Bacterial Classification and Characteristics Quiz

Bacteria: Classification, Structure, Growth and Metabolism

• Bacteria are classified, structured, grown, and metabolized.
• Objectives include describing bacterial structures and functions, different classification schemes for grouping bacteria, types of bacteria, distinguishing characteristics of Gram-positive and Gram-negative bacteria, and growth, biochemical, and genetic characteristics used to differentiate bacteria.
• Singular bacterium; common noun bacteria
• Bacteria in size range from 0.2 to 5 μm.
• Mycoplasma is the smallest, Bacillus anthracis is one of the largest.

Bacterial Structure

• Prokaryotic cells are simpler than eukaryotic cells, except their cell envelopes.
• Prokaryotic cells have a capsule, cell wall, plasma membrane, cytoplasm, ribosomes, plasmids, and pili.
• Eukaryotic cells have a nucleus, nuclear pores, nuclear envelope, chromatin, nucleolus, ribosomes, plasma membrane, mitochondria, peroxisome, cytoskeleton, free ribosomes, Golgi vesicles, lysosome, smooth endoplasmic reticulum, rough endoplasmic reticulum, centrioles, Cytoplasm, and secretory vesicles.

The Bacterial Chromosome or "Nucleoid"

• The nucleoid is the area of the cytoplasm where DNA is located most often as a single circular molecule, with exceptions such as Vibrio cholerae (two circular chromosomes), and Borrelia burgdorferi (linear chromosome with multiple circular and linear plasmids).
• Bacterial genomes vary widely in size, the smallest containing over 130 genes, the largest approximately 11,600 genes.
• Human DNA has approximately 25,000 genes.
• Bacterial nucleoid contains no nuclear membrane, no nucleolus, no mitotic spindle, and no histones.
• Haploid refers to a single set of chromosomes in an organism's cells.

Plasmids

• Small extra pieces of genetic material (5–100 genes).
• Not critical for everyday functions but can provide genetic information to promote antibiotic resistance, virulence factors, and some structures like pili.
• Involved in the transfer of genetic material between bacteria through cell-to-cell contact.

Transposons

• Pieces of DNA that move readily within or between the DNAs of bacteria, plasmids, and bacteriophages.
• Nicknamed "jumping genes."
• Can code for drug-resistant enzymes, toxins, or metabolic enzymes.
• Can cause mutations or alter the expression of nearby genes.

Cytoplasm

• Also known as protoplasm, gel-like substance in bacterial cells.
• Contains the chromosome, ribosomes, macromolecules, and small molecules in water solution.
• Location of growth, metabolism, and replication.
• When viewed under electron microscope, two distinct areas: amorphous matrix (ribosomes, nutrients, granules, metabolites, and plasmids) and nucleoid region (DNA).

Inclusions in Bacterial Cells

• Inclusion bodies store energy or structural building blocks.
• Glycogen: polymer of glucose
• Poly-betahydroxybutyric acid (PHB): lipid
• Polyphosphates: polymers of PO₄
• Sulfur globules: elemental S
• Magnetosomes: magnetite (iron oxide)
• Gas vesicles: protein shells inflated with gas
• Parasporal crystals: protein

Ribosome Function

• Ribosomes are the site of protein synthesis in all living organisms.
• Prokaryotes have 70S ribosomes, while eukaryotes, larger 80S ribosomes in their cytoplasm.
• 70S ribosomes are made up of 50S and 30S subunits.
• Ribosomes are found within the cytoplasm or attached to the plasma membrane.
• Amino acids are assembled into proteins according to the genetic code on the surfaces of ribosomes during translation.

Cytoskeleton

• Cellular "scaffolding" or "skeleton" within the cytoplasm.
• Actin homologs (MreB and Mbl) help determine cell shape, segregate chromosomes and localize proteins.
• Other nonactin homologs (e.g., FtsZ) and unique bacterial cytoskeletal proteins (e.g., SecY and MinD) are involved in shape determination and cell division and chromosome segregation.

Endospores

• Intracellular structures within the cytoplasm of certain species (notably Bacillus and Clostridium).
• Endospores provide resistance to dehydration, heat, and chemicals.
• Are not a mechanism for reproduction, but for survival in harsh environments—sporulation occurs when nutrients (carbon or nitrogen); are depleted.
• Spore formation takes about six hours.
• Have a spore coat, cortex, and core wall.
• Resting (dormant) cells show no signs of life due to lack of water in the spore, and are highly resistant to heat, acids, bases, dyes, irradiation, disinfectants, and antibiotics.
• Spores contain bacterial DNA, a small amount of cytoplasm, membrane, peptidoglycan, and keratin-like coat—responsible for remarkable resistance.
• The resistance of endospores may be related to dipicolinic acid, a calcium ion chelator found only in spores.

Medically-Important Endospore-Forming Bacteria

• Bacillus anthracis, Bacillus cereus, Clostridium tetani, Clostridium botulinum, Clostridium perfringens, and Clostridium difficile.

The Plasma Membrane (cytoplasmic membrane)

• Lies just inside the peptidoglycan layer.
• Composed of a phospholipid bilayer.
• Chemically similar to eukaryotic membranes, but prokaryotic ones don't contain sterols, except for the genus Mycoplasma.
• Composed of 60% protein and 40% phospholipid.
• Arranged as a bilayer formed by phospholipid molecules made up from glycerol and fatty acids.

Functions of the Cytoplasmic Membrane

• Selective permeability and transport of molecules into the cells.
• Electron transport and oxidative phosphorylation in aerobic species (energy generation).
• Secretion of enzymes and toxins.
• Contains enzymes and carrier molecules functioning in biosynthesis of DNA, cell wall polymers and membrane lipids (biosynthesis functions.)
• Bears the receptors and other proteins of chemotactic and other sensory transduction systems.
• Semi-permeable.
• Impermeable to macromolecules.

Transport Systems in Bacteria

• Iron (Fe) is an essential nutrient for the growth of almost all bacteria. Under anaerobic conditions, Fe is generally in the +2 oxidation state and soluble. Under aerobic conditions, iron is generally in the +3 oxidation state and insoluble.
• Some bacteria secrete siderophores—compounds that chelate Fe and promote its transport as a soluble complex—to overcome this problem.

Excretion of Hydrolytic Exoenzymes and Pathogenicity Proteins

• Organisms that obtain nutrients from macromolecules like proteins, polysaccharides, or lipids secrete hydrolytic enzymes.
• In Gram-positive bacteria, proteins are secreted across the cytoplasmic membrane.
• In Gram-negative bacteria, proteins must also traverse the outer membrane.
• Type I, II, III, IV, and V secretion systems are found in Gram-negative bacteria.

Cell Walls

• Cell wall is a structure that completely surrounds the cell protoplast.
• The cell wall is the outermost component common to virtually all bacteria(except Mycoplasma.)
• Peptidoglycan composed of murein, mucopeptide.
• Important for osmotic protection and cell division and precursor to its own biosynthesis.
• Peptidoglycan is the substance that gives cell wall strength.
• Made up of 2 amino sugars and 4 amino acids (L-alanine, D-glutamic acid, DAP, D-alanine).

Characteristics of Gram-Positive Cell Walls

• Contain teichoic acids, which may contribute to stabilization, adherence, and surface antigens.

Characteristics of Gram-Negative Cell Walls

• Include an outer membrane.
• Protect cells from permeability (penicillin and lysozyme).
• Contain lipopolysaccharide (endotoxin), which is toxic to animals.

Acid-Fast Bacteria

• Mycobacteria, such as Mycobacterium tuberculosis, have an unusual cell wall. This results in an inability to be stained using the Gram stain technique.
• Resist decolorization with acid-alcohol after being stained with carbolfuchsin.
• A high concentration of lipids (mycolic acids) in their cell wall is the factor that results in acid-fastness.
• Nocardia asteroides is a weakly acid-fast bacterium.

Glycocalyx

• Additional layer outside the cell wall; a polysaccharide coating secreted by many bacteria.
• Slime layer: glycoproteins loosely associated with the cell wall; helps adhere to solid surfaces to form biofilm or buildup of plaque.
• Capsule: polysaccharides firmly attached to the cell wall; helps with adherence to surfaces and protection against phagocytosis.

Surface Appendages

• Some bacteria have distinct appendages (flagella, axial filaments, fimbriae, and pili) for motility or adherence to surfaces.
• Flagella: long, thin extensions for motility in aqueous environments.
• Axial filaments (endoflagella): wind around bacteria, causing movement in waves.
• Fimbriae: short, fine appendages for adherence to solid surfaces, crucial for virulence.
• Pili: longer appendages, used for motility (grappling hooks) and conjugation pili (DNA transfer between cells).

Bacterial Morphology

• Size, shape, and configuration of the organisms (cocci, rods, curved, or spiral) are important for classification.
• Cocci: spherical bacteria; arranged in pairs (diplococci), chains (streptococci), clusters (staphylococci), tetrads, and cubes(Sarcina.)
• Rods can be straight (E. coli), club-shaped (Corynebacterium), branching (Actinomyces), comma-shaped (Vibrio), or spore-forming (Bacillus and Clostridium).
• Spiral: Treponema.
• Pleomorphic: variable in shape.

Binary Fission

• Bacteria divide by binary fission, which is an asexual reproduction method.
• The replication of the bacterial chromosome triggers the process.
• Metabolism produces cell materials from nutrients in the environment, essential for the progress of hundreds of independent biochemical processes (regulated).
• This leads to cell division that creates two independent living units from one.

Generation Time

• The time taken for a bacterial population to double in number (replication time.)
• Depends on the species, medium, and temperature.
• Bacteria have a relatively fast generation rate, compared to other living things.
• Typical range is 30–60 minutes (optimal conditions); examples include Escherichia coli (20 minutes), Mycobacterium tuberculosis (12–24 hours), and Mycobacterium leprae (10-30 days.)

Culture Systems and Growth Curve

• Lag phase: bacteria are readapting to a new medium and increasing their metabolic activity.
• Exponential/log phase: exponential increase in the number of bacteria.
• Stationary phase: growth rate slows as the nutrients are consumed and waste products accumulate.
• Death phase: bacterial death rate exceeds the growth rate and population size decreases.

Obligate Intracellular Growth

• Some bacteria need to grow inside living cells (host cells).
• Bacteria lack the ability to produce enough adenosine triphosphate (ATP) to support their metabolism independently and must use ATP from host cells.
• These bacteria are referred to as obligate intracellular bacteria.
• Examples: Chlamydia, Rickettsia, Ehrlichia, and Anaplasma.

Environmental Conditions

• Factors that affect bacterial growth include:
  • Acidity (pH values)
  • Water activity
  • Temperature
  • Oxygen
  • Toxins
  • Antimicrobial substances

Biochemical Reactions

• Identification and classification of bacteria involve observing traits and characteristics (phenotype).
• Biochemical tests identify and classify bacteria that involve the observation of phenotypic characteristics, such as growth on media (selective/nonselective/ differential media), characteristic colony appearance under a microscope, the Gram stain technique, biochemical tests (e.g., fermenting specific sugars or producing certain enzymes), and immunologic tests (antigenicity) and genotypic characteristics.

Criteria for Identification and Classification of Bacteria

• Phenotypic criteria are used to identify and classify bacteria, including growth on media, colony appearance, bacterial microscopy appearance, Gram stain technique, biochemical properties, and immunologic tests (antigenicity)
• Genotypic criteria include sequencing of rRNA (ribosomal RNA).