This study was performed to compare the selectivity of the fifteen-tube Standard Methods
most probable number (MPN) lactose-fermentation confirming step to the sensitivity of
the enzyme B-galactosidase which is the confirming factor for total coliforms in the
chromogenic substrate Quanti-Tray MPN method available from Colilert. This was
accomplished by running split samples utilizing both methods over a five-month period
which included different levels of stratification and algal blooms in a large reservoir. A
total of 73 samples were run in parallel over this period. The average MPN value using
the Quanti-Tray system was 2020 while the average using the Standard Methods 15-tube
MPN was 14. The highest MPN values were 17,890 with the Quanti-Tray system and 109
with the Standard Methods MPN. The average number of tubes producing gas increases
from 4 to 13 when the lauryl tryptose broth tubes and the brilliant green lactose bile
broth tubes were allowed an additional 120 hours of incubation in the Standard Methods
MPN. This would have increased the average MPN value to 900 with gas being
produced in 5, 5, and 3 tubes in the confirmation step. Isolates from tubes that produced
gas in 48 hours were speciated as Escherichia coli, Klebsiella pneumoniae, Klebsiella
oxytoca, Enterobacter cloacea, and Citrobacter freundii. Isolates from tubes producing
gas after the extended incubations were speciated as Enterobacter aerogenes, Enterobacter agglomerans, Enterobacter cloacae, and Enterobacter amnigenus. Isolates
from tubes with growth without gas were speciated as Hafnia alvei which is also a total
coliform. All isolates, that were taken from positive wells from the Quanti-Trays and
were speciated, matched those total coliforms isolated from the Standard Methods MPN
before and after the extended incubations. The results of this study show that the
chromogenic substrate Quanti-Tray method that relies on the enzyme galactosidase as the
confirming factor for total coliforms is more accurate than the lactose-fermentation
confirming step for the Standard Methods MPN. The Standard Methods 15-tube MPN
method does not enumerate the slow and non-lactose fermenting coliforms in the
environment.
Includes 14 references, tables.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 240 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 6 |
| Published : | 11/01/2002 |
