When we take a microbiological sample from a source, we want to answer the questions:
- “Is there something here?”
- “Is it a lot or a little?”
- “What are they doing and is it bad for my process?”
Understanding Sample Volume
The tools we use to answer these questions, ATP, qPCR, and sequencing, give us varying depths of information. We need to know the sample volume to test to answer our questions successfully.
- Microbial Load: Sampling too little risks missing microbes outside the assay's Limit of Detection (LoD), resulting in a false negative
- Inhibition Risk: Sampling too much can introduce inhibitors, leading to a failed assay
Balancing between these two intrinsically linked values (microbial load and inhibition risk) determines a good sample size.
Finding the Right Balance
Sampling is similar to taking a census. In rural areas, covering a large area is necessary to gather a significant sample size. In densely populated cities, less ground needs to be covered to interview the same number of people. Similarly, we don't aim to sample everyone but we aim to sample enough to represent the greater population.
Considering Inhibition Risk
In reality, the microbial load is unknown, so sample size depends on perceived inhibition risk. Factors like dissolved solids, acidity, and hydrocarbons inform this decision.
It is important to remember that the purpose of assaying your sample is not to collect as many microbes as possible but to gain enough information to draw an accurate picture of the population. We use direct experimentation for new sample types where we cannot make an educated guess about inhibition.
Sampling Strategy
- Start with a wide range of sample volumes, starting with a high volume where a detectable microbial load is likely
- Gradually decrease the volume to a point where inhibition is unlikely to occur