chapter 7

Microbial Nutrition and Growth

Chapter Overview

  • Focus on Bacterial and Archaeal Growth

Learning Outcomes

  • After reading this chapter, students should be able to:

    • Describe binary fission as observed in bacteria and archaea.

    • Summarize the three phases in a typical bacterial cell cycle.

    • Summarize molecular mechanisms for chromosome partitioning and cytokinesis.

    • State the functions of cytoskeletal filaments during cytokinesis and in determining cell shape.

    • Describe the five phases of a microbial growth curve observed when microbes are grown in batch culture.

    • Use terms that describe a microbe’s growth range or requirement for each of the factors that influence microbial growth (e.g., psychrophilic, mesophilic, thermophilic, etc.).

    • Summarize the adaptations of extremophiles to their habitats.

    • Describe enzymes that protect microbes from toxic O2 products.

    • Distinguish sessile and planktonic microbial lifestyles.

    • Define quorum sensing and provide examples of cellular processes regulated by quorum sensing.

    • Distinguish defined (synthetic) media from complex media and the uses of liquid vs. solid growth media.

    • Compare and contrast supportive (general purpose), enriched, selective, and differential media, listing examples of each and describing how each is used.

    • Discuss the use of enrichment cultures in isolating microbes.

    • Differentiate the streak-plate, spread-plate, and pour-plate methods for isolating pure cultures.

    • Evaluate direct cell counts, viable counting methods, and cell mass measurements for determining population size.

    • Explain why plate count results are expressed in terms of colony-forming units (CFU).

    • Design appropriate approaches for measuring the population size of different types of samples.

    • Distinguish batch culture and continuous culture.

    • Differentiate chemostats and turbidostats.

Microbial Nutrition and Growth

  • Reproductive Strategies

    • Eukaryotic microbes can reproduce asexually and sexually, and can be haploid or diploid.

    • Prokaryotes reproduce asexual and are haploid only; all cells must replicate and segregate the genome prior to division.

Reproductive Strategies

  • Binary Fission

    • Most bacteria and archaea divide by binary fission.

    • Some divide by budding (e.g., Actinomycetes).

Bacterial Cell Cycle

  • Key Pathways

    • Two pathways required during the cell cycle:

    • DNA replication and partitioning

    • Cytokinesis

Chromosome Replication and Partitioning

  • Bacterial Chromosomes

    • Most bacterial chromosomes are circular.

    • Origin of Replication: Site at which replication begins.

    • Terminus: Site at which replication is terminated, located opposite to the origin.

    • Replisome: Group of proteins needed for DNA synthesis.

    • DNA replication proceeds bidirectionally from the origin, with origins moving to opposite ends of the cell.

Cytokinesis - Septation

  • Septation

    • Formation of cross walls between daughter cells involves several steps:

    • Selection of site for septum formation.

    • Assembly of Z ring composed of protein FtsZ.

    • Assembly of cell wall synthesizing machinery.

    • Constriction of the cell and septum formation.

Cellular Growth and Determination of Cell Shape

  • Determination by Peptidoglycan Synthesis

    • Growth is determined by enzyme functions:

    • Penicillin Binding Proteins (PBPs): Link peptidoglycan strands and catalyze controlled degradation for new growth.

    • P: PBP enzymes that degrade peptidoglycan at sites where new units are added.

Microbial Nutritional Requirements

  • Basic Nutritional Requirements

    • Essential for growth includes carbon, energy source, electron source, and chemical makeup of cells.

  • Elements Required for Growth

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

    • Micronutrients: Required in minute amounts, act as enzyme cofactors (Cu, Zn, etc.).

Growth Factors

  • Organic compounds like vitamins, amino acids, purines, and pyrimidines that some organisms cannot synthesize.

    • Prototroph: Microbe that can synthesize all necessary molecules for growth (needs iron and other elements).

    • Auxotroph: Microbe that cannot synthesize all required growth molecules.

Influence of Environmental Factors on Growth

  • Salt Concentrations:

    • Hypotonic Solutions: Water enters the cell, cell swells and may burst.

    • Hypertonic Solutions: Water leaves the cell causing plasmolysis.

Extremophiles

  • Halophiles: Grow optimally in salt concentrations above 0.2M.

    • Extreme Halophiles: Require 2M to 6.2M of salt to maintain stability and activity.

pH Tolerance Mechanisms of Microbes

  • Internal pH is usually maintained near neutrality.

  • Microbes exchange potassium ions for protons and synthesize protective proteins.

Temperature Ranges for Microbial Growth

  • Psychrophiles: <0°C to 20°C.

  • Psychrotolerant: 0°C to 35°C.

  • Mesophiles: 20°C to 45°C.

  • Thermophiles: 55°C to 85°C.

  • Hyperthermophiles: 85°C to 113°C.

Oxygen Requirements

  • Obligate Aerobe: Requires O2 for survival.

  • Facultative Anaerobe: Can use O2 or grow without it.

  • Microaerophile: Uses low levels of O2.

  • Aerotolerant Anaerobe: Can tolerate O2 but does not use it.

  • Obligate Anaerobe: Cannot tolerate O2.

Toxic Oxygen Species

  • Generated during electron transfer reactions.

  • Must be neutralized using specific enzymes such as catalase.

Pressure and Radiation Effects on Growth

  • Barotolerant: Affected by increased pressure to some extent, but can tolerate.

  • Barophilic: Require or grow better under high pressure, having adapted membrane composition.

  • Radiation: Ionizing radiation can lead to mutations and cell death due to DNA disruption.

Culture Media for Bacterial Growth

  • Types of Growth Media:

    • Defined (synthetic) media ensure precise chemical composition.

    • Complex media composed of digests from undefined substances (e.g., yeast extract).

Supportive, Enriched, Selective, and Differential Media

  • Supportive Media: Provide nutrients for many organisms.

  • Enriched Media: General media with added nutrients.

  • Selective Media: Favor growth of certain microorganisms while inhibiting others.

  • Differential Media: Allow differentiation between groups based on biological characteristics.

Isolation of Pure Cultures

  • Techniques include spread plate, pour plate, and streak plate methods.

Measuring Microbial Growth

  • Increase in population size as well as cell growth can be measured through direct and indirect counts, turbidity assessment, and viable counting methods.

Conclusion

  • Factors influencing microbial growth include pH, salt concentration, pressure, oxygen levels, and temperature.

  • Microbial communities, such as biofilms, and phenomena like quorum sensing play crucial roles in microbial ecology.