11The partitioning of pathogenic strains isolated in environmental or human cases to their original source is 12 challenging. The pathogens usually colonize multiple animal hosts, including livestock, which contaminate 13 48 This article describes AB SA ("Accessory-Based Source Attribution method"), a novel approach for 49 source attribution based on "source enriched" accessory genomics data and unsupervised multinomial 50 logistic regression. We demonstrate that the AB SA method enables the animal source prediction of 51 large-scale datasets of bacterial populations through rapid and easy identification of source predictors 52 from the non-core genomic regions. Herein, AB SA correctly self-attribute the animal source of a set 53 of S. Typhimurium and S. 1,4,[5],12:i:-isolates and further classifies the 84% of strains contaminating 54 natural environments in the pig category (with high probability ranging between˜85 and˜99%). 55 56 2