OSAKA — Bacteria has been discovered on the production line of a Snow Brand Milk Products Co. factory here that processed low-fat milk believed to have made nearly 5,000 people ill, the company’s president said Saturday.

Tetsuro Ishikawa said company employees found staphylococcus aureus bacteria in a valve that carries low-fat milk left over in the production process to a spare tank.

“We do not know where the staphylococcus aureus originated from. Maybe the cleaning (of the production line) was insufficient. I would have to say it was an oversight on our part,” Ishikawa told reporters at the company’s West Japan branch office in Osaka.

According to Snow Brand, the valve in question was supposed to be washed once a week, but company records show it was last washed June 2.

Also, when the company staff disassembled the valve on Thursday to check for contamination, they discovered solidified milk the size of a 10 yen coin inside, company officials said.

The officials will try to determine whether the bacteria was responsible for making thousands of people in western Japan suffer diarrhea, abdominal pains and vomiting after drinking Snow Brand low-fat milk on Tuesday and Wednesday. The milk was processed at the factory in Osaka’s Miyakojima Ward.

Ishikawa said the company discovered the bacteria during its inspection of the production line. Its staff swabbed the inner parts of pipes and valves in 95 places to check for possible contamination.

Company officials said earlier Saturday that government health inspectors failed to find any bacteria capable of causing the suspected food poisoning at the Miyakojima factory.

Two officials from the Health and Welfare Ministry and three from the Osaka Municipal Government questioned Snow Brand executives about the manufacturing process, storage, and how ingredients are supplied to the factory.

Staphylococcus, or staph, is a common bacterium that can cause diarrhea and vomiting in people with a weakened immune system.

The contaminated milk, packaged in 1-liter and 500-milliliter cartons, was produced June 23 and 24 at the plant. Some 128,000 cartons were produced during the period.