What microorganism are liable to give a positive presumptive test?

Microorganisms that can yield a positive presumptive test typically include coliform bacteria, such as Escherichia coli (E. coli) and other members of the Enterobacteriaceae family. These bacteria are commonly used as indicators of fecal contamination in water and food samples. In a presumptive test, they ferment lactose, producing gas and acid, which suggests their presence. Other microorganisms that may also test positive include certain Enterococcus species, though they are not coliforms.