A silent regulatory gene cfaD' on region 1 of the CFA/I plasmid NTP 113 of enterotoxigenic Escherichia coli

A large family of antivirulence regulators modulates the effects of transcriptional activators in Gram-negative pathogenic bacteria

We have reported that transcription of a hypothetical small open reading frame (orf60) in enteroaggregative E. coli (EAEC) strain 042 is impaired after mutation of aggR, which encodes a global virulence activator. We have also reported that the cryptic orf60 locus was linked to protection against EAEC diarrhea in two epidemiologic studies. Here, we report that the orf60 product acts as a negative regulator of aggR itself. The orf60 protein product lacks homology to known repressors, but displays 44–100% similarity to at least fifty previously undescribed small (<10 kDa) hypothetical proteins found in many gram negative pathogen genomes. Expression of orf60 homologs from enterotoxigenic E. coli (ETEC) repressed the expression of the AraC-transcriptional ETEC regulator CfaD/Rns and its regulon in ETEC strain H10407. Complementation in trans of EAEC 042orf60 by orf60 homologs from ETEC and the mouse pathogen Citrobacter rodentium resulted in dramatic suppression of aggR. A C. rodentium orf60 homolog mutant showed increased levels of activator RegA and increased colonization of the adult mouse. We propose the name Aar (AggR-activated regulator) for the clinically and epidemiologically important orf60 product in EAEC, and postulate the existence of a large family of homologs among pathogenic Enterobacteriaceae and Pasteurellaceae. We propose the name ANR (AraC Negative Regulators) for this family.

We report here the identification and characterization of a new family of negative regulators in Gram-negative bacteria, including many pathotypes of diarrheagenic Enterobacteriaceae and members of the Pasteurellaceae. Members of this regulator family in enteroaggregative (EAEC) and enterotoxigenic E. coli (ETEC) and in Citrobacter rodentium downregulate the expression of positive regulator partners AggR, CfaD/Rns and RegA, respectively, all members of the AraC/XylS family of regulators. Accordingly, we propose the name ANR (AraC Negative Regulators) for this family. ANR members orf60 (termed Aar), orf02851 (Rnr), orf0450 and orf01070 (Cnr) from EAEC, C. rodentium and ETEC respectively were characterized in this study. Deletion of ANR homologs upregulated the expression of AggR and RegA in EAEC strain 042 and C. rodentium respectively; overexpression of orf60, orf02851, orf0450 and orf01070 in EAEC strain 042 down-regulated AggR. C. rodentium harboring a null mutation in orf02851 exhibited a significant increase in expression of the regA and RegA-regulated fimbriae. The orf02851 mutant showed higher levels of C. rodentium in feces and colonic contents, and greater weight loss compared to mice inoculated with the wild-type.

The complete nucleotide sequence of region 1 of the CFA/I fimbria! operon of human enterotoxigenicEscherichia coli

The production of the plasmid-encoded fimbrial antigen CFA/I of enterotoxigenic Escherichia coli requires two DNA regions: CFA/I region 1 and CFA/I region 2. These two regions are separated by about 40 kb on the wildtype plasmid. CFA/I region 1 contains the structural genes, whereas CFA/I region 2 contains a positive regulator. The first two genes (cfaA and cfaB) and the cfaD' sequence of region 1 have already been described. Here the total nucleotide sequence of region 1 is presented. Two new genes in region 1 are described, named cfaC and cfaE. The GC content of the genes in region 1 is 33.6% which is substantially lower than normally found in E. coli genes (50%). The codon usage also differs from the standard codons used in E. coli.

Genetic characterization and immunogenicity of coli surface antigen 4 from enterotoxigenic Escherichia coli when it is expressed in a Shigella live-vector strain

The genes that encode the enterotoxigenic Escherichia coli (ETEC) CS4 fimbriae, csaA, -B, -C, -E, and -D', were isolated from strain E11881A. The csa operon encodes a 17-kDa major fimbrial subunit (CsaB), a 40-kDa tip-associated protein (CsaE), a 27-kDa chaperone-like protein (CsaA), a 97-kDa usher-like protein (CsaC), and a deleted regulatory protein (CsaD'). The predicted amino acid sequences of the CS4 proteins are highly homologous to structural and assembly proteins of other ETEC fimbriae, including CS1 and CS2, and to CFA/I in particular. The csaA, -B, -C, -E operon was cloned on a stabilized plasmid downstream from an osomotically regulated ompC promoter. pGA2-CS4 directs production of CS4 fimbriae in both E. coli DH5alpha and Shigella flexneri 2a vaccine strain CVD 1204, as detected by Western blot analysis and bacterial agglutination with anti-CS4 immune sera. Electron-microscopic examination of Shigella expressing CS4 confirmed the presence of fimbriae on the bacterial surface. Guinea pigs immunized with CVD 1204(pGA2-CS4) showed serum and mucosal antibody responses to both the Shigella vector and the ETEC fimbria CS4. Among the seven most prevalent fimbrial antigens of human ETEC, CS4 is the last to be cloned and sequenced. These findings pave the way for CS4 to be included in multivalent ETEC vaccines, including an attenuated Shigella live-vector-based ETEC vaccine.

Expression of colonization factor antigen I fimbriae by enterotoxigenic Escherichia coli; influence of growth conditions and a recombinant positive regulatory gene

Enterotoxigenic Escherichia coli (ETEC) may spontaneously lose the positive regulatory cfaR gene and thereby the capacity to express colonization factor antigen I (CFA/I). A recombinant plasmid harbouring the cfaR gene was transformed into cfaR-negative mutant ETEC strains. CFA/I expression of wild-type and cfaR-transformed ETEC cultivated in different liquid media was quantified. At 37 degrees C, a high level of CFA/I expression from wild-type and cfaR-transformed strains was observed after growth in CFA broth. Transformation enhanced CFA/I expression only marginally. The transformant cultures showed a considerable variation in CFA/I expression which was paralleled by the proportion of individual bacteria producing CFA/I. This heterogeneity could be explained by a variable tendency to structural CFA/I gene loss among individual cfaR-transformed bacteria.

CooC and CooD are required for assembly of CS1 pili

Many strains of enterotoxigenic Escherichia coli (ETEC) isolated from patients with diarrhoeal disease exhibit CS1 pili on their surfaces. These appendages, which are thought to be important for colonization of the upper intestine, are composed largely of multiple identical protein subunits encoded by cooA. We have sequenced the DNA directly downstream of cooA and identified two open reading frames, cooC and cooD, transcribed in the same direction as cooB and cooA. Following cooD is DNA homologous to an insertion sequence, so cooB, A, C and D appear to encode all the information needed for E. coli K-12 to synthesize CS1 pili. Complementation analysis of mutants cloned in E. coli K-12 and constructed in an ETEC-derived strain indicates that cooC and cooD are not required for stability of the major CS1 pilin protein or for its transport to the periplasm, but, like cooB, both are needed for assembly of cooA into pili.

Induction of colonization factor antigen I (CFA/I) and coli surface antigen 4 (CS4) of enterotoxigenic Escherichia coli: relevance for vaccine production

Regulatory proteins control the expression of the fimbrial colonization factor antigens CFA/I and CS4 of enterotoxigenic Escherichia coli (ETEC). To examine the mechanism behind lack of expression of these antigens in spontaneous CFA-negative mutants, we mobilized a recombinant plasmid harbouring the cfaD gene, which encodes a positive regulator of CFA/I and CS4 expression, into such derivatives. In electron microscopy, the induced surface structures were morphologically identical to the fimbriae of the CFA/I+ and CS4+ wild type strains. Immunogold labelling with monoclonal antibodies showed that the distribution of CFA/I and CS4 specific epitopes along the induced fimbriae was indistinguishable from that of the wild type strains. The percentage of fimbriated cells was consistently higher in the cfaD transformants than in the corresponding wild type strains. The present work reports on the efficiency of the cloned cfaD gene in restoring and enhancing the production of morphologically intact CFA/I and CS4 fimbriae.

Presence of cfaD-homologous sequences and expression of coli surface antigen 4 on enterotoxigenic Escherichia coli; relevance for diagnostic procedures

We examined the ability of a colonization factor antigen I (CFA/I) polynucleotide probe to identify coli-surface antigen 4 producing (CS4+) strains of enterotoxigenic Escherichia coli (ETEC). At low stringency (LS) the probe hybridized to colony lysates of strains previously shown to produce CS4 or CFA/I fimbriae. Only DNA from CFA/I+ strains maintained a stable probe-target hybrid under high stringency (HS) conditions. On examination of several clones from three previous CS4 producers, identified as positive in LS and negative in HS colony hybridization, spontaneous loss of nucleotide sequences homologous to a gene encoding a positive CFA/I regulator, CfaD, was found to be associated with lacking expression of CS4. Our findings indicate that, on stored or subcultured isolates of ETEC, identification of CS4 strains may benefit from applying gene probe technology.