Detection of Filamentous Salmonella on Low awFood Items and Enumeration of Filamentous Salmonella
Department of Bacteriology & Food Research Institute, University of Wisconsin-Madison
Filamentous Cells are found on low aw Food Items
Enumerating Filamentous Salmonella
Under certain stresses, Salmonella can form multi-chromosomal filaments (>10μm), which can septate when the cells encounter favorable conditions (Matticket al., 2000, AEM). Salmonellawith lower infectious doses are associated with outbreaks of foods that have low water activity (aw). One potential explanation for the low infectious dose is Salmonella may form multi-chromosomal filaments on these food items, which form only a single colony forming unit (CFU) in plate count assays. Filamentous Salmonella were detected on the surface of food items with low aw after their inoculation with Salmonella in stationary-phase. We developed a method to enumerate the viable units in a population of Stress Induced Filamentous Salmonella (SIFS). By incubating filamentous Salmonella cells in 1:5 diluted Luria-Bertani broth (0.2x LB) at room temperature for 8 hours, the percentage of filamentous cells in the population decreased from 70% to 18%. Concurrently, the concentration of CFU in culture increased 1-log, while the concentration of the non-filamentous Salmonella control sample remained unchanged over the same period of time. This method could be used to monitor food contamination and measure infectious doses on food surfaces that contain SIFS.
1. Pilot study
20% peanut butter agar
50% peanut butter agar
Filamentous Salmonella can form on the surface of low-awfood items.
Filamentous cells have greater DNA/OD600 ratio than control cells
2. Enumerating Salmonella: 0.2x LB
1. Filamentous Cells on Food Items
Inoculate food items with Salmonella during stationary-phase
Peanut (Inner and outer shell, skin)
Cheese (Swiss and Cheddar)
Peanut butter (agar)
Culture for 4 days at 30°C
Harvest in PBS and look for filaments microscopically
2. Enumerating Filamentous Salmonella
Enumeration is optimal after incubation in 0.2x LB at 22°C for 8 hours.