Beta-galactosidase for distinguishing between Citrobacter and Salmonella

Mechanisms of Escherichia coli inactivation by several disinfectants

The objective of this study was to elucidate dominant mechanisms of inactivation, i.e. surface attack versus intracellular attack, during application of common water disinfectants such as ozone, chlorine dioxide, free chlorine and UV irradiation. Escherichia coli was used as a representative microorganism. During cell inactivation, protein release, lipid peroxidation, cell permeability change, damage in intracellular enzyme and morphological change were comparatively examined. For the same level of cell inactivation by chemical disinfectants, cell surface damage was more pronounced with strong oxidant such as ozone while damage in inner cell components was more apparent with weaker oxidant such as free chlorine. Chlorine dioxide showed the inactivation mechanism between these two disinfectants. The results suggest that the mechanism of cell inactivation is primarily related to the reactivity of chemical disinfectant. In contrast to chemical disinfectants, cell inactivation by UV occurred without any changes measurable with the methods employed. Understanding the differences in inactivation mechanisms presented herein is critical to identify rate-limiting steps involved in the inactivation process as well as to develop more effective disinfection strategies.

Salmonella screening procedure with tests for beta-galactosidase and flagellar antigens

Differentiation of Salmonella from other gram-negative bacilli requires several biochemical and serological tests. A simplified 24-hr screening procedure has been devised which allows discarding of large numbers of isolates (picked from selective plating media) before they are subjected to this extensive testing. Cultures of gram-negative organisms isolated to triple sugar-iron slants during routine examination of products for Salmonella were tested for the presence of beta-galactosidase and Salmonella flagellar antigens. beta-Galactosidase-positive cultures which did not agglutinate in polyvalent flagellar antiserum were considered to be nonsalmonellae. Of 1,103 Salmonella cultures tested, none of the 61 different serotypes was missed by this procedure, whereas 673 (82.3%) of 818 nonsalmonellae were excluded from further testing. This screening procedure eliminates most nonsalmonellae and augments the proportion of cultures undergoing further biochemical and serological testing which will be confirmed as Salmonella.

Identification of Enterobacteriaceae in the clinical microbiology laboratory

A total of 5,137 enterobacterial clinical isolates were examined with three series of biochemical tests. The data were analyzed with respect to the use of economical and practical procedures for the accurate identification of lactose-fermenting and -nonfermenting Enterobacteriaceae within 24 hr after isolation.

Selective inhibition of Escherichia coli in the presence of Salmonella typhimurium by phenethyl beta-D-galactopyranoside

Phenethyl beta-d-galactopyranoside (PEG) was hydrolyzed by the beta-galactosidase of Escherichia coli to form the toxic product phenethyl alcohol. Salmonella typhimurium did not hydrolyze PEG. In mixed culture, the ratio of S. typhimurium to E. coli was increased by growing the organisms in lactose broth containing 2.5% PEG. The high concentration of PEG required for inhibition of E. coli can be attributed to inadequate cell permeability rather than to prevention of beta-galactosidase induction.

Delayed Lactose Fermentation by Enterobacteriaceae

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.