Simple, Low-Cost and Effective E.coli Test Products
The simple, low-cost and effective water quality tests that we are comparing in Kathmandu, Nepal during January 2019 include Quanti-Tray® 2000 and five to six different potential products for the EC-Kit 2.0, as follows:
Quanti-Tray®
IDEXX Quanti-Tray® 2000 is an enzyme-substrate coliform test (Standard Methods 9223) that uses semi-automated quantification methods based on Most Probable Number (MPN).
The MPN method uses multiple qualitative (presence/absence) data points to generate a maximum probability coliform count per 100 mL value, given by a standard MPN table.
Colilert is the patented Defined Substrate Technology® (DST®) medium that is used Quanti-Tray® 2000 system. It allows the user to simultaneously detect total coliforms and E. coli. Two nutrient-indicators, ONPG and MUG, are the major sources of carbon in Colilert and can be metabolized by the coliform enzyme β-galactosidase and the E. coli enzyme β-glucuronidase, respectively.
Quanti-Tray® 2000 provides bacterial counts of total coliform and E.coli as low as 1 MPN/100 mL and up to 2419 MPN/100 mL. Quanti-Tray® 2000 can test both chlorinated and non-chlorinated samples. However, chlorinated samples should first be treated with sodium thiosulfate before the Quanti-Tray® reagent is added.
Quanti-Tray® 2000 is easy-to-use, rapid and accurate, and has been approved by the United States Environmental Protection Agency (EPA) for drinking, source/surface, ground, and waste- waters. However, one of its main drawbacks is its cost: Quanti-Tray® requires the use of an expensive sealer (≈$4000), and the trays and reagents are expensive ( ≈$10/test) which makes this method generally unaffordable in low income countries such as Nepal.
P/A Vial (H2S) Test
The hydrogen sulfide (H2S) bacteria test using the original M1 medium is a well-known, simple and low-cost P/A test, developed by Manja, Maurya and Rao (1982). The test identifies the presence of H2S-producing bacteria, associated with fecal contamination in a volume of water, which has been shown to correlate with the presence of fecal contamination. Venkobachar, Kumar, Talreja, Kumar, and Iyengar (1994) later developed a second test medium, M2, which consisted of the original M1 medium with the addition of L-cystine, which was shown to increase the sensitivity and reliability of the H2S test.
Since the H2S test reagent includes a chlorine-neutralizing compound, sodium thiosulfate, the H2S test is a suitable microbiological test for chlorinated water supplies. The dehydrated H2S test reagent for this program of testing will include an 8 mL and a 20-mL sample volumes. The P/A vials do not detect E.coli, however, we intend to pair the P/A vials with Compact Dry ECO 1 mL sample size.
Compact Dry ECO[1]
Compact Dry™ ECO is a ready-to-use test method recommended for the isolation and enumeration of coliforms and E. coli in raw materials, finished products, or environmental samples.
Compact Dry™ ECO uses a chromogenic medium for performing E. coli and coliform counts. It includes a dehydrated culture media with a cold water-soluble gelling agent in a non-woven cloth matrix. The medium is instantly hydrated when inoculated with a sample, and capillary action diffuses the sample evenly over the matrix to form a gel within seconds. Compact Dry™ ECO contains two chromogenic substrates, Magenta-Gal and X-Gluc that produces a color reaction allowing the differentiation of E. coli from other members of the coliform group.
Compact Dry™ EC performs comparably to the FDA MPN method for enumeration of E. coli and coliform bacteria. (6) Compact Dry™ ECO is AOAC validated (AOAC no. 110402) and the ready-to-use trays save space and greatly reduce the time needed to perform microbiological testing. Compared to other commonly used culture systems, Compact Dry ™ has a longer shelf life, can be stored at room temperature, does not require manual sample spreading, is rigid, stackable and easy to label, and allows for direct colony picking for further subculture.
Formula
Compact Dry™ EC contains dehydrated culture media, a gelling agent, and the chromogenic substrates, 5-bromo- 6-chloro-3-indoxyl-beta-D-galactopyranoside (Magenta-Gal) and 5-bromo-4-chloro-3-indoxyl-beta-D-glucuronic acid, cyclohexylammonium salt (X-Gluc) to assist in differentiating E. coli from other coliforms.
Final pH 7.0 +/- 0.2 at 25°C
Storage and Shelf Life
Storage: Upon receipt, store at 1-30ºC away from direct light. Media should not be used if there are any signs of deterioration, contamination, or if the expiration date has passed. Product is light and temperature sensitive; protect from light, excessive heat, moisture, and freezing. If foil pouch is opened and not all plates are used, return remaining plates to pouch and reseal until next use. Opened packages should be used as soon as possible.
The expiration dating on the product label applies to the product in its intact packaging when stored as directed. The product may be used and tested up to the expiration date on the product label and incubated for the recommended quality control incubation times.
Refer to the document "Storage" on the Hardy Diagnostics Technical Document website for more information: https://catalog.hardydiagnostics.com/cp_prod/Content/hugo/CompactDryEC.html.
ECC
TestDropPro
TestDropPro is a small handheld water-contaminant detecting device developed by Lishtot. It uses triboelectric fields to determine risk levels. It enables the user to test three different types of water -- bottled water, tap water and natural waters -- with different parameters for each type of test. The entire test can be performed in less than 1 minute with near-instantaneous results which compares favorably with conventional water tests which can take up to 24 hours or more to obtain results. The TestDropPro allows you to test any type of water (chlorinated, non-chlorinated etc.) when poured into a clear plastic cup or in a water bottle and swirled for 10 seconds. Selecting the correct button for bottled water, tap water or natural water, a blue or red light will flash, indicating whether that sample is safe (blue) or contaminated (red). The TestDrop Pro measures a number of contaminants concurrently including E. coli, metals and pesticides. The device has the capability to connect to a smartphone app which displays a risk scale from safe to medium risk to high risk. There is no contact of the TestDropPro directly with the water sample.
Resorufin β-D-Glucuronide (REG)
Resorufin β-D-glucuronide (REG) is a chromogenic test that produces a characteristic bright pink color after 24 hours incubation. This bright pink color change indicates the presence of E. coli. In combination with a medium to promote bacteria growth, this chromogen is only sensitive to E. coli. The synthesis protocols are open source and the resulting product is low-cost. Thus, utilizing REG will lower the cost of an E. coli specific P/A test and make it affordable in low-income countries such as Nepal.
[1] Conpact Dry text below adapted from https://catalog.hardydiagnostics.com/cp_prod/Content/hugo/CompactDryEC.html and https://hyserve.com/produktgruppe.php?lang=en&gr=1