Pseudomonas fluorescens grows better in glucose-enriched LB media than in LB media alone
Andrew Passer and Muhammad Khan
Abstract
We have demonstrated that the bacterium Pseudomonas
fluorescens grows to a higher density in broth enriched with
glucose than it does in only an amino acid broth. Optimum
growth occurs around 3.4% w/v glucose.
Method
Using a Bioscreen C growth curve analyzer, we analyzed the
growth of P. fluorescens over 3 days. We compared all growth
to growth in a standard LB solution of 1% w/v tryptone, 0.5%
w/v yeast extract, and 0.1 M NaCl dissolved in distilled water.
Varying the concentration of glucose that we added to the LB
allowed us to estimate an optimum concentration. To establish
a point of comparison for a given curve, we fitted a line to the
top the growth curve, then found where the line intersected the
time value that occurred when the growth curve’s derivative
was at a maximum. We then plotted these values, used as
proxies to represent maximum bacterial density, against the
glucose concentrations that yielded those respective values.
Finally, we fitted a parabola to the data points and computed
the maximum of that parabola.
contributed to the 18% difference between the first and second
maximum values we calculated. One final difficulty that
should be noted is that the method used to fit a line to the top
of each growth curve was rather subjective. We used Excel to
perform a linear regression on the curves, but the section of
each curve that was used for regression was subject to
interpretation. We chose the straightest segment that occurred
immediately after the exponential growth phase to represent
the top of the curve for each trial. We relied on visual, rather
than algorithmic methods to accomplish this. However, we
still feel confident that our numbers give us a reasonable value
to use for adding glucose to optimize the growth of
P. fluorescens in otherwise standard LB.
Results
Performing this experiment twice yielded similar results. The
first experiment suggested an optimum glucose concentration
of 3.1% w/v, while the second experiment suggested an
optimum concentration of 3.7%. Averaging the two gave us a
value of 3.4% w/v of glucose as the optimum.
Discussion
There were some notable difficulties in our analysis. First, the
growth of numerous samples continued past the experimental
cut-off of three days. Since Pseudomonas fluorescens is a
motile bacterium, it is unclear whether this continued rise in
optical density was from real growth, or merely a result of
dead cells settling on the bottom of the growth wells. This
contributed to our decision to use an unconventional growth
analysis. Another difficulty is that growth of P. fluoresecens
trails off more slowly at higher concentrations than it rises at
lower concentrations, suggesting a nonparabolic function of
growth. As we included some data points higher on the
concentration axis for the second experiment, this likely
Conclusion
Pseudomonas fluorescens grows well in sugarless LB
solution, yet its growth can be greatly enhanced by
supplementing the broth with moderate amounts of glucose,
around 3.4% w/v. More experiments done in this style will
allow a statistically supported value to be established.
Acknowledgements
We thank Dr. James Brown for his guidance of our research
and for suggesting the analytical protocol employed for
comparing growth.