A research group is studying a bacterium X that binds to mucosal cells in the lung and invades. Wildtype X has an LD 50 value of 10 bacteria when administered to mice by inhalation. Using transposon...

A research group is studying a bacterium X that binds to mucosal cells in the lung and invades. Wildtype X has an LD⁵⁰
value of 10 bacteria when administered to mice by inhalation. Using transposon mutagenesis, the researchers have isolated two mutants of X that they call Xmut1 and Xmut2, both of which have LD⁵⁰
values of 105 when inhaled by mice. However, in tissue culture cells, Xmut1 can invade the cells just as well as wild-type X, while Xmut2 cannot. Provide a possible explanation for these results.