Frankly I don't think they bother. It's not a huge issue. Sequencing will run to a certain coverage (or depth), aligning multiple fragments so that each nucleo

Frankly I don't think they bother. It's not a huge issue. Sequencing will run to a certain coverage (or depth), aligning multiple fragments so that each nucleotide is sampled e.g. 30 times, which would weed out contaminants. Saliva is generally considered 'good enough' (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3497576/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3497576/) They might just use an anti-microbial agent in their collection kit to prevent growth of bacteria until it reaches their lab (plus you don't want bacterial nucleases to fragment your DNA).