The ATP concentration ([ATP]) per cell is relatively constant at 1 pg (10‐15g) active cell‐1 for bacteria and 100 pg active cell‐1 for fungi.
The term active cell indicates that in the environment form which the sample was taken, the cell is carrying out metabolic reactions; it is not dormant.

Bacterial endospores, fungal spores and dormant cells are not metabolically active and consequently, the [ATP] cell is immeasurably small.
The ATP test detects all metabolically active cells in the sample.

Culture, Culture data depend on:

• Ability of cells to grow under the culture conditions (nutrients, incubation temperature and atmosphere)
• Generation (population doubling) time short enough for colonies to be visible within the test time frame

Injured cells and dormant cells that aren’t active in the environment from which sample was collected can recover and from colonies on growth medium;
Cells that were active in the environment from which the sample was taken but won’t grow under the culture conditions are missed

The potential impact of contaminants is >> for culture tests than for ATP test because the number of contaminant cells is magnified through reproduction (i.e. for each contaminant cell – introduced from source other than sample – there will be 2n cells after n generations; recall that 1 cell 109 cells after 30 generations and that generation times can range form 15 min to >30d, depending on species and growth conditions).

1. Different microbes need different types of food (nutrients) – in one manual of nutrient media there are > 3,000 recipes; each one optimized for a specific type of microbe.

2. Obligate aerobes will only grow in an oxygen rich environment (colonies are growing only near surface of growth medium).
Obligate anaerobes will only grow in an oxygen depleted environment (colonies are only growing near bottom of growth medium)
Facultative anaerobes grow like aerobes when oxygen is plentiful, but shift into anaerobic mode once oxygen is depleted (colonies grow throughout growth medium)

3. Expanding on point made on page 1, it takes 1 billion cells to form a visible colony.
• Colonies of fast growing microbes (generation time – TG 0.5 to 2h) can be visible within 1 or 2 days.
• Colony populations continue to double. This means that colony size continues to increase over time, until all colonies merge (confluent growth)
• Colonies of slow growing microbes (TG 4h; some microbes have TG 10d!) won’t become visible for a week or longer.
• If a cultured sample has both fast growing and slowing growing microbes, the colonies of the slow growers will be eclipsed by those of the fast growers; and never be detected;even though they are present.

As a consequence of these three factors (and others), only a fraction of the total number of viable cells gets detected by routine culture testing. Various authors have reported between 0.01 and 0.1% recovery of viable cells from environmental samples are detected by any single culture method. The % recovery increases to 10% for laboratory cultures. In general, it is advisable to incubate cultures longer than prescribed time periods so that the chances of a ‘false negative’ are decreased.

Consider the following example in which a user is claiming that an ATP test is giving ‘false positives’ relative to a standard Heterotrophic Plate Count (HPC): the ATP test is showing measureable quantities of microorganisms, whereas the HPC shows an absence of countable colonies.

The interpretation depends on the basis selected by the user:
• 1. Is your objective to measure only aerobic, carbon‐consuming microorganisms? If so, then you have a basis for making the argument that the ATP test is providing a false positive, in the context of measuring only a portion of the total population. But then, the ATP test is not designed to measure only specific microbes – it is designed to measure the total population.
• 2. If however your objective is to measure the total population (generally a good practice for process control), then what you are seeing is better classified as a ‘false negative’ on the part of the culture test because it does not in fact measure all of the microorganisms.

  Overall, these tests are very different and equally useful when put to use in the right ways. They should be considered complementary, not competitive.