Sialic acid and N-acetylglucosamine Regulate type 1 Fimbriae Synthesis
- Author: Ian C. Blomfield1
- Editor: Paul Cohen2
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VIEW AFFILIATIONS HIDE AFFILIATIONSAffiliations: 1: School of Biosciences, University of Kent, Kent, UK; 2: University of Rhode Island, Kingston, RI
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Received 07 April 2015 Accepted 07 April 2015 Published 04 June 2015
- Correspondence: Ian Bloomfield, [email protected]

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Abstract:
Type 1 fimbriae of E. coli, a chaperon-usher bacterial adhesin, are synthesized by the majority of strains of the bacterium. Although frequently produced by commensal strains, the adhesin is nevertheless a virulence factor in Extraintestinal Pathogenic E. coli (ExPEC). The role of the adhesin in pathogenesis is best understood in Uropathogenic E. coli (UPEC). Host attachment and invasion by type 1 fimbriate bacteria activates inflammatory pathways, with TLR4 signaling playing a predominant role. In a mouse model of cystitis, type 1 fimbriation not only enhances UPEC adherence to the surface of superficial umbrella cells of the bladder urothelium, but is both necessary and sufficient for their invasion. Moreover the adhesin plays a role in the formation of transient intracellular bacterial communities (IBCs) within the cytoplasm of urothelial cells as part of UPEC cycles of invasion. The expression of type 1 fimbriation is controlled by phase variation at the transcriptional level, a mode of gene regulation in which bacteria switch reversibly between fimbriate and afimbriate phases. Phase variation has been widely considered to be a mechanism enabling immune evasion. Notwithstanding the apparently random nature of phase variation, switching of type 1 fimbrial expression is nevertheless controlled by a range of environmental signals that include the amino sugars sialic acid and N-acetylglucosamine (GlcNAc). Sialic acid plays a pivotal role in innate immunity, including signaling by the toll-like receptors. Here how sialic acid and GlcNAc control type 1 fimbriation is described and the potential significance of this regulatory response is discussed.
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Citation: Blomfield I. 2015. Sialic acid and N-acetylglucosamine Regulate type 1 Fimbriae Synthesis. Microbiol Spectrum 3(3):MBP-0015-2014. doi:10.1128/microbiolspec.MBP-0015-2014.




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Abstract:
Type 1 fimbriae of E. coli, a chaperon-usher bacterial adhesin, are synthesized by the majority of strains of the bacterium. Although frequently produced by commensal strains, the adhesin is nevertheless a virulence factor in Extraintestinal Pathogenic E. coli (ExPEC). The role of the adhesin in pathogenesis is best understood in Uropathogenic E. coli (UPEC). Host attachment and invasion by type 1 fimbriate bacteria activates inflammatory pathways, with TLR4 signaling playing a predominant role. In a mouse model of cystitis, type 1 fimbriation not only enhances UPEC adherence to the surface of superficial umbrella cells of the bladder urothelium, but is both necessary and sufficient for their invasion. Moreover the adhesin plays a role in the formation of transient intracellular bacterial communities (IBCs) within the cytoplasm of urothelial cells as part of UPEC cycles of invasion. The expression of type 1 fimbriation is controlled by phase variation at the transcriptional level, a mode of gene regulation in which bacteria switch reversibly between fimbriate and afimbriate phases. Phase variation has been widely considered to be a mechanism enabling immune evasion. Notwithstanding the apparently random nature of phase variation, switching of type 1 fimbrial expression is nevertheless controlled by a range of environmental signals that include the amino sugars sialic acid and N-acetylglucosamine (GlcNAc). Sialic acid plays a pivotal role in innate immunity, including signaling by the toll-like receptors. Here how sialic acid and GlcNAc control type 1 fimbriation is described and the potential significance of this regulatory response is discussed.

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FIGURE 1
A model for the sialic acid Neu5Ac utilisation pathway of E. coli. Sialic acids, released from glycoconjugates in the α-anomer, cross the outer membrane by diffusion either through the sialic acid-selective channel NanC or the general porins OmpF and OmpC. The porins are presumably non-selective for the anomeric forms of Neu5Ac, so that any of the β-anomer formed spontaneously would also be taken up via this route (not shown). Periplasmic mutarotase NanM catalyses the rapid equilibrium of the α-anomer with the thermodynamically more stable β-anomer, before uptake by the inner membrane transporter NanT. Cytoplasmic Neu5Ac induces the nanATEK operon by converting NanR from the active transcriptional inhibitor (NanRA) to the inactive form (NanRI). Conversion of Neu5Ac to GlcNAc-6-P is catalysed by N-acetylneuraminate aldolase (NanA), which generates N-acetylmannosamine and pyruvate, together with N-acetylmannosamine kinase (NanK) and N-acetylmannosamine-6-phosphate-2-epimerase (NanE). To enter glycolysis, GlcNAc-6-P is converted to fructose-6-phosphate by N-acetylglucosamine-6-phosphate deacetylase (NagA), generating GlcN-6-P, followed by glucosamine-6-phosphate deaminase (NagB). The nagAB operon is induced by GlcNAc-6-P, which inactivates the transcriptional repressor NagC. GlcN-6-P can also be converted to UDP-GlcNAc, required for both peptidoglycan and LPS biosynthesis, by the sequential action of phosphoglucosamine mutase (GlmM) and the bifunctional glucosamine-1-phosphate acetyltransferase and N-acetylglucosamine-1-phosphate uridyltransferase (GlmU). Proteins are shown in boxes (enzymes and porins in black-bordered boxes, while transcriptional regulators are bordered by red). The broken arrow linking GlcN-6-P to UDP-GlcNAc indicates that more than one step is involved.

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FIGURE 2
The organisation of the 1.4 Kbp fimB-nanCMS intergenic region. The location of binding sites for NanR (O NR ), NagC (O NC1 and O NC2 ), IHF (ibs) and SlyA (ON SA12 ) and Dam methylation sites GATCNanR (GNanR) and GATCNagC (GNagC) are indicated. The transcription start sites for the nanCMS and fimB promoters (P), and the direction of transcription (arrows), are shown. The sections delineated by sloping parallel lines indicate where sections are omitted to allow the regulatory region shared by fimB and nanCMS (O NR to O NC2 ) to be shown to scale. The diagram is drawn to the scale indicated in base pairs (bp).
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