Azospirillum species bacteria are connected with rhizosphere biofilms, where they are associated with the roots of various grasses, cereals, and tuber plants. They are generally gram-negative large bacteria (2-4 micrometers in diameter), rod- or pear-shaped. They are known to fix nitrogen and their isolation is based on their ability to grow at nitrogen concentrations too low to support the growth of most microorganisms. Their greatest benefit to plants whose roots they colonize is the stimulation of their root system. Plants colonized with Azospirillum are as productive as heavily fertilized plants which were not colonized with Azospirillum. Enhanced SEM. Image width: 9.14 micrometers. Magnification 10,940x if the image is printed 10 cm wide. Bar: 2 micrometers.