Lecture 18:

Assessment of Microbiome Diversity and Function: Bioreactors, Continuous Culture, and Batch Culture

Key Concepts in Microbial Growth

Bacterial Growth – Binary Fission

• Definition: Increase in mass/size versus the number of cells.

• Key Quote: "The dream of every cell is to become two cells" — Francois Jacob, paraphrased by Jacques Monod in ‘Chance and Necessity’ 1970.

Growth Rate and Cell Composition

• The growth rate impacts both cell volume and composition.

• Understanding these changes is crucial for accurate modeling and prediction of microbial behavior in different environments.

Bacterial Growth Curve

• Phases: Lag, Exponential (Log), Deceleration, and Stationary phases.

• Representation: Can be plotted on an arithmetic or semi-logarithmic scale to visualize growth over time. Mathematical modeling of growth is essential for precise determination of microbial kinetics.

Mathematical Description of Bacterial Growth

Bacterial growth can be represented using various mathematical equations, such as:

• [ \frac{dX}{dt} = \mu X ]

• [ X = X_0 e^{\mu t} ]

• [ \mu_{max} = \frac{\ln 2}{g_d} ] These equations facilitate the calculation of growth rates and other microbial parameters.

Factors Influencing Microbial Growth

• Growth rate and microbial behavior can be influenced by several factors:

  • In Vitro vs. In Vivo Conditions: Differences in environmental conditions can have significant impacts on growth rates.

  • Culture Media Composition: Specific growth rates vary depending on the nutrients available. For example, Salmonella's growth rate in nutrient broth is higher compared to minimal media.

Microbial Cultivation Systems

Batch Culture

• Characterized by ambient changes in nutrient concentration and waste accumulation.

• Limited by the initial amount of nutrients.

Continuous Culture

• Nutrient supply and waste removal are constant, maintaining steady-state conditions.

• Dilution Rate (D): Equivalent to the specific growth rate (( \mu )) under steady-state.

Measuring Microbial Growth

Methods of Measurement

• Cell Number: Total and viable counts using direct methods (e.g., microscopic or electronic counting) and indirect methods (e.g., plate count, filtration).

• Mass: Measuring biomass through dry mass and optical density.

• Metabolic Activity: Using indicators like metabolic end products to infer microbial growth.

Optical Density

• Measurement of turbidity correlates with the microbial biomass and cell population. This non-invasive method is commonly used for monitoring microbial growth in liquid cultures.

Impact of Culture Systems on Microbial Communities

Diversity in Bioreactors

• Cultivation of fecal microbiota in bioreactors often results in a reduction of microbial diversity compared to natural environments due to the absence of host-associated variables.

Anaerobic Dynamic Membrane Bioreactor (AnDMBR)

• AnDMBR systems enhance the hydrolysis of lignocellulosic materials and the production of volatile fatty acids (VFAs) by mimicking ruminal conditions. This innovation has significant implications for the treatment of food waste and the study of gut microbial ecosystems.

Conclusion

Understanding microbial growth dynamics, culture systems, and the effect of environmental variables on microbial communities is essential for various biotechnological applications. This knowledge is applied to optimize bioreactors, improve microbial cultivation techniques, and further our understanding of microbial ecology.