Bacterial interference with coliform colony sheen production on membrane filters

Area of Science:

• Microbiology
• Environmental Science
• Water Quality Analysis

Background:

• The membrane filter (MF) method is standard for detecting coliform bacteria in drinking water.
• Coliform detection relies on colony growth and a characteristic green metallic sheen on modified Endo medium.
• Noncoliform bacteria, enumerated by standard plate count (SPC), can interfere with MF coliform detection.

Purpose of the Study:

• To quantitatively assess the interference of specific SPC bacteria on coliform sheen production using the MF method.
• To investigate the impact of bacterial density and specific species on coliform enumeration accuracy.

Main Methods:

• Laboratory experiments were conducted using the MF method with modified Endo medium.
• Specific noncoliform bacteria (Pseudomonas aeruginosa, Aeromonas hydrophila, Flavobacterium sp., Bacillus sp.) were introduced at varying concentrations.
• Escherichia coli and Enterobacter cloacae were used as target coliforms, and sheen production was quantified.

Main Results:

• Pseudomonas aeruginosa and Aeromonas hydrophila significantly reduced Escherichia coli sheen colony counts at concentrations of 3,000 and 220 per filter, respectively.
• Flavobacterium sp. and Bacillus sp. did not interfere with coliform sheen production.
• Excessive crowding of coliform colonies on MF filters also reduced sheen production, even for Enterobacter cloacae at low densities.

Conclusions:

• Specific noncoliform bacteria can interfere with the accurate enumeration of coliforms via the MF method.
• Bacterial density and species composition are critical factors affecting the reliability of coliform detection in drinking water.
• Further research may be needed to optimize MF methods for diverse water matrices with high noncoliform loads.