This HiDip slide is recommended for detection of coliforms, Salmonella and Citrobacter from water, milk & food. PA Medium : It is for detection of presence or absence of coliforms. It is based on lactose fermentation with Bromocresol purple as indicator. Gram positive bacteria are inhibited by SLS. H2S Medium : It is based on detection of H2S production. It produces black centred colonies.