PHE said 160 cases have been identified - 153 in England, six in Wales and one in Scotland – up from 151 the week before.
The agency said the figures suggest the outbreak is subsiding, but it remains vigilant to further cases and the risk of those affected passing the infection to others.
More than 60 cases have received hospital care and two with E. coli O157 infection have died.
Samples have been confirmed as STEC serogroup O157 phage type 34, positive for the eae (intimin) and verocytotoxin 2 genes but negative for the verocytotoxin 1 gene.
The ECDC told us the UK health authorities are leading the investigations and informing the European Commission, ECDC, EFSA and other Member States’ public health and food safety authorities about progress.
To date, no other EU country has reported an increase in Shiga toxin-producing E. coli (STEC) O157 or cases related to the UK outbreak through EPIS or EWRS, it added.
WHO monitoring situation
The World Health Organization (WHO), which was told about the outbreak at the start of July, said it continues to monitor the epidemiological situation and conduct risk assessment based on the latest information.
As of 14 July, four patients were in hospital and features of haemolytic uremic syndrome (HUS) were reported in seven cases.
Eating mixed salad leaves, including rocket leaves, particularly from catering establishments such as cafes and restaurants, is associated with the infection.
The outbreak is characterised by multiple small clusters linked to catering and residential care premises.
Cases are predominantly female (75%) and over the age of 18 (91%) – the age range is between one and 98 years with onset dates starting from 31 May.
The FSA’s investigations are focusing on the distribution of mixed salad leaves to wholesale and not supermarkets.
In response to the outbreak, some wholesalers have stopped adding some imported leaves to their mixed salad products, pending further investigations.
FSA said it was 'not in a position' to confirm the number of wholesalers being investigated and asked if produce could still be on sale or in restaurants, the agency added it could not be ruled out but cases appear to be subsiding.
When asked if it saw a wider problem at farm level, as an E. coli outbreak was reported from salad last year, the FSA said a source hasn’t been confirmed at this stage but it was not aware of any specific issues at farm level.
"The fresh produce industry has systems in place to assess and manage risks. However we review such incidents to see what lessons can be learned and there is also guidance being developed at an international level," said the agency.
PHE is using various approaches including whole genome sequencing (WGS) to test samples from those affected.
This has indicated the strain involved is likely to be an imported one, possibly from the Mediterranean area. Italian media have reported UK firms who import salad from the country are part of the investigation.
The strain is not related to those currently circulating among livestock in the country, but is closely related to sequences identified in people reporting recent travel to the Mediterranean region.
However, other European countries have not reported similar increases, and the strain has not been reported outside the UK.
Source unknown as outbreak slows
Dr Isabel Oliver, director of PHE’s field epidemiology service, said it was pleased to see a significant reduction in the number of cases over the past week.
“This could indicate that we are over the worst of this outbreak, with those affected reporting the last onset of symptoms on 5 July. But this is still too many, and the risk of those affected passing the infection onto others remains.”
Dr Oliver said the source remains unconfirmed and it was not ruling out other food items.
“It’s important to be aware that no individual wholesaler, supplier, retailer, or restaurant has been confirmed as the source and currently the Food Standard Agency’s investigations focus on the distribution of mixed salad leaves to wholesale and not supermarkets.
“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.”
Asked about its advice on washing, the agency said it does not recommend that washed ready-to-eat products are washed again as this is unlikely to provide additional benefits and may introduce cross contamination.
"The reason people are advised not to wash raw chicken is because cooking will kill any bugs, whereas salad obviously does not have that protection. It should also be noted that the bacterial load on raw chicken is likely to be higher than on salads so washing is more problematic."