To each sample pool, DNA internal controls (ICs) of A. hydrophila and L. pneumophila DNA (NIST RM 8376) were spiked-in at ~5.7e6 genome copy/uL each. Then, deactivated microbial pellets containing a mixture of S. aureus, S. enterica, P. aeruginosa, E. coli, and L. monocytogenes (200k cells/pellet) were added directly to each sample pool, proportional to the number of aliquots (e.g. 10 pellets for 10 aliquots). Aliquots of each sample were then made and distributed frozen. DNA was extracted from each sample using Zymo Quick-DNA Fecal/Soil Microbe Miniprep Kit using the Vortex Genie 2 protocol at speed 10 for 40 for bead beating, and eluting into 100 microL. NEBNext Ultra II FS was used to generate the DNA libraries with NEB unique dual indices.