Artifacts in dilution pharmacokinetic models caused by adherent bacteria

Area of Science:

• Microbiology
• Bacterial Physiology
• Biotechnology

Background:

• Standard interpretation of bacterial growth in liquid cultures assumes homogeneous distribution.
• Dilution rates exceeding growth rates typically lead to a decrease in colony-forming units (CFU).
• Observed increases in CFU in Pseudomonas aeruginosa cultures challenged this assumption.

Purpose of the Study:

• To investigate the phenomenon of increasing CFU in liquid cultures despite dilution rates higher than growth rates.
• To propose and validate a mathematical model explaining this unexpected bacterial behavior.
• To highlight the implications for interpreting experimental results involving liquid cultures.

Main Methods:

• Culturing Pseudomonas aeruginosa in Mueller-Hinton broth under specific dilution rates.
• Monitoring CFU over time to observe population dynamics.
• Developing a two-compartment mathematical model (homogeneous and adherent populations) to describe bacterial behavior.
• Fitting the model to experimental data.

Main Results:

• Observed an initial decrease in CFU followed by an increase, contradicting simple dilution expectations.
• The proposed two-compartment model, including a non-dilutable adherent population, accurately fit the experimental data.
• Similar CFU trends were observed with other bacterial strains, suggesting a general phenomenon.

Conclusions:

• Bacterial cultures in liquid media may contain a significant non-dilutable adherent population.
• This adherent population can lead to artificial and unexpected results when interpreting CFU counts.
• Researchers should consider the potential presence of adherent populations when designing and analyzing experiments in liquid cultures, especially when evaluating selective influences like antibiotics.