Ammonia released from hydrolysis of urine is a major reason for the stench in public toilets. Apart from being a put-off, ammonia is also hazardous. Current approaches to tackle this problem include flushing water, using microbes (BioBlocks®) that break down urea in urine to prevent its hydrolysis, and using air fresheners. The first approach requires using copious amounts of water, which is not abundantly available in many parts of the world, while the second is not cost-effective. Finally, spraying air fresheners is not a solution, as it does not get rid of ammonia. We propose to engineer Escherichia coli to assimilate ammonia and convert it into indigoidine, a blue-colored compound. Ammonia from air can be dissolved in an aqueous medium and can act as a nitrogen source for the engineered cells that maybe housed in a cassette; the synthesized indigoidine will indicate when the cassette has to be replaced.