance in the images below.

Segmentation and spore formation after three days in the fridge

Spore formation after three days in the frige

It is good to remember that not all bacteria look like E. coli!

Spores and other bacteria