Delayed Lactose Fermentation by Enterobacteriaceae

Abstract

Goodman , R. E. (University of California, Los Angeles), and M. J. Pickett . Delayed lactose fermentation by Enterobacteriaceae . J. Bacteriol. 92: 318–327. 1966.—When 171 Citrobacter freundii strains and 14 Paracolobactrum arizonae strains examined, 51 of the C. freundii strains and 13 of the P. arizonae strains were found to be delayed or negative lactose fermenters. Of the slow fermenters, 65% yielded rapidly fermenting mutants in cultures undergoing delayed fermentation. Lactose fermentation could generally be hastened by increasing lactose concentrations. Many organisms which fermented lactose slowly grew readily on a medium containing lactose as the sole carbon source. Regardless of their ability to ferment lactose, all strains of C. freundii and P. arizonae investigated could be shown to possess β-galactosidase. Delayed fermenters failed to take up lactose from the culture medium, whereas prompt fermenters did so readily. The β-galactosidases of 12 strains of enteric bacteria were studied in crude cell extracts with respect to specific activity, stability, and activity at varying substrate ( o -nitrophenyl-β- d -galactopyranoside) concentrations, at varying p H, and in the presence of sodium, potassium, and magnesium. The widely varying specific activities and the approximate similarity of the Michaelis constants (about 2 × 10 −4 m ) suggested that the strains investigated produced differing amounts of β-galactosidase. Moreover, qualitative differences in the enzymes provided evidence that these strains synthesized different molecular forms of β-galactosidase. The results suggested that organisms which ferment lactose only after a prolonged delay do so because they possess multiple defects in their lactose-metabolizing machinery.