Quantitative and Single-Cell Microbiology

Standard Microbiology Techniques

  • Bulk Measurements:

    • Used to determine bacterial growth and other microbial traits.

    • Optical Density (OD) Measurement:

    • Measurement at 600 nm (A600) used to assess the density of bacterial cells in batch culture.

    • Bacteria are cultivated in various types of containers, including culture tubes, flasks, and microplates.

Population Growth

  • Binary Fission:

    • Bacteria reproduce by a process called binary fission which involves the doubling of the population size.

    • To estimate the number of generations (n) from the cell number increase (N):
      N<em>2=N</em>1∗2nN<em>2 = N</em>1 * 2^n

    • It utilizes the natural logarithm for calculation:
      extln(N<em>2)−extln(N</em>1)=nextln(2)ext{ln}(N<em>2) - ext{ln}(N</em>1) = n ext{ln}(2)

  • Growth Rate Calculation:

    • Growth rate ($$) can be calculated from the cell number increase (N) during a time interval ($t2-t1$) in the exponential growth phase:
      N2 = N1 * e^{ t}

    • Where:

    • $N_2$ = Final number of cells

    • $N_1$ = Initial number of cells

    • $t = t2 - t1$ (time interval)

Data Distribution

  • Mean Value Analysis:

    • Understanding possible distributions of data if the mean is 1.

    • Averages can conceal significant variations within the data set.

Phenotypic Heterogeneity

  • Discussion on the concept of phenotypic heterogeneity within microbial populations.

Causes and Consequences of Phenotypic Heterogeneity

  • Non-genetic Sources:

    • Influenced by environmental stimuli.

    • Stochastic gene expression leading to variability in cellular processes among genetically identical cells.

  • Individuality in Bacteria:

    • Refers to phenotypic differences observed even in genetically homogenous bacterial environments.

  • Quantification Methods:

    • Phenotypic heterogeneity can be quantified utilizing single-cell techniques.

Examples of Phenotypic Heterogeneity

  • Dormant Cells:

    • Metabolically inactive states or growth arrest.

  • Persister Cells:

    • Cells that remain viable and tolerant to antibiotics and various stressors.

    • Persistence mechanisms are underwritten by dormancy among other molecular processes.

    • Reference: Balaban et al. 2019, Nature Reviews Microbiology.

Single-Cell Microbiology Techniques

  • Light Microscopy:

    • Types include Brightfield, Phase Contrast, and Fluorescence Microscopy.

    • Fluorescence Techniques:

    • Involves the use of fluorescent dyes/probes, e.g., fluorescence in situ hybridization (FISH), and gene reporter systems such as GFP (green fluorescent protein).

    • Magnification Levels:

    • Commonly applied magnification is 40x (up to 100x for enhanced detail).

Fluorescent Gene Reporter Systems

  • Fundamental in reporting gene expression dynamics, which can be regulated or constitutive.

    • Reporter Gene Fusions:

    • Transcriptional Fusion:

      • +1 indicates the transcription start site followed by the ribosome binding site (RBS) leading to a reporter gene.

    • Translational Fusion:

      • Integrated in-frame fusion of the promoter and reporters allowing for the visualization of protein synthesis dynamics.

Flow Cytometry

  • A single-cell analysis technique where cells travel through a narrow channel one at a time.

    • Process:

    • Light illuminates passing cells, where various sensors detect unique light scattering and fluorescent properties.

    • Capable of analyzing a large number of bacterial cells (10,000-100,000) in a mere 1 minute.

  • Data Outputs:

    • Dot plots and histograms illustrating heterogeneity in response to environmental stressors, with an emphasis on cell size and complexity through forward and side scatter plots.

NanoSIMS Technique

  • Nanoscale Secondary Ion Mass Spectrometry:

    • Cells utilize substrates that are labeled with stable isotopes (e.g., 2H^{2}H, 13C^{13}C, 15N^{15}N).

  • Usage:

    • Analyzes the metabolic activity of individual cells, such as examining the 13C/12C^{13}C/^{12}C ratio to deduce metabolic processes.

    • References: Wagner 2009, Annual Review of Microbiology; Nikolic et al. 2017, PLoS Genetics.

Measurement Capabilities of Single-Cell Techniques

  • Variability assessments in the following:

    • Gene expression, cell size (morphology), metabolism, growth (elongation rate), motility of bacteria.

Metabolic Heterogeneity

  • Example with Methylorubrum extorquens PA1:

    • Studied under a single-carbon source environment (methanol) using MxaF-GFP for fluorescence microscopy analytics.

Heterogeneity in Virulence

  • Example with Salmonella Typhimurium:

    • Utilized PfliC-gfp system to monitor virulence through time-lapse microscopy methods.

    • Reference for lineage studies: Freed et al. 2008, PLoS Genetics.

Time-Lapse Microscopy in Microfluidic Devices

  • Implementation in Research:

    • Incorporates 100x magnification within a microfluidic chip.

    • Observational capabilities of filament formation under antibiotic treatment (kasugamycin).

  • Features:

    • Bacterial growth dynamics in a confined space either in 1D or 2D, with controlled nutrient inflow and easy medium switching.

Droplet-Based Microfluidic Technology

  • Overview by Nikolic et al. 2023 in Front Microbiology.

    • Field of View:

    • Assessed using 40x microscope objective with motorized Z-focus control for precise observation.

  • Functionality:

    • Supports planktonic or aggregate bacterial life, analyzing both single cells and colonies without fresh nutrient inflow.

Additional Research Considerations

  • Examples of current research investigating responses to RNA phages, noted by Nikolic et al. 2025.