The isolation of cultures for confirmation was carried out successfully in all laboratories. The cultures from 99.95% of the observed fluorescent wells of the Colilert-18 method and from 99.61% of fluorescent tubes in the MMPN method were isolated. A total of 2,184 fluorescence-positive wells from Colilert-18 were further cultivated and tested. Of these, 2,130 were confirmed as E. coli (biotypes 1 and 2; Table 2(a)). Of the 2,507 fluorescence-positive wells of the MMPN method tested, 2,458 were confirmed as E. coli (Table 2(b)). 2.5% and 3.3% of the isolates from Colilert-18 and MMPN methods, respectively, were β-glucuronidase positive but did not produce gas from the lactose (biotype 2; Table 2). The false positive rate of both methods was low, being 2.2% and 2.0% for the Colilert-18 and MMPN methods, respectively. Most of the false positive cultures (28 out of 50 in Colilert-18 and 24 out of 49 in MMPN) were found to be indole negative but fluorescent positive (biotypes 5 and 6; Table 2).
Frequencies of the biotypes1–8 based on oxidase test (ox), indole test (ind44), β-glucuronidase test (fluor44) and gas production from lactose test (gas44) in the preliminary positive (fluorescent) wells of (a) the Colilert-18 and (b) MMPN methods
| Laboratory . | N . | 1. Ox− ind44+ fluor44+ gas44 + . | 2. Ox− ind44+ fluor44+ gas44– . | 3. Ox− ind44+ fluor44– gas44 + . | 4. Ox− ind44+ fluor44–gas44– . | 5. Ox− ind44–fluor44+ gas44 + . | 6. Ox− ind44–fluor44+ gas44– . | 7. Ox− ind44–fluor44–gas44 + . | 8. Ox− ind44–fluor44–gas44- . | False positive rate % . |
|---|---|---|---|---|---|---|---|---|---|---|
| (a) Colilert-18 method | ||||||||||
| 1 | 426 | 411 | – | – | 1 | – | 10 | – | 2 | 3.1 |
| 2 | 409 | 376 | 17 | 2 | 1 | 9 | 1 | – | 2 | 3.7 |
| 3 | 343 | 312 | 22 | 1 | 1 | 4 | 1 | – | 2 | 2.6 |
| 4 | 438 | 424 | 10 | – | – | 2 | – | – | – | 0.5 |
| 5 | 176 | 162 | 4 | – | 9 | – | 1 | – | – | 5.7 |
| 6 | 392 | 391 | 1 | – | – | – | – | – | – | 0.0 |
| Total | 2,184 | 2,076 | 54 | 3 | 12 | 15 | 13 | 0 | 6 | |
| Average false positive rate of Colilert-18 method: 2.2% | ||||||||||
| (b) Miniaturised MPN method | ||||||||||
| 1 | 490 | 481 | 3 | 5 | 1 | – | – | – | – | 1.2 |
| 2 | 465 | 405 | 42 | 2 | – | 9 | 4 | 1 | 2 | 3.9 |
| 3 | 408 | 372 | 26 | 1 | – | 5 | 3 | – | 1 | 2.5 |
| 4 | 563 | 555 | 2 | – | – | 3 | – | 2 | 1 | 1.1 |
| 5 | 176 | 163 | 4 | 8 | 1 | – | – | – | – | 5.1 |
| 6 | 405 | 402 | 3 | – | – | – | – | – | – | 0.0 |
| Total | 2,507 | 2,378 | 80 | 16 | 2 | 17 | 7 | 3 | 4 | |
| Average false positive rate of miniaturised MPN method: 2.0% | ||||||||||
| Laboratory . | N . | 1. Ox− ind44+ fluor44+ gas44 + . | 2. Ox− ind44+ fluor44+ gas44– . | 3. Ox− ind44+ fluor44– gas44 + . | 4. Ox− ind44+ fluor44–gas44– . | 5. Ox− ind44–fluor44+ gas44 + . | 6. Ox− ind44–fluor44+ gas44– . | 7. Ox− ind44–fluor44–gas44 + . | 8. Ox− ind44–fluor44–gas44- . | False positive rate % . |
|---|---|---|---|---|---|---|---|---|---|---|
| (a) Colilert-18 method | ||||||||||
| 1 | 426 | 411 | – | – | 1 | – | 10 | – | 2 | 3.1 |
| 2 | 409 | 376 | 17 | 2 | 1 | 9 | 1 | – | 2 | 3.7 |
| 3 | 343 | 312 | 22 | 1 | 1 | 4 | 1 | – | 2 | 2.6 |
| 4 | 438 | 424 | 10 | – | – | 2 | – | – | – | 0.5 |
| 5 | 176 | 162 | 4 | – | 9 | – | 1 | – | – | 5.7 |
| 6 | 392 | 391 | 1 | – | – | – | – | – | – | 0.0 |
| Total | 2,184 | 2,076 | 54 | 3 | 12 | 15 | 13 | 0 | 6 | |
| Average false positive rate of Colilert-18 method: 2.2% | ||||||||||
| (b) Miniaturised MPN method | ||||||||||
| 1 | 490 | 481 | 3 | 5 | 1 | – | – | – | – | 1.2 |
| 2 | 465 | 405 | 42 | 2 | – | 9 | 4 | 1 | 2 | 3.9 |
| 3 | 408 | 372 | 26 | 1 | – | 5 | 3 | – | 1 | 2.5 |
| 4 | 563 | 555 | 2 | – | – | 3 | – | 2 | 1 | 1.1 |
| 5 | 176 | 163 | 4 | 8 | 1 | – | – | – | – | 5.1 |
| 6 | 405 | 402 | 3 | – | – | – | – | – | – | 0.0 |
| Total | 2,507 | 2,378 | 80 | 16 | 2 | 17 | 7 | 3 | 4 | |
| Average false positive rate of miniaturised MPN method: 2.0% | ||||||||||
N, number of cultures tested;−, the isolate tested negative; +, the isolate tested positive.
Biotypes 3–8 were considered negative for E. coli (false positive).