Which of the following microbial enumeration methods is MOST suitable for detecting low concentrations of *E. coli* in a large water volume?

The most suitable microbial enumeration method for detecting low concentrations of *E. coli* in a large water volume is the Most Probable Number (MPN) method. Microbial enumeration refers to determining the number of microorganisms present in a sample. *E. coli* is a bacterium commonly used as an indicator of fecal contamination in water, meaning its presence suggests potential health risks. A large water volume necessitates a method capable of processing significant sample sizes efficiently.

The MPN method is a statistical technique, not a direct count, used to estimate the concentration of bacteria in a sample when direct plating is impractical or unsuitable, particularly when dealing with low numbers. It relies on the principle that the more bacteria present in a sample, the higher the probability of observing positive results in a series of dilutions and replicate tubes containing a growth medium. The process involves performing serial dilutions of the water sample – progressively reducing the concentration of the sample – and inoculating multiple tubes (typically five) with each dilution. A growth medium, like lactose broth, is used, which allows *E. coli* to grow and produce gas if present. After incubation at a specific temperature (usually 35°C), tubes showing gas production are considered positive.

The results are then compared to a statistical table (MPN table) to determine the most probable number of bacteria per unit volume of the original water sample. For example, if three out of five tubes are positive in a particular dilution, the MPN table would provide an estimated concentration range. The MPN method is particularly well-suited for detecting low concentrations because it can handle large volumes of water, which increases the chance of detecting even a few bacteria. Direct plating methods, like spread plating or membrane filtration, can be less sensitive when dealing with very low concentrations, as a single bacterium might not form a visible colony on the agar plate. Membrane filtration, while useful, requires filtering a large volume through a filter, which can be time-consuming and may clog with debris present in the water. The MPN method avoids this issue by using multiple tubes and dilutions, increasing the likelihood of detecting even rare bacteria. Other methods, such as direct microscopic counts, are not specific to *E. coli* and would count all bacteria present, not just the target organism. Therefore, the MPN method’s statistical approach and ability to process large volumes make it the most appropriate choice for this scenario.