**6. Entero-invasive** *E. coli* **(EIEC)**

### **6.1 An overview of EIEC**

Enteroinvasive *E. coli* (EIEC) pathotype causes bacillary dysentery in humans worldwide characterized by abdominal cramps, bloody and mucous diarrhea [159]. The incidence of EIEC-related diseases varies by geographic region but is highly frequent in developing countries. In developed countries, EIEC-related infections are mainly travelers' diarrheal cases in people with recent travel history to endemic regions [160]. The first EIEC strain belonging to serotype O124 was reported in 1947 [161]. In the later years, some of the bacterial species implicated in dysentery that were previously classified as *Shigella* were renamed as EIEC [162]. EIEC and *Shigella* spp. share several serogroups, phenotypic and other genotypic

characteristics, which often makes it challenging to discriminate between the two genera in clinical samples. Like some *Shigella spp.*, most EIEC strains are non-motile and lack the ability to decarboxylate lysine or ferment lactose [3]. EIEC invades the human intestinal epithelial layer where it induces dysentery syndrome that is characterized by watery stool containing blood, mucus, and leukocytes, symptoms that are similar to those presented by *Shigella* spp. associated infection (**Figures 2** and **3**) [159].

At least, 20 serotypes have been assigned to this pathotype [159] among which some of the EIEC-associated O antigens including O28, O112ac, O121, O124, O143, O144, O152, and O167, are identical to O antigens present in *Shigella* spp. [3, 159]. Humans are the major reservoir of EIEC strains and transmission occurs through the fecal-oral route from the ingestion of contaminated foods or water and personto-person contact [159, 160]. The incidence rate and morbidity for EIEC are less or underreported but it appears to follow a similar trend as *Shigella* [3, 159]. Although EIEC strains cause sporadic cases of infection, they are also implicated in outbreaks. Of note are outbreaks of EIEC linked to O96:H19 strain that was traced to cooked vegetables and salads that were contaminated by asymptomatic food handlers in 2012 and 2014 in Italy [163] and the United Kingdom [164], respectively. Recently, the first case of EIEC outbreak in the US in about half a century was caused by EIEC serotype O8:H19 [160].

#### **6.2 Virulome of EIEC**

EIEC strains cause infection in humans by their ability to invade the colon mucosa layer with the expression of essential virulence determinants that mediate colonization, adherence, and invasion of the intestinal epithelial cells of the host (**Table 1**). These genes that are also shared with *Shigella* are located on the chromosome or virulence plasmid.

#### *6.2.1 Colonization and adherence*

The colonization, adherence, and invasion of intestinal epithelial cells by EIEC are mediated by genetic factors encoded by genes on a plasmid, pINV. pINV is a virulence plasmid found in EIEC that encodes the type III secretion system necessary for attachment, invasion of the host cell, and intercellular spread. This plasmid is structurally and functionally similar to those in *Shigella* strains [159], and with the replication (*rep*) and conjugation (*tra*) regions in IncFIIA plasmids. pINV had large deletions in the *tra* region which makes it incapable of self-transfer by conjugation but can be mobilized by other conjugative plasmids. Among the numerous functional insertion sequences present in this plasmid is the IS*1111* family, but only defective copies of the IS family are found in *Shigella* pINV plasmids [159]. pINV carries a PAI-structure that is composed of gene clusters encoding a T3SS apparatus (Mxi and Spa), its effector proteins (IpaB, IpaC, and IpaD) with their chaperons (IpgA, IpgC, IpgE, and Spa15), and two global transcriptional regulators (VirB and MxiE) that activate and regulate the expression of most of the virulence genes [3, 159]. All T3SS effectors are carried on the pINV except for a few effector proteins of the IpaH family that are chromosomal (**Figure 3**).

Also carried on pINV are genes that encode IcsA, a protein that facilitates the bacterial movement inside the cytoplasm, VirA, a GTPase-activating protein, and RnaG, a small RNA that negatively control the expression of *icsA* gene [159, 165], as well as the gene encoding OspG and OspF proteins which facilitate the evasion of the host innate immune response. All EIEC isolates are reported to carry this plasmid as it is essential for the pathogenesis and pathoadaptation

of this pathotype. However, loss of this genetic element has been reported in some EIEC strains [159, 165].
