| Summary: | <i>Francisella tularensis</i> is a highly infectious, intracellular bacterium and it is the causative agent of tularemia. The bacterium has been isolated from more than 250 species, including protozoa. Previous studies have shown that the growth of <i>Legionella pneumophila</i> within the amoeba results in a dramatic increase in the resistance to disinfectants. Since <i>Francisella</i> persists in the environment for years, this study investigates whether <i>Acanthamoeba castellanii</i>-grown <i>F. novicida</i> exhibits an alteration in the resistance to disinfectants. The disinfectants used are didecyldimethylammonium chloride (DDAC) combined with isopropyl alcohol (D1), benzalkonium chloride combined with DDAC and formic acid (D2), and polyhexamethylene biguanide (PHMB, D3). The effect of disinfectants on the bacterial viability is determined by a colony-forming unit (CFU), by transmission electron microscopy (TEM), by fluorescence microscopy, and the damage of the bacterial membrane. Our data has shown that only a one-log<sub>10</sub> loss in bacterial viability is exhibited upon treatment of agar-grown <i>Francisella</i>, while in amoeba-grown <i>Francisella</i> there was a three-log<sub>10</sub> difference with D3. The D1 disinfectant sterilized the bacteria within 10 s. The treatment of agar-grown <i>F. novicida</i> with D2 reduces bacterial viability by seven-log<sub>10</sub> within 10 s and 15 min, respectively. Surprisingly, the treatment of amoeba-grown <i>F. novicida</i> with D2 results in a total loss of bacterial viability. In conclusion, <i>A. castellanii-</i>grown <i>F. novicida</i> is more susceptible to many disinfectants.
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