STEC: What is E. coli doing on lettuce?

In May this year the UK Health Security Agency released a general advice notice because there was a sharp spike in E. coli infections¹. This was for a Shiga-toxin producing type of E. coli (STEC), which causes gastrointestinal infections...

It was a relatively large outbreak (275 cased as of 25 June 2024) compared to about 1500 cases of STEC reported annually, triggering the alert². It turned out that the bacteria were on the lettuce used in sandwiches. The problem was that the sandwiches were distributed to multiple retail outlets across the country, rapidly spreading the contamination far and wide.

While most E. coli are completely harmless, STEC causes a range of disease ranging from diarrhoea, sometimes bloody, through to stomach cramps, vomiting and fever. In rare cases, the "Shiga toxin" produced by the bacteria can cause a kidney disease called HUS - haemolytic uraemic syndrome. It is normally transmitted in food, or via direct contact with animals.

Why lettuce?

There was considerable surprise that the STEC was on lettuce in the sandwiches, and not, for instance, on beef or cheese, although we know that STEC can be carried by lots of different food types into the food chain. It is normally associated with farmed animals, so can be a problem with undercooked meat, or in unpasteurised milk. But the bacteria can also get into soil and water from animal manure. Water also moves them around a farming environment quite easily, and if the water is used to irrigate crops, the bacteria land directly onto any growing plants and the soil.

The main thing about E. coli is it is very capable of surviving and adapting to what we regard as a "hostile environment". While it is very easy to grow the bacteria in the lab in conditions that mimic an animal gut - warm, 37^oC nutrient rich broth, it doesn’t appear do so well when its gets cooler or nutrients are less abundant. And that initially tricked us into thinking that STEC can’t survive outside the animal environment. But what it does instead is simply grow more slowly and find alternative nutrients.

When STEC bacteria land on plant leaves or in the soil and find plant roots, they shift their metabolism and growth rate to match the new environment. In terms of nutrients, we can think of plant leaves as a similar to our skin: quite dry, and nutrient poor, yet can still harbour microbes. Similarly, plant roots are like our guts: richer in nutrients, more available water, but far busier with neighbouring microbes. In fact, the diversity and density of microbes in plant roots is often greater than we find in animal guts, proving it’s a great place to hang out. Plants actively secrete nutrients, especially from their roots, which we know microbes, including STEC literally flock towards. However, once STEC are on roots, they aren’t so likely to get to the leaves, which are harvested for food. That means its unlikely that lettuce roots was the source of the current outbreak.

The leaves are quite different and nutrients are harder to come across there. Instead, what STEC can do, like many of its cousins that are normally found on plants, is sneak inside the plant tissue. Plants breath through pores on the undersides of their leaves called stomata. These provide a natural opening that microbes can pass through. Although there are gate-keepers of the pores, the guard cells, they don’t seem to deter STEC. In contrast, microbes that cause plant disease are recognised and the guard cells close the stomata to prevent them getting inside. In fact, STEC seem to find that guard cells are a good place to attach to plant leaves, giving them easy access³. The conditions inside are more favourable, with good access to nutrients, moisture, protection from UV, and fewer other competiting microbes. What we’ve seen in the glasshouse is that STEC can be present both on the outsides and insides of leaves of crops like lettuce and spinach⁴. So, it's more likely that STEC was present on lettuce leaves, on the surface and possibly inside, when they were harvested for food.

STEC can stick firmly to plant tissue, whether the leaves or roots. It uses the same mechanism as it does inside animal tissue where hair-like projections, called fimbriae, literally grab onto receptors on the cells. STEC, like other E. coli, have multiple fimbrial types, each sticking to different receptors. That gives them lots of options: if one receptor isn’t available, there is a good chance that STEC has another fimbria that will do the job. This means that they can’t get rinsed or brushed off the surface. Once bound, they attach very tightly, making it impossible to remove them with washing. The only way to get rid of them is to kill them off with a microcidal treatment.

Hygiene and monitoring of STEC

The other way that STEC can get into food is in the processing factory. The May-2024 outbreak investigations are still ongoing, so we don’t yet know where it came from. In previous STEC outbreaks though, this has been a less common route of transmission. In countries that have more plant-associated outbreaks, the evidence shows STEC is transmitted through water, from farmed (or occasionally wild) animals, into irrigation water, onto salad crops and into the food chain. It tends to be other food-poisoning bacteria, like Listeria that can be stubbornly associated with a factory and cause contamination in food products.

Farmers in the UK, like other countries, have good guidance in place for monitoring the quality of the water. They test the level of generic bacteria in irrigation water, including general E. coli, and when that reaches a threshold, they need to intervene. The food standards agency in Scotland has produced a guidance tool for salad crop producers to work out where to source their water, how often to test, and what sort of interventions are in place⁵. Equally, guidance is in place for food producers to check the water they use for washing their produce⁶.

Food safety relies on best practice and risk assessment, informed by the most recent scientific evidence. Nothing is risk free, but the UK has an excellent record for producing safe food, with high consumer confidence⁷. One of the big headaches however, is in detection of STEC and making risk-based decisions. That is because this group of E. coli is rapidly evolving. When STEC burst onto the scene in the late 1980s, the most frequent type was ‘O157’. But that is no longer the case, not only have other O-types superseded O157, including the current O145, but they seem to be much more genetically divergent. What that means is that even if E. coli are detected carrying the Shiga toxin gene, they may not cause any disease. So how do we work out which one is harmful and which can we ignore? There isn’t a straight answer to this critical question yet, but there is a lot of different work going on across the world to try and address it. The big task now is to update the risk profile for STEC, so the vital information gets out to the clinical labs, the farmers and growers, and the food retailers as quickly as possible.