Bacterial Growth, Nutrition, and Genetics

Cell Replication in Bacteria

  • Occurs asexually.
  • Involves two common mechanisms:
    • Binary fission
    • Snapping division (e.g., in Corynebacterium).

Cell Division

  • Binary Fission
    • Triggered by replication of the bacterial chromosome.
    • Requires:
      • Growth and extension of cell wall components.
      • Production of a septum (cross wall) to divide the daughter bacteria into two cells.
    • Septum consists of two membranes separated by two layers of peptidoglycan.
    • Process requires penicillin-binding proteins (PBPs) and other enzymes.

FtsZ Protein

  • Directs cytokinesis and cell division.
  • FtsZ proteins assemble to form a Z ring, anchored to the cytoplasmic membrane.
  • The Z ring pinches the cell envelope to separate the cytoplasm of new cells.
  • Additional proteins are added to the Z ring to form the divisome.

Binary Fission Process

  • (a) Young Cell: At an early phase of the cycle.
  • (b) Parent Cell Preparation: Enlarges its cell wall, cell membrane, and overall volume; DNA replication starts.
  • (c) Septum Formation: The septum begins to grow inward as chromosomes move toward opposite ends of the cell; other cytoplasmic components are distributed to the two developing cells.
  • (d) Complete Septum Synthesis: The septum is synthesized completely through the cell center, creating two separate cell chambers.
  • (e) Daughter Cell Division: Daughter cells divide completely in some species, while others remain attached, forming chains, doublets, or other cellular arrangements.

Snapping Division

  • Only the inner layer grows inward to generate a septum dividing new cells.
  • Results in a palisade arrangement of cells.
  • Involves the ruptured remains of the old cell wall.

Generation Time

  • Each division adds two new cells.

Generation Time Defined

  • The time it takes for the population to double through one round of binary fission.
  • Also called the doubling time.
  • Bacterial doubling times vary enormously.
  • Nn = N0 * 2^n
    • Where:
      • N_n is the number of cells after n generations.
      • N_0 is the initial number of cells.
      • n is the number of generations.
  • Most pathogens grow rapidly, but there are exceptions.
  • Examples:
    • E. coli in the laboratory: 15-20 minutes.
    • Mycobacterium tuberculosis: 15-20 hours.
    • M. leprae: 14 days.

Growth Curve of Bacteria

  • Microorganisms grown in a closed culture (batch culture) follow a reproducible growth pattern called the growth curve.
  • Phases:
    • Lag phase (遲緩期)
    • Logarithmic (log)/Exponential phase (對數生長期)
    • Stationary phase (穩定期)
    • Decline phase/Death phase (衰退期)

Growth Curve Phases

  • Lag Phase:
    • Cells are gearing up for the next phase of growth.
    • The number of cells does not change significantly.
    • Cells grow larger and are metabolically active, synthesizing needed proteins.
  • Logarithmic (Log)/Exponential Phase:
    • Cells are actively dividing by binary fission, and their number increases exponentially.
    • The generation time under specific growth conditions is genetically determined and is called the intrinsic growth rate.
  • Stationary Phase:
    • Cells switch to a survival mode of metabolism.
    • The number of new cells created by cell division is equivalent to the number of cells dying.
    • Waste products accumulate, and nutrients are gradually used up.
    • Oxygen depletion begins to limit aerobic cell growth.
    • Endospore production may occur.
    • Expression of virulence factors.
  • Decline Phase/Death Phase:
    • Cells lyse and release nutrients into the medium.
    • The number of dying cells exceeds the number of dividing cells.
    • Persister cells characterized by a slow metabolic rate may be present and are associated with certain chronic infections.

Growth Rate by Phase

  • Lag: Zero growth rate.
  • Exponential: Constant growth rate.
  • Maximum Stationary: Zero growth rate.
  • Decline: Negative growth rate (death).

Requirements for Growth

  • Physical Requirements:
    • Temperature
    • pH
    • Osmotic pressure
    • Light
  • Chemical Requirements:
    • Carbon
    • Nitrogen, sulfur, and phosphorus
    • Trace elements
    • Oxygen
    • Organic growth factors

Temperature

  • Microorganisms grow well at temperatures that humans favor.
  • Certain bacteria can grow at extreme temperatures.
  • Temperature affects many metabolic factors in a cell, including enzyme function, ribosomes, cell membrane, and transport.

Temperature: Categories of Microbes

  • Psychrophiles (嗜冷菌):
    • 0°C to 15°C
  • Psychrotrophs (耐冷菌):
    • 20°C to 30°C
    • Associated with low-temperature food spoilage.
  • Mesophiles (嗜溫菌):
    • 25°C to 40°C
    • Optimum temperature for many pathogenic bacteria is about 37°C.
  • Thermophiles (嗜熱菌):
    • 55°C to 65°C
    • Found in hot springs.
  • Hyperthermophiles (極端嗜熱菌):
    • 70°C to 100°C
    • Archaea

Food Preservation Temperatures

  • Temperatures in the range of 60-130°F (15-55°C) allow for rapid bacterial growth; some may produce toxins.
  • Refrigeration temperatures (around 4°C) may allow slow growth of spoilage bacteria, very few pathogens.

pH

  • Most bacteria are neutrophiles, growing optimally at a pH near 7.
  • Acidophiles grow optimally at pH less than 5.5 (e.g., Lactobacillus).
  • Alkaliphiles grow best at pH between 8.0 and 11.5 (e.g., Bacillus).

Osmotic Pressure

  • Halophiles (嗜鹽菌): Require high salt concentrations.
  • Halotolerant (耐鹽菌): Do not need high salt concentrations for growth but can survive and divide in high salt conditions.

Oxygen Requirements

  • Obligate Aerobes (絕對需氧菌): Require oxygen to live.
  • Facultative Anaerobes (兼性厭氧菌): Thrive in the presence of oxygen but can also grow in its absence by fermentation or anaerobic respiration.
  • Obligate Anaerobes (絕對厭氧菌): Killed by oxygen.
  • Aerotolerant Anaerobes (耐氧厭氧菌): Do not use oxygen but are not harmed by its presence.
  • Microaerophiles (微需氧菌): Require a minimum level of oxygen (1-10%).

Oxygen Stress

  • Toxic oxygen derivatives are formed when cellular proteins transfer electrons to O_2.
  • Reactive oxygen species (ROS) can damage proteins, lipids, and nucleic acids.
  • Examples of ROS:
    • O_2^-(Superoxide radical 超氧自由基)
    • H2O2 (Hydrogen peroxide 過氧化氫)
    • OH (Hydroxyl radical 羥基自由基)

Detoxification of ROS

  • Superoxide dismutase (SOD) (超氧化物歧化酶) detoxifies superoxide radicals.
  • Catalase (過氧化氫酶) detoxifies hydrogen peroxide.
  • Peroxidase (過氧化酶) also detoxifies hydrogen peroxide.

Catalase Test

  • Mixing a culture sample in 3% hydrogen peroxide will release bubbles if the culture is catalase-positive.

Microbial Responses to Environmental Factors

  • Table summarizes various microbial responses to solute & water activity, pH, temperature, and oxygen concentration.
    • e.g. Osmotolerant, Halophile, Acidophile, Neutrophile, Alkaliphile, Psychrophile, Psychrotroph, Mesophile, Thermophile, Hyperthermophile, Obligate aerobe, Facultative anaerobe, Aerotolerant anaerobe, Obligate anaerobe, Microaerophile

Carbon

  • One of the most important requirements for microbial growth.
  • Autotrophs (自營): Use an inorganic source of carbon (carbon dioxide).
  • Heterotrophs (異營): Use reduced organic molecules (proteins, carbohydrates, amino acids, and fatty acids).

Energy Source

  • Chemotrophs (化學營養生物): Acquire energy from redox reactions.
  • Phototrophs (向光性生物): Use light as an energy source.

Microbial Nutritional Categories

  • Photoautotrophs: Use light and CO_2. (e.g., plants, algae)
  • Chemoautotrophs: Use chemical compounds and CO_2. (e.g., Hydrogen, sulfur, and nitrifying bacteria)
  • Photoheterotrophs: Use light and organic compounds. (e.g., Green nonsulfur bacteria)
  • Chemoheterotrophs: Use chemical compounds and organic compounds. (e.g., most animals, fungi, and protozoa, and many bacteria)

Other Nutrients

  • Microorganisms need other elements to synthesize cellular material, including nitrogen, sulfur, and phosphorus.

Culture Media

  • A nutrient material prepared for the growth of microorganisms in a laboratory.
  • Inoculum (接種物): Microbes introduced into a culture medium to initiate growth.
  • Culture (培養物): Microbes that grow and multiply in or on a culture medium.
  • Agar / broth medium; Petri dish / test tubes; Plate, slant, or deep.

Culture Media Requirements

  • Right nutrients, energy source, carbon source, nitrogen source, minerals, water, properly adjusted pH, suitable level of oxygen, sterile, proper temperature.

Types of Culture Media

  • Basic Medium (基礎培養基): Supports growth of a large variety of organisms.
  • Differential Medium (鑑別培養基): Used in the identification of bacteria by supplementing with dyes, pH indicators, or antibiotics.
  • Enriched Media (滋養培養基): Contains growth factors, vitamins, and other essential nutrients to promote the growth of fastidious organisms.
  • Enrichment Medium (增殖培養基): Promotes the growth of a particular organism.
  • Selective Medium (選擇培養基): Inhibits the growth of unwanted microorganisms and supports the growth of the organism of interest.
  • Transport Medium (傳送培養基): Preserves a specimen and minimizes bacterial overgrowth.

Differential Medium Example

  • Blood agar and Mannitol salt agar.

Microbial Genetics

  • Genetic information is used within a cell to produce proteins needed for cell function.
  • Genetic information can be transferred horizontally between cells of the same generation.
  • Genetic information can be transferred vertically to the next generation of cells.

Overview of Molecular Biology

  • DNA --> mRNA --> Protein --> Function.

Mutations

  • Base substitution mutations: A single DNA base pair is altered.
  • Frameshift mutations: DNA base pairs are added or removed from the sequence, causing a shift in the sequence reading.

DNA Replication

  • Enzymes unwind the parental double helix; proteins stabilize the unwound DNA.
  • The leading strand is synthesized continuously by DNA polymerase.
  • The lagging strand is synthesized discontinuously; DNA ligase joins the discontinuous fragments.

Transcription

  • RNA polymerase binds to the promoter, and DNA unwinds at the beginning of a gene.
  • RNA is synthesized by complementary base pairing of free nucleotides with the nucleotide bases on the template strand of DNA.

Translation

  • Components needed to begin translation come together (ribosome, mRNA, tRNA).
  • tRNA brings amino acids to the ribosome to build a polypeptide chain based on the mRNA sequence.

Regulation of Gene Expression

  • Pre-transcriptional control: Regulate the transcription of mRNA.
    • Induction: Turns on the transcription of genes; inducer initiates transcription.
    • Repression: Inhibits gene expression; repressors are regulatory proteins.

Operon Regulation

  • Clusters of genes that share the same promoter and are transcribed as a single large mRNA that contains multiple genes.

Inducible Operon

  • Repressor active, operon off. The repressor protein binds with the operator, preventing transcription from the operon.
  • When an inducer binds to the repressor protein, the inactivated repressor can no longer block transcription.

Repressible Operon

  • Repressor inactive, operon on. The repressor is inactive, and transcription and translation proceed.
  • When the corepressor binds to the repressor protein, the activated repressor binds with the operator, preventing transcription.

lac Operon

  • Example of gene regulation in bacteria with cAMP and CAP influencing expression based on glucose availability.

Recombination

  • The exchange of genes between two DNA molecules to form new combinations of genes on a chromosome.

Genetic Transfer

  • Transformation
  • Conjugation
  • Transduction

Transformation

  • Genes are transferred from one bacterium to another as "naked" DNA in solution.
  • Example: Griffith's experiment.

Conjugation

  • Genetic material is transferred from a donor cell to a recipient cell through direct contact.
  • Involves F factors (plasmids).

Transduction

  • DNA is transferred from a donor cell to a recipient cell inside a virus that infects bacteria (bacteriophage, or phage).

Mobile Gene Elements (MGEs)

  • Genetic material that can move around within a genome or be transferred from one species to another.
  • Examples: prophage, plasmid, transposon.

Transposon

  • Small segments of DNA that can move from one region of a DNA molecule to another.
  • Transposition is the movement.

Genetic Transfer and Recombination Overview

  • Summary of transformation, transduction, conjugation and transposition.