Interpret  Results with Coliscan S Easygel

This photo shows three different bacterial species as they appear at 24 hrs incubation time on Coliscan® S Easygel® medium.  The three bacteria are Escherichia coli ATCC 25922, Escherichia coli 0157:H7 ATCC 700728, and Citrobacter freundii ATCC 8090.  

                            1 - E. coli 0157:H7         2 - E. coli         3 - Other coliform 

 

 

Each of the quadrants is from a separate dish of Coliscan® S which has been inoculated with one or more of the three listed bacteria.  Each dish of Coliscan® S was from the same batch and lot number of the medium, so were identical in composition and pH.  Therefore, Coliscan® S medium color changes and colony colors are due entirely to the presence and growth of the bacteria with which that dish was inoculated.

 

 

 

A.  The upper right quadrant was inoculated with a mix of all three bacterial types to illustrate how a test sample containing all three types might appear in a single dish of Coliscan® S medium. 

 

 

 

B.  The lower right quadrant was inoculated with only E. coli 0157:H7.  Note that since this bacterium is sorbitol negative, it produces no acid of sorbitol fermentation and grows as dark pink CFUs without any yellow halos or yellowing of the medium in general.  In fact, it apparently produces basic byproducts which raise the pH of the medium (making it more pink).  This bacterium is glucuronidase negative and galactosidase positive, so CFUs are pink in this medium (the same as in standard Coliscan® Easygel®).

 

 

 

D.  The upper left quadrant was inoculated with only Citrobacter freundii, a general coliform.  Since it ferments sorbitol, the pink CFUs are surrounded by a yellow halo and the entire medium has a color shift towards yellow due to the acid from the numerous bacteria growing in that dish. This bacterium is glucuronidase negative and galactosidase positive, so CFUs are pink in this medium (the same as in standard Coliscan® Easygel®). 

 

 

 

C.  The lower left quadrant was inoculated with only E. coli 25922, which is glucuronidase positive and galactosidase positive (representative of 93-97% of all E. coli types).  Since it ferments sorbitol, the blue/purple CFUs are surrounded by a yellow halo and (in this dish since there are so many colonies) the entire medium has assumed a yellow color due to the acid from the fermentation of the sorbitol. 

 

 

 

It is important to understand the basic principles involved when using Coliscan® S.  First, if the dish is crowded with large numbers of sorbitol fermenters, the entire dish color will rapidly turn to yellow and it will be very difficult to acertain if any sorbitol negative colonies (E. coli 0157) are present in the test sample.  Therefore, the sample must be diluted sufficiently that the resulting CFUs in the dish are separated by at least several millimeters of space.  Also, the dishes should be incubated at 35-37 C and should be checked as early as 18-20 hours as early colony appearance and differentiation are often the most distinctive.  If colonies are well separated upon early examination, the allowance of further incubation time (to 24-30 hrs) will result in a size increase and colors will intensify. 

 

Coliscan® S Easygel® is meant to be selective for gram negative coliform bacteria and  differentiates between glucuronidase and glucuronidase bacteria as does standard Coliscan® Easygel®.  Therefore, it can be used for the same purpose as standard Coliscan® Easygel®.  In addition, Coliscan® S Easygel® differentiates sorbitol negative from sorbitol positive bacteria and so is useful in screening for sorbitol negative toxigenic (E. coli 0157 and others) coliform bacteria.  Coliscan® S cannot differentiate specific different toxigenic coliform strains, and is not claimed to be an absolute confirmation method for E. coli 0157 and similar toxigenic coliforms.   It is meant to be a useful screening tool for the presence of sorbitol negative coliform bacteria and if such organisms show up in a test sample and further information on their specific identity is desired, the cultures should be sent to a diagnostic laboratory equipped to provide that information. 

 

This photo shows three different bacterial species as they appear at 24 hrs incubation time on Coliscan® S Easygel® medium.  The three bacteria are Escherichia coli ATCC 25922, Escherichia coli 0157:H7 ATCC 700728, and Citrobacter freundii ATCC 8090. 

 

 

 

Each of the quadrants is from a separate dish of Coliscan® S which has been inoculated with one or more of the three listed bacteria.  Each dish of Coliscan® S was from the same batch and lot number of the medium, so were identical in composition and pH.  Therefore, Coliscan® S medium color changes and colony colors are due entirely to the presence and growth of the bacteria with which that dish was inoculated.

 

 

 

The upper left quadrant was inoculated with a mix of all three bacterial types to illustrate how a test sample containing all three types might appear in a single dish of Coliscan® S medium. 

 

 

 

The lower left quadrant was inoculated with only E. coli 0157:H7.  Note that since this bacterium is sorbitol negative, it produces no acid of sorbitol fermentation and grows as dark pink CFUs without any yellow halos or yellowing of the medium in general.  In fact, it apparently produces basic byproducts which raise the pH of the medium (making it more pink).  This bacterium is glucuronidase negative and galactosidase positive, so CFUs are pink in this medium (the same as in standard Coliscan® Easygel®).

 

 

 

The upper right quadrant was inoculated with only Citrobacter freundii, a general coliform.  Since it ferments sorbitol, the pink CFUs are surrounded by a yellow halo and the entire medium has a color shift towards yellow due to the acid from the numerous bacteria growing in that dish. This bacterium is glucuronidase negative and galactosidase positive, so CFUs are pink in this medium (the same as in standard Coliscan® Easygel®). 

 

 

 

The lower right quadrant was inoculated with only E. coli 25922, which is glucuronidase positive and galactosidase positive (representative of 93-97% of all E. coli types).  Since it ferments sorbitol, the blue/purple CFUs are surrounded by a yellow halo and (in this dish since there are so many colonies) the entire medium has assumed a yellow color due to the acid from the fermentation of the sorbitol. 

 

 

 

It is important to understand the basic principles involved when using Coliscan® S.  First, if the dish is crowded with large numbers of sorbitol fermenters, the entire dish color will rapidly turn to yellow and it will be very difficult to acertain if any sorbitol negative colonies (E. coli 0157) are present in the test sample.  Therefore, the sample must be diluted sufficiently that the resulting CFUs in the dish are separated by at least several millimeters of space.  Also, the dishes should be incubated at 35-37 C and should be checked as early as 18-20 hours as early colony appearance and differentiation are often the most distinctive.  If colonies are well separated upon early examination, the allowance of further incubation time (to 24-30 hrs) will result in a size increase and colors will intensify. 

 

Coliscan® S Easygel® is meant to be selective for gram negative coliform bacteria and  differentiates between glucuronidase and glucuronidase bacteria as does standard Coliscan® Easygel®.  Therefore, it can be used for the same purpose as standard Coliscan® Easygel®.  In addition, Coliscan® S Easygel® differentiates sorbitol negative from sorbitol positive bacteria and so is useful in screening for sorbitol negative toxigenic (E. coli 0157 and others) coliform bacteria.  Coliscan® S cannot differentiate specific different toxigenic coliform strains, and is not claimed to be an absolute confirmation method for E. coli 0157 and similar toxigenic coliforms.   It is meant to be a useful screening tool for the presence of sorbitol negative coliform bacteria and if such organisms show up in a test sample and further information on their specific identity is desired, the cultures should be sent to a diagnostic laboratory equipped to provide that information.