Introduction to Microorganisms

Introduction to Microorganisms

• Common diseases in the oral cavity are caused by microorganisms.
• Understanding microbiology is important for understanding diseases, etiology, progression, diagnosis, and treatment.
• Microorganisms are minute living organisms, too small to be seen by the naked eye.

Cellular Microorganisms

• Generally unicellular, the whole organism is one cell.
• A single microbial cell performs all functions required to maintain growth and reproduction.

Eukaryotes

• Eu=true; karyote=nucleus
• Characterized by the presence of a distinct nucleus.
• Nucleus surrounded by a nuclear membrane that separates it from other cell contents.
• Fungi are an example.

Prokaryotes

• Pro=before
• Represent a simpler form of cellular organization.
• No membrane-bound nucleus.
• Hereditary material is suspended in the cytoplasm, called the nucleoid or nuclear region.
• Devoid of mitochondria and membrane-bound organelles.
• Bacteria are an example.

Acellular Microorganisms

• Viruses are acellular microorganisms.
• They have no cell structure and are obligate intracellular parasites.
• They depend on the host cell machinery for survival and reproduction.

Nomenclature

• The system of naming organisms in use today was established more than two centuries ago.
• Scientific names are Latinized because Latin was the language traditionally used by scholars.
• Scientific nomenclature assigns each organism two names.
• The genus name is the first name and is always capitalized.
• The species name follows and is not capitalized.
• After mentioning the scientific name once, it can be abbreviated with the genus initial followed by the specific species.

Scientific Names

• Describe an organism.
• Honor a researcher.
• Identify the habitat of a species.

Bacterial Morphology

• Bacteria are differentiated into major categories based on shape, size, arrangement, and staining characteristics.

Differential Stains

• Composed of more than one dye to classify bacteria into different groups.
• Gram stain classifies bacteria according to cell wall structure into Gram-positive (violet) and Gram-negative (pink).
• Ziehl-Neelsen stain is used to identify Mycobacteria.

Gram Stain Steps

• Fixation
• Primary stain: Crystal violet
• Mordant: Iodine (fixes the dye)
• Decolorizing agent: Alcohol/Acetone
• Counterstain: Safranin

Bacterial Ultrastructure and Functions

• All Bacteria have a nucleoid, ribosomes, and a cytoplasmic membrane.
• Most bacteria have a cell wall.
• Some bacteria are further enveloped by a capsule or slime layer.
• Some bacteria have appendages, e.g., flagella and pili.

Bacterial Cytoplasm

• Nucleoid (bacterial DNA): Genetic information contained in a single chromosome formed of supercoiled double-stranded DNA.
• Plasmids: Additional genetic information is contained on plasmids, small circular ds-DNA molecules that can replicate independently of the chromosome.
• Ribosomes: Site of protein synthesis; prokaryotic ribosomes have a sedimentation constant of 70S, smaller than the 80S ribosomes of eukaryotes.
• Inclusion granules: Nutrient granules, e.g., carbohydrates and lipids.
• Mesosomes: Invaginations of the cytoplasmic membrane into the cytoplasm involved in cell division and sporulation.

Cytoplasmic Membrane

• A phospholipid-protein bilayer similar to that in eukaryotes, but it lacks sterols in bacteria.
• Functions: selective transport, excretion of extracellular enzymes, cell wall biosynthesis, respiration, reproduction, and chemotactic system.

Bacterial Cell Wall

• Strong, relatively rigid structure surrounding the cytoplasmic membrane.
• Composed mainly of peptidoglycan, which gives the cell wall its strength, in addition to other components.

Gram-positive Cell Wall

• Composed of peptidoglycan (50% of cell wall material) and teichoic acid.

Gram-negative Cell Wall

• Composed of a much thinner peptidoglycan layer (5-10% of cell wall material) and an outer membrane.
• The outer membrane is a phospholipid-protein bilayer external to peptidoglycan layer; lipopolysaccharide (LPS) is present on the outer surface.
• Lipid A of the LPS forms the endotoxin of Gram-negative bacteria
• Periplasmic space: the space between the cytoplasmic membrane and outer membrane, contains the peptidoglycan layer and a gel-like solution of proteins.

Cell Wall Functions

• Maintains the characteristic shape of the bacterium.
• Supports the weak cytoplasmic membrane against the high internal osmotic pressure of the protoplasm.
• Responsible for the staining affinity of the organism.
• Plays an important role in cell division.

Wall-deficient Variants

• Mycoplasma: the only bacteria that exist naturally without a cell wall, do not have a defined shape, and are naturally resistant to cell wall inhibitors like penicillin and cephalosporins.
• L-Forms: Develop from cells that normally possess a cell wall, develop when exposed to hydrolysis by lysozyme, or by blocking peptidoglycan biosynthesis with antibiotics like penicillin; can survive antibiotic therapy, and their reversion to the walled state can produce relapses of the overt infection.

Glycocalyx

• A polysaccharide material outside the cell, formed in vivo only; can exist as a capsule or a slime layer.
• Capsule: A layer that adheres to the surface of the cell and forms a well-defined halo when stained.
• Slime layer: Surface layer, loosely distributed around the cell.
• Functions: Protects against antibacterial agents and phagocytosis; some bacteria attach to the target surface by their capsules or glycocalyx to establish infection.

Appendages: Flagella

• Can be seen only with the electron microscope.
• Flagella consist of a single type of protein called flagellin, which differs in different bacterial species; flagellins are highly antigenic (H antigen).
• Function: Motility (chemotaxis) toward nutrients and away from disinfectants.
• Distribution: may be Monotrichous, Amphitrichous, Lophotrichous, or Peritrichous.

Appendages: Pili (fimbriae)

• Shorter and thinner than flagella.
• Can be seen only by the electron microscope.
• Composed of protein subunits termed pilin.
• Functions: Adherence, which is considered a virulence factor, and conjugation between bacteria.

Flagella vs Pili

• Flagella Morphology: Whip-like appendages, long
• Pili Morphology: Hair-like appendages and thin
• Flagella Composition: Flagellin
• Pili Composition: Pilin
• Flagella Antigenicity: Highly antigenic
• Pili Antigenicity: Non-antigenic
• Flagella Functions: Motility
• Pili Functions: Adherence and conjugation

Bacterial Spores (Endospores)

• A highly resistant resting phase that does not grow or reproduce and is formed in vitro by some bacteria, e.g., Bacillus and Clostridium.
• Sporulation is triggered by unfavorable environmental conditions, e.g., depletion of nutrients, accumulation of metabolites, or changes in the growth requirements.
• Spores are much more resistant than vegetative bacteria to disinfectants, dryness, and heating.
• Moist heat at 121° C for 15-30 min is needed to kill spores, while 60° C is sufficient to kill the vegetative forms.
• Endospores can respond quickly to environmental changes, returning to the vegetative state within 15 minutes.