Purpose: A study was conducted to evaluate survival of 22 strains of avirulent E. coli and Salmonella in crop soil with 10% fast-pyrolysis switchgrass biochar. The goal was to validate effective surrogate strains of bacteria to use in future studies.
Methods: Twenty-two, individual, 90 g soil samples were amended with 10% biochar and inoculated with 5 strains of non-toxigenic E. coli O157:H7, 10 strains of non-pathogenic E. coli, and 7 attenuated vaccine strains of Salmonella, respectively. Samples were held at 21°C for up to nine weeks and populations determined on antibiotic- and chromogen-supplemented Tryptic Soy Agar. Three strains of non-pathogenic E. coli were resistant to 50ppm rifampicin, and all other 19 strains were resistant to 100ppm nalidixic acid.
Results: Initial day 0 populations averaged 5.31 log CFU/g of soil. By 7 days, two salmonellae and four E. coli were undetectable by direct plating (minimum detection limit = 100 CFU/g), while 4 strains were still >4.1 log. Eight strains were undetectable by week three, while non-pathogenic E. coli strain TVS355 was still at 4.67 log CFU/g. Twelve strains were undetectable by week 7, including all non-toxigenic O157:H7. However, three strains of non-pathogenic E. coli (TVS-353, TVS-354, and TVS-355), were all >4.1 log. At nine weeks of storage, only 7 of 22 strains were still detectable by direct plating, including (log CFU/g in parenthesis) the non-pathogenic E. coli TVS-354 (4.22), TVS-355(3.36), TVS 353 (3.06), and the attenuated salmonellae ATCC-53467 (4.44), Chi8089 (3.38), Chi4096 (2.78), and Chi3985 (2.0).
Significance: These results identify suitable surrogate bacteria for use in future biochar-soil decontamination studies and, potentially, other evaluations predicting pathogen persistence in soils.