Ongoing investigations into the deadly salmonella outbreak which has spread to 10 facilities across the region, south eastern Sydney and the ACT have so far failed to uncover the source of infection.

Two aged care residents have died after contracting the unusual salmonella bovismorbificans strain, and there remains 16 suspected cases.

Aged care provider IRT has confirmed that 25 of the confirmed cases have occurred at seven of its care centres, and it is working with NSW Health and the NSW Food Authority to track the source.

Illawarra Shoalhaven Local Health District public health director Curtis Gregory said the outbreak seemed to have been contained within the aged care facilities.

‘‘We believe there’s no ongoing risk to the broader community, although there has been enhanced surveillance in place to monitor any other suspicious cases of salmonella,’’ Mr Gregory said.

‘‘To date 26 of the 28 confirmed cases have the same serotype - or genetic marker - with the results yet to come in for the two latest cases.

‘‘This rare bovismorbificans strain may occur via contaminated manure being put on gardens, and that’s why leafy vegetables and salad items popped up early as a risk food.

‘‘So far though the epidemiological  investigation hasn’t shown that to be the issue however.’’

The NSW Food Authority’s continued investigations of IRT suppliers led to the seven-hour shutdown of Unanderra wholesale bakery Betta Maid on Wednesday, due to hygiene and pest control issues. These were rectified later in the day and the business reopened.

However the food authority has advised IRT to withhold products from Betta Maid pending the results of further testing of its products, with results due early next week.

Mr Gregory said the public health unit had also taken lengthy food histories from the confirmed patients which were being analysed.

‘‘Normally with food-borne illness we can pinpoint it to one meal or function - our investigations here are being complicated by the fact that the outbreak is across 10 facilities spread over a wide geographical area,’’ he said. ‘‘And we are dealing with an outbreak period of two to three weeks.’’

IRT has pulled high-risk foods off the menu and conducted industrial cleaning at all its sites.