Bacterial strains and preparation

The 212 bacterial strains used in this study were obtained from cosmetics, food, environmental sources, as well as from the American Type Culture Collection (Table 1).

All 212 strains used in this study were maintained at − 80ºC in broth supplemented with 20% glycerol. Each strain was aseptically streaked onto Tryptic soy agar (TSA) (Difco™, Franklin Lakes, NJ) and incubated for 24 h at 30ºC, from which an isolated colony was sub-cultured in Nutrient Broth (NB, pH 7.2) (Difco™), then incubated at 30ºC for 24 h.

Inclusivity testing was performed using 143 members of the B. cereus group. One of these strains, B. cereus “3A”, previously obtained from eye shadow (Yossa and Jo Huang, personal communication), was used to establish the limit of detection (LOD) for our assays. Exclusivity testing was performed using a panel of 69 strains: 38 non-cereus strains of Bacillus, and 31 strains of non-Bacillus bacteria.

Cosmetic products

There were two types of cosmetics used in this study: liquid and powder. To assess how well our assays could detect the presence of B. cereus 3A in artificially contaminated samples of liquid-type cosmetics, we purchased facial toner products [N = 3], all of which were labeled “alcohol free”. These facial toners were primarily composed of water and plant-derived compounds and preserved with phenoxyethanol, an antibacterial…