Multidrug-resistant strain of Mycobacterium tuberculosis (MDR-Tb) and Multidrug -sensitive Mycobacterium tuberculosis (MDS-Tb) isolated from humans were injected subcutaneously into the guinea pig. One set of infected lungs of animals was fixed in 10% formalin pH 7.2, while the other set was treated with 75% ethanol, 2 hours prior to fixation in 10% formalin.
After six months of fixation, MDS-Tb strains were completely inactivated by both methods of fixation. MDR-Tb strains resisted inactivation by 10% formalin, but were inactivated by the treatment with 75% ethanol 2 hours prior to 10% formalin fixation.