Direct toxicity assessment (DTA) techniques seek to measure the impact of toxic chemicals on biological materials resident in the environment. This study features the use of freeze-dried bacterial cells in combination with a rapid DTA analyser, SciTOX™. The effects of three factors—cryoprotectant type, bacterial strain, and storage temperature— were tested in order to validate the shelf life of the freeze-dried cells. Three freeze-dried Gram-negative bacterial strains, Acinetobacter calcoaceticus, Escherichia coli and Pseudomonas putida, were tested by using the bacteria in the SciTox™ DTA assay and recording their responses to two standard toxicants: 2,4-dicholorophenol and 3,5-
dichlorophenol. Each freeze-dried strain of bacteria was prepared in two forms—either pre-treatment with polyethylene glycol (PEG) or with sucrose/Tween 80—prior to storing at either 4 or −20 °C for three different storage periods (1, 2 or 3 months). While the sucrose/Tween 80
pre-treated freeze-dried cells exhibited better cell viability, we concluded that PEG was a more suitable cryoprotectant for the bacteria used in the DTA assay because of EC50 parity with fresh cell and zero-time freeze-dried cell assays. The results showed that freeze-dried cells, with appropriate materials and conditions, can give reproducible DTA results
for up to 3 months. The availability of a biocomponent that can be activated by simple rehydration makes the deployment of this technology much easier for an end user
