Most cosmetics have great potential for microbial contamination and growth, especially creams and lotions that are packed in jars, opened frequently, and applied to the skin with the fingers. Brushes that are used to apply makeup around the eyes or other parts of the face touch the skin and the cosmetic repeatedly. Each use increases the chance for contamination. Several cases of eye ulceration and partial or complete blindness have been attributed to mascaras contaminated with pseudomonas.
Cosmetic contamination may also occur because consumers leave the containers open for a period of time. Moreover, most cosmetics are stored at room temperature and the warm temperatures stimulate the growth of microorganisms. In addition, the ingredients in cosmetics contain all the things microorganisms like—water, oils, peptides, and a variety of carbohydrates.
Source

Factors that influence the effectiveness of preservatives in your skin care products

Initial Contamination Level
■ Raw materials
■ Water supply
■ Processing sanitation—equipment and premises
Composition of Cosmetic/Personal Care Product
■ pH of the product
■ Water content
■ Antimicrobial effects of other ingredients
Distribution and Use
■ Packaging
■ Storage temperature
■ Shelf life expectancy
■ Potential for contamination by consumer

Ingredients that enhance preservative efficacy: solutes (salts and high concentration of sugars), esters, cationic and anionic surfactants, humectants (glyerin, propylene glycol), phenolic antioxidants (BHT), chelators (EDTA), fragrances. – Source