Dilutions for the GEL EC CFU Kit

Add 10 mL sample and 90 mL E. coli free water to Thio-Bag

Pour Aquagenx EC+TC growth medium into Thio-Bag

The Aquagenx GEL EC CFU Kit was originally designed to test drinking water sources. However, the GEL Kit is easily adapted for more contaminated waters with higher concentrations of bacteria such as surface water and wastewater, and for waters with higher allowable limits of bacteria.

This is accomplished by diluting GEL samples. The sections below provide guidance on the sample volume and diluent volume per 100 mL test.

Any dilution beyond 1:100 with high CFU counts means the water is very high risk and is not suitable for drinking without treatment to eliminate the presence of E. coli, or to reduce E. coli  to very low numbers.

Diluent

​Dilution water needs to be free of E. coli, preferably sterile water, such as:

 

  • Standard Methods phosphate buffer
  • Autoclaved or boiled reagent water
  • Autoclaved or boiled tap water
  • High quality, certified bottled water. Some bottled waters are produced locally and are microbiologically unsafe.
  • Distilled water from a commercial source and sold in properly sealed bottles. While not necessarily sterile, it should be free of E. coli.

Surface and Recreational Waters

The Aquagenx Compartment Bag can be used to test rivers, streams, creeks, marine waters and fresh recreational waters for E. coli and total coliforms using a 1:10 dilution.

1:10 Dilution

Steps for 1:10 dilution:

  • Collect 10 mL sample with a pipette
  • Add 10 mL sample to Thio-Bag
  • Add 90 mL diluent to Thio-Bag (fill Thio-Bag to 100 mL fill line)
  • Add our E. coli (EC) growth medium to Thio-Bag and dissolve the medium
  • Pour contents of Thio-Bag into GEL bag
  • Proceed with remainder of testing procedures

→ Use our 1:10 Dilution Instructions for Use 

Wastewater

E. coli can be detected and quantified by the GEL EC CFU Kit in municipal and urban wastewater. 

For such urban wastewater, the dilution factor may need to be 1:100, 1:1,000, 1:10,000 or even more, depending on the anticipated bacteria concentration. When the sample is diluted, any toxic chemicals present in the sample are also diluted. Dilution of those chemicals does not interfere with the growth of the E. coli or total coliforms present in the GEL sample.

1:100 Dilution

If the expected results are in the range of 900-11,000 E. coli or total coliforms/100 mL, use a 1:100 dilution by doing two serial 1:10 dilutions.

Steps for 1:100 dilution:

  • Collect 10 mL sample with a pipette
  • Add 10 mL sample to Thio-Bag
  • Add 90 mL dilutent to Thio-Bag
  • Take 10 mL from the above diluted sample, add it to a second 100 mL Thio-Bag. Add 90 mL of diluent.
  • Add our E. coli (EC) growth medium to the second Thio-Bag and dissolve the medium
  • Pour contents of the second Thio-Bag into the GEL bag and proceed with testing procedures

1:1000 Dilution

If the expected results are in the range of 9000-110,000 E. coli or total coliforms/100 mL, use a 1:1000 dilution by doing three serial 1:10 dilutions.

Steps for 1:1000 dilution:

 

  • Collect 10 mL sample with a pipette
  • Add 10 mL sample to Thio-Bag
  • Add 90 mL dilutent to Thio-Bag
  • Take 10 mL from the above diluted sample, add it to a second 100 mL Thio-Bag, and then add 90 mL of diluent
  • Take a 10 mL sample from the second Thio-Bag, add it to a third 100 mL Thio-Bag, and then add 90 mL of diluent. 
  • Add our E. coli (EC) growth medium to the third Thio-Bag and dissolve the medium
  • Pour contents of the third Thio-Bag into the GEL bag and proceed with testing procedures