Alginate microbeads as inoculant carriers for plant growth-promoting bacteria

A method of inoculating wet and dry seeds with plant growth-promoting bacteria (PGPB) using alginate microbeads as a substrate and Azospirillum brasilense as the model PGPB was developed. The microbeads were produced by low pressure spraying of an alginate solution mixed with liquid bacterial culture suspended in a very rich medium through a small nozzle resulting in small-diameter droplets. These droplets, when sprayed into a slowly stirred solution of CaCl2, immediately hardened into microbeads at diameters ranging between 100 and 200 µm. Although the process killed part of the entrapped bacteria, the remaining bacteria residing in the microbeads were sufficient [>1011 colony-forming units (CFU) g–1 inoculant] for seed inoculation. Further, it was found that the bacterial population in the inoculant could be enhanced by secondary multiplication in the same medium for an additional 16 h. It was found that the microbeads could be used either wet or dry. Dry inoculant was produced using dry air at 38°C, creating a powdery substance loaded with >109 CFU g–1 beads. Alternatively, dry microbeads were produced using a standard freeze-drying procedure. This dry preparation was easily attached to dry seed surfaces with the addition of 1% alcohol-diluted lecithin or with 0.5% synthetic paper adhesive (Resistol). The bacteria were slowly released from the microbeads in amounts ranging from 104 to 106 CFU g–1 depending on the type (wet or dry, with or without skim milk) and the time of incubation (the longer the incubation period, the smaller the amount of bacteria released with time). The wet and dry inoculants enhanced the development of wheat and tomato seedlings growing in unfertile soil, and biodegraded within 15 days in moist soil.