usher, and chaperone protein [11]. However, the CU pili biosynthetic pathway at the

outer membrane involved two assembly proteins (i.e., chaperones and ushers) [10].

11.2.2 Curli

Curli pili are a class of thin, irregular extracellular amyloid fibers produced by Salmonella

and E. coli species. Curli was first isolated and characterized in the late 1980s as it was

responsible for causing bovine mastitis. Curli has been considered as another bacterial

filaments class of fibers termed amyloids. They have been involved in various pathogenic

patterns of many Gram-negative bacteria. The two species belonging to this group such as

E. coli and Salmonella express proteinaceous appendages termed curli pili. The curli are

implicated in cellular communication, colonization, and biofilm formation. The mor­

phology of curli can be inspected through an electron microscope, which usually presents

a coiled surface structure, thin, irregular, highly aggregated, and wired fibers, which can

have a diameter of 2 nm composed of a single type of curli as a subunit [12].

11.2.3 Type III Secretion System (T3SS)

Several pathogenic bacteria like Salmonella, enteropathogenic E. coli (EPEC), Shigella, and

Pseudomonas possess the ability to deliver effector proteins directly to eukaryotic host-cell via

T3SS also called injectisome [13]. It has been recently discovered in Gram-positive bacteria,

particularly in clostridia species. The essential part of the T3SS injectisome is a hollow needle

channel used by bacteria to cause infections into the eukaryotic host cells. Some of the human

pathogenic bacteria that present T3SS injectisome are Salmonella spp., Shigella spp.,

Burkholderia spp., Yersinia pestis, Ps. aeruginosa, Chlamydia spp., V. cholerae, enteropathogenic

and enterohemorrhagic E. coli, Xanthomonas spp., plantlike Ps. syringae, Erwinia spp.,

Xanthomonas, E. amylovora, Ps. syringae, Ralstonia solanacearum, and Xanthomonas spp. [14].

The T3SS is composed of more than 20 different proteins that oligomerize to form three

components consisting of the basal body, needle filament, and translocation. In bacteria, the

mechanism of injectisome assembly is similar to flagellum assembly.

11.2.4 Type IV Secretion System (T4SS)

The T4SSs is a secretion systems type, described in Gram-negative or positive pathogenic

bacteria and some of the archaea species. Many bacteria use T4SSs to secrete a large number

of virulent protein substrates like protein–DNA complexes. Within this line, the T4SSs fa­

mily can be classified into four groups: effector protein translocation into host cells; con­

jugative transfer of chromosomal and plasmid DNA from a cell-to-cell or transposons of

E. coli and Agrobacterium tumefaciens; and some Gram-positive bacteria [15]. However, for

many Gram-negative bacteria such as Helicobacter pylori, Bordetella pertussis, L. pneumophila,

and Brucella spp., the T4SSs have a role in mediating the transfer of virulent proteins and

transferring toxins into the host cell [16]. The architecture of T4SSs can be assembled mostly

by the type IVa and IVb [17].

11.2.5 Type IV Pili

The T4P is a filamentous structure that extends from the cell’s surface, usually from a cell

pole. The type IV (T4P) pili were first described in the 1970s for P. aeruginosa [18] and

afterward were characterized on many human pathogenic bacteria such as Vibrio cholerae,

174

Bioelectronics