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 ZymoBIOMICS DNA Microprep Kit, and eluting into 30 microL. NEBNext Ultra II FS was used to generate the DNA libraries with NEB unique dual indices.