For decades, public health officials and clinicians primarily hunted for one notorious pathogen: E. coli O157. Its unique biochemical characteristics made it relatively easy to identify in laboratories, and its association with severe, sometimes fatal, foodborne illness was well-established. This focus, however, created a blind spot. Lurking in the shadows were hundreds of other E. coli serotypes capable of producing the same dangerous Shiga toxins, a diverse family of pathogens now known as non-O157 Shiga toxin-producing E. coli (STEC). Thanks to a revolution in diagnostic technology, the true scale of this hidden threat is finally coming into focus, revealing a significant and growing public health challenge that extends far beyond the classic O157 strain.
The shift to advanced molecular testing is fundamentally changing our understanding of the epidemiology of these infections. While STEC O157 cases have held steady or even decreased in some regions, reported cases of non-O157 STEC are rising dramatically. In England, the number of diagnosed non-O157 STEC infections skyrocketed from 297 in 2016 to 2,341 in 2023 – a near ten-fold increase that public health experts attribute largely to improved detection through PCR testing. This trend is not isolated; similar increases are being documented in the United States and other parts of the world as laboratories modernize their methods. These bacteria are now recognized as a major cause of foodborne illness globally, with some of the most common serogroups—O26, O103, O111, O121, O45, and O145—being designated the “Big Six” due to their public health significance.
The Diagnostic Revolution: Shining a Light on the Invisible
The primary reason non-O157 STEC remained in the shadows for so long is fundamentally technical. The traditional method for detecting E. coli O157 involves culturing a stool sample on a special agar plate called sorbitol MacConkey (SMAC) agar. O157 bacteria have a unique trait – they do not ferment sorbitol – which makes them stand out as colorless colonies on this plate. In contrast, most non-O157 STEC strains do ferment sorbitol, making them indistinguishable from the harmless E. coli that are normally present in the human gut. Without a specific test to find them, they were simply missed, and illnesses were often misclassified as generic gastroenteritis.
The game-changer has been the widespread adoption of culture-independent diagnostic tests (CIDT), primarily polymerase chain reaction (PCR) tests. Instead of looking for a particular type of bacterium, these molecular tests scan a stool sample for the genetic fingerprints of the Shiga toxin genes themselves (stx1 and stx2). This allows laboratories to detect the presence of any STEC, regardless of its serogroup. As more hospitals and labs have implemented this technology, the number of identified non-O157 infections has surged. In England, the proportion of diagnostic laboratories using PCR for gastrointestinal pathogens rose from about 20% in 2018 to 40% in 2023, directly paralleling the increase in cases. A study in a Canadian pediatric hospital demonstrated the stark superiority of this method, where PCR identified 21 STEC-positive samples, while traditional culture on SMAC agar detected only 5 O157 STEC isolates.
This diagnostic shift has revealed that the challenge is not with a single new pathogen, but with a vast and diverse family. From 2016 to 2023, English surveillance identified 9,378 non-O157 STEC isolates, which comprised 338 different serotypes. This diversity complicates public health responses, as officials must now track a much wider array of pathogens, each with its own characteristics, rather than focusing on a single well-known enemy.
A Varied Threat: Major Pathogenic Strains and Virulence Factors
Not all non-O157 STEC strains pose an equal risk to human health. Scientists have learned that the severity of illness is closely linked to the specific combination of virulence factors a strain carries. Among the hundreds of serotypes, a handful have emerged as the most common and concerning.
The “Big Six” non-O157 STEC serogroups – O26, O45, O103, O111, O121, and O145 – have been identified by regulatory agencies as the most significant contributors to foodborne illness. In the United States, these serogroups are estimated to cause nearly 170,000 illnesses each year. They share many virulence and epidemiological features with E. coli O157:H7 and have been declared adulterants in raw beef by the U.S. Department of Agriculture, meaning their presence triggers a mandatory recall. A study of commercial ground beef in the U.S. found that these serogroups were responsible for 71% of non-O157 STEC diseases, with O26 being the most prevalent at 22%, followed by O111 (16%), O103 (12%), O121 (8%), O45 (7%), and O145 (5%).
The following table outlines the most common non-O157 STEC serogroups and their key characteristics based on recent surveillance data:
| Serogroup | Prevalence & Key Characteristics |
| O26 | Most common non-O157 group in many regions (16% of isolates in England); associated with a high risk of hemolytic uremic syndrome (HUS), particularly in children under 5 |
| O1O3 | One of the top six non-O157 serogroups identified in the U.S. and Europe; frequently implicated in human infections |
| O111 | A major cause of non-O157 illness and outbreaks globally; associated with severe disease including HUS |
| O145 | Associated with a higher rate of hospitalization than other non-O157 STEC cases; one of the serotypes most likely to be linked to HUS |
| O121 | A regulated adulterant in the U.S.; known to cause severe disease and outbreaks |
| O45 | One of the “Big Six” non-O157 serogroups of concern in North America |
| O146 | Second most common serotype in England (12% of isolates); more frequently isolated from adults |
| O91 | Third most common serotype in England (11% of isolates) |
The severity of disease caused by these strains is largely determined by the specific Shiga toxin type and other virulence factors they possess. There are two main types of Shiga toxins, Stx1 and Stx2, with several subtypes. Stx2, and particularly the Stx2a and Stx2c subtypes, is much more strongly associated with severe disease and the development of HUS. Another critical virulence factor is the eae gene, which codes for a protein called intimin that helps the bacteria attach tightly to the human intestinal wall. Strains that carry both the stx2a gene and the eae gene are considered the most virulent and are most likely to cause outbreaks and severe outcomes like HUS. Other factors, such as toxins like subtilase (SubA) and genes carried on a large virulence plasmid, also contribute to the pathogenicity of these strains.
Persistent Challenges in Detection and Surveillance
Despite the advances in PCR technology, significant challenges remain in the detection and surveillance of non-O157 STEC. One major hurdle is that a positive PCR test, which detects the Shiga toxin gene, is only the first step. To fully characterize the strain for public health purposes – determining its serotype, specific toxin subtypes, and other virulence factors – the bacteria must be isolated through culture. However, this isolation is not always successful. In the English data, a full third of PCR-positive samples could not be cultured, meaning that while officials knew a patient was infected with a Shiga toxin-producing bacterium, they could not identify the specific strain, creating a gap in surveillance.
This detection challenge is a key reason why non-O157 STEC is considered a “hidden” pathogen. Unlike O157, most non-O157 STEC serogroups lack specific biochemical markers that make them easy to identify on culture plates. This complicates efforts to accurately and efficiently detect them in food, stool, and environmental samples, likely leading to a significant underestimation of their true impact on public health.
The following table compares the traditional and modern methods for detecting STEC:
| Method | Mechanism | Advantages | Disadvantages |
| Culture on SMAC Agar | Relies on O157’s inability to ferment sorbitol. | Inexpensive; identifies O157 for further confirmation. | Misses virtually all non-O157 STEC strains |
| Enzyme Immunoassay (EIA) | Detects Shiga toxins directly. | Broader detection than O157-specific culture. | Less sensitive than PCR; can yield false negatives |
| PCR (Molecular Testing) | Detects Shiga toxin genes (stx1, stx2). | Highly sensitive; can detect all STEC types; fast. | Requires confirmatory culture for strain characterization; can be more expensive. |
The epidemiology of these infections is further complicated by the diverse reservoirs and transmission routes. Cattle are recognized as the primary reservoir for many STEC serotypes, and studies show they are widespread in herds. One study in Spain found STEC in 63.5% of beef cattle herds and 56.5% of sheep flocks. A U.S. study of ground beef found Shiga toxin genes in 24.3% of samples tested, though a much smaller percentage contained strains considered a significant food safety threat. Transmission to humans occurs through the fecal-oral route, most commonly by consuming contaminated food or water, or through direct contact with animals and their environments, such as at petting zoos. As of this writing, an investigation is underway to see if an outbreak of E. coli could be linked to the petting zoo at the recent Arizona State Fair. The infectious dose is frighteningly low, as few as 10 to 100 bacterial cells are sufficient to cause an infection, making stringent hygiene and food safety practices critical.
Public Health Impact and Clinical Consequences
The rising detection of non-O157 STEC has significant implications for public health, revealing a substantial burden of illness that was previously unaccounted for. The clinical symptoms of a non-O157 STEC infection are similar to those caused by O157 and can range from unpleasant to life-threatening. Patients often suffer from diarrhea, severe stomach cramps, and vomiting. A significant proportion develop bloody diarrhea (hemorrhagic colitis). The most feared complication is hemolytic uremic syndrome (HUS), a serious condition that causes kidney failure, hemolytic anemia, and thrombocytopenia. HUS is more common in young children and can lead to permanent kidney damage or death.
Data from England provides a stark picture of the clinical impact. Where clinical details were available, 27% of non-O157 cases required admission to the hospital, and 6% developed HUS. The serotypes most likely to be associated with progression to HUS were O26:H11 (9% of cases) and O145:H28 (7% of cases). Furthermore, strains harboring the stx2a toxin subtype were most frequently isolated from patients who developed HUS, confirming the critical role of this virulence factor in disease severity. This demonstrates that while non-O157 STEC infections are often perceived as less severe, certain strains can cause outcomes just as serious as those caused by O157.
Certain demographic groups are disproportionately affected. Children, the elderly, and individuals with weakened immune systems are at greatest risk for severe infection. Surveillance data from England showed that STEC O26:H11 was more frequently reported in children under five than in any other age group. This heightened vulnerability is due to developing immune systems in the young and more frequent hand-to-mouth behavior, which facilitates transmission. For all patients, treatment is primarily supportive; the use of antibiotics is generally not recommended for STEC infections because certain antimicrobials can induce increased production of Shiga toxin, potentially worsening the patient’s condition and raising the risk of developing HUS.
Analysis & Next Steps
The landscape of STEC infection is being fundamentally reshaped by diagnostic technology, revealing a public health challenge that is both broader and more complex than previously understood. What is new is the realization that the classic O157 serogroup represents only a fraction of the total threat, with numerous other strains now known to be circulating widely and causing a significant portion of severe illnesses. This matters because our historical focus on a single pathogen has left systems and the public potentially underestimating the risk from others, which can be equally, if not more, dangerous. The population most affected continues to be young children, who bear the greatest burden of the most severe complications like HUS due to their developing immune systems and hygiene practices.
Moving forward, several paths are critical. Continuous improvement in diagnostic capabilities remains a cornerstone, ensuring that all clinical laboratories can not only detect the presence of a Shiga toxin but also successfully isolate and fully characterize the strain to guide public health action. For food safety and public health officials, the necessary response is to expand surveillance and intervention strategies to explicitly account for the major non-O157 serogroups, recognizing that control measures designed for O157 may not be sufficient. This includes adopting a “One Health” approach that recognizes the interconnection between human, animal, and environmental health. As one study noted, improving surveillance programs by testing matrices like raw milk filters and calf feces on farms, rather than just finished food products, could provide earlier detection and prevent outbreaks.
Finally, for the public, the essential task is to maintain alertness and practice food safety techniques . The same food safety practices that protect against O157 are effective against non-O157 STEC: thoroughly cooking meat, especially ground beef; washing hands carefully after contact with animals or their environments; and properly washing fresh fruits and vegetables. The rising detection of non-O157 STEC is not necessarily a sign of a new epidemic, but rather the long-overdue unveiling of a hidden one, providing the clear picture needed to mount a more effective and comprehensive defense for global public health.
