Abstract
Different Salmonella serovars generally display different antigenic formulae, but there are some exceptions. For instance, the same antigenic formula, 6,7:c:1,5, is shared by Salmonella enterica serovar, Paratyphi C, Typhisuis, and Choleraesuis. Moreover, three biotypes have been described within the S. Choleraesuis serovar. A distinction among such biotypes can only be based on biochemical behaviors (biotyping) posing serious concerns when rapid characterization is required. The study of an outbreak of severe epizootic salmonellosis in wild boars occurred in Italy between 2012 and 2014 and the typing of the isolates recovered from the outbreak were used to test different approaches for serovar identification. A number of 30 S. Choleraesuis var. Kunzendorf isolates from the outbreak were typed by means of four different methods to derive serovar and biotype: (i) slide agglutination method followed by biochemical tests, (ii) suspension array xMAP® Salmonella Serotyping Assay (SSA), (iii) whole genome sequencing (WGS) and data analysis using SeqSero tool, and (iv) WGS and data analysis using Salmonella TypeFinder tool. Slide agglutination, xMAP® SSA and WGS, followed by SeqSero analysis, are methods that infer the serovars according to the White-Kauffmann-Le Minor (WKL) scheme, based exclusively on antigens. Using these methods, isolates with incomplete antigenic formulae could be misleadingly excluded from an outbreak. On the contrary, WGS followed by Salmonella TypeFinder data analysis, which predicts the serotype on the basis of Multilocus sequence typing (MLST), might be able to cluster together isolates belonging to the same outbreak irrespective of the antigenic formula. Results suggest the benefit of routine use of a combination of in silico MLST and antigenic formula analysis to solve specific ambiguous case studies for outbreak investigation purposes.
Highlights
The White-Kauffmann-Le Minor (WKL) scheme summarizes antigenic formulae of all known Salmonella serovars, on the basis of antigenic variability in the outer membrane lipopolysaccharides (O antigen), flagellar proteins (H1 and H2 antigens) and capsular polysaccharide (Vi antigen) [1]
All Salmonella isolates were serotyped by slide agglutination with Salmonella antisera (Statens Serum Institut, Copenhagen, Denmark) and serovar names assigned according to the WKL; distinction between the biotypes of S
Kunzendorf by phenotypic serotyping and biochemical tests. All these isolates were assigned to the antigenic formula 6,7: c: 1,5 and showed
Summary
The White-Kauffmann-Le Minor (WKL) scheme summarizes antigenic formulae of all known Salmonella serovars, on the basis of antigenic variability in the outer membrane lipopolysaccharides (O antigen), flagellar proteins (H1 and H2 antigens) and capsular polysaccharide (Vi antigen) [1]. Traditional serotyping is labor intensive, and it requires trained technicians to provide valuable data [3] Another limitation of this method, which leads to inconclusive results, is a possible loss of expression of antigens required for definitive serovar identification (for example rough strains) [4]. Serovar Paratyphi C is associated with enteric fever in humans; serovar Typhisuis is associated with chronic paratyphoid/caseous lymphadenitis in swine [8] and serovar Choleraesuis may cause serious outbreaks of salmonellosis and paratyphoid in pigs [9], with clinical outcomes, such as enterocolitis and septicemia [10], often resulting in fatal systemic disease [11]. Results suggest the benefit of a combination of in silico MLST and antigenic formula detection to deep insight into a specific case of uncertainty in Salmonella serovar attribution
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