The manager of an 19th-century Japanese guesthouse has apologised after its 158-year-old spa bath was found to be teeming with poisonous bacteria.

The Daimaru Besso guesthouse in Fukuoka Prefecture advertises its spa baths, filled with water from local thermal springs. A recent inspection revealed the water contained legionella bacteria 3700 times the permitted level, Asahi TV reported.

After the onsen was opened to local health inspectors last year, the hotel’s staff admitted to only changing the water once every six months.

According to local regulations the water should have been replaced weekly. The management for the historic hotel admitted to only doing this task just twice a year.

At a conference on Tuesday the president of the hotel group owning the ryokan inn, Makoto Yamada, told local media he was “very sorry”.

The Daimaru Besso Hot Spring Resort has since reopened to visitors. Photo / Fukuoka Prefecture Tourism Association, Supplied

The bacteria in the hot pools is known to cause Legionnaires’ disease and respiratory problems. While the disease is treated with antibiotics, it is very serious.

Legionella bacteria are normally found in baths and damp surfaces which are not regularly cleaned.

Yamada told the Asahi Shimbun that he was not aware of the laws regarding changing spa water and that it was not uncommon to find legionella bacteria in pools.

The ryokan was operating under the impression that its onsen pools were safe, “because the large baths were free-flowing so the water was changed quite often.”

The onsen water was also found to be untreated, which Yamada said was because staff “disliked the smell of chlorine”.

The baths were closed last year, following this visit from health inspectors. They are now open again to guests.

On the hotel’s website it states: “Our bath waters come from the Futsukaichi Onsen, which has been flowing for over 1,300 years, a symbol of the unchanging history of the area.”

The hotel claims its spa water has health benefits, particularly in the treatment of sore skin and joints.