Klebsiella pneumoniae are gram-negative facultative anaerobes that are found within host-associated commensal microbiomes, but they can also cause a wide range of infections that are often difficult to treat. These infections are caused by different pathotypes of K. pneumoniae, called either classical and hypervirulent strains. These two strain groups are genetically distinct, inhabit non-overlapping geographies, and cause different types of harmful infections in humans. These distinct bacterial groups have also been found to interact differently with the host immune system. Initial innate immune defenses against K. pneumoniae infection include complement, macrophages, neutrophils, and monocytes; these defenses are primary strategies employed by the host to clear infections. K. pneumoniae pathogenesis depends upon the interactions between the microbe and each of these host defenses, and it is becoming increasingly apparent that bacterial genetic diversity impacts the outcomes of these interactions. Herein we highlight recent advances in our understanding of K. pneumoniae pathogenesis, with a focus on how bacterial evolution and diversity impact K. pneumoniae interactions with mammalian innate immune host defenses. We also discuss outstanding questions regarding how K. pneumoniae can frustrate normal immune responses, capitalize upon states of immunocompromise, and cause infections with high mortality.

Summary Small RNAs are principal elements of bacterial gene regulation and physiology. Two small RNAs in Brucella abortus, AbcR1 and AbcR2, are required for wild-type virulence. Examination of the abcR loci revealed the presence of a gene encoding a LysR-type transcriptional regulator flanking abcR2 on chromosome 1. Deletion of this lysR gene (bab1_1517) resulted in the complete loss of abcR2 expression while no difference in abcR1 expression was observed. The B. abortus bab1_1517 mutant strain was significantly attenuated in macrophages and mice, and bab1_1517 was subsequently named vtlR for virulence-associated transcriptional LysR-family regulator. Microarray analysis revealed three additional genes encoding small hypothetical proteins also under the control of VtlR. Electrophoretic mobility shift assays demonstrated that VtlR binds directly to the promoter regions of abcR2 and the three hypothetical protein-encoding genes, and DNase I footprint analysis identified the specific nucleotide sequence in these promoters that VtlR binds to and drives gene expression. Strikingly, orthologs of VtlR are encoded in a wide range of host-associated α-proteobacteria, and it is likely that the VtlR genetic system represents a common regulatory circuit critical for host–bacterium interactions.

Metal homeostasis in bacterial cells is a highly regulated process requiring intricately coordinated import and export, as well as precise sensing of intracellular metal concentrations. The uptake of zinc (Zn) has been linked to the virulence of Brucella abortus; however, the capacity of Brucella strains to sense Zn levels and subsequently coordinate Zn homeostasis has not been described. Here, we show that expression of the genes encoding the zinc uptake system ZnuABC is negatively regulated by the Znsensing Fur family transcriptional regulator, Zur, by direct interactions between Zur and the promoter region of znuABC. Moreover, the MerR-type regulator, ZntR, controls the expression of the gene encoding the Zn exporter ZntA by binding directly to its promoter. Deletion of zur or zntR alone did not result in increased zinc toxicity in the corresponding mutants; however, deletion of zntA led to increased sensitivity to Zn but not to other metals, such as Cu and Ni, suggesting that ZntA is a Zn-specific exporter. Strikingly, deletion of zntR resulted in significant attenuation of B. abortus in a mouse model of chronic infection, and subsequent experiments revealed that overexpression of zntA in the zntR mutant is the molecular basis for its decreased virulence. IMPORTANCEThe importance of zinc uptake for Brucella pathogenesis has been demonstrated previously, but to date, there has been no description of how overall zinc homeostasis is maintained and genetically controlled in the brucellae. The present work defines the predominant zinc export system, as well as the key genetic regulators of both zinc uptake and export in Brucella abortus. Moreover, the data show the importance of precise coordination of the zinc homeostasis systems as disregulation of some elements of these systems leads to the attenuation of Brucella virulence in a mouse model. Overall, this study advances our understanding of the essential role of zinc in the pathogenesis of intracellular bacteria.T he pathogenic alphaproteobacterium Brucella abortus preferentially infects cattle, bison, and elk (1), but the bacteria are also highly efficient at infecting humans. In order to establish a chronic infection in these hosts, the brucellae must survive and replicate within host macrophages (2). While the macrophage serves as the niche for Brucella during a chronic infection, the intracellular environment of these phagocytic immune cells is inhospitable as the bacteria are bombarded with a variety of environmental stresses, including exposure to reactive oxygen species (ROS), low pH, limited oxygen availability, and nutrient deprivation (3). Notwithstanding, the brucellae have evolved multiple strategies to cope with the harsh intramacrophagic environment and ultimately establish a replicative niche in these cells.With regard to the nutrient limitation experienced by the brucellae within macrophages, metal cations are likely found in extremely low concentrations, and, in fact, macrophages produce transporters, such as the NRAMP family of transp...