Bacteria can be divided into two distinct groups:
- Gram-positive (Gram+)
- Gram-negative (Gram-)
Gram(+) bacteria (mg PLFA per kg) 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 favourable.
Gram- (mg PLFA per kg) dominant populations (<1) are often associated with forms of stress, such as ploughing and pesticide use. Gram- bacteria can better tolerate these types of disturbances due to the presence of an outer membrane.
Gram (+)/Gram (-) ratio
The Gram (+)/Gram (-) ratio (expressed in mg C/kg) is an indication of disturbances and stress. At a lower ratio, Gram(-) bacteria are dominant. These bacteria are better resistant to disruptions like ploughing and pesticides. A high ratio means that Gram(+) bacteria are dominant, these bacteria more resistant to drought and water stress. A balanced ratio is achieved when conditions are more favourable.