Prior to the epidemic that emerged in Haiti in October of 2010, cholera had not been documented in this country. After its introduction, a strain of Vibrio cholerae O1 spread rapidly throughout Haiti, where it caused over 600,000 cases of disease and >7,500 deaths in the first two years of the epidemic. We applied whole-genome sequencing to a temporal series of V. cholerae isolates from Haiti to gain insight into the mode and tempo of evolution in this isolated population of V. cholerae O1. Phylogenetic and Bayesian analyses supported the hypothesis that all isolates in the sample set diverged from a common ancestor within a time frame that is consistent with epidemiological observations. A pangenome analysis showed nearly homogeneous genomic content, with no evidence of gene acquisition among Haiti isolates. Nine nearly closed genomes assembled from continuous-long-read data showed evidence of genome rearrangements and supported the observation of no gene acquisition among isolates. Thus, intrinsic mutational processes can account for virtually all of the observed genetic polymorphism, with no demonstrable contribution from horizontal gene transfer (HGT). Consistent with this, the 12 Haiti isolates tested by laboratory HGT assays were severely impaired for transformation, although unlike previously characterized noncompetent V. cholerae isolates, each expressed hapR and possessed a functional quorum-sensing system. Continued monitoring of V. cholerae in Haiti will illuminate the processes influencing the origin and fate of genome variants, which will facilitate interpretation of genetic variation in future epidemics.
Vibrio cholerae is the cause of substantial morbidity and mortality worldwide, with over three million cases of disease each year. An understanding of the mode and rate of evolutionary change is critical for proper interpretation of genome sequence data and attribution of outbreak sources. The Haiti epidemic provides an unprecedented opportunity to study an isolated, single-source outbreak of Vibrio cholerae O1 over an established time frame. By using multiple approaches to assay genetic variation, we found no evidence that the Haiti strain has acquired any genes by horizontal gene transfer, an observation that led us to discover that it is also poorly transformable. We have found no evidence that environmental strains have played a role in the evolution of the outbreak strain.
Salmonella enterica serovar Heidelberg has caused numerous outbreaks in humans. Here, we report draft genomes of five isolates of serovar Heidelberg associated with the recent (2011) multistate outbreak linked to ground turkey in the United States. Isolates 2011K-1110 and 2011K-1132 were recovered from humans, while isolates 2011K-1138, 2011K-1224, and 2011K-1225 were recovered from ground turkey. Whole-genome sequence analysis of these isolates provides a tool for studying the short-term evolution of these epidemic clones.
To increase understanding of drug-resistant Vibrio cholerae, we studied selected molecular mechanisms of antimicrobial drug resistance in the 2010 Haiti V. cholerae outbreak strain. Most resistance resulted from acquired genes located on an integrating conjugative element showing high homology to an integrating conjugative element identified in a V. cholerae isolate from India.
Vibrio cholerae; cholera; antimicrobial drug resistance; bacteria; Haiti; outbreak; strain; O1
We characterized 20 Shigella isolates with decreased susceptibility to fluoroquinolones. Most patients (80%) from whom a travel history was obtained reported travel to South or Southeast Asia. Mutations within the quinolone resistance determining regions of gyrA and parC and plasmid-mediated resistance determinants (qnrB, qnrS, and aac(6′)-Ib-cr) were identified. The rise in antimicrobial resistance among Shigella isolates may necessitate the increased use of extended-spectrum cephalosporins or macrolides in some patients.
CTX-M–type β-lactamases are increasing among US Enterobacteriaceae isolates. Of 2,165 non-Typhi Salmonella isolates submitted in 2007 to the National Antimicrobial Resistance Monitoring System, 100 (4.6%) displayed elevated MICs (>2 mg/L) of ceftriaxone or ceftiofur. Three isolates (serotypes Typhimurium, Concord, and I 4,5,12:i:–) contained blaCTX-M-5, blaCTX-M-15, and blaCTX-M-55/57, respectively.
Salmonella; antimicrobial resistance; CTX-M beta-lactamase; bacteria; United States; dispatch
Plasmid-mediated quinolone resistance determinants are emerging among gram-negative pathogens. Here we report results of a retrospective study investigating the prevalence of aac(6′)-Ib-cr, qepA, and qnr genes among 19,010 human isolates of non-Typhi Salmonella enterica collected in the United States from 1996 to 2006.
The MtrR transcriptional-regulatory protein is known to repress transcription of the mtrCDE operon, which encodes a multidrug efflux pump possessed by Neisseria gonorrhoeae that is important in the ability of gonococci to resist certain hydrophobic antibiotics, detergents, dyes, and host-derived antimicrobials. In order to determine whether MtrR can exert regulatory action on other gonococcal genes, we performed a whole-genome microarray analysis using total RNA extracted from actively growing broth cultures of isogenic MtrR-positive and MtrR-negative gonococci. We determined that, at a minimum, 69 genes are directly or indirectly subject to MtrR control, with 47 being MtrR repressed and 22 being MtrR activated. rpoH, which encodes the general stress response sigma factor RpoH (sigma 32), was found by DNA-binding studies to be directly repressed by MtrR, as it was found to bind to a DNA sequence upstream of rpoH that included sites within the rpoH promoter. MtrR also repressed the expression of certain RpoH-regulated genes, but this regulation was likely indirect and a reflection of MtrR control of rpoH expression. Inducible expression of MtrR was found to repress rpoH expression and to increase gonococcal susceptibility to hydrogen peroxide (H2O2) and an antibiotic (erythromycin) recognized by the MtrC-MtrD-MtrE efflux pump system. We propose that, apart from its ability to control the expression of the mtrCDE-encoded efflux pump operon and, as a consequence, levels of gonococcal resistance to host antimicrobials (e.g., antimicrobial peptides) recognized by the efflux pump, the ability of MtrR to regulate the expression levels of rpoH and RpoH-regulated genes also modulates levels of gonococcal susceptibility to H2O2.
Neisseria gonorrhoeae utilizes the mtrCDE-encoded efflux pump system to resist not only host-derived, hydrophobic antimicrobials that bathe mucosal surfaces, which likely aids in its ability to colonize and infect numerous sites within the human host, but also antibiotics that have been used clinically to treat infections. Recently, overexpression of the MtrC-MtrD-MtrE efflux pump was shown to be critically involved in the capacity of gonococci to develop chromosomally mediated resistance to penicillin G, which for over 40 years was used to treat gonococcal infections. Mutations in either the promoter or the coding sequence of the mtrR gene, which encodes a repressor of the efflux pump operon, decrease gonococcal susceptibility to penicillin. We now describe the capacity of MtrR to directly or indirectly influence the expression of two other loci that are involved in gonococcal susceptibility to penicillin: ponA, which encodes penicillin-binding protein 1 (PBP 1), and the pilMNOPQ operon, which encodes components of the type IV pilus secretion system, with PilQ acting as a channel for entry for penicillin. We determined that MtrR increases the expression of ponA directly or indirectly, resulting in increased levels of PBP 1, while repressing the expression of the divergently transcribed pilM gene, the first gene in the pilMNOPQ operon. Taken together with other studies, the results presented herein indicate that transcriptional regulation of gonococcal genes by MtrR is centrally involved in determining levels of gonococcal susceptibility to penicillin and provides a framework for understanding how resistance developed over the years.
The obligate human pathogen Neisseria gonorrhoeae uses the MtrC-MtrD-MtrE efflux pump to resist structurally diverse hydrophobic antimicrobial agents (HAs), some of which bathe mucosal surfaces that become infected during transmission of gonococci. Constitutive high-level HA resistance occurs by the loss of a repressor (MtrR) that negatively controls transcription of the mtrCDE operon. This high-level HA resistance also requires the product of the mtrF gene, which is located downstream and transcriptionally divergent from mtrCDE. MtrF is a putative inner membrane protein, but its role in HA resistance mediated by the MtrC-MtrD-MtrE efflux pump remains to be determined. High-level HA resistance can also be mediated through an induction process that requires enhanced transcription of mtrCDE when gonococci are grown in the presence of a sublethal concentration of Triton X-100. We now report that inactivation of mtrF results in a significant reduction in the induction of HA resistance and that the expression of mtrF is enhanced when gonococci are grown under inducing conditions. However, no effect was observed on the induction of mtrCDE expression in an MtrF-negative strain. The expression of mtrF was repressed by MtrR, the major repressor of mtrCDE expression. In addition to MtrR, another repressor (MpeR) can downregulate the expression of mtrF. Repression of mtrF by MtrR and MpeR was additive, demonstrating that the repressive effects mediated by these regulators are independent processes.
ChiA, an 88-kDa endochitinase encoded by the chiA gene of the gram-negative enteropathogen Vibrio cholerae, is secreted via the eps-encoded main terminal branch of the general secretory pathway (GSP), a mechanism which also transports cholera toxin. To localize the extracellular transport signal of ChiA that initiates transport of the protein through the GSP, a chimera comprised of ChiA fused at the N terminus with the maltose-binding protein (MalE) of Escherichia coli and fused at the C terminus with a 13-amino-acid epitope tag (E-tag) was expressed in strain 569B(chiA::Kanr), a chiA-deficient but secretion-competent mutant of V. cholerae. Fractionation studies revealed that blockage of the natural N terminus and C terminus of ChiA did not prevent secretion of the MalE-ChiA-E-tag chimera. To locate the amino acid sequences which encoded the transport signal, a series of truncations of ChiA were engineered. Secretion of the mutant polypeptides was curtailed only when ChiA was deleted from the N terminus beyond amino acid position 75 or from the C terminus beyond amino acid 555. A mutant ChiA comprised of only those amino acids was secreted by wild-type V. cholerae but not by an epsD mutant, establishing that amino acids 75 to 555 independently harbored sufficient structural information to promote secretion by the GSP of V. cholerae. Cys77 and Cys537, two cysteines located just within the termini of ChiA(75-555), were not required for secretion, indicating that those residues were not essential for maintaining the functional activity of the ChiA extracellular transport signal.
The chiA gene of Vibrio cholerae encodes a polypeptide which degrades chitin, a homopolymer of N-acetylglucosamine (GlcNAc) found in cell walls of fungi and in the integuments of insects and crustaceans. chiA has a coding capacity corresponding to a polypeptide of 846 amino acids having a predicted molecular mass of 88.7 kDa. A 52-bp region with promoter activity was found immediately upstream of the chiA open reading frame. Insertional inactivation of the chromosomal copy of the gene confirmed that expression of chitinase activity by V. cholerae required chiA. Fluorescent analogues were used to demonstrate that the enzymatic activity of ChiA was specific for β,1-4 glycosidic bonds located between GlcNAc monomers in chitin. Antibodies against ChiA were obtained by immunization of a rabbit with a MalE-ChiA hybrid protein. Polypeptides with antigenic similarity to ChiA were expressed by classical and El Tor biotypes of V. cholerae and by the closely related bacterium Aeromonas hydrophila. Immunoblotting experiments using the wild-type strain 569B and the secretion mutant M14 confirmed that ChiA is an extracellular protein which is secreted by the eps system. The eps system is also responsible for secreting cholera toxin, an oligomeric protein with no amino acid homology to ChiA. These results indicate that ChiA and cholera toxin have functionally similar extracellular transport signals that are essential for eps-dependent secretion.
In May of 2011, an enteroaggregative Escherichia coli O104:H4 strain that had acquired a Shiga toxin 2-converting phage caused a large outbreak of bloody diarrhea in Europe which was notable for its high prevalence of hemolytic uremic syndrome cases. Several studies have described the genomic inventory and phylogenies of strains associated with the outbreak and a collection of historical E. coli O104:H4 isolates using draft genome assemblies. We present the complete, closed genome sequences of an isolate from the 2011 outbreak (2011C–3493) and two isolates from cases of bloody diarrhea that occurred in the Republic of Georgia in 2009 (2009EL–2050 and 2009EL–2071). Comparative genome analysis indicates that, while the Georgian strains are the nearest neighbors to the 2011 outbreak isolates sequenced to date, structural and nucleotide-level differences are evident in the Stx2 phage genomes, the mer/tet antibiotic resistance island, and in the prophage and plasmid profiles of the strains, including a previously undescribed plasmid with homology to the pMT virulence plasmid of Yersinia pestis. In addition, multiphenotype analysis showed that 2009EL–2071 possessed higher resistance to polymyxin and membrane-disrupting agents. Finally, we show evidence by electron microscopy of the presence of a common phage morphotype among the European and Georgian strains and a second phage morphotype among the Georgian strains. The presence of at least two stx2 phage genotypes in host genetic backgrounds that may derive from a recent common ancestor of the 2011 outbreak isolates indicates that the emergence of stx2 phage-containing E. coli O104:H4 strains probably occurred more than once, or that the current outbreak isolates may be the result of a recent transfer of a new stx2 phage element into a pre-existing stx2-positive genetic background.