Transient infection of eukaryotic cells with commensal and extraintestinal pathogenic Escherichia coli of phylogenetic group B2 blocks mitosis and induces megalocytosis. This trait is linked to a widely spread genomic island that encodes giant modular nonribosomal peptide and polyketide synthases. Contact with E. coli expressing this gene cluster causes DNA double-strand breaks and activation of the DNA damage checkpoint pathway, leading to cell cycle arrest and eventually to cell death. Discovery of hybrid peptide-polyketide genotoxins in E. coli will change our view on pathogenesis and commensalism and open new biotechnological applications.

The mammalian gut harbors a dense microbial community interacting in multiple ways, including horizontal gene transfer (HGT). Pangenome analyses established particularly high levels of genetic flux between Gram-negative Enterobacteriaceae . However, the mechanisms fostering intraenterobacterial HGT are incompletely understood. Using a mouse colitis model, we found that Salmonella -inflicted enteropathy elicits parallel blooms of the pathogen and of resident commensal Escherichia coli . These blooms boosted conjugative HGT of the colicin-plasmid p2 from Salmonella enterica serovar Typhimurium to E. coli . Transconjugation efficiencies of ∼100% in vivo were attributable to high intrinsic p2-transfer rates. Plasmid-encoded fitness benefits contributed little. Under normal conditions, HGT was blocked by the commensal microbiota inhibiting contact-dependent conjugation between Enterobacteriaceae . Our data show that pathogen-driven inflammatory responses in the gut can generate transient enterobacterial blooms in which conjugative transfer occurs at unprecedented rates. These blooms may favor reassortment of plasmid-encoded genes between pathogens and commensals fostering the spread of fitness-, virulence-, and antibiotic-resistance determinants.

A genomic island encoding the biosynthesis and secretion pathway of putative hybrid nonribosomal peptide-polyketide colibactin has been recently described in Escherichia coli . Colibactin acts as a cyclomodulin and blocks the eukaryotic cell cycle. The origin and prevalence of the colibactin island among enterobacteria are unknown. We therefore screened 1,565 isolates of different genera and species related to the Enterobacteriaceae by PCR for the presence of this DNA element. The island was detected not only in E. coli but also in Klebsiella pneumoniae , Enterobacter aerogenes , and Citrobacter koseri isolates. It was highly conserved among these species and was always associated with the yersiniabactin determinant. Structural variations between individual strains were only observed in an intergenic region containing variable numbers of tandem repeats. In E. coli , the colibactin island was usually restricted to isolates of phylogenetic group B2 and inserted at the asnW tRNA locus. Interestingly, in K. pneumoniae , E. aerogenes , C. koseri , and three E. coli strains of phylogenetic group B1, the functional colibactin determinant was associated with a genetic element similar to the integrative and conjugative elements ICE Ec1 and ICE Kp1 and to several enterobacterial plasmids. Different asn tRNA genes served as chromosomal insertion sites of the ICE-associated colibactin determinant: asnU in the three E. coli strains of ECOR group B1, and different asn tRNA loci in K. pneumoniae . The detection of the colibactin genes associated with an ICE-like element in several enterobacteria provides new insights into the spread of this gene cluster and its putative mode of transfer. Our results shed light on the mechanisms of genetic exchange between members of the family Enterobacteriaceae .

Nonpathogenic Escherichia coli strain Nissle 1917 (O6:K5:H1) is used as a probiotic agent in medicine, mainly for the treatment of various gastroenterological diseases. To gain insight on the genetic level into its properties of colonization and commensalism, this strain's genome structure has been analyzed by three approaches: (i) sequence context screening of tRNA genes as a potential indication of chromosomal integration of horizontally acquired DNA, (ii) sequence analysis of 280 kb of genomic islands (GEIs) coding for important fitness factors, and (iii) comparison of Nissle 1917 genome content with that of other E. coli strains by DNA-DNA hybridization. PCR-based screening of 324 nonpathogenic and pathogenic E. coli isolates of different origins revealed that some chromosomal regions are frequently detectable in nonpathogenic E. coli and also among extraintestinal and intestinal pathogenic strains. Many known fitness factor determinants of strain Nissle 1917 are localized on four GEIs which have been partially sequenced and analyzed. Comparison of these data with the available knowledge of the genome structure of E. coli K-12 strain MG1655 and of uropathogenic E. coli O6 strains CFT073 and 536 revealed structural similarities on the genomic level, especially between the E. coli O6 strains. The lack of defined virulence factors (i.e., alpha-hemolysin, P-fimbrial adhesins, and the semirough lipopolysaccharide phenotype) combined with the expression of fitness factors such as microcins, different iron uptake systems, adhesins, and proteases, which may support its survival and successful colonization of the human gut, most likely contributes to the probiotic character of E. coli strain Nissle 1917.

Uropathogenic Escherichia coli (UPEC) strain 536 (O6:K15:H31) is one of the model organisms of extraintestinal pathogenic E. coli (ExPEC). To analyze this strain's genetic basis of urovirulence, we sequenced the entire genome and compared the data with the genome sequence of UPEC strain CFT073 (O6:K2:H1) and to the available genomes of nonpathogenic E. coli strain MG1655 (K-12) and enterohemorrhagic E. coli . The genome of strain 536 is ≈292 kb smaller than that of strain CFT073. Genomic differences between both UPEC are mainly restricted to large pathogenicity islands, parts of which are unique to strain 536 or CFT073. Genome comparison underlines that repeated insertions and deletions in certain parts of the genome contribute to genome evolution. Furthermore, 427 and 432 genes are only present in strain 536 or in both UPEC, respectively. The majority of the latter genes is encoded within smaller horizontally acquired DNA regions scattered all over the genome. Several of these genes are involved in increasing the pathogens' fitness and adaptability. Analysis of virulence-associated traits expressed in the two UPEC O6 strains, together with genome comparison, demonstrate the marked genetic and phenotypic variability among UPEC. The ability to accumulate and express a variety of virulence-associated genes distinguishes ExPEC from many commensals and forms the basis for the individual virulence potential of ExPEC. Accordingly, instead of a common virulence mechanism, different ways exist among ExPEC to cause disease.