The objectives were to examine effects of adding E. coli O157:H7 with or without chemical or microbial additives on the bacterial diversity of alfalfa silage, and to examine associations between the abundance of bacterial taxa and silage fermentation quality indices. Alfalfa forage was harvested at 54% DM, chopped to 19-mm lengths, and ensiled in quadruplicate in lab silos after treatment with the following: 1, distilled water (Control); 2, 1 × 10⁵ cfu/g of E. coli O157:H7 (EC); 3, EC and 1 × 10⁶ cfu/g of L. plantarum (EC+LP); 4, EC and 1 × 10⁶ cfu/g of L. buchneri (EC+LB); and 5, EC and 2.2 g/kg of propionic acid (EC+PA). After 100 d of ensiling, silage samples were chemically characterized and analyzed for bacterial composition by sequencing the V3 - V4 region of the 16S rRNA gene using the Illumina MiSeq platform. The experiment had a completely randomized design. Data were analyzed using the GLIMMIX procedure of SAS. Significant differences were declared at P < 0.05. Pearson correlation coefficients were generated between taxa and chemical components using R software version 3.2.2. Significant correlations were declared at P ≤ 0.10. Relative to the Control, adding EC+LP or EC+LB reduced (P < 0.05) the Shannon index, a measure of species diversity, but adding EC alone did not (P > 0.05). Treatment with EC+LP increased (P < 0.05) the abundance of Lactobacillus, Sphingomonas, and Pantoea while that of Weissella and Methylobacterium was reduced (P < 0.05) in the EC+LB silage compared with the Control. Lactate concentration correlated positively (P = 0.04) with the abundance of Lactobacillus. Negative correlations were detected between NH₃-N concentration and abundance of Sphingomonas and Pantoea., Silage pH was negatively correlated (P < 0.10) with abundance of Lactobacillus and Pantoea. Abundance of some unidentified species belonging to genus Rhodococcus and Pseudomonas correlated positively (P < 0.05) with lactate or acetate concentrations, whereas those of unidentified species belonging to Salana, Pantoea, and Rhodococcus correlated negatively (P < 0.05) with NH₃-N concentration. Future studies should aim to speciate and determine the functions of the unidentified bacteria detected in this study.