In vitro assessment of the effectiveness of powder disinfectant (Stalosan® F) against Lawsonia intracellularis using two different assays

The objective of this study was to determine the in vitro efficacy of Stalosan® F, a mixed chemical and heavy metal disinfectant, against two strains of Lawsonia intracellularis using both a modified tissue culture and a direct count method. For testing as a powder, 1 g, 0.5 g, or 0.25 g of Stalosan F was applied to bacterial solutions spread into sterile dishes. For use as an aqueous suspension, Stalosan F was prepared to final concentrations of 1%, 4%, 8%, 16%, and 32%. In both applications, L. intracellularis was exposed to Stalosan F for 0.5 h, 1 h, 2 h, and 4 h. The results showed that both strains were similar in their susceptibilities to Stalosan F. The modified tissue culture assay showed no detectable L. intracellularis in cell culture after exposure to all levels of Stalosan F powder for 0.5 h. Furthermore, the number of viable bacteria was markedly reduced in the aqueous concentration of 4% and no L. intracellularis was detected at concentrations of ≥8% for 0.5 h. Using the direct count method, detection of live bacteria was less than 1% after exposure to the powder for 0.5 h. After exposure to the aqueous form, the number of viable bacteria killed was over 99% in concentrations of ≥16% compared to controls. Our results indicate that Stalosan F in both powder and suspension forms is able to inactivate over 99% of L. intracellularis after 30 min of exposure. Furthermore, both laboratory methods can be used to determine the effect of disinfectants on L. intracellularis viability.