Appendix 



2. On an Agar Plate: 



a. The agar plate should contain the test organism in pure culture. 



b. Place a drop (approximately 0.05 ml) of the cephalosporin substrate on an area 

 of bacterial growth, and tilt the plate slightly so that the drop drains across the 

 surface of the growth. 



c. If the test organism produces j3-lactamase, the substrate along the path will turn 

 red; if it is j3-lactamase negative, the substrate along the path will be yellow. 



d. Although the red reaction on the plate is not as easily seen as that in the micro- 

 dilution well, it is generally quite obvious. If the analyst is unsure about the reac- 

 tion, the test should be repeated in a microdilution well or small tube. 



e. Usually a positive reaction can be read immediately, but the plate should be 

 reexamined after 10 min. 



C. Precautions 



1. Primary isolation media (e.g., modified Thayer-Martin medium) may grow (3-lactamase- 

 producing bacteria in addition to the organism of interest (e.g.. Legionnaires' disease 

 bacterium), which may lead to false-positive results. 



2. Clinical specimens (e.g.. pleural fluids) cannot be evaluated directly with the chro- 

 mogenic cephalosporin test. 



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