Abstract:

In this study， we investigated the effects of different lactic acid bacteria （LAB） on the fermentation quality and tetracycline resistance genes （TRGs） in elephant grass （Pennisetum purpureum） silage. Elephant grass was ensiled with six different treatments applied in a completely randomized block design. The six treatments were as follows： a control group （CK） without bacterial inoculant， and five LAB-treatment groups： a Lactiplantibacillus plantarum treatment group （strain A，LA）， a L. plantarum treatment group （strain B，LB）， a Lacticaseibacillus yichunensis treatment group （LC）， a Pediococcus pentosaceus treatment group （LD）， and the Loigolactobacillus coryniformis treatment group （LE）. Following 30 days of ensiling， bags were opened and the contents were sampled to analyze the nutritional parameters of the silage and the abundance of antibiotic resistance genes （ARGs）. It was found that， compared with CK， all the LAB-treatment groups exhibited significantly lower pH （P<0.05） and elevated lactic acid content. Notably， the LC group demonstrated a marked increase in lactic acid content （P<0.05）. Compared with the CK， some treatment groups showed significantly decreased contents of neutral detergent fiber and acid detergent fiber （ADF）. Specifically， the ADF content was significantly lower in the LA， LB， LC， and LD groups than in CK （P<0.05）. The ammonia-nitrogen content was significantly lower in all LAB treatments than in CK after the 30-day ensiling period （P<0.05）. Importantly， LAB supplementation significantly decreased the absolute abundance of both TRG and mobile genetic elements （MGEs）， compared with CK （P<0.05）. We detected strong positive correlations between the expression of the integron gene intl1 and the absolute expression levels of tetA， tetX， tetM， tetW， and tetK （P<0.01）. The LC and LB groups showed the largest reductions in the absolute abundance of TRGs， compared with CK （P<0.05）， indicating that L. yichunensis and L. plantarum are particularly effective in suppressing TRGs. In conclusion， specific lactic acid bacteria strains enhance the nutritional quality of elephant grass silage while mitigating contamination with TRGs.

Key words: elephant grass, lactic acid bacteria, silage, tetracycline resistance genes