Escherichia coli (E. coli): biology, roles, and public health

E. coli has the shape of straight, cylindrical rods with round ends. The diameter is 1.1-1.5 µm and the length is 2.0-6.0 µm. They occur in pairs or singly. In Gram stain they behave negatively (gram negative). It does not form bacterial spores. The cells consist mainly (70-85 %) of water whereas the dry mass consists of 96 % of polymers, among which the proteins dominate. There are 4288 different proteins annotated. In the cytoplasm as well as in the cell envelope (consisting of cell membrane, periplasm, outer membrane) they fulfil structural, enzymatic and regulatory functions. The genome comprises about 4600 kilobase pairs and occurs as a covalently self-contained bacterial chromosome.

Fimbriae

Many strains possess fimbriae (pili). A cell of strain K-12 typically contains about 100-500 type 1 fimbriae with a length of 0.2-2.0 µm and a diameter of about 7 nm. There are more than 30 different types of fimbriae, which are divided into two according to their adhesive properties to red blood cells: MS (mannose-sensitive), which cannot clump red blood cells in the presence of mannose (hemagglutination), and MR (mannose-resistant), which do not mind the presence of the sugar. Type 1 fimbriae, which are MS fimbriae, are found in both symbiotic and pathogenic strains and are therefore not used for differentiation. MR fimbriae are serologically diverse and often function as virulence factors. Their attachment is both species- and organ-specific. In addition, E. coli also forms a sex pilus (also F-pilus, F for fertility) with which cell-cell contacts for the exchange of genetic information (conjugation) are possible. Furthermore, the F-pilus also serves as a receptor for some bacteriophages after which the viral DNA is introduced (transduction).

Move

Cells of E. coli can move actively by peritrichous flagellation (they are motile) or, more rarely, they are incapable of active movement. Motile E. coli move along with their proteinaceous flagellum, repeatedly changing direction: A bacterium moves along in one direction as the flagella bunch up and work together. Locomotion is intermittently interrupted briefly by tumbling, as the flagellar bundle disintegrates and the individual flagella turn in different directions. Thereafter, the flagellar bundle reforms and accelerates the bacterium in a new direction. The stability of the bundle is reinforced by chemoreceptors. If a nutrient is offered to the bacteria, the stability of the flagellar bundle is further enhanced and the bacteria accumulate.

E. coli is chemotactic: if individuals swim up a concentration gradient of an attractant, they change direction less frequently. Swimming down a concentration gradient, their movement pattern is indistinguishable from that in an isotropic solution, and they change direction more frequently. In addition to positive chemotaxis, E. coli can also actively move away from contaminants (negative chemotaxis), with low concentrations of contaminants not being attractants and high concentrations of nutrients not being repellents. There are mutants that do not recognize certain pollutants and non-chemotactic mutants that also cannot recognize attractants. The process requires L-methionine.

Signal transduction for accurate chemotactic responses has evolved over time for optimal work output with minimal protein expression. Due to high selection pressure, chemotaxis in E. coli is highly sensitive, has a rapid response and is perfectly adapted. Moreover, the arrangement within the bacterial chemosensory system appears to be highly conserved.

Membrane proteins

For mass transfer, E. coli possesses transport proteins in the cell membrane. Among the porins, outer-membrane proteins OmpF and OmpC dominate, which are not substrate-specific but prefer cationic and neutral ions and do not accept hydrophobic compounds. Copy number depends on the osmolarity of the environment and serves to adapt to the habitat. Under conditions in the colon (hyperosmolarity, higher temperature), OmpC channels predominate. If the bacterium leaves its host and finds itself in a less preferred habitat, e.g. a body of water (lower osmolarity and temperature), OmpF synthesis is promoted. For substrates that are not transported at all or insufficiently by the non-specific porins, there are substrate-specific porins. In the case of phosphate deficiency, E. coli expresses the protein PhoE. Together with maltodextrins, this produces maltoporins, which also act as a receptor for the lambda phage and are therefore also called LamB. Strains that can metabolize sucrose take it up via the channel protein ScrY. Long-chain fatty acids are transported into the cell by FadL.

Metabolism

E. coli is heterotrophic, facultatively anaerobic and has the ability to obtain energy through both the respiratory chain and "mixed acid fermentation". The fermentation balance in E. coli is as follows:

Glucose is fermented by E. coli to form acid, which can be detected with methyl red as a pH indicator. Besides acid, E. coli also forms gas from glucose. The indole test for tryptophanase is positive. The Voges-Proskauer reaction to detect acetoin formation is negative. No discoloration is visible on Simmons citrate agar because E. coli cannot use citrate as its sole energy source. In addition, it cannot utilize malonate. Acetate and tartrate can be metabolized (test with Jordan's methyl red). Nitrate can be reduced to nitrite. No hydrogen sulfide is formed on Triple Sugar Iron agar. E. coli cannot hydrolyze urea or gelatin, but some strains can hydrolyze esculin. Lysine is decarboxylated by many strains, ornithine by only a few. In the potassium cyanide growth test, E. coli does not grow. It has no phenylalanine deaminase, no lipase and no DNase in the strict sense. The oxidase test with Kovacs reagent is always negative. Furthermore, most strains can ferment L-arabinose, lactose, maltose, D-mannitol, D-mannose, mucic acid, D-sorbitol, trehalose and D-xylose.

Serotypes

Serotyping is a useful way to classify E. coli based on the numerous differences in antigen structure on the bacterial surface.

Four groups of serotypes are distinguished:

• flagellar H-antigens for the flagella, derived from "bacteria growing with puffs", as their active locomotion on an agar plate produces a matt ripple pattern that looks like a tarnished glass plate. They are protein antigens.
• somatic O-antigens, derived from "without puff" for the lipopolysaccharides located on the surface of the cell wall. Their specificity is determined by carbohydrate side chains. Currently, about 190 different O antigens are known.

Rarely used for diagnostic purposes:

• K-antigens for the capsule, which are composed of polysaccharides
• fimbrial F antigens for the fimbriae

E. coli occurs as a universal and commensal companion in the lower intestinal tract of warm-blooded animals (including humans). There are about ^108-109 colony forming units per g in the stool. It can also survive in other habitats. In newborns, it plays an important role as an initial colonizer. Although it is present in low numbers itself, it serves to colonize obligate anaerobes that have physiological importance in digestion. Despite its low intestinal content, E. coli occupies a dominant position in the intestine, which it colonizes in humans within 40 hours of birth via food, water, or other individuals. The ability to adhere to the mucus allows E. coli to remain in the intestine for long periods of time. Although a great deal is known about the organism, relatively little is known about its ecology in the gut.

Food hygiene

Sporadic outbreaks of enterotoxic strains (ETEC) transmitted through drinking water are known. In addition, ETEC are transmitted through consumption of soft cheeses and raw vegetables. Outbreaks of enteropathogenic strains (EPEC) are commonly associated with contaminated drinking water and some meat products. Infections with enterohemorrhagic E. coli (EHEC) often originate from food or occur via water. Commonly infected foods include undercooked ground beef, raw milk, cold sandwiches, water, non-pasteurized apple juice, sprouts, and raw vegetables. In addition, epidemics have been linked to hamburgers, roast beef, cabbage rolls, and raw sausage (Teewurst).

The gastric acid-resistant strain Escherichia coli O157:H7 (EHEC), which is harmless to cattle, can be detected in 1-2 % of cattle faeces. This strain can also contaminate meat during slaughter and cause severe food poisoning in humans. The reason for this is that starchy cereals are often fed, which are incompletely broken down in the rumen and fermented to acid, so that acidophilic bacteria accumulate there. Feeding hay or grass reduces the number of human pathogenic strains.

Since a complete EHEC sanitation of livestock is not possible, prophylaxis must start with slaughter hygiene. Beef products should be cooked through at a minimum of 70 °C for at least 10 minutes. Due to the high environmental resistance of the pathogens, food producers should carry out exposure tests and HACCP analyses. Risk groups (children under 6 years of age and immunocompromised persons) should not consume raw products.

Bathing water

According to the EU Bathing Water Regulations of 2008, the following limit values apply for E.coli:

• "Excellent quality" up to 500 CFU / 100 ml
• "Good quality" up to 1000 CFU / 100 ml

For drinking water, on the other hand, a restrictive limit of 0 CFU / 100 ml applies.

Sandy beaches can be particularly affected by E.coli because it takes longer for sewage bacteria to break down in sand than in seawater.