On the possibility of using radioactive labeling and gamma camera technique to study Salmonella typhimurium in the blue mussel, Mytilus edulis

This paper presents a radiolabeting method for Salmonella typhimurium to be used for in vivo studies on the kinetics of uptake in blue mussels with a gamma camera technique. S. typhimurium bacteria were labeled with technetium-99m (⁹⁹Tc^m) a[ conditions preserving the viability in seawater and the cell surface properties of the bacteria. Stannous fluoride (SnF₂) was used to facilitate the binding of ⁹⁹Tc^m to the bacteria. The toxicity of SnF₂ could inhibit the growth of bacteria, and it can also bind extracellularly and reduce the negative cell surface charge of the bacteria. Additionally, SnF₂ can cause radiochemical by-products such as Tc-stannous colloids, which might interact with the image analysis. To optimize the labeling, two different concentrations of SnF₂ used in the labeling process were evaluated. Neither the efficiency nor the stability of the binding of ⁹⁹Tc^m to the bacteria increased significantly, when the amount of SnF₂ was increased 10-fold. Both treatments of bacteria reduced the viable counts, whereas the viability assessed microscopically with fluorescent probe was affected only little. However, after incubation in seawater, the viability was reduced for cells treated with the highest concentration of SnF₂. Still, approximately 60% remained viable. Presence of radioactivity, not bound to bacteria, was investigated by centrifugation in Percoll. Less than 4% of probable Tc-stannous colloids were found, and they were therefore not considered to be disturbing the imaging analysis. The net negative surface charge of the bacteria, examined by measuring the electrophoretic mobility, was significantly reduced when the concentration of SnF₂ increased, but was still negative. Radioactive particles, formed by mixing ⁹⁹Tc^m and SnF₂ in the absence of bacteria, were rapidly taken up by mussels in a way similar to that of the more heavily labeled bacteria. When less SnF₂ was used for labeling of the bacteria, different uptake and processing kinetics were seen. Thus, to keep the natural conditions, the concentration of the labeling compounds have to be minimized. The study showed that it is possible to balance the labeling method and get a valuable tool for following the uptake and fate of S. typhimurium in blue mussels.