Two functional O-polysaccharide polymerase wzy (rfc) genes are present in the rfb gene cluster of Group E1 Salmonella enterica serovar Anatum

Resuscitation of viable but nonculturable bacteria promoted by ATP-mediated NAD+ synthesis

INTRODUCTION

Entry into the viable but nonculturable (VBNC) state is a survival strategy adopted by bacteria to survive harsh environment. Although VBNC cells still have metabolic activity, they lose the ability to form colonies on nonselective culture media. Thus, conventional bacterial detection methods, such as plate counting, are unable to detect the presence of VBNC cells. When the environmental conditions are appropriate, VBNC cells can initiate resuscitation, posing a great risk to the safety of public health. The study of the VBNC resuscitation mechanism could provide new insights into the prevention and control of VBNC resuscitation.

OBJECTIVES

Uncovering the molecular mechanism of VBNC cell resuscitation by investigating the role of O-antigen ligase (RfaL) in inhibiting the resuscitation of Escherichia coli O157:H7 in the VBNC state.

METHODS

RfaL was screened and verified as a resuscitation inhibitor of VBNC Escherichia coli O157:H7 by detecting resuscitation curve and time-lapse microscopy. The mechanism of RfaL impacts VBNC E. coli resuscitation was investigated by detecting the single cell ATP content, metabolomic changes, NAD(H) content and new protein biosynthesis of WT and ΔrfaL at different stage of resuscitation.

RESULTS

Mutation of rfaL, which encoded an O-antigen ligase, markedly shortened the resuscitating lag phase. Further studies indicated that ΔrfaL VBNC cells contained higher ATP levels, and ATP consumption during the resuscitating lag phase was highly correlated with resuscitation efficiency. Metabolomic analysis revealed that ATP was utilized to activate the Handler and salvage pathways to synthesize NAD+, balancing redox reactions to recover cell activity and promote cell resuscitation.

CONCLUSION

Our findings revealed a strategy employed by VBNC cells for revival, that is, using residual ATP to primarily recover metabolic activity, driving cells to exit dormancy. The synthesis pathway of lipopolysaccharide (LPS) in rfaL null mutant was inhibited and could supply more ATP to synthesis NAD+ and promote resuscitation.

Involvement of a Phage-Encoded Wzy Protein in the Polymerization of K127 Units To Form the Capsular Polysaccharide of Acinetobacter baumannii Isolate 36-1454

A comprehensive understanding of capsular polysaccharide (CPS) diversity is critical to implementation of phage therapy to treat panresistant Acinetobacter baumannii infections. Predictions from genome sequences can assist identification of the CPS type but can be complicated if genes outside the K locus (CPS biosynthesis gene cluster) are involved. Here, the CPS produced by A. baumannii clinical isolate 36-1454 carrying a novel K locus, KL127, was determined and compared to other CPSs. KL127 differs from KL128 in only two of the glycosyltransferase (gtr) genes. The K127 unit in 36-1454 CPS was the pentasaccharide β-d-Glcp-(1→6)-d-β-GalpNAc-(1→6)-α-d-Galp-(1→6)-β-d-Glсp-(1→3)-β-d-GalpNAc in which d-Glcp at position 4 replaces d-Galp in K128, and the glycosyltransferases encoded by the different gtr genes form the surrounding linkages. However, although the KL127 and KL128 gene clusters encode nearly identical Wzy polymerases, the linkages between K units that form the CPS chains are different, i.e., β-d-GalpNAc-(1→3)-d-Galp in 36-1454 (K127) and β-d-GalpNAc-(1→4)-d-Galp in KZ-1093 (K128). The linkage between K127 units in 36-1454 is the same as the K-unit linkage in five known CPS structures, and a gene encoding a Wzy protein related to the Wzy of the corresponding K loci was found encoded in a prophage genome in the 36-1454 chromosome. Closely related Wzy proteins were encoded in unrelated phage in available KL127-carrying genomes. However, a clinical isolate, KZ-1257, carrying KL127 but not the prophage was found, and K127 units in the KZ-1257 CPS were β-d-GalpNAc-(1→4)-d-Galp linked, confirming that Wzy_KL127 forms this linkage and thus that the phage-encoded Wzy_Ph1 forms the β-d-GalpNAc-(1→3)-d-Galp linkage in 36-1454. IMPORTANCE Bacteriophage therapy is an attractive innovative treatment for infections caused by extensively drug resistant Acinetobacter baumannii, for which there are few effective antibiotic treatments remaining. Capsular polysaccharide (CPS) is a primary receptor for many lytic bacteriophages, and thus knowledge of the chemical structures of CPS produced by the species will underpin the identification of suitable phages for therapeutic cocktails. However, recent research has shown that some isolates carry additional genes outside of the CPS biosynthesis K locus, which can modify the CPS structure. These changes can subsequently alter phage receptor sites and may be a method utilized for natural phage resistance. Hence, it is critical to understand the genetics that drive CPS synthesis and the extent to which genes outside of the K locus can affect the CPS structure.

The WbaK acetyltransferase of Salmonella enterica group E gives insights into O antigen evolution

O antigens are polysaccharides consisting of repeat units of three to eight sugars, generally assembled by genes in a discrete O antigen gene cluster. Salmonella enterica produces 46 forms of O antigen, and most of the variation is determined by genes in the gene cluster. However in some cases the structures are modified by enzymes encoded outside of the gene cluster, and several such modifications have been reported for Salmonella enterica group E, some with the genes on bacteriophages and one gene at a distant chromosomal site. We identified the enzyme, WbaK, that is responsible for O-acetylating the subgroup E1 O antigen, and found that the gene is located just downstream of the gene cluster as currently known. The wbaK gene appears to have been imported by a recombination event that also replaced the last 37 bp of the wbaP gene, indicating that homologous recombination was involved. Some of the group E strains we studied must have the original gene cluster, as they lack wbaK and the sequence downstream of wbaP is very similar to that in several other S. enterica O antigen gene clusters. In effect the gene cluster was extended by one gene in subgroup E1. It appears that a function that is usually encoded by a gene outside of the gene cluster has been added to the gene cluster, in this case giving an example of how such gene clusters can evolve.

Characterization of the Caulobacter crescentus holdfast polysaccharide biosynthesis pathway reveals significant redundancy in the initiating glycosyltransferase and polymerase steps

Caulobacter crescentus cells adhere to surfaces by using an extremely strong polar adhesin called the holdfast. The polysaccharide component of the holdfast is comprised in part of oligomers of N-acetylglucosamine. The genes involved in the export of the holdfast polysaccharide and the anchoring of the holdfast to the cell were previously discovered. In this study, we identified a cluster of polysaccharide biosynthesis genes (hfsEFGH) directly adjacent to the holdfast polysaccharide export genes. Sequence analysis indicated that these genes are involved in the biosynthesis of the minimum repeat unit of the holdfast polysaccharide. HfsE is predicted to be a UDP-sugar lipid-carrier transferase, the glycosyltransferase that catalyzes the first step in polysaccharide biosynthesis. HfsF is predicted to be a flippase, HfsG is a glycosyltransferase, and HfsH is similar to a polysaccharide (chitin) deacetylase. In-frame hfsG and hfsH deletion mutants resulted in severe deficiencies both in surface adhesion and in binding to the holdfast-specific lectin wheat germ agglutinin. In contrast, hfsE and hfsF mutants exhibited nearly wild-type levels of adhesion and holdfast synthesis. We identified three paralogs to hfsE, two of which are redundant to hfsE for holdfast synthesis. We also identified a redundant paralog to the hfsC gene, encoding the putative polysaccharide polymerase, and present evidence that the hfsE and hfsC paralogs, together with the hfs genes, are absolutely required for proper holdfast synthesis.

The genome of epsilon15, a serotype-converting, Group E1 Salmonella enterica-specific bacteriophage

The genome sequence of the Salmonella enterica serovar Anatum-specific, serotype-converting bacteriophage epsilon15 has been completed. The nonredundant genome contains 39,671 bp and 51 putative genes. It most closely resembles the genome of phiV10, an Escherichia coli O157:H7-specific temperate phage, with which it shares 36 related genes. More distant relatives include the Burkholderia cepacia-specific phage, BcepC6B (8 similar genes), the Bordetella bronchiseptica-specific phage, BPP-1 (8 similar genes) and the Photobacterium profundum prophage, P Pphipr1 (6 similar genes). epsilon15 gene identifications based on homologies with known gene families include the terminase small and large subunits, integrase, endolysin, two holins, two DNA methylase enzymes (one adenine-specific and one cytosine-specific) and a RecT-like enzyme. Genes identified experimentally include those coding for the serotype conversion proteins, the tail fiber, the major capsid protein and the major repressor. epsilon15's attP site and the Salmonella attB site with which it interacts during lysogenization have also been determined.

Coexistence of two distinct versions of O-antigen polymerase, Wzy-alpha and Wzy-beta, in Pseudomonas aeruginosa serogroup O2 and their contributions to cell surface diversity

Assembly of B-band lipopolysaccharide (LPS) in Pseudomonas aeruginosa follows a Wzy-dependent pathway, requiring the O-antigen polymerase Wzy and other proteins. The peptide sequences of the wzy(alpha) product from strains of serotypes O2, O5, and O16 are identical, but the O units in O5 are alpha-glycosidically linked, while those in O2 and O16 are beta-linked. We hypothesized that a derivative of the D3 bacteriophage wzy(beta) is present in the chromosomes of O2 and O16 and that this gene is responsible for the beta-linkage. By a combination of PCR and primer walking, wzy(beta) genes of both serotypes have been amplified and cloned. They are identical but share only 87.42% sequence identity with their xenolog in D3. A chromosomal knockout mutant of O16 wzy(beta) was made, and it produces semirough LPS devoid of B-band O antigen. The cloned wzy(beta) is capable of complementing the O16 wzy(beta) mutant, as well as cross-complementing a wzy(alpha) knockout mutant. However, in the latter case, the restored O antigen was beta-linked. Using reverse transcription-PCR, we showed that wzy(alpha) was transcribed in O2 and O16 strains and was functional, since both of these genes could complement the wzy(alpha) mutant of O5. With the coexistence of wzy(alpha) and wzy(beta) in O2 and O16 and the B-band O polysaccharides in these being beta-linked, we hypothesized that iap, an inhibitor of the alpha-polymerase gene, must be present in these serotypes. Indeed, through PCR, TOPO-cloning, and nucleotide-sequencing results, we verified the presence of iap in both O2 and O16 serotypes.