Fundamentals of Microbiology: Microbial Cell Structure, Classification, and Interactions

Microbial Diversity and Classification

Key Groups of Microbial Organisms and Their Prevalence

Microorganisms, or microbes, are microscopic living organisms that exist in diverse environments. They include bacteria, archaea, fungi, protists, viruses, and prions. Microbes are essential for ecological balance, human health, and biotechnology.

• Definition of Microbe: A microbe is a microscopic organism, which may be unicellular, multicellular, or acellular (as in viruses and prions).

• Examples: Escherichia coli (bacterium), Saccharomyces cerevisiae (fungus), Amoeba proteus (protist), Influenza virus.

• Classification: Microbes are classified as prokaryotes (bacteria, archaea), eukaryotes (fungi, protists), or non-cellular entities (viruses, prions).

Additional info: Microbes can be beneficial (e.g., gut flora) or pathogenic (e.g., Staphylococcus aureus).

How Microbes Are Classified

Microbes are classified based on cellular structure, genetic makeup, and mode of reproduction.

• Prokaryotes vs. Eukaryotes:

  • Prokaryotes lack a nucleus and membrane-bound organelles; examples include bacteria and archaea.

  • Eukaryotes possess a nucleus and organelles; examples include fungi, protists, plants, and animals.

• Binomial Naming System: Microbes are named using the genus and species (e.g., Staphylococcus aureus).

Human-Microbe Interactions

Types of Symbiotic Interactions

Microbes interact with humans in various symbiotic relationships:

• Mutualism: Both host and microbe benefit (e.g., gut bacteria synthesizing vitamins).

• Commensalism: Microbe benefits, host is unaffected (e.g., skin flora).

• Parasitism: Microbe benefits at the host's expense (e.g., pathogenic bacteria).

True Pathogen vs. Opportunistic Pathogen: True pathogens cause disease in healthy hosts; opportunistic pathogens cause disease in immunocompromised hosts.

Establishment of Microbiome and Disease Causation

• Dysbiosis: Imbalance in the microbiome can lead to disease.

• Host Factors: Genetics, immunity, and environment affect susceptibility.

• Microbe Location: Pathogenicity can depend on where microbes colonize.

Biofilms and Microbial Communities

Importance of Biofilms

Biofilms are structured communities of microbes attached to surfaces and embedded in a self-produced matrix.

• Planktonic: Free-floating microbial cells.

• Biofilm: Sessile, surface-attached communities; more resistant to antibiotics and immune responses.

Prokaryotic Cell Structure and Function

Limitations on Prokaryotic Cell Sizes and Shapes

Prokaryotic cells are generally small (0.2–2.0 μm in diameter) due to surface area-to-volume constraints.

• Common Shapes: Cocci (spherical), bacilli (rod-shaped), spirilla (spiral), vibrio (comma-shaped).

• Example: Helicobacter pylori changes shape under stress.

Domains of Prokaryotes

• Bacteria

• Archaea

Cell Division in Prokaryotes

Prokaryotes reproduce primarily by binary fission, an asexual process.

• Binary Fission: Cell duplicates DNA, elongates, and divides into two identical cells.

• Equation: $N = N_0 \times 2^n$ (where $N$ is final cell number, $N_0$ is initial, $n$ is number of generations)

Gram-Negative vs. Gram-Positive Bacteria

Gram staining differentiates bacteria based on cell wall structure.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present
LPS (Lipopolysaccharide)	Absent	Present
Teichoic Acids	Present	Absent
Exotoxins	Common	Common
Endotoxins	Absent	Present

Transport of Materials in Bacteria

Bacteria transport materials via their cell wall and plasma membrane using various mechanisms.

• Passive Transport: No energy required (diffusion, osmosis).

• Active Transport: Requires energy (primary and secondary active transport, phosphotransferase systems).

Structure and Function of Prokaryotic Cell Components

• Plasma Membrane: Hydrophilic exterior, hydrophobic interior; proteins facilitate transport.

• Cell Wall: Provides shape and protection; composition varies among bacteria.

• Other Structures: Capsule (protection), pili/fimbriae (attachment), ribosomes (protein synthesis).

Endospores

Endospores are dormant, resistant structures formed by certain bacteria (e.g., Bacillus, Clostridium) under adverse conditions.

• Function: Survival in extreme environments.

• Medically Relevant Genera: Bacillus, Clostridium

True Statements about Prokaryotic Cells

• Prokaryotic cells have 70S ribosomes.

• Prokaryotic cells reproduce by binary fission.

• Prokaryotic cells can store nutrients in inclusion bodies.

• Prokaryotic cells lack a nucleus.

• Fimbriae are used for motility and attachment.

• Archaea and bacteria can be classified using the Gram stain.

Eukaryotic Cell Structure and Evolution

Basic Description and Differences from Prokaryotes

	Eukaryotes	Prokaryotes
Organisms	Fungi, protists, plants, animals	Bacteria, archaea
Size	10–100 μm	0.2–2.0 μm
Cell Division	Mitosis/meiosis	Binary fission
Plasma Membrane	Often contain sterols	Rarely contain sterols
Cell Wall	Present in fungi/plants	Present in most
Nucleus	Present	Absent

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic organelles (mitochondria, chloroplasts) from prokaryotic ancestors.

• Sequence of Events:

  1. Primitive eukaryote engulfs aerobic bacterium.

  2. Bacterium becomes mitochondrion.

  3. Engulfment of photosynthetic bacterium leads to chloroplasts.

  4. Symbiosis becomes permanent.

• Evidence:

  • Double membranes around organelles.

  • Own circular DNA in mitochondria/chloroplasts.

  • Ribosomes similar to prokaryotes (70S).

  • Replication by binary fission.

Four Kingdoms of Eukaryotes

• Animals: Multicellular, no cell wall, sexual reproduction, includes helminths (parasitic worms).

• Fungi: Unicellular (yeasts) or multicellular (molds), cell wall of chitin, reproduce sexually/asexually, form hyphae.

• Protists: Mostly unicellular, diverse modes of reproduction, basic definition: eukaryotic microbes not classified as animals, plants, or fungi.

• Plants: Not covered in exam context.

Comparison Table: Microbial Groups

Cell Type	Bacteria	Archaea	Protist	Fungi	Helminths	Viruses	Prions
Prokaryote/Eukaryote/Neither	Prokaryote	Prokaryote	Eukaryote	Eukaryote	Eukaryote	Neither	Neither
Cell Wall Composition	Peptidoglycan	Pseudopeptidoglycan	Varies	Chitin	None	Protein coat	Protein only
Pathogenic (Y/N)	Y	Y/N	Y/N	Y/N	Y	Y	Y

Additional info: Viruses and prions are acellular and require host cells for replication.

Cellular Components: Structure and Function

Structure	Present in Prokaryotes/Eukaryotes/Neither	Function and Notes
Plasma Membrane	Both	Selective barrier; contains sterols in eukaryotes
Cell Wall	Most prokaryotes, fungi, plants	Structural support; composition varies
Glycocalyx	Both	Protection, adhesion
Flagella	Both	Motility
Cilia	Eukaryotes	Motility, feeding
Ribosomes	Both	Protein synthesis; 70S in prokaryotes, 80S in eukaryotes
Cytoskeleton	Eukaryotes	Shape, movement
Nucleus	Eukaryotes	Contains DNA
Pili	Prokaryotes	Attachment, conjugation
Fimbriae	Prokaryotes	Attachment
Capsule	Prokaryotes	Protection
Nucleoid	Prokaryotes	Region containing DNA
Endoplasmic Reticulum	Eukaryotes	Protein/lipid synthesis
Golgi Apparatus	Eukaryotes	Protein modification, sorting
Mitochondria	Eukaryotes	ATP production
Chloroplasts	Eukaryotes (plants, algae)	Photosynthesis
Vesicles/Vacuoles	Eukaryotes	Storage, transport
Lysosomes	Eukaryotes	Digestion
Endospores	Prokaryotes (some)	Dormancy, resistance
Chromosome	Both	Genetic information

Additional Eukaryotic Cell Features

• Eukaryotic cells sexually reproduce by mitosis.

• Eukaryotic cells can be unicellular or multicellular.

• Eukaryotic cells always have a cell wall (in fungi/plants).

• Eukaryotic cells can be photosynthetic (plants, algae).

• Eukaryotic cells may use fimbriae for motility (rare).

• Yeast is a unicellular eukaryote.

Example Classification

If you observe a single-celled organism with a cell wall, nucleus, is unicellular, nonmotile, and lacks chloroplasts, it is most likely a fungus (yeast).