Characterization of Antifungal Bacteria in Switchgrass Seed.

Switchgrass seed is often plagued by poor germination, some of which is due to seed rotting fungi. Switchgrass seed treated with streptomycin sulfate resulted in a total loss of germination due to seed rot. Streptomycin is a strong bactericidal antibiotic that affects protein translation. We hypothesize that there must be some streptomycin sulfate sensitive bacteria in the switchgrass seed that checks the growth of phytopathogenic fungi. Our goal was to isolate and identify antifungal bacteria in switchgrass seed. A total of 106 bacterial isolates were obtained from switchgrass seeds. These isolates were screened against our antifungal assay using three seed rotting fungi: Alternaria sp. (slow grower), Fusarium /Gibberella (medium grower), and Fusarium sporotrichioides (fast grower). Bacteria were grown in 1X TSB for three days and supernatants obtained after cell removal by centrifugation. Fungal containing agar plugs were place on top of filter paper saturated in bacterial supernatant placed inside of a 1X TSA agar Petri plate. Radial growth was measured daily, and growth rates determined and compared with untreated controls. No bacterial supernatants stopped fungal growth completely over the 6 to 14 day measurement period. Ten bacteria (4, 20, 51, 96, 10B, 11A, 15A, 28A, 35B, 64A) significantly inhibited all three fungi, with three bacterial isolates (28A, 20, and 11A) showing very strong inhibition. Inhibition was greatest with Alternaria and least on Fusarium sporotrichioides. Some bacteria appeared to stimulate fungal growth, but only with the slower growing fungus. Antifungal bacteria were identified through their 16s rRNA gene sequence. It is concluded that resistance to seed rot fungi in switchgrass seed may be partially due to switchgrass seed bacteria producing antifungal compounds.