Comparison of classical serotyping and PremiTest assay for routine identification of common Salmonella enterica serovars

Abstract

The commercial PremiTest Salmonella kit uses a multiplexed DNA typing test aimed at identifying common serovars of Salmonella enterica. It was used in assays over a 9-month period in the Belgian reference laboratory that performs the routine identification of Salmonella strains of animal origin. A blind analysis of 754 strains was conducted in parallel by classical serotyping and the PremiTest assay. Full results were available for 685 strains (90.8%) by serotyping, while the remaining 69 strains were found to be nontypeable due to either a lack of surface antigen expression or autoagglutination properties. When the PremiTest assay (version 4.2) was performed with crude bacterial extracts, it identified 658 strains (87.3%), including most strains found to be nontypeable by serotyping. In contrast, it gave no, wrong, dual, or noninterpretable results for 96 strains, for which 23 were caused by assay failures. When purified DNA instead of crude extracts were tested, the number of strains successfully identified to the serovar level increased to 714 (94.7%), while all assay failures were cleared. Our conclusion is that, in its actual development stage, the application of the investigated kit to purified DNA samples offers a valuable alternative to classical serotyping for laboratories performing the routine identification of Salmonella strains belonging to commonly encountered serovars and isolated from a given geographical area, assuming that the system has been validated beforehand with a significant number of strains originating from that particular area.

FIG. 1.

Typical microarray results obtained by PT. The boxed microarray spots are used by the PT software (version 4.2) to calculate a PT signature that is used as serovar identifier. (A to F) Strains identified correctly only if purified DNA was assayed; (G) a strain yielding the correct result when crude material was assayed; (H) incorrect identification by PT; (I and J) a single PT signature matching two possible serovars; (K) a strain found to be nontypeable by classical serotyping and identified as Salmonella serovar Paratyphi B by PT; (L) uncommon serovar yielding a PT signature not recognized by the system. ST, serotyping; NT, nontypeable; Purif., purified.