A random selection of the presumptive E. coli-negative but coliform bacteria-positive wells in the Colilert-18 trays was isolated for confirmation. In total, 1,244 of the 5,706 yellow but not fluorescent wells were tested (22%). Of these, most cultures were indole and fluorescent negative upon confirmation, but 91 cultures (7.3%) were classified into biotypes 1 and 2 and regarded as positive for E. coli (Table 3). The false negative rate was higher than 10% in laboratories 1 and 4, which analysed only natural samples (Table 3). In other laboratories, which also analysed samples spiked with waste water, the false negative rate remained below 10%.
Frequency of the biotypes 1–8 based on oxidase test (ox), indole test (ind44), β-glucuronidase test (fluor44) and gas production from lactose test (gas44) in the preliminary negative results of the Colilert-18 method
| 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– . | Non-coliform (ox+) . | False negative rate (%) . | Natural samples (%) . |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 236 | 27 | 1 | 1 | 15 | – | 2 | 2 | 115 | 73 | 11.9 | 100 |
| 2 | 223 | 13 | – | 2 | 25 | 1 | 1 | 8 | 135 | 38 | 5.8 | 37 |
| 3 | 238 | 7 | – | 5 | 28 | – | 3 | 10 | 149 | 36 | 2.9 | 4 |
| 4 | 180 | 20 | 3 | 3 | 26 | – | – | 1 | 90 | 37 | 12.8 | 100 |
| 5 | 176 | 6 | 0 | 6 | 10 | 3 | 3 | 2 | 75 | 71 | 3.4 | 78 |
| 6 | 191 | 14 | 0 | 1 | 6 | 2 | – | 1 | 64 | 103 | 7.3 | 17 |
| Total | 1,244 | 87 | 4 | 18 | 110 | 6 | 9 | 24 | 628 | 358 | 7.3 | 55 |
| 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– . | Non-coliform (ox+) . | False negative rate (%) . | Natural samples (%) . |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 236 | 27 | 1 | 1 | 15 | – | 2 | 2 | 115 | 73 | 11.9 | 100 |
| 2 | 223 | 13 | – | 2 | 25 | 1 | 1 | 8 | 135 | 38 | 5.8 | 37 |
| 3 | 238 | 7 | – | 5 | 28 | – | 3 | 10 | 149 | 36 | 2.9 | 4 |
| 4 | 180 | 20 | 3 | 3 | 26 | – | – | 1 | 90 | 37 | 12.8 | 100 |
| 5 | 176 | 6 | 0 | 6 | 10 | 3 | 3 | 2 | 75 | 71 | 3.4 | 78 |
| 6 | 191 | 14 | 0 | 1 | 6 | 2 | – | 1 | 64 | 103 | 7.3 | 17 |
| Total | 1,244 | 87 | 4 | 18 | 110 | 6 | 9 | 24 | 628 | 358 | 7.3 | 55 |
N, number of cultures tested; −, the isolate tested negative; +, the isolate tested positive.
Biotypes 1 and 2 were considered positive for E. coli (false negative).