Pellicle formation in a hay infusion

Cultures growing on the surface of liquid media often form a more or less continuous film called a pellicle. This film consists of micrbial cells and their exracellular products. Pellicles, therefore represent liquid/air interface biofilms. Complex pellicles can be formed on the surface of an aqueous culture in which organic matter is decomposing. Infusions made with hay in water quickly generate pellicles which are easy to harvest and study. One of the most spectacular pellicles we have observed was formed on the surface of a finger bowl of water being used to produce onion root tips for a mitosis lab.

One onion, suspended in the water, failed to produce roots or stalks. As it decomposed, a rich, not too smelly, pellicle formed on the surface of the water. This film contained numerous gram negative and gram positive rods, the latter including a large number of endospore forming cells.


By suspending a clean microscope slide below the surface of the liquid in the hay infusion culture, (see microfishing exercise) one can also sample the biofilm formed in a less aerobic portion of the same microbial environment.