Introduction to Microbiology

Introduction to Microbiology

• Medical microbiology is the study of microorganisms (bacteria, viruses, fungi, and parasites) that cause diseases in humans
• It includes pathogenesis, disease pathology, immunology, and epidemiology of diseases

Branches of Microbiology

• Bacteriology: The study of bacteria (the smallest, simplest single-celled organisms)
• Mycology: The study of fungi (including microscopic forms—molds and yeasts—and larger forms—mushrooms)
• Parasitology: The study of parasites (including pathogenic protozoa and helminths)
• Virology: The study of viruses (minute particles that parasitize living things)
• Immunology: The study of the immune systems that protect against infection
• Microbial taxonomy: The classification, naming, and identification of microorganisms
• Biotechnology: Any process in which humans use the systems or processes of organisms to create a desired product

Genetic engineering and recombinant DNA technology

• Techniques that alter the genetic makeup of organisms to introduce new compounds, different genetic combinations, and/or unique organisms.

The Universal Tree of Life

• Based on ribosomal RNA (rRNA) analysis, life is divided into three domains: Archaea, Bacteria, and Eukarya
• Bacteria and Archaea are prokaryotic but not closely related
• Eukarya includes protists, fungi, plants, and animals

Members of the domain Bacteria

• Unicellular and prokaryotic
• Thick, rigid cell walls surrounding a cell membrane containing peptidoglycan
• Diverse ecologically (free-living soil organisms to deadly parasites); some photosynthesize, others require oxygen, and still others are killed by oxygen

Ranks or Levels of Bacterial Taxonomy (descending order)

• Domain: Collection of similar kingdoms
• Kingdom: Collection of similar phyla or divisions
• Phylum/Division: Collection of similar classes
• Classes: Collection of similar orders
• Orders: Collection of similar families
• Families: Collection of similar genera
• Genera: Collection of related species
• Species: Group of related isolates or strains

Binomial Nomenclature

• Organisms are named using binomial nomenclature (e.g., Escherichia coli)
• Genus name comes first, is capitalized, and italicized or underlined
• Species name comes second, is not capitalized, and italicized or underlined
• The genus name may be used alone; the species name cannot

Bacterial Morphology

• Bacteria are classified by shape: cocci (round), bacilli (rods), and spirochetes (spiral)
• Bacteria can be pleomorphic (variable in shape)
• Arrangement (e.g., clusters, chains) is important for identification

Structure of Bacteria

• Prokaryotic cells have three architectural regions: cell envelope (capsule, cell wall, plasma membrane), cytoplasmic region (DNA, ribosomes, inclusions), and appendages (flagella, pili)
• The cell wall, composed of peptidoglycan (except in Mycoplasma), is the outermost component.
• Mycoplasma have no cell wall

Cell Walls of Gram-Positive and Gram-Negative Bacteria

• Gram-positive bacteria have a thicker peptidoglycan layer and teichoic acids
• Gram-negative bacteria have a complex outer layer (lipopolysaccharide, lipoprotein, phospholipid). The outer membrane contains endotoxin (lipopolysaccharide)
• The periplasmic space between the outer and cytoplasmic membrane is in gram-negative bacteria

Cell Walls of Acid-Fast Bacteria

• Mycobacteria have an unusual cell wall with high lipid content, making them acid-fast
• Acid-fast bacteria resist decolorization with acid-alcohol after staining with carbolfuchsin

Peptidoglycan

• Found in bacterial cell walls; provides structural support. Synonyms include murein and mucopeptide.
• Target for antibacterial drugs (e.g., penicillins, cephalosporins, vancomycin)

Lipopolysaccharide (LPS)

• Component of the outer membrane of gram-negative bacteria; is endotoxin

Teichoic Acid

• Found in gram-positive bacteria, part of the cell wall
• Causes septic shock

Cytoplasmic Membrane

• Phospholipid bilayer
• Important for active transport, energy generation, cell wall synthesis, and secretion of enzymes/toxins
• sterols are absent, except in Mycoplasma.

Capsule

• Organized layer of glycocalyx
• Attached to the cell wall; comprised of polysaccharide or polypeptide
• Important for adherence, resistance to phagocytosis, and infection

Slime Layer

• Unorganized layer surrounding bacterial cell
• Polysaccharide coating aiding adherence and biofilm formation

Ribosomes

• Site of protein synthesis
• Bacterial ribosomes (70S) differ in size and composition from eukaryotic ribosomes (80S)
• Target for some antibiotics

Granules

• Cytoplasmic storage areas for nutrients

Nucleoid

• Region of the cytoplasm where bacterial DNA is located; single, circular molecule

Plasmids

• Small, circular DNA molecules replicating independently of the chromosome
• Carry genes for antibiotic resistance, heavy metal resistance, UV light resistance, pili formation, toxins
• Transmissible vs. non-transmissible plasmids

Transposons

• Pieces of DNA that can move from one site to another (jumping genes)

Flagella

• Long, whiplike appendages for chemotaxis (movement toward nutrients)
• Different arrangements (e.g., monotrichous, lophotrichous, amphitrichous, peritrichous, atrichous)

Pili (Fimbriae)

• Hairlike filaments extending from the cell surface
• Mediate bacterial attachment to host cells (important for infection)

Bacterial Spores

• Highly resistant structures formed under adverse conditions by Bacillus and Clostridium
• Contain bacterial DNA, little water, and a thick, keratin-like coat

Conclusion

• Different species of bacteria have various structures and properties, making them important in medicine and ecology
• Their structures are a crucial aspect of bacterial classification, identification and function.