A letting agency is suing a former tenant for at least $50,000 for a tweet criticising her mouldy appartment.

Amanda Bonnen from Chicago, moved out of her rented flat on June 30 but not before using Twitter, the microblogging site, to complain: "Who said sleeping in a mouldy apartment was bad for you? Horizon realty thinks it's okay."

Horizon, a letting agency based in the city, took offence and sued for damages. It claims that the company's good name had been tarnished "throughout the world" as a result of the slur.

The offending tweet was posted by "abonnen" - later identified as Ms Bonnen - on May 12. It referred to damage to the apartment caused by an overnight leak two months earlier.

But although Ms Bonnen's account was set to public, meaning any Twitter user could see it, a Google cache of the now deactivated account shows she had just 17 followers.

Other flats were also affected by the leak but according to Jeff Michael, whose family have run Horizon for 25 years, all other tenant grievances were resolved.

As a result it filed a lawsuit on Monday against the former tenant without contacting her first. "We're a 'sue first, ask questions later' kind of an organisation, Mr Michael told the Chicago Sun-Times.

He added: "The statements are obviously false, and it is our intention to prove that."

The firm, which manages some 1,500 apartments in Chicago, is now facing a barrage of bad publicity.

Mr Michael released a statement defending its legal action. He said: "I would first like to take this opportunity to apologise for tongue in cheek comments that were made previously regarding our approach to litigation. This statement is not in line with our philosophy towards property management and was taken out of context."

Ms Bonnen has since counter-sued the company over a separate issue. She claims that Horizon violated city laws by failing to pay tenants interest on security deposits. The company says the claims are baseless.

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