2.2.2. Escherichia coli

Escherichia coli is a bacterium that is commonly found in the gastrointestinal tract of humans and warm-blooded animals. Due to its high prevalence in the gut, E. coli is used as the preferred indicator to detect and measure faecal contamination in the assessment of food and water safety. Considered as harmless commensals, E. coli strains constitute about 1 percent of the normal gut microbial population. While most of the strains within the gut are beneficial for human gastrointestinal function, others are harmful. Pathogenic E. coli strains are distinguished from others by their ability to cause serious illness as a result of their genetic elements for toxin production, adhesion to and invasion of host cells, interference with cellular metabolism and tissue destruction. To be transmitted from one host to another, these bacteria must leave their host and get into the surrounding environment. There is evidence that some E. coli can survive for several weeks outside the host, and even grow in water or soil. But it is on plant matter that E. coli colonization has become a concern, since although most types of E. coli are harmless, the presence of pathogenic strains on fruit and vegetables presents a food safety risk. Produce-associated outbreaks of E. coli O157 infections linked to consumption of leafy green vegetables have been increasingly recognized. However, several E. coli clones have acquired pathogenicity islands via horizontal gene transfer that enable them to cause urinary tract infections and diarrheal diseases. Diarrheal E. coli are divided into six categories that can cause illness ranging from moderate diarrhea to severe systemic diseases (e.g. hemolytic-uremic syndrome).

Shiga toxin-producing E. coli

Shiga toxin-producing E. coli (STEC) is a zoonotic pathogen colonizing mainly cattle and small ruminants. Although cattle products, principally beef, are the most commonly recognized sources of E. coli O157 infections, fruits and vegetables consumed raw are also an important source. Three leaf attachment mechanisms have been described in E. coli O157. First, contrary to nonpathogenic E. coli, the pathogenic strain STEC O157:H7 adheres strongly to tomato skin, spinach leaves, and roots of alfalfa sprouts. Adhesion to these surfaces is mediated by curli. The adhesion of E. coli O157, as well as the related enteropathogenic E. coli (EPEC), to a variety of salad leaves is mediated by the filamentous type secretion system (T3SS). The E. coli O157 secretion system is designed to translocate effector proteins into mammalian cells; protein translocation is mediated by an ATPase and is dependent on a translocation pore that is inserted into the plasma membrane. The flagella also play a role in E. coli O157 leaf attachment. Taken together, these data suggest that E. coli O157 employ multiple mechanisms to colonize plants and are well adapted to this biosphere. Like Salmonella, E. coli O157 can also reach the sub-stomatal cavity and the spongy mesophyll and survive in this environment.

Enteroaggregative E. coli and enterotoxigenic E. coli

Enteroaggregative E. coli (EAEC) may be an important cause of bacterial gastroenteritis. However, the reservoir of EAEC is unknown. There have been observed the following two patterns of bacterial distribution following 1 h incubation of EAEC with lettuce leaves: (i) diffuse adherence to the epidermis and (ii) localized adhesion to the guard cell of the stomata. The binding to the epidermis is mediated by the pilus, which is known to play a role in colonization of the human gut, whereas aggregation around the stomata is mediated by flagellae.

Enterotoxigenic E. coli (ETEC) is an important cause of infantile and travelers’ diarrhea and causes severe watery diarrhea in calves and piglets. The flagellum is the main adhesin mediating the attachment of ETEC to the epidermis of lettuce leaves.