A Degenerate Type III Secretion System from Septicemic Escherichia coli Contributes to Pathogenesis

Abstract

The type III secretion system (T3SS) is an important virulence factor used by several gram-negative bacteria to deliver effector proteins which subvert host cellular processes. Enterohemorrhagic Escherichia coli O157 has a well-defined T3SS involved in attachment and effacement (ETT1) and critical for virulence. A gene cluster potentially encoding an additional T3SS (ETT2), which resembles the SPI-1 system in Salmonella enterica, was found in its genome sequence. The ETT2 gene cluster has since been found in many E. coli strains, but its in vivo role is not known. Many of the ETT2 gene clusters carry mutations and deletions, raising the possibility that they are not functional. Here we show the existence in septicemic E. coli strains of an ETT2 gene cluster, ETT2 sepsis , which, although degenerate, contributes to pathogenesis. ETT2 sepsis has several premature stop codons and a large (5 kb) deletion, which is conserved in 11 E. coli strains from cases of septicemia and newborn meningitis. A null mutant constructed to remove genes coding for the putative inner membrane ring of the secretion complex exhibited significantly reduced virulence. These results are the first demonstration of the importance of ETT2 for pathogenesis.Type III secretion systems (T3SSs) are utilized by gramnegative pathogenic bacteria to deliver target-specific effector proteins into eukaryotic cells (14). This energy-requiring system is composed of more than 20 largely conserved proteins, which form a secretion apparatus for direct transfer of effector proteins from bacterial cytoplasm into the host cell or the medium. These effectors interfere with specific cellular processes and facilitate bacterial survival and replication. Typical features of the T3SS are the lack of an obvious secretion signal in the delivered proteins and the requirement for binding of a chaperone to the substrate before secretion (11,16).Previously characterized T3SSs in members of the family Enterobacteriaceae are composed of connected inner and outer membrane rings and a needle, through which the effector proteins are most likely passed into the host cytosol. Proteins translocated by this system promote critical processes such as bacterial internalization by mammalian cells in Salmonella and Shigella spp. (34, 36), induction of macrophage apoptosis in Salmonella, Shigella, and Yersinia spp. (24,27), and creation of attachment and effacement lesions in enteropathogenic and enterohemorrhagic Escherichia coli (18).In enteropathogenic and enterohemorrhagic E. coli, a chromosomal pathogenicity island (PAI) called the locus of enterocyte effacement contains genes encoding the components of T3SS which are involved in the formation of attaching and effacing lesions (18).The published sequences of the enterohemorrhagic E. coli O157:H7 genome revealed the presence of an additional putative T3SS (13, 30), which has not been previously described. This type III secretion system was termed ETT2, for E. coli type III secretion system 2 (13), to distinguish it from the locus of e...

Sequencing and Functional Annotation of Avian Pathogenic Escherichia coli Serogroup O78 Strains Reveal the Evolution of E. coli Lineages Pathogenic for Poultry via Distinct Mechanisms

“…Among the virulence-associated loci present in 7122 and IMT2125 but absent from APEC O1 is the degenerate type III secretion locus ETT2, located between yqeG and the glyU tRNA locus in many E. coli strains (63). The ETT2 locus in 7122 and IMT2125 is truncated from within eivA to the end of the locus described for E. coli O157:H7 strain Sakai, consistent with the truncation described for another sepsis-associated APEC O78 strain, termed 789 (30). Within ETT2, the predicted spaS and eivJ genes contain distinct frameshift mutations in 7122 and IMT2125, consistent with the varied patterns of mutational attrition of the locus reported among E. coli strains (30,63).…”

“…The ETT2 locus in 7122 and IMT2125 is truncated from within eivA to the end of the locus described for E. coli O157:H7 strain Sakai, consistent with the truncation described for another sepsis-associated APEC O78 strain, termed 789 (30). Within ETT2, the predicted spaS and eivJ genes contain distinct frameshift mutations in 7122 and IMT2125, consistent with the varied patterns of mutational attrition of the locus reported among E. coli strains (30,63). The ymcDCBA-yccZ-etp-etk genes, which determine O-antigen capsule biosynthesis (located between appA and yccM) (64), were conserved in both 7122 and IMT2125 but absent in APEC O1.…”

“…Analysis of the same APEC O2: K1:H5 mutant library in a low-dose model of chicken lung colonization identified a novel fimbrial locus, encoding ExPEC adhesin I (yqi), which influences virulence (20,21). A number of other virulence factors have been identified by using defined mutants in poultry models, including the autotransported proteins AatA (22), Tsh (23,24), and Vat (25), fimbrial gene clusters pap (26), fim (27,28), stg (29), and csg (28), a degenerate inv/mxi-spa-like type III secretion locus termed ETT2 (30), and factors mediating iron uptake (31)(32)(33) and phosphate transport (34). Flagellin contributes to persistence and invasion after oral dosing of chicks with APEC O78 (28), and ExPEC invasins (Ibe proteins) also facilitate APEC invasion (35,36).…”

Colibacillosis in poultry: unravelling the molecular basis of virulence of avian pathogenicEscherichia coliin their natural hosts

“…Subtractive hybridization studies with an APEC O78 strain first revealed a gene of the degenerate Type III secretion system (ETT2) (Mokady et al, 2005). Subsequent work established the presence of an ETT2 cluster among APEC strains that underwent mutational attrition but remained essential for APEC virulence (Ideses et al, 2005b). In E. coli O157:H7, the cryptic ETT2 island does not function as a protein secretion system but as a regulator of other virulence determinants (Zhang et al, 2004).…”

Harpin Mediates Cell Aggregation in Erwinia chrysanthemi 3937

“…Pellicle formation in 3937 requires a functional type III secretion system (T3SS) (23). A similar phenomenon has been observed in Escherichia coli, where a degenerate T3SS is required for both virulence and bacterial aggregation (10). However, based on experiments with another plant pathogen, Ralstonia solanacearum, it does not appear that the T3SS of plant pathogens contributes to bacterial adhesion to host cells (20).…”