Bacteria play a significant role in plant's growth by:
They act as decomposers and circulate nutrients into the soil hence maintaining and improving soil quality.
They also help in forming manure(organic matter used as fertilizer) via the composting (decomposition of plant and animal waste) process.
The soil microbes perform nitrogen fixation (conversion of atmospheric N2, intoNH3, NO3-, etc.), hence enriching the soil with nitrates(NO3-), ammonia (NH3), and other nutrients.
Example of microbes used as biofertilizers is Azospirillum, Rhizobium, Azotobacter, Mycorrhizativity, etc.
Microbes replenish the soil nutrients keeping the soil fertile for flourishing with flora.
Bacteria such as Rhizobium form symbiotic associations with root nodules of leguminous plants and help in nitrogen fixation.
These are the ways by which bacteria help grow plants.