The Bacterium: Escherichia coli

Escherichia coli (E. coli) is a Gram-negative, rod-shaped bacterium that belongs to the family Enterobacteriaceae. It is one of the most extensively studied and well-known bacteria and serves as a model organism in various areas of biology and biotechnology. We will also discuss pathogenic forms of E. coli and some of the food poisoning situations that have arisen as a result of its presence in our produce and food products.

Structure

E. coli cells are typically rod-shaped and range in size from 1 to 5 micrometers in length. They have a double-layered cell envelope consisting of an inner cytoplasmic membrane and an outer membrane, separated by a periplasmic space. The outer membrane contains lipopolysaccharides (LPS) that play a role in the bacterium’s interactions with the environment and host organisms.

Motility

E. coli exhibits motility through the use of flagella. These long, whip-like appendages allow the bacterium to move in liquid environments and colonize different niches. The arrangement and number of flagella can vary among different strains of E. coli.

Growth and Metabolism

E. coli is a facultative anaerobe, meaning it can grow in the presence or absence of oxygen. It can utilize a wide range of carbon sources for energy and carbon metabolism, including sugars, organic acids, and alcohols. E. coli is known for its ability to rapidly multiply, with a typical doubling time of 20-30 minutes under optimal growth conditions. The bacterium, like many will form biofilms if left unchallenged in growth. This type of colony is regularly associated with food poisoning issues on vegetable surfaces.

Genetic Features

E. coli has a circular chromosome consisting of approximately 4.6 million base pairs. It possesses a remarkable genetic repertoire and serves as a host for genetic engineering and recombinant DNA technology. It was studied in the 60s because it is an easy microorganism to ferment and whose genetic structure can be explored relatively well. Jacob and Monod examined the lac operon which is a characteristic type of gene structure in prokaryotes. Many well-characterized plasmids, such as pUC and pBR322, have been derived from E. coli and are widely used in molecular biology research.

Ecological Role

While E. coli is primarily associated with the gastrointestinal tract of warm-blooded animals, it can also be found in various environments, including soil, water, and plants. Most strains of E. coli are harmless and part of the normal gut microbiota. However, some pathogenic strains can cause illness, including gastrointestinal infections and urinary tract infections.

Importance in Research and Biotechnology

E. coli has been instrumental in advancing our understanding of fundamental biological processes. It has served as a model organism for studying gene expression, DNA replication, protein synthesis, and metabolic pathways. E. coli is also widely used in biotechnology for the production of recombinant proteins, enzymes, and metabolites of industrial and pharmaceutical importance.

Safety Considerations

While most strains of E. coli are harmless, there are pathogenic strains, such as enterohemorrhagic E. coli (EHEC) or certain serotypes of enteropathogenic E. coli (EPEC), that can cause severe illness. Proper handling and safety precautions should be taken when working with pathogenic strains to prevent contamination and ensure the safety of researchers and the public.

The Diarrheagenic E. coli (DEC)

Diarrheagenic Escherichia coli (DEC) is a group of pathogenic strains of the bacterium Escherichia coli that cause gastrointestinal infections and are a significant global health concern. These bacteria are responsible for various forms of diarrhea, ranging from mild and self-limiting to severe and life-threatening. In this comprehensive essay, we will explore DEC, its various subtypes, modes of transmission, symptoms, and the impact on public health.

I. Introduction to Diarrheagenic Escherichia coli (DEC)

While most E. coli strains are harmless and even beneficial, some strains have evolved to become pathogens, causing a range of infections, including urinary tract infections, sepsis, and gastrointestinal illnesses. DEC is a subset of pathogenic E. coli that specifically targets the digestive system.

II. Classification of DEC

DEC is a diverse group of bacteria, and different strains within this group are classified based on their virulence factors and the clinical symptoms they induce. The following are some of the major subtypes of DEC:-

1. Enterotoxigenic E. coli (ETEC): ETEC is one of the most common causes of traveler’s diarrhea. It produces heat-labile and heat-stable enterotoxins that lead to the overproduction of fluid in the small intestine, resulting in watery diarrhea (Ahmed et al., 2013).
2. Enteropathogenic E. coli (EPEC): EPEC is primarily associated with pediatric diarrhea in developing countries. It adheres to the lining of the small intestine, causing a characteristic “attaching and effacing” lesion that leads to diarrhea (Deborah & Frankel, 2005).
3. Enterohemorrhagic E. coli (EHEC): EHEC is perhaps the most well-known DEC due to outbreaks linked to contaminated food, particularly ground beef. EHEC strains, such as E. coli O157:H7, produce Shiga toxins, leading to bloody diarrhea and sometimes severe complications like hemolytic-uremic syndrome (HUS).
4. Enteroinvasive E. coli (EIEC): EIEC can invade the intestinal lining, leading to an inflammatory response, which results in diarrhea similar to shigellosis.
5. Enteroaggregative E. coli (EAEC): EAEC adheres to the intestinal lining and produces toxins, which can result in persistent diarrhea, particularly in children and immunocompromised individuals.
6. Diffusely Adherent E. coli (DAEC): DAEC strains adhere diffusely to the intestinal mucosa and are associated with cases of persistent diarrhea in infants.

III. Transmission of DEC

DEC infections are often transmitted through the consumption of contaminated food and water, contact with infected individuals, and exposure to animal reservoirs. The following are common modes of transmission:

1. Contaminated Food and Water: Consumption of undercooked or raw food, especially ground beef and unpasteurized milk, as well as water contaminated with fecal matter, can introduce E. coli into the digestive system.
2. Person-to-Person Transmission: Direct contact with infected individuals or contaminated surfaces can lead to the transmission of DEC. This is particularly relevant in crowded settings, like childcare facilities and households.
3. Zoonotic Transmission: Many DEC strains can be transmitted from animals to humans, especially in settings where humans and animals share living spaces, as seen in some agricultural and farming communities.
4. Faecal-Oral Route: DEC is transmitted through the fecal-oral route, which means that ingesting fecal matter containing the bacteria can lead to infection.

IV. Clinical Presentation of DEC Infections

DEC infections can result in a range of clinical symptoms, from mild and self-limiting diarrhea to severe and life-threatening conditions.

1. Watery Diarrhea: ETEC and EAEC are associated with watery diarrhea, often accompanied by abdominal cramps and low-grade fever.
2. Bloody Diarrhea: EHEC, particularly E. coli O157:H7, can cause bloody diarrhea, which may progress to hemolytic-uremic syndrome (HUS) in severe cases. HUS can lead to kidney failure and other complications (Smith et al., 2014).
3. Persistent Diarrhea: EAEC, DAEC, and other strains can result in persistent diarrhea, particularly in children and immunocompromised individuals.
4. Pediatric Diarrhea: EPEC is a common cause of pediatric diarrhea in developing countries. It can lead to diarrhea, vomiting, and dehydration.
5. Shigellosis-Like Diarrhea: EIEC infections can mimic the symptoms of shigellosis, including fever, abdominal cramps, and bloody diarrhea.

V. Diagnosis and Detection

Diagnosing DEC infections typically involves clinical evaluation and laboratory testing. Stool cultures and molecular techniques, such as polymerase chain reaction (PCR), are commonly used to identify the presence of DEC strains in stool samples. Testing for specific virulence factors, such as Shiga toxins in EHEC, can help confirm the subtype of DEC.

VI. Treatment and Management

The management of DEC infections depends on the specific subtype and the severity of the symptoms. In most cases, supportive care is essential, focusing on rehydration to prevent and treat dehydration. Antibiotics are generally not recommended for ETEC and EAEC infections, as they may not significantly alter the course of the illness and can potentially worsen antibiotic resistance.

For severe cases of EHEC infection, particularly when associated with HUS, hospitalization and supportive care are necessary. In these cases, blood transfusions and dialysis may be required to manage complications.

VII. Prevention and Public Health Concerns

Preventing DEC infections is a critical public health concern, as they can lead to significant morbidity and mortality, especially in vulnerable populations. Some preventive measures include:

1. Food Safety: Cooking meat, particularly ground beef, to a safe temperature (160°F or 71°C) can help kill DEC bacteria. Avoiding consumption of raw or undercooked foods, including unpasteurized milk and juices, is also crucial.
2. Hand Hygiene: Practicing good hand hygiene, especially after using the restroom and before preparing or consuming food, can reduce the risk of transmission.
3. Safe Water: Ensuring access to safe drinking water and proper sanitation facilities can prevent the contamination of water sources with fecal matter.
4. Childcare and Healthcare Facilities: Implementing strict infection control measures in childcare and healthcare facilities can help contain outbreaks and prevent person-to-person transmission.
5. Vaccination: In some cases, vaccines are being developed for specific DEC subtypes, such as ETEC, which can be particularly beneficial for travelers to high-risk regions.
6. Animal Husbandry Practices: Implementing safe animal husbandry practices in agricultural and farming settings can reduce the transmission of DEC strains from animals to humans.

VIII. Recent Food Poisoning Cases

 Escherichia coli is regularly implicated in food poisoning cases. Unwashed salads are one of the worst examples with lettuce in particular being an issue (Ackers et al., 1998). This food item regularly turns up as the source of E. coli. A recent study by the University of Illinois recently showed that lettuce is more susceptible to contamination than brassicas and spinach. They exposed the vegetables to E. coli and kept them at various temperatures. It was shown that lettuce kept at room temperature could support the bug better than these other vegetables. However, refrigeration at 4 °C (39 °F) reduced the growth rate.

May 2024 saw a number of people in the UK struck down by this bacterium. It is likely to have been through consumption of a ‘ready-to-eat’ food rather than one that was cooked according to the lead microbiologists Hugh Pennington, emeritus professor of bacteriology at the University of Aberdeen. In this case it is the Shiga toxigenic (STEC) bacterium. The source of the infection has yet to be identified but it has struck down 113 people, mostly young adults between May 25th and June 4th. Just 37 people have needed hospitalization. More cases are likely.

The distribution of cases in the United Kingdom is broad; 81 cases in England, 13 in Scotland and 18 in Wales with just one in Northern Ireland. The  UK Health Security Agency (UKHSA) thought the wave of cases stemmed from a ‘nationally distributed food item’ or ‘multiple food items’. One likely source would be a nationally distributed cheese that had not been pasteurised properly.

As well as practicing good hygiene mentioned earlier, a number of processors regularly use a variety of processes to treat food produce to reduce the numbers of bacteria growing on that food. Pasteurisation is an effective method to minimise growth as is refrigeration. We can now find treatment systems using antibiotics such as nisin, antimicrobials such as benzoic acid and sorbic acid, washing with chlorinated compounds, peroxides etc. There are also methods based on pulsed light, other non-thermal methods including ionization and cold plasma energy. 

IX. Global Impact

DEC infections, particularly in developing countries with limited access to clean water and sanitation, are a significant public health concern. They are a leading cause of diarrhea-related morbidity and mortality, particularly among children. In such settings, EPEC, ETEC, and EAEC are major contributors to the burden of diarrheal diseases.

In developed countries, EHEC outbreaks are a major concern due to their potential for severe complications like HUS. These outbreaks are often associated with contaminated food, such as ground beef and leafy greens.

Diarrheagenic E. coli (DEC) is a diverse group of pathogenic Escherichia coli strains responsible for a range of gastrointestinal infections. They cause a significant global health burden, with symptoms that vary from watery diarrhea to severe, bloody diarrhea and life-threatening complications like hemolytic-uremic syndrome (HUS). Prevention through safe food handling, hand hygiene, access to clean water, and vaccination in some cases is crucial for reducing the impact of DEC on public health, particularly in vulnerable populations. Continued research, surveillance, and public health measures are essential for combating DEC infections and their consequences worldwide.

Escherichia coli is a versatile bacterium with a long history of scientific exploration and practical applications. Its well-characterized genetics, rapid growth, and ability to produce proteins of interest have made it an invaluable tool in molecular biology, biotechnology, and microbiology research.

References

Ackers, M. L., Mahon, B. E., Leahy, E., Goode, B., Damrow, T., Hayes, P. S., … & Slutsker, L. (1998). An outbreak of Escherichia coli O157: H7 infections associated with leaf lettuce consumption. Journal of infectious Diseases, 177(6), pp. 1588-1593 (Article)

Ahmed, T., Bhuiyan, T. R., Zaman, K., Sinclair, D., & Qadri, F. (2013). Vaccines for preventing enterotoxigenic Escherichia coli (ETEC) diarrhoea. Cochrane Database of Systematic Reviews, (7).

Deborah Chen, H., & Frankel, G. (2005). Enteropathogenic Escherichia coli: unravelling pathogenesis. FEMS Microbiology Reviews, 29(1), pp. 83-98 (Article).

Smith, J. L., Fratamico, P. M., & Gunther IV, N. W. (2014). Shiga toxin-producing Escherichia coli. Advances in Applied Microbiology, 86, pp. 145-197