Many of those who have the E coli bug have eaten pre-packed salad, including rocket leaves, and the authorities have ordered wholesalers supplying UK restaurant chains and supermarkets to stop importing rocket from the Mediterranean, where the salad has thought to have originated from.
Dr Isobel Oliver from PHE, said: "All food sample results to date have been negative for E. coli O157 - but it's important to be aware that where food has been contaminated with E. coli O157, it is not always possible to identify the bacteria on food testing.
"As an additional precautionary measure, we have advised a small number of wholesalers to cease adding some imported rocket leaves to their mixed salad products pending further investigations."
Outbreaks of O157 are rare compared with other food-borne diseases.
PHE was first alerted to the outbreak at the end of June.
Washing salad and vegetables: What is the best technique
Health officials admit that washing foods is not 100% fail-safe, but it gives people the best chance against infection.
But what exactly are the best ways to wash your greens? The BBC asked two food industry professionals for the best practice in food safety and hygiene when it comes to salad and vegetables.
Camilla Schneideman worked in the food industry for 18 years having run her own restaurants abroad and in the UK, and is currently the managing director of the Leiths Cookery School in London.
Rosalind Rathouse is a professional cook with more than 50 years of experience, who runs the Cookery School in central London.
It's important to bear in mind that no method is absolutely certain to remove all E. coli bacteria when it comes to raw food - but there are things you can do to reduce the risk as far as possible.
"Let things soak," says Ms Schneideman, whose first job in a restaurant was solely to wash salad.
"Submerge it in cold water and leave it there for up to 20 minutes to let the soil particles come loose, then take it out and rinse it again," she adds.
She recommends the submersion technique for anything that grows in the ground, and may still be harbouring soil particles, in particular things like lettuce - for which you should break leaves off before soaking - and radishes, leeks and herbs.
However, depending on how you are cooking them, Ms Schneideman cautions against soaking mushrooms because, although they can often be dirty with soil, they "absorb like sponges".
Instead she suggests brushing the soil off and rinsing, and "if you intend to saute the mushrooms the heat will kill the bacteria regardless".
Ms Rathouse's advice is to wash all fruit and vegetables "very, very thoroughly" under running water, having first removed and thrown away any roots with soil or sand. The only vegetables she would soak are potatoes, to loosen any soil before thoroughly washing them.
With a whole lettuce, she advises first taking off the outside leaves - which are most likely to come into contact with bacteria - and either throwing them away, or using them in a cooked dish such as soup.
When it comes to mushrooms, Ms Rathouse washes them in a big bowl of water, before rinsing them well in running water and drying them with kitchen towel.
Her main advice is to be scrupulous about making sure hands and surfaces are clean to avoid transferring bacteria into and around the kitchen.
