Ethanolamine metabolism through two genetically distinct loci enables Klebsiella pneumoniae to bypass nutritional competition in the gut

Successful microbial colonization of the gastrointestinal (GI) tract hinges on an organism's ability to overcome the intense competition for nutrients in the gut between the host and the resident gut microbiome. Enteric pathogens can exploit ethanolamine (EA) in the gut to bypass nutrient competition. However, Klebsiella pneumoniae (K. pneumoniae) is an asymptomatic gut colonizer and, unlike well-studied enteric pathogens, harbors two genetically distinct ethanolamine utilization (eut) loci. Our investigation uncovered unique roles for each eut locus depending on EA utilization as a carbon or nitrogen source. Murine gut colonization studies demonstrated the necessity of both eut loci in the presence of intact gut microbiota for robust GI colonization by K. pneumoniae. Additionally, while some Escherichia coli gut isolates could metabolize EA, other commensals were incapable, suggesting that EA metabolism likely provides K. pneumoniae a selective advantage in gut colonization. Molecular and bioinformatic analyses unveiled the conservation of two eut loci among K. pneumoniae and a subset of the related taxa in the K. pneumoniae species complex, with the NtrC-RpoN regulatory cascade playing a pivotal role in regulation. These findings identify EA metabolism as a critical driver of K. pneumoniae niche establishment in the gut and propose microbial metabolism as a potential therapeutic avenue to combat K. pneumoniae infections.

Klebsiella pneumoniae l-Fucose Metabolism Promotes Gastrointestinal Colonization and Modulates Its Virulence Determinants

Colonization of the gastrointestinal (GI) tract by Klebsiella pneumoniae is generally considered asymptomatic. However, gut colonization allows K. pneumoniae to either translocate to sterile site within the same host or transmit through the fecal-oral route to another host. K. pneumoniae gut colonization is poorly understood, but knowledge of this first step toward infection and spread is critical for combatting its disease manifestations. K. pneumoniae must overcome colonization resistance (CR) provided by the host microbiota to establish itself within the gut. One such mechanism of CR is through nutrient competition. Pathogens that metabolize a broad range of substrates have the ability to bypass nutrient competition and overcome CR. Herein, we demonstrate that in response to mucin-derived fucose, the conserved fucose metabolism operon (fuc) of K. pneumoniae is upregulated in the murine gut, and we subsequently show that fucose metabolism promotes robust gut colonization. Growth studies using cecal filtrate as a proxy for the gut lumen illustrate the growth advantage that the fuc operon provides K. pneumoniae. We further show that fucose metabolism allows K. pneumoniae to be competitive with a commensal Escherichia coli isolate (Nissle). However, Nissle is eventually able to outcompete K. pneumoniae, suggesting that it can be utilized to enhance CR. Finally, we observed that fucose metabolism positively modulates hypermucoviscosity, autoaggregation, and biofilm formation but not capsule biogenesis. Together, these insights enhance our understanding of the role of alternative carbon sources in K. pneumoniae gut colonization and the complex relationship between metabolism and virulence in this species.

MgrB-Dependent Colistin Resistance in Klebsiella pneumoniae Is Associated with an Increase in Host-to-Host Transmission

Due to its high transmissibility, Klebsiella pneumoniae is one of the leading causes of nosocomial infections. Here, we studied the biological cost of colistin resistance, an antibiotic of last resort, in this opportunistic pathogen using a murine model of gut colonization and transmission. Colistin resistance in K. pneumoniae is commonly the result of the inactivation of the small regulatory protein MgrB. Without a functional MgrB, the two-component system PhoPQ is constitutively active, leading to an increase in lipid A modifications and subsequent colistin resistance. Using an isogenic mgrB deletion mutant (MgrB-), we demonstrate that the mutant's colistin resistance is not associated with a fitness defect under in vitro growth conditions. However, in our murine model of K. pneumoniae gastrointestinal (GI) colonization, the MgrB- colonizes the gut poorly, allowing us to identify a fitness cost. Moreover, the MgrB- mutant has higher survival outside the host compared with the parental strain. We attribute this enhanced survivability to dysregulation of the PhoPQ two-component system and accumulation of the master stress regulator RpoS. The enhanced survival of MgrB- may be critical for its rapid host-to-host transmission observed in our model. Together, our data using multiple clinical isolates demonstrate that MgrB-dependent colistin resistance in K. pneumoniae comes with a biological cost in gut colonization. However, this cost is mitigated by enhanced survival outside the host and consequently increases its host-to-host transmission. Additionally, it underscores the importance of considering the entire life cycle of a pathogen to determine the actual biological cost associated with antibiotic resistance. IMPORTANCE The biological cost associated with colistin resistance in Klebsiella pneumoniae was examined using a murine model of K. pneumoniae gut colonization and fecal-oral transmission. A common mutation resulting in colistin resistance in K. pneumoniae is a loss-of-function mutation of the small regulatory protein MgrB that regulates the two-component system PhoPQ. Even though colistin resistance in K. pneumoniae comes with a fitness defect in gut colonization, it increases bacterial survival outside the host enabling it to transmit more effectively to a new host. The enhanced survival is dependent upon the accumulation of RpoS and dysregulation of the PhoPQ. Hence, our study expands our understanding of the underlying molecular mechanism contributing to the transmission of colistin-resistant K. pneumoniae.

A human herpesvirus 7 glycoprotein, U21, diverts major histocompatibility complex class I molecules to lysosomes

All members of the herpesvirus family persist in their host throughout life. In doing so, herpesviruses exploit a surprising number of different strategies to evade the immune system. Human herpesvirus 7 (HHV-7) is a relatively recently discovered member of the herpesvirus family, and little is known about how it escapes immune detection. Here we show that HHV-7 infection results in premature degradation of major histocompatibility complex class I molecules. We identify and characterize a protein from HHV-7, U21, that binds to and diverts properly folded class I molecules to a lysosomal compartment. Thus, U21 is likely to function in the normal course of HHV-7 infection to downregulate surface class I molecules and prevent recognition of infected cells by cytotoxic T lymphocytes.

Identification of a nonneuronal isoform of synaptotagmin

Synaptotagmins, which have been found exclusively in neuroendocrine or exocrine tissues, have been implicated in the regulation of secretory vesicle fusion with the plasma membrane. The present paper describes a synaptotagmin isoform (synaptotagmin-5) which exhibits 49% amino acid identity to synaptotagmin-1 and -2. Synaptotagmin-5 mRNA is expressed in rat kidney, adipose tissue, lung, and heart, as well as at higher levels in brain and PC12 cells. Antiserum specific for the synaptotagmin-5 isoform recognizes a protein of about 50 kDa which is about 6-fold more abundant in brain synaptic vesicles than in whole brain membranes.

Targeting of the "insulin-responsive" glucose transporter (GLUT4) to the regulated secretory pathway in PC12 cells

Insulin-activated glucose transport depends on the efficient sorting of facilitated hexose transporter isoforms to distinct subcellular locales. GLUT4, the "insulin-responsive" glucose transporter, is sequestered intracellularly, redistributing to the cell surface only in the presence of hormone. To test the hypothesis that the biosynthesis of the insulin-responsive compartment is analogous to the targeting of proteins to the regulated secretory pathway, GLUT4 was expressed in the neuroendocrine cell line, PC12. Localization of the transporter in differentiated PC12 cells by indirect immunofluorescence revealed GLUT4 to be in the perinuclear region and in the distal processes. Although, by immunofluorescence microscopy, GLUT4 co-localized with the endosomal protein transferrin receptor and the small synaptic vesicle (SSV) marker protein synaptophysin, fractionation by velocity gradient centrifugation revealed that GLUT4 was excluded from SSV. Immunoelectron microscopic localization indicated that GLUT4 was indeed targeted to early and late endosomes, but in addition was concentrated in large dense core vesicles (LDCV). This latter observation was confirmed by the following experiments: (a) an antibody directed against GLUT4 immunoadsorbed the LDCV marker protein secretogranin, as assayed by Western blot; (b) approximately 85% of secretogranin metabolically labeled with 35S-labeled sulfate and allowed to progress into secretory vesicles was coadsorbed by an antibody directed against GLUT4; and (c) GLUT4 was readily detected in LDCV purified by ultracentrifugation. These data suggest that GLUT4 is specifically sorted to a specialized secretory compartment in PC12 cells.

Isoform-specific subcellular targeting of glucose transporters in mouse fibroblasts

GLUT1, the erythrocyte glucose transporter, and GLUT4, the adipose/muscle transporter, were each expressed in NIH-3T3 cells by retrovirus-mediated gene transfer. In fibroblasts overexpressing GLUT1, basal as well as insulin-stimulated deoxyglucose uptake was increased. Expression of GLUT4 was without affect on either basal or hormone stimulated hexose uptake. Localization of each of the transporters by indirect immunofluorescence revealed that, whereas GLUT1 was found primarily on the cell surface, GLUT4 was directed to vesicles in a perinuclear distribution and throughout the cytoplasm. The GLUT4-containing compartment represented neither Golgi complex nor lysosomes, as evidenced by the failure of lgp110 or Golgi mannosidase to co-localize. However, there was substantial overlap between the distribution of GLUT4 and the transferrin receptor, and some colocalization of the transporter isoform with the manose-6-phosphate receptor. In addition, when FITC-wheat germ agglutinin bound to the cell surface was allowed to internalize at 37 degrees C, it concentrated in vesicular structures coincident with GLUT4 immunoreactivity. These data establish that GLUT1 and GLUT4 contain within their amino acid sequences information which dictates targeting to distinct cellular compartments. Moreover, GLUT4 can be recognized by those cellular factors which direct membrane proteins to the endosomal pathway.