Foodborne illnesses affect millions of people annually and account for thousands of deaths. It is estimated that about a million people per year are infected with salmonella infections in the United States alone. Salmonella and Listeria are among the most widely distributed and deadliest causes of foodborne infections. Their rapid and reliable detection on food and industrial food processing equipment is paramount. Now, scientists at Tel Aviv University have introduced a new, ultrasensitive, chemiluminescence-based method for the direct detection of Salmonella and Listeria monocytogenes. Because of the simplicity and sensitivity, this test is significantly faster than conventional methods and can be carried out in the field.
Findings from the new study were published recently in Angewandte Chemie through an article titled “Ultrasensitive Detection of Salmonella and Listeria monocytogenes by Small‐Molecule Chemiluminescence Probes.” Current testing methods usually require the growth of bacterial cultures in a containment laboratory. A conclusive result based on standard diagnostic techniques generally takes two to six days.
The new method is based on chemiluminescence—the emission of light resulting from a chemical process. The simplicity of the tests allows for both enrichment of the bacteria and their detection in a test tube, with no further sample preparation, so no containment laboratory is required. The chemiluminescence probes have proven to be about 600 times more sensitive than conventional fluorescence probes.
“We report new chemiluminescence probes for the ultrasensitive direct detection of viable pathogenic bacteria,” the authors wrote. “The probes are composed of a bright phenoxy‐dioxetane luminophore masked by a triggering group, which is activated by a specific bacterial enzyme, and could detect their corresponding bacteria with an LOD value of about 600‐fold lower than that of fluorescent probes. Moreover, we were able to detect a minimum of 10 Salmonella cells within 6 h incubation. The assay allows for bacterial enrichment and detection in one test tube without further sample preparation.”
The success of this technique is due to two specially developed probe molecules made by combining a luminescent substance (a phenoxy-dioxetane) with a “trigger”. In this form, the probe does not light up. The trigger is tailored to the bacteria to be detected: it is recognized by a specific enzyme produced by the pathogen—a special esterase in the case of Salmonella and a special phospholipase C for Listeria—that splits it from the luminescent portion.
This initiates a chemical reaction that causes the luminescent molecule to split off more pieces. The energy released by the reaction is emitted in the form of a very intense green glow. Tests with various bacteria demonstrated that the probe tailored to Listeria test only reacts to Listeria monocytogenes, not to other, non-pathogenic, strains of listeria. The intensity of the glow can be used to quantify the concentration of bacteria. The tests are so sensitive that, for example, a count of ten Salmonella can be detected within six hours of enrichment. Even dried bacteria can be swabbed from surfaces and be detected.
The study authors were excited by their findings and are confident that their new method can be used more broadly to develop specific chemiluminescence probes for other bacteria. “We anticipate that this design strategy will be used to prepare analogous chemiluminescence probes for other enzymes relevant to specific bacteria detection and point‐of‐care diagnostics,” the authors concluded.