Main Themes of Microbiology

Introduction to Microbiology

• Microbiology is defined as the study of organisms that cannot be seen without external magnification.

  • "Micro" signifies small, and "Biology" is the study of living things.

  • It encompasses living things invisible to the unaided eye.

Sizes of Microorganisms

• The smallest size visible to the unaided human eye is approximately $1 \text{ mm}$.

• Microorganisms exist on a much smaller scale:

  • Red blood cell: $10,000 \text{ nm}$ in diameter

  • E. coli (bacterium): $1,000 \text{ nm} \times 3,000 \text{ nm}$

  • Smallpox virus: $200 \text{ nm} \times 300 \text{ nm}$

  • Poliovirus: $30 \text{ nm}$

  • Bacterial ribosomes: $25 \text{ nm}$

  • Bacteriophage MS2: $24 \text{ nm}$

  • Bacteriophage T4: $50 \text{ nm} \times 225 \text{ nm}$

  • Tobacco mosaic virus: $15 \text{ nm} \times 300 \text{ nm}$

The Ubiquitous Nature of Microbes

• Organisms studied in microbiology are collectively referred to as microorganisms or microbes, not "germs."

• They are the dominant form of life on Earth, both in terms of:

  • Numbers

  • Biomass

  • Overall effects (including Algae and Archaea)

• Despite their prevalence, microbes often receive negative attention due to their association with disease.

  • Medical microbiology is the field that specifically studies microbes capable of causing disease.

  • It's important to note that only approximately $1\%$ of all microbes cause disease.

Positive Uses and Benefits of Microbes

Microbes play crucial and beneficial roles in various aspects of life and industry:

• Food Production: Essential for making products like cheese, bread, and beer.

• Medical Applications:

  • Source of antibiotics.

  • Tools in genetic engineering.

• Ecology:

  • Vital components in aquatic food chains.

  • Crucial for water purification processes.

  • Facilitate nutrient cycles in soil, contributing to ecosystem health.

• Industry: Used in industrial waste removal.

• Human Health: Crucial for maintaining healthy human life (e.g., gut microbiome).

Types of Microbes: Cells

Microbes can be single-celled or multicellular and are primarily categorized into two main types of cells:

Prokaryotic Cells

• Examples: Bacteria and Archaea.

• Characteristics:

  • Generally smaller in size ($0.1 \text{ µm} - 10 \text{ µm}$).

  • Genetic material (DNA) is typically a single, long strand.

  • Lack a true nucleus.

  • Do not possess membrane-bound organelles.

Eukaryotic Cells

• Examples: Fungi, Protozoa, and Algae.

• Characteristics:

  • Generally larger in size ($10 \text{ µm} - 100 \text{ µm}$).

  • Genetic material (DNA) is organized into chromosomes.

  • Contain a true nucleus.

  • Possess various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes).

Similarities Between Prokaryotic and Eukaryotic Cells

Despite their differences, both cell types share fundamental components:

• Nucleic Acids (DNA/RNA): Serve as the genetic material of the cells.

• Ribosomes: Granules responsible for assisting in protein synthesis.

• Plasma Membrane: The outer border of the cell that controls the passage of substances into and out of the cell.

Kinds of Microorganisms: Detailed Overview

Bacteria

• Description: Unicellular prokaryotes.

• Visibility: Individual cells (e.g., Escherichia coli) are microscopic and require viewing under a microscope.

Fungi

• Description: Eukaryotic organisms.

• Structure: Can be either multicellular (e.g., mushrooms, molds) or unicellular (e.g., yeasts).

Protozoa

• Description: Unicellular eukaryotes.

• Note: The term "protozoa" is a broad classification and is sometimes considered obsolete in modern taxonomy (further discussed in Topic 2).

Viruses

• Nature: Not composed of cells; they are considered intracellular parasites.

• Replication: Can only replicate inside a living host cell.

• **Basic Structure (all viruses have):

  • Capsid: A protein coat.

  • Nucleic Acid: Genetic material, which can be either DNA or RNA.

  • The combination of nucleic acid and protein coat is termed a nucleocapsid.

• **Optional Structures (some viruses also have):

  • Envelope: An outer membrane derived from the host cell.

  • Spike proteins: Proteins embedded in the envelope, involved in host cell recognition.

Virus Survival and the Definition of Life

• Survival Outside the Body: Viruses can survive outside a host body for varying durations depending on the specific virus.

  • Some viruses (e.g., HIV) die within seconds outside the body.

  • Others (e.g., Influenza virus) can live for about 48 hours.

  • Some viruses may remain dormant in environmental factors like dust or water molecules for many years.

  • Crucially, even if they can survive, viruses can only replicate when inside a host cell.

• Are Viruses Alive? This is a complex philosophical question:

  • Viruses possess some structures and exhibit some activities common to organic life, but they lack many others.

  • They lack most of the internal structure and biosynthetic machinery necessary for independent reproduction.

  • If the criterion for defining life is simplified to "the ability to move a genetic blueprint into future generations, thereby regenerating your likeness," then viruses could be considered alive by this definition. However, their obligate parasitic nature makes their classification unique.

Measuring Microorganisms: The Metric System

• Microbes are measured using the metric system, which is essential for working with extremely small scales.

• The metric system is a decimal-based system, with units differing by factors of $10$, simplifying conversions.

Metric System Units

• Conversions (in relation to 1 meter):

  • $1 \text{ meter} = 100 \text{ cm}$

  • $1 \text{ meter} = 1,000 \text{ mm}$

  • $1 \text{ meter} = 1,000,000 \text{ µm}$

  • $1 \text{ meter} = 1,000,000,000 \text{ nm}$

• Step-wise Conversions:

  • $1 \text{ cm} = 10 \text{ mm}$

  • $1 \text{ mm} = 1,000 \text{ µm}$

  • $1 \text{ µm} = 1,000 \text{ nm}$

• Most Helpful Units for Microbiologists: Micrometer ($\mu \text{m}$) and Nanometer (nm) are the most useful units for microbiologists due to the minuscule size of microbes.

Dimensional Analysis and Conversion Factors

• Converting between units often uses dimensional analysis and conversion factors.

• Example: Converting $5 \text{ cm}$ to millimeters (mm):

  • Given $1 \text{ cm} = 10 \text{ mm}$:

  • $5 \text{ cm} \times \frac{10 \text{ mm}}{1 \text{ cm}} = 50 \text{ mm}$

• Example: Converting $5 \text{ mm}$ to centimeters (cm):

  • $5 \text{ mm} \times \frac{1 \text{ cm}}{10 \text{ mm}} = 0.5 \text{ cm}$

Scientific Notation

• Purpose: A useful way to express very large or very small numbers concisely.

• Format: A "single-digit value" (between 1 and 10, not including 10) multiplied by an exponent (a power of $10$).

  • Example: $6,500,000 = 6.5 \times 10^{6}$

• Converting Large Numbers: Move the decimal point leftward until only one non-zero digit remains to the left of the decimal. The number of places moved is the positive exponent.

  • Example: $1,650,000 \text{ mm} = 1.65 \times 10^{6} \text{ mm}$

• Converting Small Numbers: Move the decimal point rightward until only one non-zero digit remains to the left of the decimal. The number of places moved is the negative exponent.

  • Example: $0.00000165 \text{ m} = 1.65 \times 10^{-6} \text{ m}$

• Practice Examples:

  • $652,000 \text{ µm} = 6.52 \times 10^{5} \text{ µm}$

  • $0.000917 \text{ m} = 9.17 \times 10^{-4} \text{ m}$

  • $45,300 = 4.53 \times 10^{4}$

  • $0.0000657 = 6.57 \times 10^{-5}$

  • $5.01 \times 10^{3} = 5,010$

  • $2.75 \times 10^{-6} = 0.00000275$

Visualizing Microorganisms

• Naked Eye Limit: The human eye can typically resolve objects down to approximately $1 \text{ mm}$.

Light Microscope

• Range: Useful for viewing objects from roughly $1 \text{ mm}$ down to $1 \text{ µm}$.

• Examples: Amoeba ($100 \text{ µm}$), Red blood cells ($10 \text{ µm}$), Bacteria ($1-10 \text{ µm}$).

Electron Microscope

• Range: Essential for visualizing objects smaller than $1 \text{ µm}$, providing much higher magnification and resolution.

• Examples: DNA ($1 \text{ nm}$ diameter), HIV virus ($100 \text{ nm}$), Flagellum ($10 \text{ nm}$).

Historical Discoveries in Microbiology

Robert Hooke ($1635-1703$)

• First to observe microbes (though not bacteria) using a crude microscope.

• Published Micrographia in $1665$, where he coined the term "cell" based on the appearance of cork tissue.

Anton van Leeuwenhoek ($1632-1723$)

• Considered the first Microbiologist.

• Developed high-quality single-lens microscopes capable of $300-500\text{x}$ magnification.

• Was the first to meticulously observe and describe bacteria, which he referred to as "animalcules."

The Disproof of Abiogenesis (Spontaneous Generation)

• Abiogenesis (Spontaneous Generation): A widely held belief prior to the mid-1800s that living organisms could spontaneously arise from non-living matter (e.g., mice from stored grain, maggots from uncovered meat).

Francesco Redi ($1626-1697$)

• One of the first to challenge the abiogenesis hypothesis.

• Through controlled experiments, he demonstrated that maggots did not spontaneously appear on meat but developed from eggs laid by flies.

Lazzaro Spallanzani ($1729-1799$)

• Further deepened doubts about spontaneous generation.

• Hypothesized that microbes were present in the air and could be killed by boiling.

• Proved that no microorganisms would "spontaneously" appear in nutrient broth that had been boiled and then sealed, preventing air contamination.

Louis Pasteur ($1822-1895$)

• Credited with definitively disproving spontaneous generation.

• Conducted famous experiments using swan-neck flasks.

  • Broth in these flasks was boiled to sterilize it.

  • The swan neck allowed air exchange but trapped airborne dust and microbes, preventing them from reaching the broth.

  • The broth remained sterile unless the neck was broken, allowing microbes to enter.

• Theory of Biogenesis (developed by Pasteur): Established the principle that "Living things come from other living things" and that life forms do not appear spontaneously but originate from pre-existing "parents."

Pasteurization and Germ Theory

• Pasteurization: Pasteur discovered that heating wine prevented it from souring due to microbial activity.

  • This process, now known as pasteurization, involves heating liquids to a specific temperature for a set time to kill harmful microbes.

• Germ Theory of Disease: Pasteur's work with wine spoilage and his swan-neck flask experiments firmly established the Germ Theory of Disease, which states that microorganisms cause disease.

Recent Discoveries in Microbiology

• Development of the Electron Microscope: Revolutionized microbiology by allowing detailed visualization of viruses and intricate cellular components previously invisible.

• Vaccinations: Significant advancements in preventing infectious diseases (further discussed in Topic 19).

• Genomic Sequencing and Bioengineering: Modern techniques enabling the study of microbial genetics and the manipulation of microorganisms for various applications.