Technical paper 01-032016 Morten Miller, PhD., and Jan Nielsen, MSc., Bactiquant Page 1:4
Plate counts and Bactiquant HETEROTROPHIC BACTERIA: Heterotrophic bacteria are broadly defined as bacte-
HPC TEST VARIABLES SPREAD PLATING:
ria that require organic carbon for growth.
The advantage of the spread plating technique is that
These bacteria are traditionally quantified using het-
the bacterial colonies rapidly become visible. They
erotrophic plate counts (HPC).
grow on the surface and not into the agar. The disadvantage is that it selects for motile and rapid growing
HPC METHODS:
bacteria.
There is no universal “HPC measurement.” Although
Another drawback is that the spread plating is not
standardized methods have been formalized, HPC
compatible with long incubation times as it becomes
test methods involve a wide variety of test conditions
difficult to differentiate between individual colonies
that lead to a wide range of quantitative and qualita-
after more than three days of incubation.
tive results.
The consequence is that slow growing bacteria are
Temperatures employed typically range from around
not detected due to substrate competition, inhibition
20°C to 40°C, incubation times from a few hours to
and over growth.
seven days or a few weeks, and nutrient conditions from low to high.
POUR PLATING:
The HPC methods do not indicate the specific het-
The advantage of the pour plating technique is that
erotrophic bacteria present or their sources. Instead,
colony interaction is significantly reduced.
HPC testing indicates the culturable organisms pre-
This makes it possible to differentiate colonies even
sent, which could be as low as
after long incubations times (2-3 weeks).
0,1-1% of the total bacteria present in a water sample.
The pour plate technique allows for detection of slow
The result will differ significantly according to which
growing bacteria as well as rapid growing bacteria.
method is used.
The pour plate technique in combination with longer
The actual organisms recovered in HPC testing can
incubation times provides a more true representation
also vary widely between locations, between seasons
of the bacterial presence in water samples as com-
and between consecutive samples at a single loca-
pared to spread plating.
tion (WHO, Heterotrophic Plate Counts and Drinking
The disadvantage is that it is more elaborate and
Water Safety, Editors; J. Bartram et al., 2003)
time consuming protocol.
THE CONSEQUENCE IS THAT:
PLATE COUNT AGAR (PCA):
1 Plate counts are not a quantitative measure of total
Contains high concentrations of yeast extract and
bacteria in a water sample 2 Results on identical samples using different HPC methods are often not correlated 3 The HPC method measures the presence of bacteria that can grow under a specific set of growth
tryptone or peptone. The colonies will often become very big due to the high substrate load. Small colonies will rapidly become overgrown by larger colonies which make it difficult to count the smaller colonies.
conditions; temperature, incubation time, pH, substrate media etc.
Bactiquant ®water - Heterotrophic Plate Counts (HPC); Recommendations for protocols when comparing with Bactiquant ®-water analysis results Copyright © Bactiquant, March 2016, first edition, version 2016.3 • www.bactiquant.com