NlpI facilitates deposition of C4bp on Escherichia coli by blocking classical complement-mediated killing, which results in high-level bacteremia

Structural basis for recruitment of peptidoglycan endopeptidase MepS by lipoprotein NlpI

Peptidoglycan (PG) sacculi surround the cytoplasmic membrane, maintaining cell integrity by withstanding internal turgor pressure. During cell growth, PG endopeptidases cleave the crosslinks of the fully closed sacculi, allowing for the incorporation of new glycan strands and expansion of the peptidoglycan mesh. Outer-membrane-anchored NlpI associates with hydrolases and synthases near PG synthesis complexes, facilitating spatially close PG hydrolysis. Here, we present the structure of adaptor NlpI in complex with the endopeptidase MepS, revealing atomic details of how NlpI recruits multiple MepS molecules and subsequently influences PG expansion. NlpI binding elicits a disorder-to-order transition in the intrinsically disordered N-terminal of MepS, concomitantly promoting the dimerization of monomeric MepS. This results in the alignment of two asymmetric MepS dimers respectively located on the two opposite sides of the dimerization interface of NlpI, thus enhancing MepS activity in PG hydrolysis. Notably, the protein level of MepS is primarily modulated by the tail-specific protease Prc, which is known to interact with NlpI. The structure of the Prc-NlpI-MepS complex demonstrates that NlpI brings together MepS and Prc, leading to the efficient MepS degradation by Prc. Collectively, our results provide structural insights into the NlpI-enabled avidity effect of cellular endopeptidases and NlpI-directed MepS degradation by Prc.

Determinants of bacterial survival and proliferation in blood

Bloodstream infection is a major public health concern associated with high mortality and high healthcare costs worldwide. Bacteremia can trigger fatal sepsis whose prevention, diagnosis, and management have been recognized as a global health priority by the World Health Organization. Additionally, infection control is increasingly threatened by antimicrobial resistance, which is the focus of global action plans in the framework of a One Health response. In-depth knowledge of the infection process is needed to develop efficient preventive and therapeutic measures. The pathogenesis of bloodstream infection is a dynamic process resulting from the invasion of the vascular system by bacteria, which finely regulate their metabolic pathways and virulence factors to overcome the blood immune defenses and proliferate. In this review, we highlight our current understanding of determinants of bacterial survival and proliferation in the bloodstream and discuss their interactions with the molecular and cellular components of blood.

NhaA facilitates the maintenance of bacterial envelope integrity and the evasion of complement attack contributing to extraintestinal pathogenic Escherichia coli virulence

Extraintestinal pathogenic Escherichia coli (ExPEC) is responsible for severe bloodstream infections in humans and animals. However, the mechanisms underlying ExPEC's serum resistance remain incompletely understood. Through the transposon-directed insertion-site sequencing approach, our previous study identified nhaA, the gene encoding a Na+/H+ antiporter, as a crucial factor for infection in vivo. In this study, we investigated the role of NhaA in ExPEC virulence utilizing both in vitro models and systemic infection models involving avian and mammalian animals. Genetic mutagenesis analysis revealed that nhaA deletion resulted in filamentous bacterial morphology and rendered the bacteria more susceptible to sodium dodecyl sulfate, suggesting the role of nhaA in maintaining cell envelope integrity. The nhaA mutant also displayed heightened sensitivity to complement-mediated killing compared to the wild-type strain, attributed to augmented deposition of complement components (C3b and C9). Remarkably, NhaA played a more crucial role in virulence compared to several well-known factors, including Iss, Prc, NlpI, and OmpA. Our findings revealed that NhaA significantly enhanced virulence across diverse human ExPEC prototype strains within B2 phylogroups, suggesting widespread involvement in virulence. Given its pivotal role, NhaA could serve as a potential drug target for tackling ExPEC infections.

Biomimetic Lipopolysaccharide-Free Bacterial Outer Membrane-Functionalized Nanoparticles for Brain-Targeted Drug Delivery

The blood-brain barrier (BBB) severely blocks the intracranial accumulation of most systemic drugs. Inspired by the contribution of the bacterial outer membrane to Escherichia coli K1 (EC-K1) binding to and invasion of BBB endothelial cells in bacterial meningitis, utilization of the BBB invasion ability of the EC-K1 outer membrane for brain-targeted drug delivery and construction of a biomimetic self-assembled nanoparticle with a surface featuring a lipopolysaccharide-free EC-K1 outer membrane are proposed. BBB penetration of biomimetic nanoparticles is demonstrated to occur through the transcellular vesicle transport pathway, which is at least partially dependent on internalization, endosomal escape, and transcytosis mediated by the interactions between outer membrane protein A and gp96 on BBB endothelial cells. This biomimetic nanoengineering strategy endows the loaded drugs with prolonged circulation, intracranial interstitial distribution, and extremely high biocompatibility. Based on the critical roles of gp96 in cancer biology, this strategy reveals enormous potential for delivering therapeutics to treat gp96-overexpressing intracranial malignancies.

Peptidoglycan Endopeptidase Spr of Uropathogenic Escherichia coli Contributes to Kidney Infections and Competitive Fitness During Bladder Colonization

Uropathogenic Escherichia coli (UPEC) is the most common pathogen of urinary tract infections (UTIs). Antibiotic therapy is the conventional measure to manage such infections. However, the rapid emergence of antibiotic resistance has reduced the efficacy of antibiotic treatment. Given that the bacterial factors required for the full virulence of the pathogens are potential therapeutic targets, identifying such factors may facilitate the development of novel therapeutic strategies against UPEC UTIs. The peptidoglycan (PG) endopeptidase Spr (also named MepS) is required for PG biogenesis in E. coli. In the present study, we found that Spr deficiency attenuated the ability of UPEC to infect kidneys and induced a fitness defect during bladder colonization in a mouse model of UTI. Based on the liquid chromatography (LC)/mass spectrometry (MS)/MS analysis of the bacterial envelope, spr deletion changed the levels of some envelope-associated proteins, suggesting that Spr deficiency interfere with the components of the bacterial structure. Among the proteins, FliC was significantly downregulated in the spr mutant, which is resulted in reduced motility. Lack of Spr might hinder the function of the flagellar transcriptional factor FlhDC to decrease FliC expression. The motility downregulation contributed to the reduced fitness in urinary tract colonization. Additionally, spr deletion compromised the ability of UPEC to evade complement-mediated attack and to resist intracellular killing of phagocytes, consequently decreasing UPEC bloodstream survival. Spr deficiency also interfered with the UPEC morphological switch from bacillary to filamentous shapes during UTI. It is known that bacterial filamentation protects UPEC from phagocytosis by phagocytes. In conclusion, Spr deficiency was shown to compromise multiple virulence properties of UPEC, leading to attenuation of the pathogen in urinary tract colonization and bloodstream survival. These findings indicate that Spr is a potential antimicrobial target for further studies attempting to develop novel strategies in managing UPEC UTIs.

An ArcA-Modulated Small RNA in Pathogenic Escherichia coli K1

Escherichia coli K1 is the leading cause of meningitis in newborns. Understanding the molecular basis of E. coli K1 pathogenicity will help develop treatment of meningitis and prevent neurological sequelae. E. coli K1 replicates in host blood and forms a high level of bacteremia to cause meningitis in human. However, the mechanisms that E. coli K1 employs to sense niche signals for survival in host blood are poorly understood. We identified one intergenic region in E. coli K1 genome that encodes a novel small RNA, sRNA-17. The expression of sRNA-17 was downregulated by ArcA in microaerophilic blood. The ΔsRNA-17 strain grew better in blood than did the wild-type strain and enhanced invasion frequency in human brain microvascular endothelial cells. Transcriptome analyses revealed that sRNA-17 regulates tens of differentially expressed genes. These data indicate that ArcA downregulates the sRNA-17 expression to benefit bacterial survival in blood and penetration of the blood–brain barrier. Our findings reveal a signaling mechanism in E. coli K1 for host adaptation.

cjrABC-senB hinders survival of extraintestinal pathogenic E. coli in the bloodstream through triggering complement-mediated killing

BACKGROUND

Extraintestinal pathogenic E. coli (ExPEC) is a common gram-negative organism causing various infections, including urinary tract infections (UTIs), bacteremia, and neonatal meningitis. The cjrABC-senB gene cluster of E. coli contributes to ExPEC virulence in the mouse model of UTIs. Consistently, the distribution of cjrABC-senB is epidemiologically associated with human UTIs caused by E. coli. cjrABC-senB, which has previously been proposed to encode an iron uptake system, may facilitate ExPEC survival in the iron availability-restricted urinary tract. Given that the bloodstream is also an iron limited environment to invading bacteria, the pathogenic role of cjrABC-senB in ExPEC bacteremia, however, remains to be investigated.

METHODS

The ability of ExPEC RS218 strains with and without cjrABC-senB to survive in the mouse bloodstream and human serum was evaluated. Subsequently, the role of this gene cluster in the ExPEC interaction with the complement system was evaluated. Finally, the distribution of cjrABC-senB in human clinical E. coli isolates was determined by PCR. The frequency of cjrABC-senB in bacteremia isolates that were not associated with UTIs (non-UTI bacteremia isolates) was compared with that in UTI-associated isolates and fecal isolates.

RESULTS

Expression of cjrABC-senB attenuated the survival of RS218 in the mouse bloodstream and human serum. The cjrABC-senB-harboring strains triggered enhanced classical- and alternative-complement pathway activation and became more vulnerable to complement-mediated killing in serum. cjrA was identified as the major gene responsible for the attenuated serum survival. Expressing cjrABC-senB and cjrA increased bacterial susceptibility to detergent and induced periplasmic protein leakage, suggesting that the expression of these genes compromises the integrity of the outer membrane of ExPEC. In addition, the frequency of cjrABC-senB in non-UTI bacteremia isolates was significantly lower than that in UTI-associated isolates, while the frequencies in non-UTI bacteremia isolates and fecal isolates showed no significant difference. Consistently, this epidemiological investigation suggests that cjrABC-senB does not contribute to E. coli bacteremia in humans.

CONCLUSION

The contribution of cjrABC-senB to the pathogenesis of ExPEC is niche dependent and contradictory because the genes facilitate ExPEC UTIs but hinder bacteremia. The contradictory niche-dependent characteristic may benefit the development of novel strategies against E. coli-caused infections.

Glycine, serine and threonine metabolism confounds efficacy of complement-mediated killing

Serum resistance is a poorly understood but common trait of some difficult-to-treat pathogenic strains of bacteria. Here, we report that glycine, serine and threonine catabolic pathway is down-regulated in serum-resistant Escherichia coli, whereas exogenous glycine reverts the serum resistance and effectively potentiates serum to eliminate clinically-relevant bacterial pathogens in vitro and in vivo. We find that exogenous glycine increases the formation of membrane attack complex on bacterial membrane through two previously unrecognized regulations: 1) glycine negatively and positively regulates metabolic flux to purine biosynthesis and Krebs cycle, respectively. 2) α-Ketoglutarate inhibits adenosine triphosphate synthase, which in together promote the formation of cAMP/CRP regulon to increase the expression of complement-binding proteins HtrE, NfrA, and YhcD. The results could lead to effective strategies for managing the infection with serum-resistant bacteria, an especially valuable approach for treating individuals with weak acquired immunity but a normal complement system.

Serum-resistant bacteria can escape complement killing in the bloodstream. Here, using metabolomics and metabolite perturbations, the authors describe an altered metabolic state in serum-resistant Escherichia coli and show that exogenous glycine potentiates elimination of pathogenic bacteria in vivo.

Molecular Epidemiology of Extraintestinal Pathogenic Escherichia coli

Extraintestinal pathogenic Escherichia coli (ExPEC) are important pathogens in humans and certain animals. Molecular epidemiological analyses of ExPEC are based on structured observations of E. coli strains as they occur in the wild. By assessing real-world phenomena as they occur in authentic contexts and hosts, they provide an important complement to experimental assessment. Fundamental to the success of molecular epidemiological studies are the careful selection of subjects and the use of appropriate typing methods and statistical analysis. To date, molecular epidemiological studies have yielded numerous important insights into putative virulence factors, host-pathogen relationships, phylogenetic background, reservoirs, antimicrobial-resistant strains, clinical diagnostics, and transmission pathways of ExPEC, and have delineated areas in which further study is needed. The rapid pace of discovery of new putative virulence factors and the increasing awareness of the importance of virulence factor regulation, expression, and molecular variation should stimulate many future molecular epidemiological investigations. The growing sophistication and availability of molecular typing methodologies, and of the new computational and statistical approaches that are being developed to address the huge amounts of data that whole genome sequencing generates, provide improved tools for such studies and allow new questions to be addressed.

Bordetella pertussis isolates vary in their interactions with human complement components

Whooping cough is a re-emerging respiratory tract infection. It has become clear that there is a need for better understanding of protective immune responses and variation between Bordetella pertussis strains to aid the development of improved vaccines. In order to survive in the host, B. pertussis has evolved mechanisms to evade complement-mediated killing, including the ability to bind complement-regulatory proteins. Here we evaluate the variation in interactions with the complement system among recently isolated strains. Isolates whose genomes appear highly similar and cluster together on a SNP-based dendrogram were found to vary significantly in resistance to complement-mediated killing and in the deposition of C3b/iC3b, C5b-9 and C1 esterase inhibitor (C1-INH). The key role of Vag8 as a receptor for C1-INH was confirmed and its expression was shown to vary in a panel of isolates. A Vag8 knockout mutant showed increased sensitivity to complement-mediated killing. Antibodies in convalescent sera blocked C1-INH binding to B. pertussis and may play an important role in natural immunity.

Structural basis of adaptor-mediated protein degradation by the tail-specific PDZ-protease Prc

Peptidoglycan (PG) is a highly cross-linked, protective mesh-like sacculus that surrounds the bacterial cytoplasmic membrane. Expansion of PG is tightly coupled to growth of a bacterial cell and requires hydrolases to cleave the cross-links for insertion of nascent PG material. In Escherichia coli, a proteolytic system comprising the periplasmic PDZ-protease Prc and the lipoprotein adaptor NlpI contributes to PG enlargement by regulating cellular levels of MepS, a cross-link-specific hydrolase. Here, we demonstrate how NlpI binds Prc to facilitate the degradation of its substrate MepS by structural and mutational analyses. An NlpI homodimer binds two molecules of Prc and forms three-sided MepS-docking cradles using its tetratricopeptide repeats. Prc forms a monomeric bowl-shaped structure with a lid-like PDZ domain connected by a substrate-sensing hinge that recognizes the bound C terminus of the substrate. In summary, our study reveals mechanistic details of protein degradation by the PDZ-protease Prc bound to its cognate adaptor protein.

Human Meningitis-Associated Escherichia coli

Escherichia coli is the most common Gram-negative bacillary organism causing meningitis, and E. coli meningitis continues to be an important cause of mortality and morbidity throughout the world. Our incomplete knowledge of its pathogenesis contributes to such mortality and morbidity. Recent reports of E. coli strains producing CTX-M-type or TEM-type extended-spectrum β-lactamases create a challenge. Studies using in vitro and in vivo models of the blood-brain barrier have shown that E. coli meningitis follows a high degree of bacteremia and invasion of the blood-brain barrier. E. coli invasion of the blood-brain barrier, the essential step in the development of E. coli meningitis, requires specific microbial and host factors as well as microbe- and host-specific signaling molecules. Blockade of such microbial and host factors contributing to E. coli invasion of the blood-brain barrier is shown to be efficient in preventing E. coli penetration into the brain. The basis for requiring a high degree of bacteremia for E. coli penetration of the blood-brain barrier, however, remains unclear. Continued investigation on the microbial and host factors contributing to a high degree of bacteremia and E. coli invasion of the blood-brain barrier is likely to identify new targets for prevention and therapy of E. coli meningitis.

How Escherichia coli Circumvent Complement-Mediated Killing

Complement is a crucial arm of the innate immune response against invading bacterial pathogens, and one of its main functions is to recognize and destroy target cells. Similar to other pathogens, Escherichia coli has evolved mechanisms to overcome complement activation. It is well known that capsular polysaccharide may confer resistance to complement-mediated killing and phagocytosis, being one of the strategies adopted by this bacterium to survive in serum. In addition, proteases produced by E. coli have been shown to downregulate the complement system. Pic, an autotransporter secreted by different pathogens in the Enterobacteriaceae family, is able to cleave C2, C3/C3b, and C4/C4b and works synergistically with human Factor I and Factor H (FH), thereby promoting inactivation of C3b. Extracellular serine protease P, a serine protease of enterohemorrhagic E. coli (EHEC), downregulates complement activation by cleaving C3/C3b and C5. StcE, a metalloprotease secreted by EHEC, inhibits the classical complement-mediated cell lysis by potentiating the action of C1 inhibitor, and the periplasmic protease Prc contributes to E. coli complement evasion by interfering with the classical pathway activation and by preventing membrane attack complex deposition. Finally, it has been described that E. coli proteins interact with negative complement regulators to modulate complement activation. The functional consequences resulting from the interaction of outer membrane protein A, new lipoprotein I, outer membrane protein W, and Stx2 with proteins of the FH family and C4b-binding protein (C4BP) are discussed in detail. In brief, in this review, we focused on the different mechanisms used by pathogenic E. coli to circumvent complement attack, allowing these bacteria to promote a successful infection.

Multifunctional and Redundant Roles of Borrelia burgdorferi Outer Surface Proteins in Tissue Adhesion, Colonization, and Complement Evasion

Borrelia burgdorferi is the causative agent of Lyme disease in the U.S., with at least 25,000 cases reported to the CDC each year. B. burgdorferi is thought to enter and exit the bloodstream to achieve rapid dissemination to distal tissue sites during infection. Travel through the bloodstream requires evasion of immune surveillance and pathogen clearance in the host, a process at which B. burgdorferi is adept. B. burgdorferi encodes greater than 19 adhesive outer surface proteins many of which have been found to bind to host cells or components of the extracellular matrix. Several others bind to host complement regulatory factors, in vitro. Production of many of these adhesive proteins is tightly regulated by environmental cues, and some have been shown to aid in vascular interactions and tissue colonization, as well as survival in the blood, in vivo. Recent work has described multifaceted and redundant roles of B. burgdorferi outer surface proteins in complement component interactions and tissue targeted adhesion and colonization, distinct from their previously identified in vitro binding capabilities. Recent insights into the multifunctional roles of previously well-characterized outer surface proteins such as BBK32, DbpA, CspA, and OspC have changed the way we think about the surface proteome of these organisms during the tick-mammal life cycle. With the combination of new and old in vivo models and in vitro techniques, the field has identified distinct ligand binding domains on BBK32 and DbpA that afford tissue colonization or blood survival to B. burgdorferi. In this review, we describe the multifunctional and redundant roles of many adhesive outer surface proteins of B. burgdorferi in tissue adhesion, colonization, and bloodstream survival that, together, promote the survival of Borrelia spp. throughout maintenance in their multi-host lifestyle.

Innate immune evasion of Escherichia coli clinical strains from orthopedic implant infections

Escherichia coli is one of the first causes of Gram-negative orthopedic implant infections (OII). Those infections, usually hematogenous, mostly originate from the urinary tract. We investigated the strategies developed by E. coli in this context to evade host innate immune responses, i.e. complement and polymorphonuclear neutrophils (PMN). Twenty strains from OII were compared with 20 strains from bacteremia in patients with non-infected orthopedic implant. In both groups, 6/20 (30 %) strains lysed PMNs, due to the production of the pore-forming toxin α-hemolysin (HlyA). For the others, resistance to phagocytic killing by PMN was not significantly different between both groups. In contrast, resistance to complement-mediated serum killing was significantly higher in OII strains than in the others (65 % vs 10 %; P <0.001). In E. coli, different mechanisms have been involved in complement resistance. Here, serum resistance was not linked to a group 2 capsule, or a loss of outer membrane permeability, or the recruitment of the complement inhibitor C4bp, but was significantly associated with the synthesis of long-chain LPS, regardless of the O-antigen. Thus, serum resistance could promote seeding of peri-implant tissues by helping E. coli to either persist in blood and reach the site of infection or overcome localized complement activation.

C4b-binding protein: The good, the bad and the deadly. Novel functions of an old friend

C4b-binding protein (C4BP) is best known as a potent soluble inhibitor of the classical and lectin pathways of the complement system. This large 500 kDa multimeric plasma glycoprotein is expressed mainly in the liver but also in lung and pancreas. It consists of several identical 75 kDa α-chains and often also one 40 kDa β-chain, both of which are mainly composed of complement control protein (CCP) domains. Structure-function studies revealed that one crucial binding site responsible for inhibition of complement is located to CCP1-3 of the α-chain. Binding of anticoagulant protein S to the CCP1 of the β-chain provides C4BP with the ability to strongly bind apoptotic and necrotic cells in order to prevent inflammation arising from activation of complement by these cells. Further, C4BP interacts strongly with various types of amyloid and enhances fibrillation of islet amyloid polypeptide secreted from pancreatic beta cells, which may attenuate pro-inflammatory and cytotoxic effects of this amyloid. Full deficiency of C4BP has not been identified but non-synonymous alterations in its sequence have been found in haemolytic uremic syndrome and recurrent pregnancy loss. Furthermore, C4BP is bound by several bacterial pathogens, notably Streptococcus pyogenes, which due to inhibition of complement and enhancement of bacterial adhesion to endothelial cells provides these bacteria with a survival advantage in the host. Thus, depending on the context, C4BP has a protective or detrimental role in the organism.

Edwardsiella tarda evades serum killing by preventing complement activation via the alternative pathway

Edwardsiella tarda is a Gram-negative bacterium with a broad host range that includes a wide variety of farmed fish as well as humans. E. tarda has long been known to be able to survive in host serum, but the relevant mechanism is unclear. In this study, we investigated the fundamental question, i.e. whether E. tarda activated serum complement or not. We found that (i) when incubated with flounder serum, E. tarda exhibited a high survival rate (87.6%), which was slightly but significantly reduced in the presence of Mg(2+); (ii) E. tarda-incubated serum possessed strong hemolytic activity and bactericidal activity, (iii) compared to the serum incubated with a complement-sensitive laboratory Escherichia coli strain, E. tarda-incubated serum exhibited much less chemotactic activity, (iv) in contrast to the serum incubated with live E. tarda, the serum incubated with heat-inactivated E. tarda exhibited no apparent hemolytic capacity. Taken together, these results indicate for the first time that E. tarda circumvents serum attack by preventing, to a large extent, complement activation via the alternative pathway, and that heat-labile surface structures likely play an essential role in the complement evasion of E. tarda.

Heat shock proteins IbpA and IbpB are required for NlpI-participated cell division in Escherichia coli

Lipoprotein NlpI of Escherichia coli is involved in the cell division, virulence, and bacterial interaction with eukaryotic host cells. To elucidate the functional mechanism of NlpI, we examined how NlpI affects cell division and found that induction of NlpI inhibits nucleoid division and halts cell growth. Consistent with these results, the cell division protein FtsZ failed to localize at the septum but diffused in the cytosol. Elevation of NlpI expression enhanced the transcription and the outer membrane localization of the heat shock protein IbpA and IbpB. Deletion of either ibpA or ibpB abolished the effects of NlpI induction, which could be restored by complementation. The C-terminus of NlpI is critical for the enhancement in IbpA and IbpB production, and the N-terminus of NlpI is required for the outer membrane localization of NlpI, IbpA, and IbpB. Furthermore, NlpI physically interacts with IbpB. These results indicate that over-expression of NlpI can interrupt the nucleoids division and the assembly of FtsZ at the septum, mediated by IbpA/IbpB, suggesting a role of the NlpI/IbpA/IbpB complex in the cell division.

Bacterial self-defence: how Escherichia coli evades serum killing

The ability to survive the bactericidal action of serum is advantageous to extraintestinal pathogenic Escherichia coli that gain access to the bloodstream. Evasion of the innate defences present in serum, including complement and antimicrobial peptides, involves multiple factors. Serum resistance mechanisms utilized by E. coli include the production of protective extracellular polysaccharide capsules and expression of factors that inhibit or interfere with the complement cascade. Recent studies have also highlighted the importance of structural integrity of the cell envelope in serum survival. These survival strategies are outlined in this review with particular attention to novel findings and recent insights into well-established resistance mechanisms.

Microbial biosynthesis of designer outer membrane vesicles

Outer membrane vesicles (OMVs) are nanoscale proteoliposomes that are ubiquitously secreted by Gram-negative bacteria. Interest in these bioparticles has escalated over the years, leading to discoveries regarding their composition, production, and vaccine potential. Given that many steps in vesicle biogenesis are 'engineerable,' it is now possible to tailor OMVs for specific applications. Such tailoring involves modifying the OMV-producing bacterium through protein, pathway, or genome engineering in a manner that specifically alters the final OMV product. For example, targeted deletion or upregulation of genes associated with the cell envelope can modulate vesicle production or remodel the composition of vesicle components such as lipopolysaccharide. Likewise, bacteria can be reprogrammed to incorporate heterologously expressed proteins into either the membrane or lumenal compartment of OMVs. We anticipate that further research in the field of OMV engineering will enable continued design and biosynthesis of specialized vesicles for numerous biotechnological purposes ranging from the delivery of vaccines to the deconstruction of cellulosic substrates.

The complex pathogenesis of bacteremia: from antimicrobial clearance mechanisms to the genetic background of the host

Bacteremia develops when bacteria manage to escape the host immune mechanisms or when the otherwise well-orchestrated immune response fails to control bacterial spread due to inherent or acquired immune defects that are associated with susceptibility to infection. The pathogenesis of bacteremia has some characteristic features that are influenced by the genetic signature of the host. In this review, the host defense mechanisms that help prevent bacteremia will be described and the populations who are at risk because of congenital or acquired deficiencies in such mechanisms will be defined. A special mention will be made to novel insights regarding host immune defense against the most commonly isolated organisms from patients with community-acquired bloodstream infections.