They also posted a photo of some 5-year-old Maybelline mascara, saying, "From the looks of it, there are a lot more colonies but less growth."
The Redditor offered up this explanation:
"Bacteria grows in nutrient rich environments, and agar plates are a nutrient rich environment. Each colony represents at least one bacterium that reproduced to form its own little family, so the plate shows an approximation of bacterial population on the old mascara."
Semicolon_Expected went on to claim that they were looking to refine their process, saying that the flawed petri dish might not be showing the extent of the bacteria. Meaning, they suspect there's much more. Gross, right?
No matter what brand you buy, bacteria will naturally build up over time as the opened product comes into contact with germs.
You should always follow the expiration instructions on the bottle or packaging. As a good rule of thumb, you should replace your mascara every three months, eyeliner and eye shadow every six to 12 months and lipstick every one and a half years. The average life of your foundation and face powders is 12 months. However, most of these products will provide a date on the back of the bottle. A small illustration of a bottle with either 6m, 12m, 24m etc. will appear on the back, indicating how many months it should last once opened.