The agency and Environmental Health Officers from Belfast City Council have linked the outbreak to Flicks restaurant in Belfast which voluntarily closed and is cooperating with the investigation.
Eight people had been hospitalised since the start of the outbreak but the PHA said it understood most have now gone home, according to its latest update yesterday. A further update will be announced tomorrow.
Monitoring the situation
The PHA said it has activated a full public health response to the outbreak and the situation is being managed and monitored closely, in what it described as Northern Ireland's worst ever E.coli outbreak.
It has advised anyone who ate at Flicks restaurant since 24 September and has symptoms of diarrhoea and/or abdominal pain to contact their GP urgently for medical advice.
Investigations are continuing and final conclusions will emerge when all cases have been identified and analysed to identify any common links.
Dr Michael Devine, consultant in health protection at the PHA, said: "It is vital that everyone follows some simple rules to prevent the spread of E coli – not just those with symptoms, but everyone should wash their hands after using the toilet and before eating or preparing food.
“Raw and cooked food should be kept completely separate, raw meat should be cooked through and vegetables and fruit should be washed thoroughly before eating."
He added there are many different types of E.coli, and while some live in the intestine quite harmlessly, others may cause a variety of diseases.
Previous negative cases
The PHA is also aware of four cases of E.coli O157 among adults in August which were linked to the same restaurant, which were investigated at the time and all tests came back negative.
In addition, further environmental health inspections were conducted, all of which were satisfactory, and enhanced surveillance to monitor for cases was also put in place.
The current cases came to light through that enhanced monitoring. As part of the investigation into the current outbreak, any potential link with the previous cases will be examined, although it is too early to comment further on this at such an early stage, said the PHA.