The form variation of the capsular polysaccharide K1 is not a critical virulence factor of Escherichia coli in a neonatal mouse model of infection

O-Acetylation of Capsular Polysialic Acid Enables Escherichia coli K1 Escaping from Siglec-Mediated Innate Immunity and Lysosomal Degradation of E. coli-Containing Vacuoles in Macrophage-Like Cells

Escherichia coli K1 causes bacteremia and meningitis in human neonates. The K1 capsule, an α2,8-linked polysialic acid (PSA) homopolymer, is its essential virulence factor. PSA is usually partially modified by O-acetyl groups. It is known that O-acetylation alters the antigenicity of PSA, but its impact on the interactions between E. coli K1 and host cells is unclear. In this study, a phase variant was obtained by passage of E. coli K1 parent strain, which expressed a capsule with 44% O-acetylation whereas the capsule of the parent strain has only 3%. The variant strain showed significantly reduced adherence and invasion to macrophage-like cells in comparison to the parent strain. Furthermore, we found that O-acetylation of PSA enhanced the modulation of trafficking of E. coli-containing vacuoles (ECV), enabling them to avoid fusing with lysosomes in these cells. Intriguingly, by using quartz crystal microbalance, we demonstrated that the PSA purified from the parent strain interacted with human sialic acid-binding immunoglobulin-like lectins (Siglecs), including Siglec-5, Siglec-7, Siglec-11, and Siglec-14. However, O-acetylated PSA from the variant interacted much less and also suppressed the production of Siglec-mediated proinflammatory cytokines. The adherence of the parent strain to human macrophage-like cells was significantly blocked by monoclonal antibodies against Siglec-11 and Siglec-14. Furthermore, the variant strain caused increased bacteremia and higher lethality in neonatal mice compared to the parent strain. These data elucidate that O-acetylation of K1 capsule enables E. coli to escape from Siglec-mediated innate immunity and lysosomal degradation; therefore, it is a strategy used by E. coli K1 to regulate its virulence.

IMPORTANCEEscherichia coli K1 is a leading cause of neonatal meningitis. The mortality and morbidity of this disease remain significantly high despite antibiotic therapy. One major limitation on advances in prevention and therapy for meningitis is an incomplete understanding of its pathogenesis. E. coli K1 is surrounded by PSA, which is observed to have high-frequency variation of O-acetyl modification. Here, we present an in-depth study of the function of O-acetylation in PSA at each stage of host-pathogen interaction. We found that a high level of O-acetylation significantly interfered with Siglec-mediated bacterial adherence to macrophage-like cells, and blunted the proinflammatory response. Furthermore, the O-acetylation of PSA modulated the trafficking of ECVs to prevent them from fusing with lysosomes, enabling them to escape degradation by lysozymes within these cells. Elucidating how subtle modification of the capsule enhances bacterial defenses against host innate immunity will enable the future development of effective drugs or vaccines against infection by E. coli K1.

Differential expression of the polysialyl capsule during blood-to-brain transit of neuropathogenic Escherichia coli K1

Escherichia coli K1 isolates synthesize a polysialic acid (polySia) capsule, are components of the adult gastrointestinal microbiota and may cause lethal bacteraemia and meningitis if acquired maternally by newborn infants. We used a neonatal rat pup K1 infection model to establish that prompt administration of a selective capsule depolymerase reverses the bacteraemic state and prevents death of almost all pups. In untreated animals, bacteria colonize the gastrointestinal tract and gain entry to the blood compartment, where they express the non-O-acetylated form of polySia. The bacteria invade the major organs of the host; histological and histochemical analysis of brain sections revealed that at least some bacteria enter the central nervous system through the blood-cerebrospinal fluid barrier at the choroid plexus prior to colonization of the meninges. Once in this location, they cease expression of polySia. The unexpected abrogation of polySia, a factor associated with the pathogenesis of meningitis and essential for transit through the blood, suggests that the neuropathogen dispenses with its protective capsule once it has colonized protected niches. Thus, systemic infections due to encapsulated pathogens may be resolved by capsule depolymerization only if the enzyme modifies the bacteria whilst they are in the blood compartment.

Going for baroque at the Escherichia coli K1 cell surface

Phase variation is usually thought of as the stochastic switching between alternatively expressed ('on') and unexpressed ('off') phenotypic states. However, coupling synthesis of a monotonous homopolysaccharide to a mechanism of random but incomplete chemical modification produces almost infinite structural variation. Potentially limitless variability implies that evolution can produce highly ornate or extravagant flourishes reminiscent of the baroque style. Here, we describe an analysis of capsular polysialic acid form variation in Escherichia coli K1, demonstrating that the large number of variant structures is controlled by a single contingency locus. The mechanism for generating maximum structural diversity from maximal genetic parsimony is conferred by a simple translational switch carried on a K1-specific prophage.

Mobile contingency locus controlling Escherichia coli K1 polysialic acid capsule acetylation

Escherichia coli K1 is part of a reservoir of adherent, invasive facultative pathogens responsible for a wide range of human and animal disease including sepsis, meningitis, urinary tract infection and inflammatory bowel syndrome. A prominent virulence factor in these diseases is the polysialic acid capsular polysaccharide (K1 antigen), which is encoded by the kps/neu accretion domain inserted near pheV at 67 map units. Some E. coli K1 strains undergo form (phase) variation involving loss or gain of O-acetyl esters at carbon positions 7 or 9 of the individual sialic acid residues of the polysialic acid chains. Acetylation is catalysed by the receptor-modifying acetyl coenzyme-A-dependent O-acetyltransferase encoded by neuO, a phase variable locus mapping near the integrase gene of the K1-specific prophage, CUS-3, which is inserted in argW at 53.1 map units. As the first E. coli contingency locus shown to operate by a translational switch, further investigation of neuO should provide a better understanding of the invasive K1 pathotype. Minimal estimates of morbidity and economic costs associated with human infections caused by extraintestinal pathogenic E. coli strains such as K1 indicate at least 6.5 million cases with attendant medical costs exceeding 2.5 billion US dollars annually in the United States alone.

Escherichia coli K1 polysialic acid O-acetyltransferase gene, neuO, and the mechanism of capsule form variation involving a mobile contingency locus

Potential O-acetylation of the sialic acid residues of Escherichia coli K1, groups W-135, Y, and C meningococci, and group B Streptococcus capsular polysaccharides modifies their immunogenicity and susceptibility to glycosidases. Despite the biological importance of O-acetylation, no sialic or polysialic acid O-acetyltransferase has been identified in any system. Here we show that the E. coli K1 O-acetyltransferase encoded by neuO is genetically linked to the endo-neuraminidase tail protein gene of a chromosomal accretion element, designated CUS-3, with homology to lambdoid bacteriophage. Molecular epidemiological analysis established concordance between O-acetyltransferase and CUS-3 in a set of E. coli K1 strains. Deleting neuO eliminated enzymatic activity, which was restored by complementation in trans, and confirmed by (13)C-NMR analysis of the acetylated product. Analysis of mutants that accumulate intracellular polysialic acid because of export defects (kpsM and kpsS) or an inability to synthesize the sialic acid precursor, N-acetylmannosamine (neuC), indicated that NeuO does not require constant association with its substrate for activity. DNA sequencing and PCR analysis of neuO from strains that had undergone random capsule form variation showed that slip strand DNA mispairing or unequal recombination resulted in gain or loss of (5'-AAGACTC-3')(n) heptanucleotide repeats (where n approximately equals 14-39) located in the neuO 5' region. These repeats code for a previously undescribed structure designated the poly(Psi) motif. The unexpected discovery of the neuO contingency locus (hypervariable gene controlling expression of a surface epitope) in E. coli, and of a potential phage for redistributing variant neuO alleles, provides a robust system for investigating the functions of localized hypermutability in pathogen evolution.

Preferential use of lambda light chains is associated with defective mouse antibody responses to the capsular polysaccharide of Neisseria meningitidis group B

The capsular polysaccharide of Neisseria meningitidis group B (CpsB) is a very poor immunogen in mammals; this has been considered to be due to the induction of tolerance to cross-reactive host glycoconjugates. It has hampered the development of an effective vaccine against this meningococcal group for many years. Syngeneic populations have a similar tolerogenic background. Thus, we used the variability in ability to mount CpsB-specific immunoglobulin (Ig) responses of individuals from these populations to reveal underlying mechanisms to tolerance contributing to the poor immunogenicity of CpsB. Here we analyze by ELISA, the individual CpsB-specific Ig response of BALB/c and other syngeneic mice to immunization with intact bacteria, using the distribution of light chains as a direct indicator of the repertoire dynamics of the response. Although approximately 96% of anti-CpsB Ig bear kappa-light chains, BALB/c mouse populations were heterogeneous in the light chain composition of their individual anti-CpsB Ig responses. The proportion of kappa and lambda-light chains used for anti-CpsB Ig was a private characteristic that remained relatively constant, for each individual, through repetitive immunizations regardless of the bacterial stimuli size. Despite the prevalence of individual use of kappa-light chains, 5% of BALB/c mice showed restricted usage of lambda-light chains in their CpsB-specific Ig responses, and an additional 11% use them significantly. The preferential use of lambda-light chains in these mice was strongly associated with defective IgM, and absent or barely detectable IgG anti-CpsB responses even after repetitive bacterial immunization. We conclude that differences in the private repertoire of specific Ig also contribute to mouse unresponsiveness to CpsB.

Bacterial penetration across the blood-brain barrier during the development of neonatal meningitis

Bacterial pathogens may breach the blood-brain barrier (BBB) and invade the central nervous system through paracellular and/or transcellular mechanisms. Transcellular penetration, e.g., transcytosis across the BBB has been demonstrated for Escherichia coli K1, group B streptococcus, Listeria monocytogenes, Citrobacter freundii and Streptococcus pneumonia strains. Genes contributing to invasion of brain microvascular endothelial cells include E. coli K1 genes ompA, ibeA, ibeB, and yijP. Understanding the mechanisms of bacterial penetration across the BBB may help develop novel approaches to preventing bacterial meningitis.