The significance of lactose fermentation and its relationship to resistance in Klebsiella pneumoniae.

In experiments similar to those of Humphries (1944), in which we used brilliant green to obtain smooth and rough phase variants from mucoid strains of Friedlander's bacillus, it was found that in nearly all of the field strains a correlation existed between resistance to the dye and ability to ferment lactose. It was found also that this correlation extended to growth rate and to growth in extreme ranges of pH. The correlation was such that lactose positive strains of Friedlander's bacillus were more resistant to brilliant green, had a faster growth rate, and were better able to grow in extreme pH ranges than were the lactose negative strains which were studied, In view of this correlation and with an eye toward its practical application, the lactose positive and negative strains were compared in respect to their sensitivity to various antibiotics. A limited investigation of the comparative pathogenicity of the strains also was undertaken. It was found also that when serologically typeable lactose negative strains of KlebsieUa pneumoniae were grown in the presence of increasing concentrations of brilliant green, two patterns of resistance appeared. In the first, which occurred quite commonly, the resistance increased without any increased growth rate or increased tolerance to pH changes; in fact, in some cases a decrease in growth rate was observed. There was also no change toward an ability to ferment lactose. The second pattern occurred quite rarely; indeed, it was observed only five times in the course of these studies. In this pattern, the resistance increased quite rapidly, and with this increase there was also a gain in the ability to ferment lactose, an increased growth rate, and a greater tolerance to growth in extreme pH ranges.