Impact of Strain Variation on the Ability of Biosensor Technology to Detect Salmonella enterica

Introduction: It is important to develop methods that can quickly and accurately detect the presence of bacteria in the food supply that cause disease. Salmonella enterica is often associated with contamination of food.  Strains vary in their ability to cause illness and to spread. All methods that target cell surface structures for finding Salmonella entericashould be evaluated for their ability to detect different strains, because targets could be missing, masked or have alternative structures.

Purpose: This study tested the ability of biosensor technology to detect strains of Salmonella entericaknown to vary in growth properties and cell surface structures.  Some of the strains tested had mutations that altered the appearance of only one cell surface structure. Other strains had mutations in metabolic pathways, which could more globally affect the appearance of multiple structures on the outer membrane as well as growth properties.

Methods: CANARY^® biosensors are B-cell lines that express monoclonal antibodies and the aequorin protein, so that luminescence is generated when the antibodies bind to their antigen. We tested PathSensor's CANARY^® biosensor based technology for the ability to detect 18 different strains of Salmonella enterica serovar Enteritidis (S. Enteritidis). Nine other serotypes of Salmonella entericawere also tested. Strain variation within the set was maximized by deliberate mutation or by selection for the absence of cell surface molecules.

Results: Of the 27 diverse Salmonella strains tested, one S. Enteritidis strain was found that was difficult to detect.  It overexpressed a gene contributing to pili formation. In addition, Salmonella entericathat grew to high cell density impeded detection, because a high organic load interfered with signal development.  Serial dilutions of organically loaded samples increased sensitivity for these strains. Strains varied in the lower limit of detection, which was between 30 and 300 CFU per sample aliquot.

Significance: Samples that contain Salmonella enterica can appear negative or weakly positive if cell concentrations are very high (> 10⁷ CFU) or very low (< 10² CFU).  In the case of highly concentrated samples, serial dilution increased signal. Only one strain was found that confounded detection at all cell concentrations and it is not found in nature.  High organic load, originating from either a high target cell concentration or from an external mileau that dilutes target cells to less than a lower limit of detection, may interfere with detection by biosensor based technologies. CANARY^® biosensor based technology accurately detected Salmonella enterica even when exceptional strain variation was present, but within parameters of optimal cell concentration.