Introduction to Microbiology

Definition and Scope

Microbiology is the study of microorganisms—organisms and agents that are too small to be seen with the naked eye. These include bacteria, archaea, viruses, fungi, protozoa, and algae. Microbiology explores their structure, function, diversity, evolution, and their roles in health, industry, and the environment.

• Microorganisms are often unicellular and lack differentiated tissues.

• They are found in diverse and extreme environments, from polar ice caps to deep-sea hydrothermal vents.

Importance of Microorganisms

Microorganisms play essential roles in the circle of life and have profound impacts on human society and the environment.

• Recycling of essential elements: Microbes decompose organic matter, releasing nutrients back into ecosystems.

• Source of nutrients: Some microbes fix nitrogen or produce vitamins.

• Photosynthesis: Certain bacteria and algae contribute to global oxygen production.

• Industrial benefits: Microbes are used in the production of cheese, bread, beer, vaccines, and antibiotics.

• Bioremediation: Microbes can clean up environmental pollutants, such as oil spills.

• Pathogenic impact: Some microbes cause diseases that have shaped human history (e.g., plague, AIDS, malaria, COVID-19).

The Human Microbiome

The human body hosts trillions of microbes, collectively known as the human microbiome. These microbes are found on the skin, in the mouth, gut, and other body sites, and play crucial roles in health and disease.

• Normal microbiota: Includes bacteria, archaea, and eukaryotic microbes that train the immune system, produce vitamins, and aid digestion.

• Microbiome profiles can influence susceptibility to chronic diseases.

• Babies are colonized by microbes during delivery and early life.

Diversity of Microorganisms

Types of Microorganisms

Microorganisms are classified into several major groups based on their cellular structure and function.

• Bacteria: Prokaryotic, unicellular organisms with diverse shapes (cocci, bacilli, spirilla).

• Archaea: Prokaryotic, often found in extreme environments.

• Fungi: Eukaryotic, includes yeasts and molds.

• Protozoa: Eukaryotic, unicellular, often motile.

• Algae: Eukaryotic, photosynthetic organisms.

• Viruses: Acellular, require host cells for replication.

• Prions: Infectious proteins causing neurodegenerative diseases.

Microbial Morphology

Bacterial classification is based on morphology (shape, size, arrangement) and physiological features.

• Spherical (cocci)

• Rod-shaped (bacilli)

• Helical (spirilla)

Examples of Microbial Groups

• Fungi: Aspergillus fumigatus, Rhizopus

• Helminths: Diphyllobothrium latum, Schistosoma mansoni

• Protozoa: Ciliates, flagellates, amoebas, Trypanosoma brucei, Plasmodium falciparum

• Algae: Green algae, Volvox

• Viruses: Bacteriophage, H1N1, Ebola, COVID-19

• Prions: Infectious protein conformers

Microbial Evolution and History

Origin and Evolution

Microbial evolution is studied using the scientific method and molecular techniques. Microfossils and stromatolites provide evidence for ancient microbial life, dating back 3.5–3.7 billion years.

• Bacterial ancestors were among the first living cells on Earth.

• Microbial DNA has been found in ancient mummies, indicating long-standing human-microbe interactions.

Discovery and Early History

Before microorganisms were described, philosophers and physicians speculated about their existence. Key figures in the history of microbiology include:

• Robert Hooke: Early microscopic observations.

• Antony van Leeuwenhoek: First accurate observations of microorganisms.

• Francesco Redi: Disproved spontaneous generation for large animals.

• John Needham & Lazzaro Spallanzani: Experiments on spontaneous generation in microbes.

• Louis Pasteur: Swan-neck flask experiments disproved spontaneous generation for microbes; developed aseptic techniques.

Golden Age of Microbiology

Major Discoveries and Techniques

Between 1857–1914, many disease-producing organisms were discovered, and microbiological techniques were refined.

• Silkworm disease: Fungal infection

• Anthrax: Bacterial infection by Bacillus anthracis

• Malaria: Parasitic infection by Plasmodium falciparum

• Development of pure cultures, agar media, Petri dish

• Understanding immunity: phagocytosis, vaccination

Koch’s Postulates

Robert Koch established criteria to link a specific microorganism to a specific disease. These postulates are still used today:

1. The organism must be present in every case of the disease, but not in healthy individuals.

2. The organism must be isolated and grown in pure culture.

3. The cultured organism should cause the disease when inoculated into a susceptible host.

4. The organism must be re-isolated from the inoculated, diseased host.

Aseptic Techniques and Medical Impact

Hand Hygiene and Aseptic Techniques

Aseptic techniques are essential in healthcare to prevent healthcare-acquired infections (HAIs). Key contributors include:

• Ignaz Semmelweis: Advocated hand washing in hospitals.

• Joseph Lister: Developed aseptic surgery techniques.

• Florence Nightingale: Established aseptic techniques in nursing.

• Types of aseptic techniques: hand washing, wearing gloves, sterilizing instruments, decontaminating surfaces.

Microbial Classification and Taxonomy

Prokaryotic vs Eukaryotic Cells

Microorganisms are classified based on cellular structure:

• Prokaryotic cells: Lack a membrane-enclosed nucleus; include bacteria and archaea.

• Eukaryotic cells: Have a membrane-delimited nucleus; include fungi, protozoa, algae.

Taxonomic Hierarchy

Microbial taxonomy organizes organisms into hierarchical groups:

• Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

• Bacterial and archaeal species: collection of strains with stable properties and 70% DNA similarity

• Microbes have Latin names: Genus (capitalized) + species (lowercase), e.g., Staphylococcus aureus

Host–Microbe Interactions

Symbiotic Relationships

Microbes and humans have evolved various symbiotic relationships:

• Parasitism: Microbe harms the host

• Mutualism: Both benefit

• Commensalism: No perceived benefit or cost

Pathogens have a parasitic relationship with their host. The term "parasite" often refers to helminths and protozoans.

Biofilms

Formation and Impact

Biofilms are sticky communities of microbes that attach to surfaces and are highly resistant to antibiotics and immune responses.

• Biofilms can form on teeth, contact lenses, medical devices, and internal tissues.

• 60–80% of infectious diseases in humans are due to biofilm-creating microbes.

• Biofilm formation involves attachment, growth, and detachment of planktonic cells.

Normal Microbiota and the Human Microbiome

Functions and Distribution

Normal microbiota includes bacteria, archaea, and eukaryotic microbes that inhabit various regions of the human body.

• Functions: train immune system, produce vitamins, aid digestion, protect against pathogens.

• Populations vary by skin region, mouth, gut, and genital/urinary tract.

• Some normal microbiota are opportunistic pathogens.

Establishing and Disrupting Microbiota

Babies are colonized by microbes during delivery and early life. Disruptions in normal microbiota, such as antibiotic therapy, can lead to opportunistic infections.

• Antibiotics can reduce normal microbiota, allowing pathogens like Candida albicans to cause infections.

• Gut microbiome disruption can lead to diarrhea.

Transient Microbiota

Transient microbiota are temporary passengers picked up from the environment and removed through hygiene practices.

• Proper hand-washing can remove transient microbes.

Environmental and Industrial Uses for Microbes

Bioremediation

Bioremediation uses microbes to clean up toxic waste and environmental pollutants.

• Certain microbes can metabolize toxic substances into harmless intermediates.

• Hundreds of microbial species can degrade petroleum oil spills into CO2.

Summary Table: Microbial Groups and Examples