Bacteria can be divided into two distinct groups:
- Gram-negative (Gram+)
- Gram-positive (Gram-)
Gram+ bacteria are generally larger than Gram- bacteria and can form spores. This makes them more resistant to drought and water stress. Gram+ dominant populations (>1) are more common at the beginning of the growing season and return to balance when soil conditions become more favorable.
Gram- dominant populations (<1) are often associated with other forms of stress, such as ploughing and pesticide use. Gram bacteria can better tolerate these types of disturbance due to the presence of an outer membrane.