Horizontally Acquired Glycosyltransferase Operons Drive Salmonellae Lipopolysaccharide Diversity

Abstract

The immunodominant lipopolysaccharide is a key antigenic factor for Gram-negative pathogens such as salmonellae where it plays key roles in host adaptation, virulence, immune evasion, and persistence. Variation in the lipopolysaccharide is also the major differentiating factor that is used to classify Salmonella into over 2600 serovars as part of the Kaufmann-White scheme. While lipopolysaccharide diversity is generally associated with sequence variation in the lipopolysaccharide biosynthesis operon, extraneous genetic factors such as those encoded by the glucosyltransferase (gtr) operons provide further structural heterogeneity by adding additional sugars onto the O-antigen component of the lipopolysaccharide. Here we identify and examine the O-antigen modifying glucosyltransferase genes from the genomes of Salmonella enterica and Salmonella bongori serovars. We show that Salmonella generally carries between 1 and 4 gtr operons that we have classified into 10 families on the basis of gtrC sequence with apparent O-antigen modification detected for five of these families. The gtr operons localize to bacteriophage-associated genomic regions and exhibit a dynamic evolutionary history driven by recombination and gene shuffling events leading to new gene combinations. Furthermore, evidence of Dam- and OxyR-dependent phase variation of gtr gene expression was identified within eight gtr families. Thus, as O-antigen modification generates significant intra- and inter-strain phenotypic diversity, gtr-mediated modification is fundamental in assessing Salmonella strain variability. This will inform appropriate vaccine and diagnostic approaches, in addition to contributing to our understanding of host-pathogen interactions.

Bacteriophage-mediated Glucosylation Can Modify Lipopolysaccharide O-Antigens Synthesized by an ATP-binding Cassette (ABC) Transporter-dependent Assembly Mechanism

“…What is clear from NMR spectra is that the recombinant system in E. coli lacks the phase variation that results in a combination of modified and unmodified chains in some Salmonella isolates (8,55).…”

“…Depending on the serotype, any of the sugars in the tetrasaccharide repeat unit may be modified. Salmonella isolates can contain up to four glucosylation systems, modifying different parts of the O-antigen structures (8). Clearly, these modification processes have a significant impact on diversity.…”

Salmonella enterica Serovar Typhi Lipopolysaccharide O-Antigen Modification Impact on Serum Resistance and Antibody Recognition

“…O-antigen structures can be modified through several processes, and recently, we identified and characterized numerous Salmonella gtr (glycosyltransferase) operons (15). Using the amino acid sequence identity of the GtrC O-antigen-modifying proteins, we were able to group the gtr operons into 10 different “families” and proposed that each family performs a different O-antigen modification (15).…”

“…O-antigen structures can be modified through several processes, and recently, we identified and characterized numerous Salmonella gtr (glycosyltransferase) operons (15). Using the amino acid sequence identity of the GtrC O-antigen-modifying proteins, we were able to group the gtr operons into 10 different “families” and proposed that each family performs a different O-antigen modification (15). We additionally noted that a single Salmonella isolate may harbor multiple gtr operons, and several families of these gtr operons can undergo phase variation (15, 16), thus generating further potential complexity of the O-antigen presented by a population.…”

CRISPR-Cas Defense System and Potential Prophages in Cyanobacteria Associated with the Coral Black Band Disease

“…reptotaenium AO1 these were: GDP-D-mannose 4,6-dehydratase (Webb et al, 2004), phosphoheptose isomerase, ADP-L-glycero-D-mannoheptose-6-epimerase and an O -antigen export system (Brooke and Valvano, 1996); and for Geitlerinema sp. BBD_1991: GDP-L-fucose synthase (Mäki and Renkonen, 2004), GDP-D-mannose 4,6-dehydratase (Webb et al, 2004), glycosyl transferase (Davies et al, 2013). In Geitlerinema sp.…”