The World Health Organization has declared the emergence of antibiotic resistance to be a global threat to human health. Broad-host-range plasmids have a key role in causing this health crisis because they transfer multiple resistance genes to a wide range of bacteria. To limit the spread of antibiotic resistance, we need to gain insight into the mechanisms by which the host range of plasmids evolves. Although initially unstable plasmids have been shown to improve their persistence through evolution of the plasmid, the host, or both, the means by which this occurs are poorly understood. Here, we sought to identify the underlying genetic basis of expanded plasmid host-range and increased persistence of an antibiotic resistance plasmid using a combined experimental-modeling approach that included whole-genome resequencing, molecular genetics and a plasmid population dynamics model. In nine of the ten previously evolved clones, changes in host and plasmid each slightly improved plasmid persistence, but their combination resulted in a much larger improvement, which indicated positive epistasis. The only genetic change in the plasmid was the acquisition of a transposable element from a plasmid native to the Pseudomonas host used in these studies. The analysis of genetic deletions showed that the critical genes on this transposon encode a putative toxin-antitoxin (TA) and a cointegrate resolution system. As evolved plasmids were able to persist longer in multiple naïve hosts, acquisition of this transposon also expanded the plasmid's host range, which has important implications for the spread of antibiotic resistance.

c Vibrio cholerae, an etiological agent of cholera, circulates between aquatic reservoirs and the human gastrointestinal tract. The type II secretion (T2S) system plays a pivotal role in both stages of the lifestyle by exporting multiple proteins, including cholera toxin. Here, we studied the kinetics of expression of genes encoding the T2S system and its cargo proteins. We have found that under laboratory growth conditions, the T2S complex was continuously expressed throughout V. cholerae growth, whereas there was growth phase-dependent transcriptional activity of genes encoding different cargo proteins. Moreover, exposure of V. cholerae to different environmental cues encountered by the bacterium in its life cycle induced transcriptional expression of T2S. Subsequent screening of a V. cholerae genomic library suggested that E stress response, phosphate metabolism, and the second messenger 3=,5=-cyclic diguanylic acid (c-di-GMP) are involved in regulating transcriptional expression of T2S. Focusing on E , we discovered that the upstream region of the T2S operon possesses both the consensus E and 70 signatures, and deletion of the E binding sequence prevented transcriptional activation of T2S by RpoE. Ectopic overexpression of E stimulated transcription of T2S in wild-type and isogenic ⌬rpoE strains of V. cholerae, providing additional support for the idea that the T2S complex belongs to the E regulon. Together, our results suggest that the T2S pathway is characterized by the growth phase-dependent expression of genes encoding cargo proteins and requires a multifactorial regulatory network to ensure appropriate kinetics of the secretory traffic and the fitness of V. cholerae in different ecological niches.

The direct major histocompatibility complex (MHC) class I antigen presentation pathway ensures intracellular peptides are displayed at the cellular surface for recognition of infected or transformed cells by CD8 ؉ cytotoxic T lymphocytes. Chlamydia spp. are obligate intracellular bacteria and, as such, should be targeted by CD8 ؉ T cells. It is likely that Chlamydia spp. have evolved mechanisms to avoid the CD8 ؉ killer T cell responses by interfering with MHC class I antigen presentation. Using a model system of self-peptide presentation which allows for posttranslational control of the model protein's stability, we tested the ability of various Chlamydia species to alter direct MHC class I antigen presentation. Infection of the JY lymphoblastoid cell line limited the accumulation of a model host protein and increased presentation of the model-protein-derived peptides. Enhanced selfpeptide presentation was detected only when presentation was restricted to defective ribosomal products, or DRiPs, and total MHC class I levels remained unaltered. Skewed antigen presentation was dependent on a bacterial synthesized component, as evidenced by reversal of the observed phenotype upon preventing bacterial transcription, translation, and the inhibition of bacterial lipooligosaccharide synthesis. These data suggest that Chlamydia spp. have evolved to alter the host antigen presentation machinery to favor presentation of defective and rapidly degraded forms of self-antigen, possibly as a mechanism to diminish the presentation of peptides derived from bacterial proteins.

The direct MHC class I antigen presentation pathway presents endogenous peptides at the cell surface, allowing the body to mount a CD8+ T cell response in order to eliminate diseased or damaged cells. In order to target this pathway for disease treatment, we must first understand the efficiency of antigen presentation, or the number of proteins that must be degraded in order to present one peptide-MHC complex at the cell surface. Previous studies have primarily examined the efficiency of proteins produced by infectious agents with values ranging from 0.03%-25. In this study, we investigated antigen presentation efficiency of different recombinant fusion proteins expressed in a human lymphoblastoid cell line expressed as self-proteins. These proteins are composed of a destabilization domain, an antigenic peptide that is presented on MHC class I HLA-A2 molecules, and a fluorescent reporter. We calculated the efficiency of antigen presentation in JY cells to be 0.35%. This value is considerably lower than what was previously observed with similar constructs in murine EL4 cells. We also found that treatment of the cells with either TLR agonists or IFNg, which have both been shown to increase MHC class I expression, did not alter the efficiency of antigen presentation. These findings suggest that the efficiency of MHC class I antigen presentation may be mechanistically distinct from common immune signaling pathways.

The direct MHC class I antigen presentation pathway presents cytosolic proteins for immune surveillance. While this pathway is an attractive target for therapeutics, we must first understand the efficiency of antigen presentation. Previous studies have shown the presentation efficiency from pathogen-derived antigens ranges from 0.03–25%. The intracellular lifestyle of Chlamydia makes them an attractive model to study class I presentation. It is likely that Chlamydia have developed mechanisms to evade CD8+ T cells through manipulation of this pathway. Here, we investigate the impact of Chlamydial infection on a recombinant fusion protein expressed in a human lymphoblastoid cell line (JY). The fusion protein contains a destabilization domain, an antigenic peptide, and a reporter. Chlamydial infection led to decreased accumulation of the model protein and increased presentation of model protein-derived antigens. This enhanced self-peptide presentation was only observed when antigen presentation was restricted to the DRiP form of the protein. We also investigated the antigen presentation efficiency of different model recombinant proteins in JY cells. We calculated the antigen presentation efficiency to be 0.35%, much lower than was previously observed in murine cells with a similar protein. The skewed antigen presentation phenotype observed following Chlamydial infection was not statistically different from the efficiency observed in the absence of infection. These data suggest that Chlamydia have evolved a mechanism to skew the host antigen presentation machinery toward presentation of DRiPs, and that the efficiency of direct class I presentation may be mechanistically unique among common immune signaling pathways.