ZymoBIOMICS™ Spike-in Control I (High Microbial Load) consists of equal cell numbers of two bacteria strains, Imtechella halotolerans and Allobacillus halotolerans. When spiked into an unknown sample, this product will serve as an in situ positive control for DNA-sequencing-based (e.g. NGS-based) microbiome measurements. These two bacteria, Imtechella halotolerans (Gram-negative) and Allobacillus halotolerans (Gram-positive) represent different cell recalcitrance and can expose potential bias during DNA extraction. Moreover, with accurately quantified cell numbers, this standard enables absolute cell number quantification in microbiome measurements.