Microbiology: What and Why?

Introduction to Microbiology

Microbiology is the study of microbes (microorganisms), which are tiny organisms visible only under a microscope. Microbes include bacteria, fungi, protozoa, algae, and viruses. While most microbes are harmless or beneficial, some can cause disease. Understanding microbes is essential for topics such as infection, antibiotic production, genetic engineering, and research.

• Microbes play vital roles in fermentation, antibiotic production, and disease.

• Studying microbes helps us understand, prevent, and treat disease.

Microbes and Humans

Normal Flora and Pathogens

Most microbes cannot invade the human body. Many are free-living in soil and water. Some microbes live harmlessly on skin, respiratory tract, and bowel (normal flora), but can become opportunistic pathogens if the environment changes (e.g., immunosuppression, antibiotic use).

• Normal Flora: Microbes that live harmlessly on or in the human body.

• True Pathogens: Microbes that overcome defenses, invade cells, and produce toxins causing disease.

• Carriers: Healthy people who harbor and spread pathogens.

Cell Types

Prokaryotic vs. Eukaryotic Cells

Microbes are either prokaryotic or eukaryotic. Viruses do not fit neatly into either group.

Prokaryotic Cells (pro = before, karyon = nucleus)

• Most are single-celled.

• Nucleoid: DNA not separated from cytoplasm by a membrane.

• No nucleus or membrane-bound organelles.

• May have one circular chromosome (DNA only).

• Cell wall is present, often with a glycoprotein layer (glycocalyx).

• May have flagella, pili for movement and attachment.

• May form spores or endospores for survival.

Eukaryotic Cells (eu = true, karyon = nucleus)

• Nucleus enclosed by nuclear membrane.

• Chromosomes: both DNA and RNA.

• Many organelles (e.g., mitochondria, endoplasmic reticulum).

• Cell division by mitosis and meiosis.

• No slime layer.

Classification of Microbes

Taxonomy and Nomenclature

Biological classification allows for orderly grouping of organisms. Binomial nomenclature refers to bacteria and living things by two names: genus and species (e.g., Staphylococcus aureus).

• Viruses are not true living cells; classified as particles.

Bacteria

General Characteristics

Bacteria are single-celled, prokaryotic organisms. They lack a nucleus and membrane-bound organelles.

• Identified by:

  • Morphology: Shape and arrangement

  • Staining properties: Gram stain

  • Motility: Flagella/axial filaments

  • Growth patterns

  • Biochemical and metabolic activities

  • Pathogenicity

  • Genetic composition

• Absorb nutrients; some can synthesize their own (e.g., photosynthesis).

• Growth rates are important for classification.

• Require C, H, O, S, P, N to grow.

Shapes and Arrangements

• Coccus (plural: cocci) – spherical-shaped

• Bacillus (plural: bacilli) – rod-shaped

• Spirillum (plural: spirilla) – spiral-shaped

• Filamentous forms: Some bacteria grow as long thread-like chains.

Cell Wall and Staining

• Most have a rigid cell wall.

• Gram Staining:

  • Gram-positive: retain crystal violet (purple)

  • Gram-negative: lose crystal violet, take up counterstain (pink)

  • Some antibiotics target Gram-positive or Gram-negative bacteria specifically.

  • Broad-spectrum antibiotics target both.

Motility

• Some bacteria are motile via flagella or axial filaments.

Algae

General Characteristics

Algae may be single-celled or multicellular, and are eukaryotic. They perform photosynthesis and produce food for themselves and other organisms.

• Algae are sources of:

  • Gelling agents (e.g., agar)

  • Stabilizers (used in ice cream)

  • Emulsifiers (used in puddings)

Fungi

Beneficial and Harmful Roles

• Beneficial: Used in food production (bread, cheese), drug manufacturing (antibiotics, enzymes).

• Harmful: Cause food spoilage, some are pathogenic (mycoses).

Basic Characteristics

• Eukaryotic (true nucleus and organelles).

• Some are single-celled (yeasts); others are multicellular (molds).

• Grow as filaments (hyphae) forming networks called mycelium.

• Reproduce by budding, hyphal extension, or spore formation.

• Resist heat, acid, alkalis, bases, and chemicals.

• Cell wall contains cellulose (not peptidoglycan).

• Cannot photosynthesize; must absorb nutrients from environment.

• Generally non-motile.

Ecological Role

• Most fungi are saprophytes – live on dead organic matter and decompose it.

Classes of Fungi

Fungal Infections (Mycoses)

• Superficial: Affect skin or mucous membranes (e.g., Candida).

• Deep-seated: Inhaled spores or enter through wounds.

• Systemic: Spread through the bloodstream (often start as superficial or deep infections).

Protozoa

General Characteristics

• Eukaryotic, single-celled, animal-like organisms.

• No cell wall, no chlorophyll (cannot photosynthesize).

• Usually free-living but some are parasitic or pathogenic.

Nutrition

• Ingest nutrients via:

  • Phagocytosis (“cell eating”)

  • Pinocytosis (“cell drinking”)

Clinical Importance

• Some are pathogens (e.g., Plasmodium spp. cause malaria).

• Amoebic dysentery (Entamoeba histolytica).

• Others live in symbiotic relationships with hosts.

Classification Based on Locomotion

Viruses

General Characteristics

• Extremely small; only visible with electron microscope.

• Not considered true cells; not classified as living organisms.

• Contain DNA or RNA (never both).

• Cannot synthesize proteins or reproduce energy independently.

Replication

• Must invade a host cell to replicate.

• Hijack host cell’s machinery to produce new viral particles.

• May change host cell’s genetic composition.

Clinical Importance

• Cause many human, animal, and plant diseases.

• Bacteriophages: viruses that infect bacteria (species and strain-specific).

• Useful in genetic research.

Treatment & Control

• Antibiotics are ineffective (target living cells, not viruses).

• Antiviral drugs can inhibit replication of some viruses.

In Reflection

• Why is it important to know this?

• Does it contain DNA or RNA?

References

• Herlihy, B. (2011). The human body in health and illness (4th ed.). Elsevier.

• Martini, F. H., Nath, J. L., & Bartholomew, E. F. (2018). Essentials of anatomy & physiology (8th ed.). Pearson.

• Scanlon, V., & Sanders, T. (2011). Essentials of anatomy and physiology (6th ed.). F. A. Davis Company.