Microbial Growth, Control, and Genetics: Study Guide for BIO 203 Exam 2

Microbial Growth

Classification of Microbes by Temperature Preference

Microorganisms are classified into five groups based on their preferred temperature ranges, which influence their growth and ecological niches.

• Psychrophiles: Prefer cold environments; optimal growth at 15°C or lower. Found in polar regions, deep ocean waters.

• Psychrotrophs: Grow between 0°C and 30°C; often responsible for food spoilage in refrigerators.

• Mesophiles: Optimal growth at 25–40°C; includes most human pathogens and organisms found in soil and water.

• Thermophiles: Prefer hot environments; optimal growth at 50–60°C. Found in hot springs, compost piles.

• Hyperthermophiles: Grow at temperatures above 80°C; often found in volcanic and deep-sea hydrothermal vents.

Growth rate drops quickly above the optimum temperature due to denaturation of enzymes and structural proteins, which impairs cellular functions.

• Psychrotrophs may spoil refrigerated food.

• Mesophiles often contain animal pathogens.

Osmotic Pressure and Microbial Growth

Osmotic pressure affects water movement across cell membranes, influencing microbial survival and growth.

• Hypertonic environments: Water leaves the cell, causing plasmolysis (cell shrinkage). Used to preserve foods (e.g., salted meats, jams).

• Hypotonic environments: Water enters the cell, potentially causing cell lysis.

• Isotonic environments: No net movement of water; ideal for most cells.

Fermentation preserves food by producing acids, lowering pH and inhibiting microbial growth. Acid acts as a preservative by creating an environment unsuitable for many bacteria.

pH Preferences

• Most bacteria prefer pH 6.5–7.5.

• Many fungi prefer pH 5–6.

• Acidophiles thrive in acidic environments.

Halophiles and Osmotic Adaptations

• Obligate halophiles: Require high salt concentrations for growth.

• Facultative halophiles: Can tolerate high salt but do not require it.

Elements Required for Microbial Growth

• Carbon: Structural backbone of organic molecules; used for energy and biosynthesis.

• Nitrogen: Needed for proteins, nucleic acids; used in amino acid and nucleotide synthesis.

• Sulfur: Component of some amino acids and vitamins.

• Phosphorus: Essential for nucleic acids, ATP, and phospholipids.

Oxygen Requirements and Tolerance

Microbes are classified based on their oxygen requirements and tolerance:

• Obligate aerobes: Require oxygen for growth.

• Facultative anaerobes: Can grow with or without oxygen.

• Anaerobes: Do not use oxygen; may be harmed by it.

• Aerotolerant anaerobes: Do not use oxygen but tolerate its presence.

• Microaerophiles: Require low levels of oxygen.

Aerobes avoid damage by toxic forms of oxygen (reactive oxygen species) using enzymes such as superoxide dismutase, catalase, and peroxidase.

Biofilms

Biofilm formation provides advantages for bacteria, such as protection from environmental stresses and antibiotics, but poses disadvantages for humans by contributing to persistent infections and contamination.

Key Vocabulary

• Psychrophile, Psychrotroph, Mesophile, Thermophile, Hyperthermophile

• Acidophile, Halophile, Hypertonic, Plasmolysis, Isotonic, Hypotonic

• Cofactor, Obligate aerobe, Facultative anaerobe, Anaerobe, Aerotolerant anaerobe, Microaerophile

• Reactive oxygen species, Superoxide dismutase, Catalase, Peroxidase, Quorum sensing

Microbial Growth: Binary Fission and Growth Curves

Bacterial Growth and Binary Fission

Bacterial growth refers to an increase in the number of cells, not cell size. Most bacteria reproduce by binary fission, a process in which a single cell divides into two identical daughter cells.

• Binary fission: DNA replication, cell elongation, septum formation, and cell separation.

• Budding: Some bacteria and yeasts reproduce by budding, forming a new cell from a parent cell.

Bacterial Growth Curve

Bacterial populations exhibit characteristic growth curves with four distinct phases:

• Lag phase: Cells adapt to environment; no increase in cell number.

• Log (exponential) phase: Rapid cell division; population doubles at regular intervals.

• Stationary phase: Growth rate slows; cell death equals cell division due to nutrient depletion and waste accumulation.

• Death phase: Cell death exceeds cell division; population declines.

Generation Time Calculation

Generation time is the time required for a cell to divide or for a population to double.

• Formula:

$N = N_0 \times 2^n$ Where: $N$ = final cell number $N_0$ = initial cell number $n$ = number of generations

Key Vocabulary

• Binary fission, Budding, Growth curve, Lag phase, Log phase, Stationary phase, Death phase, Generation time

Microbial Growth: Counting Microbes

Direct Methods of Measuring Cell Growth

Direct methods involve counting individual cells or colonies.

• Plate count: Serial dilution and spread plate method to count colony-forming units (CFUs).

• Filtration: Used for low cell concentrations; cells are trapped on a filter and then counted.

• Direct microscopic count: Cells are counted directly under a microscope using a counting chamber.

Indirect Methods of Measuring Cell Growth

Indirect methods estimate cell numbers based on cell activity or turbidity.

• Turbidity: Measures cloudiness of a culture using a spectrophotometer.

• Metabolic activity: Measures production of metabolic products (e.g., CO2, acids).

Key Vocabulary

• Plate count, Serial dilution, Spread plate method, Filtration, Direct microscopic count, Turbidity, Metabolic activity

Control of Microbial Growth

Terminology and Physical Methods

Microbial control involves reducing or eliminating microorganisms using physical or chemical methods.

• Sepsis: Presence of pathogens.

• Asepsis: Absence of pathogens.

• Sterilization: Destruction of all microbial life.

• Commercial sterilization: Kills Clostridium botulinum endospores in food.

• Disinfection: Removal of pathogens from surfaces.

• Antisepsis: Removal of pathogens from living tissue.

• Degerming: Removal of microbes from a limited area.

• Sanitization: Lowering microbial counts to safe levels.

• Biocide: Agent that kills microbes.

• Bacteriostasis: Inhibits growth but does not kill.

Factors Affecting Microbial Death Rate

• Number of microbes

• Environmental conditions (temperature, pH)

• Time of exposure

• Microbial characteristics (spore-forming, biofilm, etc.)

Effects of Control Agents on Cellular Structures

• Damage to cell membrane

• Denaturation of proteins

• Disruption of nucleic acids

Physical Methods of Microbial Control

• Moist heat: Includes boiling, autoclaving, pasteurization. More effective than dry heat.

• Dry heat: Includes flaming, incineration, hot-air sterilization. Kills by oxidation.

• Filtration: Removes microbes from liquids or air.

• Low temperatures: Inhibit microbial growth.

• High pressure: Alters protein structure.

• Desiccation: Removes water, inhibiting growth.

• Osmotic pressure: High salt or sugar concentrations preserve food.

• Radiation: Ionizing radiation (X-rays, gamma rays) damages DNA; nonionizing radiation (UV) causes thymine dimers.

Key Vocabulary

• Thermal death point, Thermal death time, Decimal reduction time, Moist heat, Autoclave, Pasteurization, Thermoduric, Dry heat, Oxidation, Flaming, Incineration, Hot-air sterilization, Filtration, Desiccation, Ionizing radiation, Nonionizing radiation

Microbial Genetics: Overview

DNA as Genetic Information

DNA stores genetic information, which is expressed as traits through the processes of transcription and translation.

• Central dogma: DNA → RNA → Protein

• Genotype: Genetic makeup

• Phenotype: Observable traits

• Genome: Complete set of genes

• Gene: Segment of DNA coding for a product

• Genetic code: Set of rules for translating DNA into proteins

Vertical vs. Horizontal Gene Transfer

• Vertical gene transfer: Genes passed from parent to offspring.

• Horizontal gene transfer: Genes transferred between organisms, not by descent.

Physical Structure of DNA

• Double helix composed of nucleotides (adenine, cytosine, guanine, thymine)

• Phosphodiester linkages connect nucleotides

Key Vocabulary

• Central dogma, DNA, RNA, Protein, Genotype, Phenotype, Mutations, Operons, Genetics, Genome, Chromosome, Gene, Genetic code, Genomics, Gene expression, Nucleotide, Nucleic acid, Adenine, Cytosine, Guanine, Thymine, Uracil, Phosphodiester linkage

Microbial Genetics: DNA Replication

Process of DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division.

• Topoisomerase/gyrase: Relieves supercoiling

• Helicase: Unwinds DNA

• Replication fork: Site of active replication

• DNA polymerase: Synthesizes new DNA

• Primase: Synthesizes RNA primer

• Single-stranded DNA binding proteins: Stabilize unwound DNA

• Leading strand: Synthesized continuously

• Lagging strand: Synthesized in fragments (Okazaki fragments)

• DNA ligase: Joins Okazaki fragments

Energy for replication is supplied by nucleoside triphosphates.

Replication is highly accurate due to proofreading by DNA polymerase.

Key Vocabulary

• Topoisomerase/gyrase, Helicase, Replication fork, DNA polymerase, Primase, Single-stranded DNA binding proteins, Leading strand, Lagging strand, Okazaki fragments, DNA ligase

Microbial Genetics: Transcription and Translation

Protein Synthesis

Protein synthesis involves transcription (DNA to RNA) and translation (RNA to protein).

• Transcription: RNA polymerase synthesizes mRNA from DNA template.

• RNA processing: In eukaryotes, introns are removed and exons joined.

• Translation: Ribosome reads mRNA and assembles amino acids into protein.

Prokaryotes lack a nucleus; transcription and translation occur simultaneously. Eukaryotes separate these processes.

Key Vocabulary

• Gene expression, rRNA, tRNA, mRNA, microRNAs, transcription, RNA polymerase, initiation, promoter, elongation, termination, genetic code, codon, anticodon, exon, intron, snRNPs, splice, translation, ribosome

Microbial Genetics: Gene Regulation

Operons and Regulation

Gene expression in bacteria is regulated at transcriptional and post-transcriptional levels.

• Operon: Group of genes regulated together

• Constitutive genes: Always expressed

• Inducible operon: Turned on by inducer

• Repressible operon: Turned off by repressor

• Catabolite repression: Inhibition of operon by preferred energy source

• Epigenetic control: Methylation affects gene expression

• Riboswitch: RNA element regulates gene expression

• microRNA: Regulates gene expression post-transcriptionally

Key Vocabulary

• Constitutive, Inducible, Repressible, Catabolite, Inducer, Repressor, Promoter, Operator, Operon, Inducible operon, Repressible operon, Epigenetic control, Methylation, Riboswitch, microRNA

Microbial Genetics: Genetic Transfer and Recombination

Genetic Diversity and Transfer

Microbes increase genetic diversity through mutations and genetic recombination.

• Mutation: Change in DNA sequence

• Mutagen: Agent causing mutations

• Genetic recombination: Exchange of genetic material

• Vertical gene transfer: Parent to offspring

• Horizontal gene transfer: Between organisms

• Mobile genetic elements: Plasmids, transposons

• Plasmids: Small, circular DNA; often carry resistance factors

• Transposons: DNA segments that move within genome

• Transformation: Uptake of naked DNA

• Conjugation: Transfer via direct contact

• Transduction: Transfer via bacteriophage

Key Vocabulary

• Mutation, Mutagen, Genetic diversity, Natural selection, Mutation rate, Genetic recombination, Crossing over, Vertical gene transfer, Horizontal gene transfer, Mobile genetic elements, Plasmids, Resistance factors, Transposons, Transformation, Conjugation, Transduction, Bacteriophage

Summary Table: Microbial Temperature Groups

Group	Optimum Temperature Range	Typical Environment
Psychrophile	≤ 15°C	Polar regions, deep ocean
Psychrotroph	0–30°C	Refrigerated foods
Mesophile	25–40°C	Soil, water, animals
Thermophile	50–60°C	Hot springs, compost
Hyperthermophile	≥ 80°C	Volcanic vents

Summary Table: Oxygen Requirements

Type	Oxygen Requirement	Enzymes Present
Obligate aerobe	Requires O2	Superoxide dismutase, catalase
Facultative anaerobe	With or without O2	Superoxide dismutase, catalase
Anaerobe	No O2	None
Aerotolerant anaerobe	Tolerates O2	Superoxide dismutase
Microaerophile	Low O2	Some enzymes

Summary Table: Methods of Microbial Control

Method	Mechanism	Application
Moist heat (autoclave)	Protein denaturation	Sterilization of media, instruments
Dry heat (incineration)	Oxidation	Sterilization of loops, waste
Filtration	Physical removal	Heat-sensitive liquids
Low temperature	Inhibits growth	Food preservation
Desiccation	Removes water	Preservation of foods
Osmotic pressure	Plasmolysis	Preservation (jams, salted meats)
Radiation	DNA damage	Sterilization of medical supplies

Additional info: Academic context and explanations have been expanded for clarity and completeness. Tables have been recreated and summarized for exam preparation.