Microbial Growth and Its Control

Introduction

This study guide summarizes key concepts from Chapter 4 of Brock Biology of Microorganisms, focusing on the principles and laboratory techniques for culturing, measuring, and understanding microbial growth. Topics include cell nutrition, growth media, cell counting methods, and the dynamics of microbial growth.

I. Culturing Microbes and Measuring Their Growth

4.1 Feeding the Microbe: Cell Nutrition

Microbial growth depends on the availability of essential nutrients, which are classified as macronutrients and micronutrients.

• Nutrients: Elements required by cells for growth.

• Macronutrients: Required in large amounts (e.g., C, O, N, H, P, S).

• Micronutrients: Required in minute amounts (e.g., trace metals, growth factors).

Chemical Makeup of a Cell

• Major elements: C, O, N, H, P, S (~96% of dry weight).

• Minor elements: K, Na, Ca, Mg, Cl, Fe (~3.7% of dry weight).

• Macromolecules: Proteins, lipids, polysaccharides, nucleic acids (~96% of dry weight; mostly proteins and RNA).

Carbon, Nitrogen, and Other Macronutrients

• Carbon: Heterotrophs obtain organic carbon from polymers or monomers; autotrophs synthesize organics from CO2.

• Nitrogen: Used in proteins and nucleic acids; sources include NH3, NO3-, N2.

• Oxygen and Hydrogen: Primarily from water.

• Phosphorus: Needed for nucleic acids and phospholipids; usually as PO43-.

• Sulfur: Required for sulfur-containing amino acids and vitamins; assimilated as SO42-, H2S, or organics.

• Potassium: Required by several enzymes.

• Magnesium: Stabilizes ribosomes, membranes, nucleic acids; enzyme cofactor.

• Calcium and Sodium: Required by some microbes (e.g., marine species).

Micronutrients: Trace Metals and Growth Factors

• Trace metals: Required in small amounts, often as enzyme cofactors (e.g., Fe for cellular respiration).

• Growth factors: Organic micronutrients such as vitamins, amino acids, purines, pyrimidines; most function as coenzymes.

4.2 Growth Media and Laboratory Culture

Culture Media

• Culture media: Nutrient solutions for growing microbes; sterilized in an autoclave.

Classes of Culture Media

• Defined media: Exact chemical composition is known.

• Complex media: Composed of digests of microbial, animal, or plant products (e.g., yeast, meat extracts).

• Selective medium: Contains compounds that inhibit growth of some microbes but not others.

• Differential medium: Contains indicators (usually dyes) to detect specific metabolic reactions.

Laboratory Culture Techniques

• Liquid or solid media: Solidified with agar; colonies form on solid media.

• Colony morphology: Used to identify microorganisms and assess culture purity.

• Aseptic technique: Prevents contamination during transfer; streak plate method with an inoculating loop is common for isolating pure cultures.

Table: Examples of Culture Media

4.3 Microscopic Counts of Microbial Cell Numbers

Total Cell Count

• Microscopic cell count: Direct observation and enumeration of cells on slides or in liquid samples using counting chambers (e.g., Petroff-Hausser chamber).

• Limitations: Cannot distinguish live from dead cells; small cells may be missed; debris may be counted as cells.

Microscopic Cell Counts in Microbial Ecology

• Used for natural samples; stains (e.g., DAPI) bind DNA for visualization.

• Fluorescent stains differentiate live and dead cells.

• Phylogenetic stains identify proportions of Bacteria and Archaea.

4.4 Viable Counting of Microbial Cell Numbers

Viable (Living) Counts

• Measures living, reproducing populations.

• Two main methods: spread-plate and pour-plate techniques.

• Colonies counted on plates with 30–300 colonies; results reported as colony-forming units (CFU) to account for clumps.

Diluting a Sample

• Samples often require serial (successive) dilutions for accurate counting.

• Ten-fold dilutions are common.

Applications of the Plate Count

• Quick and easy; used in food, dairy, medical, and aquatic microbiology.

• High sensitivity; can target specific species in mixed samples (e.g., Staphylococcus).

• Common in water quality analyses.

The Great Plate Count Anomaly

• Direct microscopic counts reveal more organisms than those recoverable on plates.

• Different organisms have unique growth requirements; plate counts may underestimate actual numbers by orders of magnitude.

4.5 Turbidimetric Measures of Microbial Cell Numbers

Turbidity Measurements

• Cell suspensions scatter light, causing turbidity (cloudiness).

• More cells result in greater turbidity; measured rapidly for cell estimates.

Optical Density and Its Relationship to Cell Numbers

• Measured with a spectrophotometer; unit is optical density (OD) at a specified wavelength (e.g., OD540).

• For unicellular organisms, OD is proportional to cell number within limits.

• Standard curve must be established to relate OD to direct cell counts.

Other Issues with Turbidimetric Growth Estimates

• Quick and easy but does not distinguish live vs. dead cells.

• Does not require destruction or disturbance of sample; same sample can be checked repeatedly.

• Problematic for microbes that form clumps or biofilms in liquid medium.

II. Dynamics of Microbial Growth

4.6 Binary Fission and the Microbial Growth Cycle

• Growth: Increase in cell number.

• Binary fission: Cell division following enlargement to twice original size; septum forms and separates daughter cells.

• Generation (doubling) time: Time required for cells to double in number; varies by species and conditions (e.g., Escherichia coli = 20 min).

• Each daughter cell receives a chromosome and sufficient cellular constituents.

The Microbial Growth Cycle

• Batch culture: Closed-system culture of fixed volume.

• Typical growth curve has four phases: lag phase, exponential phase, stationary phase, death phase.

Growth Phases

• Lag phase: Interval after inoculation before growth begins; cells adapt and synthesize necessary enzymes.

• Exponential phase: Cells double at regular intervals; growth rate influenced by medium, conditions, and organism.

• Stationary phase: Growth rate is zero due to nutrient depletion or waste accumulation; metabolism continues at reduced rate.

• Death phase: Cell numbers decline due to cell death; some subpopulations may adapt (cryptic growth).

4.7 Quantitative Aspects of Microbial Growth

• Exponential growth: Cell numbers double at regular intervals; growth rate can be plotted to analyze population dynamics.

• Equation for exponential growth: where is the final cell number, is the initial cell number, and is the number of generations.

4.8 Continuous Culture

• Continuous culture: Open system where fresh medium is added and spent medium is removed at the same rate.

• Chemostat: Most common device; maintains steady state (constant cell density and substrate concentration).

• Controls both specific growth rate () and yield (biomass); dilution rate (D) is defined as (F = flow rate, V = culture volume).

• Growth yield is controlled by the concentration of a limiting nutrient.

• Allows maintenance of exponential growth phase for extended periods; used in physiology, ecology, and evolution studies.

4.9 Biofilm Growth

• Planktonic growth: Free-floating cells in suspension.

• Sessile growth: Cells attached to surfaces, forming biofilms.

• Biofilm: Cells enmeshed in a polysaccharide matrix attached to a surface; form in stages: attachment, colonization, development, dispersal.

• Biofilms have distinct properties from planktonic cells; important in medical and industrial contexts (e.g., joint infections, dental cavities, pipe corrosion).

• Microbial mats: Multilayered sheets with different organisms in each layer (e.g., hot springs).

4.10 Alternatives to Binary Fission

• Balanced growth: Binary fission produces nearly identical cells.

• Budding cell division: Unequal cell growth forms different daughter cells.

• Hyphal growth: Long, thin filaments (hyphae) in actinomycetes; growth occurs at filament tip, not linked directly to division.

• Mycelia: Weaved hyphae; arthrospores are survival structures formed from mycelia.

• Multiple fission: Hyphal filament forms many septa simultaneously.

Additional info: These notes provide a comprehensive overview of microbial growth, laboratory techniques, and quantitative analysis, suitable for exam preparation in a college-level microbiology course.