LaMotte EZ 3-0034 - Manual (Page 4)

11. Colonies will begin to appear after 24 hours. The best time to count colonies is
after 30 hours of incubation. Colonies should not be counted after 72 hours.
Count the colonies with the dish upside down. Colonies are normally reported as
Colony Forming Units (CFU) per 100 milliliter of water. Colonies may appear
inside the medium. Count all colonies growing on the surface and within the
medium.
12. To determine the result as E. coli or Fecal Coliform count the dark blue to
purple colonies. Disregard any light-blue, blue-green or white colonies. Refer to
the ColiQuant EZ Colony Color Guide.
• To report the result in terms of E. coli or Fecal Coliform per mL of water: Divide
the number of colonies by the sample size
• To report the results in terms of E. coli or Fecal Coliform per 100 mL of water
(Most state agencies use this measurement): Multiply the number of colonies
by 100. Then divide by the sample size.
13. To determine the result as Total Coliforms (E. coli + non-fecal coliforms) count
the number of pink and purple colonies.
• To report the result in terms of Total Coliforms per mL of water: Divide the
number of colonies by the sample size
• To report the results in terms of Total Coliforms per 100 mL of water: Multiply
the number of colonies by 100. Then divide by the sample size.
14. If there are more than 300 CFU (colonies) of one color, the result should be
recorded as Too Numerous To Count (TNTC). If the E. coli purple colony count is
less than 300, even though there are more than 300 total colonies of all colors
and types, the purple colonies can be counted and a result given for E. coli.
Record TNTC for coliforms or non-coliforms that are present at greater than 300
CFU.
Total Coliform Colonies
per mL (CFU/mL) # Colonies
Amount of sample used
=
Total Coliform Colonies
per 100mL (CFU/100mL) # Colonies x 100
Amount of sample used
=
E. Coli or Fecal Coliform
Colonies per mL (CFU/mL) # Colonies
Amount of sample used
=
E. Coli or Fecal Coliform
Colonies per 100mL
(CFU/100mL)
# Colonies x 100
Amount of sample used
=
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