Growing a water sample in a culture is performed for two interconnected reasons:
- Amplification
Surface water from which our samples are taken contain bacteria, but usually at very low concentrations. Directly identifying and counting such tiny numbers under a microscope is practically impossible and anyway highly unreliable. Provided a nutrient-rich medium and optimal temperature, each individual bacterial cell in the sample can multiply into a visible colony containing millions of cells. This transforms an invisible target into something that can be seen and counted.
- Selective Detection
A water sample contains a mixed community of bacteria, most of which are naturally occurring and harmless. Our goal in testing for bacteria in water is to find specific indicators of fecal contamination.- Selective. Media like mFC agar for fecal coliforms or MI agar for enterococcus contain antibiotics or other chemicals that inhibit the growth of most non-target environmental bacteria while allowing the target bacteria to grow.
- Differential. Media often include dyes or other color-changing indicators, such as enzymes, that make the target bacteria easily identifiable. For example, E. coli colonies turn a distinctive blue on MI agar.
Serial dilutions and counting colonies permit estimation of the original number of bacteria in the water sample.
Q-Tray / MPN vs Petri dish / CFU
At some point in the early 2020's CARO (the lab) began using Q-Trays instead of Pertri dishes (?) for our coliform cultures. This makes no difference for our confidence in the analyses the company performs, but it changes the names of bacteriological analytes, by adding "(Q-Tray)" to them. This would have the effect of separating the results posted to this database into 2 groups when reported by analyte. So in order to keep them together, "(Q-Tray)" has been removed prior to uploading; it can still be seen in the units as MPN/100 mL rather than CFU/mL, and of course in the original PDF reports.
Both Q-Trays and Petri dishes are used to grow bacteria, but they serve different purposes and scales in the microbiology lab. The fundamental difference is available surface area.
A Petri dish is used when there is a need to see, isolate, and work with individual bacterial colonies.
A Q-Tray (or Quanti-Tray) is used when there is a need to accurately count (especially low numbers of) bacteria in a large volume of liquid, such as when testing drinking water for fecal contamination.
FEATURE | Petri DIsh | Q-Tray |
Output | Colony Forming Units (CFU/100 mL) | Statistically Most Probable Number (MPN/100 mL) |
Primary Use | General microbiology: isolation, identification; research, clinical diagnostics | Quantitative water/food microbiology: counting initially low concentrations of bacteria in liquids. |
Format | Single, round plate (typically 90-100 mm diameter). | Rectangular, compartmentalized tray with 49 or 97 small wells |
Inoculation | Small sample volume (0.1-1 mL) onto solid agar | Large sample volume (~100 mL) mixed with liquid medium, poured into tray, heat-sealed |
Counting Method | Individual colonies (CFUs) on agar surface. Limited to ~30 - 300 colonies for accuracy. | Most probable number of fluorescent or coloured wells. Statistics convert the pattern of positive wells into MPN/100 mL. Much higher upper limit. |
Volume | Small (e.g. 0.1-1 mL) | Large (e.g., 100 mL) |
Throughput | Manual: each dish poured, inoculated, and counted individually. | Automated: faster for large volume. filling / sealing semi-automated; counting done with UV light. |
Modern Context & Alternatives
While culture has been the gold standard for over a century, it is slow (takes 18-48 hours) and only detects bacteria that can grow under the provided conditions.
Molecular methods such as PCR and qPCR detect specific bacterial DNA/RNA in a few hours, so they are faster and can detect viable, but-non-culturable, cells. But they are more expensive, complex, and currently don't distinguish as easily between live and dead cells (which is crucial for assessing recent contamination).
Enzyme Substrate Tests use specific nutrients that produce a color change when metabolized by target bacteria (e.g., Colilert®). These are still culture-based but are faster and simpler.
Culture remains the legally defined, standardized, and most widely used method for regulatory compliance worldwide.