An inducible lambdoid prophage encoding cytolethal distending toxin (Cdt-I) and a type III effector protein in enteropathogenic Escherichia coli

doi:10.1073/pnas.0706695104

An inducible lambdoid prophage encoding cytolethal distending toxin (Cdt-I) and a type III effector protein in enteropathogenic Escherichia coli

*Graduate School of Life and Environmental Sciences, Osaka Prefecture University, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan;
^‡College of Light Industry and Food Technology, South China University of Technology, Guangzou 510640, Peoples Republic of China;
^§National Institute of Cholera and Enteric Diseases, Kolkata 700010, India; and
^†Molecular Genetics Laboratory, International Center for Diarrhoeal Disease Research, Dhaka-1212, Bangladesh

Contributed by G. Balakrish Nair, July 24, 2007 (received for review May 19, 2007)

Abstract

Cytolethal distending toxins (CDTs) are inhibitory cyclomodulins, which block eukaryotic cell proliferation and are produced by a diverse group of Gram-negative bacteria, including Escherichia coli strains associated with intestinal and extraintestinal infections. However, the mode of transmission of the toxin gene clusters among diverse bacterial pathogens is unclear. We found that Cdt-I produced by enteropathogenic E. coli strains associated with diarrhea is encoded by a lambdoid prophage, which is inducible and infectious. The genome of Cdt-I converting phage (CDT-1Φ) comprises 47,021 nucleotides with 60 predicted ORFs organized into six genomic regions encoding the head and tail, virulence, integrase, unknown functions, regulation, and lysis. The genomic organization of CDT-1Φ is similar to those of SfV, a serotype-converting phage of Shigella flexneri, and UTI89, a prophage identified in uropathogenic E. coli. Besides the cdtI gene cluster, the virulence region of CDT-1Φ genome contains sequences homologous to a truncated cycle inhibiting factor and a type 3 effector protein. Mutation analysis of susceptible E. coli strain C600 suggested that the outer membrane protein OmpC is a putative receptor for CDT-1Φ. CDT-1Φ genome was also found to integrate into the host bacterial chromosome forming lysogens, which produced biologically active Cdt-I. Furthermore, phage induction appeared to cause enhanced toxigenicity of the E. coli strains carrying lysogenic CDT-1Φ. Our results suggest that CDT-1Φ is the latest member of a growing family of lambdoid phages encoding bacterial cyclomodulins and that the phage may have a role in horizontal transfer of these virulence genes.

Footnotes

^¶To whom correspondence may be addressed. E-mail: gbnair{at}icddrb.org or shinji{at}vet.osakafu-u.ac.jp
Author contributions: S.M.F. and S.Y. designed research; M.A., A.H., M.S.A., K.S., S.H.Z., L.S., N.S., A.N.G., T.R., and S.Y. performed research; M.A., A.H., M.S.A., A.N.G., S.M.F., G.B.N., and S.Y. analyzed data; and S.M.F., G.B.N., and S.Y. wrote the paper.
The authors declare no conflict of interest.
Data deposition: The sequence reported in this paper has been deposited in the DNA Data Base of Japan (accession no. AB285204).
This article contains supporting information online at www.pnas.org/cgi/content/full/0706695104/DC1.
Abbreviations:

CDT,

cytolethal distending toxin;

CDT-1Φ,

Cdt-I converting phage;

Stx,

Shiga toxin;

Cif,

cycle inhibiting factor;

EPEC,

enteropathogenic Escherichia coli;

LEE,

locus of enterocyte effacement;

CF,

cell-associated fraction;

SF,

supernatant fraction;

CTEC,

CDT-producing Escherichia coli.