As of last week there have been 160 cases in this outbreak, predominantly in three clusters in the South West, North West and South East of England. Sixty two cases have been admitted to hospital, 7 have developed haemolytic uraemic syndrome and 2 have died. There is an ongoing investigation to find the source of this bacterium which is thought to be mixed salad leaves supplied to the wholesale catering industry; the source has not yet been identified but the focus is on rocket (posh greens).
So what is E. coli O157?
E. coli O157 causes bloody diarrhoea by producing a shiga-toxin (a “Shigella-like” toxin similar to a toxin normally the cause of bacterial dysentery from Shigella dysenteriae). The O157 stands for “O antigen” number 157; the O antigen is a molecule expressed in the outer membrane of the bacterium which can be used to distinguish different strains of bacteria. For example in 2011 an outbreak in Germany was due to E. coli O104, a different O antigen, which was thought to be caused by contaminated bean sprouts imported from Egypt.
The most important environmental source of E. coli O157 is the gastrointestinal tract of cattle, sheep and goats which all carry this bacterium normally in their guts. Any food that has been contaminated with cattle faeces is a potential source of infection and outbreaks have occurred with contaminated meat, salad onions, spinach, lettuce, apple juice, unpasteurised milk and even cookie dough! Another reason not to eat biscuits :-(
Why all the fuss?
The E. coli bacterium sits in the gut but the shiga-toxin invades through the gut wall into the bloodstream and throughout the body causing all of the usual symptoms of gastric infection. However, it can also cause a condition known as haemolytic uraemic syndrome (HUS) in 5-10% of patients. HUS can affect both adults and children but is much more common in children and the elderly. HUS is the combination of haemolytic anaemia, thrombocytopaenia and acute kidney injury which in 5-25% of children can result in severe complications including dialysis dependence, renal transplant and even death.
Added to this is the fact that the infectious dose of E. coli O157 is very low, only 10-100 bacteria, compared to more common bacterial causes of gastroenteritis like Campylobacter jejuni (1,000-100,000) and Salmonella enteritidis (10,000-10,000,000). It is therefore very easy to spread from the original source and between people.
Identifying E. coli O157 in the laboratory
The traditional laboratory method of identifying E. coli O157 in stool has been to use a selective and identifier media called Sorbitol MacConkey. The MacConkey agar contains bile which stops many bacteria growing allowing only tough gut bacteria to survive. It also contains sorbitol (those artificial sweeteners get everywhere!) The sorbitol specifically helps to identify E. coli O157 as most E. coli can ferment sorbitol to create acid and cause a colour change whereas O157 does not; E. coli O157 is called a “sorbitol non-fermenter”. Any bacterium growing on the MacConkey that does not ferment sorbitol is then further identified biochemically or by using MaldiTOF to ensure it is definitely E. coli. There are also commercial kits that can then identify the O157 antigen using latex agglutination (a method where the sample containing antigens or antibodies is mixed with latex beads coated with a specific antigen or antibody see video which shows the process using S. aureus). This MacConkey to agglutination method is a bit of a fuss and poses a risk of laboratory acquisition, especially when you factor in that all of this is done in a biological safety cabinet making it even harder.
Nowadays the preferred method of detecting E. coli O157 is to identify the gene for producing the shiga-toxin using PCR. This is quick, simple and safe but there are some drawbacks:
- It is expensive
- You can identify the toxin gene in bacteria that do not produce toxin (the gene is present but not switched on or active) thereby giving false assurance as to the “cause” of a patient’s diarrhoea when there may be something else going on
- The PCR detects the gene for the toxin, not the specific antigen e.g. O157, and this has shown that there are more than just O157 strains out there able to cause disease… e.g. O104 found in Germany which was not previously known to cause illness. No one yet knows what should be done about these strains but PHE have been inundated with results because of the introduction of PCR tests
So I have a bacterial infection, where are my antibiotics?!?
Antibiotics are not the answer in infection with E. coli O157; in fact they make the situation worse. They do not reduce the duration of gastrointestinal symptoms BUT they do increase the incidence of HUS. Twenty five percent of children with E. coli O157 who get given antibiotics develop HUS compared to only 5-10% who are not.
Antibiotics should not be given in suspected E. coli O157 infection or HUS and if they have been started they should be STOPPED.
The treatment of E. coli O157 is supportive encouraging adequate hydration and monitoring to identify and deal with HUS. Children may be encouraged to drink if given a flattened version of their favourite fizzy drink, maybe just sometimes sugary drinks are a health benefit!
Prevention of infection
Of course the ideal situation is to not acquire the infection in the first place. A number of steps can be taken to reduce the risk of infection and prevent spread of bacteria:
- Wash your hands with soap and water::
- After using the toilet
- Before and after handling food
- After contact with any animals including pets and farm animals
- Encourage others to wash their hands with soap and water, especially young children who may need help and supervision to do this properly
- Remove soil and wash any vegetables, salad (I’d still include “pre-washed” salads!) or fruit that is going to be eaten raw
- In hospital, isolate any patient with diarrhoea
- Observe strict hand hygiene when visiting farms and petting zoos as per PHE guidance
Is this big outbreak ongoing and should I be afraid?
The number of cases being notified each week is reducing and so the outbreak may be coming to an end. However, as the source hasn’t yet been identified there could be a resurgence of cases, especially if people become less careful with food and hand hygiene. There is also the risk of further cases as the primary cases infect secondary cases, rather than them being exposed to the original source (person-to-person transmission is said to occur in 10-20% of cases).
Having said all of this the risk of infection is actually low! In the total population the number of cases is still only 160 and a laboratory, like the one I work in, processes about 800 stool specimens a week, that’s 40,000 specimens a year. Eeeww, that’s nearly ½ a ton of poo and we don’t actually find many cases of E. coli O157!!!