Cultivation of Paenibacillus polymyxa by solid-state fermentation of pear residues

Abstract: Pear juice is one of main processed products of pear fruit in China. A lot of pear residues are produced during pear juice processing, and the waste is up to 40%-50% of pear fruit weight. Because pear residues contain many stone cells and can not be used directly as animal feed, most of them have to be discarded in fields or near factories. Paenibacillus polymyxa is a widely distributed bacterial species in the environment, and has no pathogen to humans, animals and plants. P. polymyxa strain produces several antibiotics and hydrolytic enzymes, which play an important role in the biocontrol of plant pathogens and the promotion of plant growth. It is reported to be both a plant-growth-promoting and effective biocontrol agent. P. polymyxa D1, a naturally occurring biocontrol agent isolated from soil, can stimulate plant growth and effectively suppresse several soil-borne diseases. Many researches on submerged fermentations of P. polymyxa have been done, but study on its solid-state fermentation has not been reported at the present. In this work, fresh Yali pear’s residues were used to cultivate P. polymyxa D1 in a solid-state fermentaion. Fresh Yali pear’s residues contained 76.2% moisture, 5.5% residual sugars and 0.32% proteins. The effects of cultivation conditions on its cell growth and sporulation were investigated. The results showed that pear residues were good material for solid-state fermentation and very suitable to cultivate P. polymyxa D1. The mositure of fresh Yali pear’s residues was very high, which might not be in favour of the cell growth during solid-state fermentation. Addition of wheat bran into pear residues improved significantly the cell growth. The possible reason of this result was that oxygen supply and nutritional status of the culture medium were improved obviously when pear residues were mixed with wheat bran. When the ratio between pear residue and wheat bran was 5:2, the highest viable cell number was achieved, 2.5×109 cfu/g. Spore number was an important parameter of microbial agent. Cultivation conditions could influence bacterial sporulation. The viable cell number and spore yield of P. polymyxa D1 obviously increased when urea, sodium nitrate and soybean meal were added into the cultural medium of pear residues, and soybean meal had the best stimulation effect. However, NH4NO3, NH4Cl and degossypolized cottonseed protein inhibited the cell growth and spore formation. Mineral salts also influenced the cell growth and sporulation of P. polymyxa D1. Supplement of NaCl, MgSO4, CaCO3 and FeSO4 into the medium largely inhibited the spore formation of P. polymyxa D1. Addition of KH2PO4 greatly promoted the cell growth and sporulation, and its spore yield and sporulation efficiency were 4.52×109 cfu/g and 95.6%, respectively. P. polymyxa is a highly anaerobic bacterium. The spore yield of P. polymyxa D1 decreased obviously as the medium volume in the fermenter increased. The optimal cultivation temperature of P. polymyxa D1 was 34°C, which was higher than that in submerged frementation. The optimal cultivation time of P. polymyxa D1 was 120 h. It was found that the spores were formed very slowly by the cells of P. polymyxa D1 when cultivated with pear residues in solid-state fermentaion, compared with submerged fermentation.