Saline is used in microbiology for numerous thins because it does not interfere with test methods and staining practices. Saline is sterile (deionized water), aka super super clean water to think of it as one way. Since this is clean it prevents microbial growth to occur and thus doesn't allow contamination. If this were to happen and say you were gram staining a slide, you would obtain a false positive slide showing organisms because you'll be staining the organisms from your sample but also those contained in your contaminated water which can not only cause confusion but have serious consequences if working in the medical field and reporting out preliminary gram stain results for board spectrum antibiotic treatment while organism identification is being down days later.
