The virulence of isoniazid resistant tubercle bacilli, recovered from patients during treatment.

For infection of animals control-cultures from the sensitivity tests were used. These were always the first subcultures on solid egg medium about four weeks old. The cultures were resistant to isoniazid, but susceptible to streptomycin, para-aminosalicylic acid and conteben. They were weighed wet, suspended in normal saline and injected subcutaneously in the right inguinal region. The guinea pigs were killed after different periods. If the degree of severity of tuberculosis for each organ Is designated with the numbers 0.5-4, it is possible, to lay down an index of the macroscopic tuberculosis for each guinea pig, which represents the sum of the numbers obtained from the different organs: inguinal nodes right and left, iliac nodes, spleen, liver, lungs and paratracheal nodes. (Fust a. Studer, Meissner, Steenken a. Wolinsky.) If the same seven organs are cultivated separately, it is possible, to lay down a cultural index for tuberculosis for each guinea pig, which represents the sum of the positive organs. (Meissner.) The isoniazid susceptibility of each culture (the primary cultures from the sputum as well as each culture from the different organs from guinea pigs) was tested on solid egg medium (modified Hohn IV, diluted with liquid Sauton medium) as indicated by the “Deutsches Zentralkomitee zur Bekaempfung der Tuberkulose”. From the primary egg medium culture about 0.5 mg. (wet weight) per ml. were suspended in normal saline (turdibity comparison), one loopful (3 mm.) from this dilution was inoculated on each medium. The tubes were plugged with cotton wool, which was dipped in molted paraffin. Readings were taken after three weeks incubation at 37 degrees C. The different degrees of susceptibility were defined as follows: Susceptible =no growth on solid medium, containing 0.1 mcgm. isonlazid per ml. Nearly susceptible= growth on medium, contaIning 0.1 mcgm., no growth on medium, containing 1.0 mcgm. isoniazid per ml.