Bacterial Overview

Bacteria Overview

• Simplest prokaryotic microorganisms.
• Composed of DNA, RNA, and protein.
• Among the first living organisms on Earth.
• Found in diverse environments, including soil, water, hot springs, radioactive waste, and deep portions of the earth's crust
• Can thrive in symbiotic and parasitic relationships with plants and animals.
• Have flourished in manned spacecraft
• Adaptable to temperature, pH, oxygen, and pressure extremes.
• Have an estimated biomass of 5 x 10^30 on Earth.
• Biomass exceeds all plants and animals.
• Antonie Van Leeuwenhoek discovered bacteria in 1676.
• Used a single-lens microscope of his own design.
• Christian Gottfried Ehrenberg introduced the term "bacterium" in 1828.
• Pasteur established bacteriology, highlighting the importance of bacteria as pathogens.
• Bacterial cell lengths typically range from 0.5 to 5.0 μm.
• Over 2,000 species exist and belong to the kingdom Monera.

Characteristics of Bacteria

• Single-celled organisms lacking organelles like chloroplasts and mitochondria
• Lack a true nucleus, are prokaryotic
• Possess double-stranded, continuous, and circular DNA in a nucleoid, without a nuclear membrane
• Have a cell membrane and a peptidoglycan cell wall
• Possess a large surface area to volume ratio that allows them to thrive in nutrient-poor environments
• Grow in numbers through binary fission, not in size
• Can exist as single cells or in clusters.
• Exhibit heterotrophic (parasitic or saprophytic) or autotrophic lifestyles
• Reproduce by binary fission after reaching a specific size
• Lack true sexual reproduction.
• Undergo genetic recombination via conjugation, transformation, and transduction.
• Their plasma membrane invaginates to form mesosomes.
• Do not experience natural death

Bacterial Shapes

• Exhibit various shapes, including spheres, rods, and spirals.
• Shape influences nutrient acquisition, surface attachment, movement, and predator avoidance.
• Most bacterial species are spherical (cocci) or rod-shaped (bacilli).
• Some are curved rods (vibrio).
• Others are spiral-shaped (spirilla or spirochetes).
• Some possess tetrahedral or cuboidal shapes (Arcula sp).
• Some elongate into filaments.
• Some alter their shape based on environmental conditions (e.g., Deinococcus sp., Helicobacter pylori, Mycosplasma sp).
• Cocci can form Staphylococcus/Micrococcus (irregular clusters), Diplococcus (pairs), Streptococcus (chains), Sarcina (cubical packets of 8), or Tetrad (groups of 4).

Motility

• Can be motile or non-motile, moving via gliding.
• Motile bacteria have flagella, which are whip-like structures arising from one or both ends or all over the cell
• Bacteria lacking flagella are termed atrichous.
• Flagellar arrangements include monotrichous (single polar flagellum, e.g., Xanthomonas sp.), lophotrichous (multiple flagella at one or both ends, e.g., Pseudomonas fluorescens), peritrichous (flagella over the entire surface, e.g., Escherichia coli), and amphitrichous (flagella at both ends, e.g., Alcaligenes faecalis).

Bacterial Cell Structure

• Typical cells have surface appendages, surface adherents, a cell wall, a plasma membrane, and cytoplasm
• Surface appendages include flagella, fimbriae, and pili.
• Flagella are rigid structures (20 nm diameter, up to 20 µm length) used for motility.
• Flagella are composed of flagellin.
• Consist of a basal body, hook, and filament.
• The hook connects the filament to the basal body's motor portion.
• Basal bodies are anchored in the cytoplasmic membrane and cell wall, using energy from ion transfer across the cell membrane,
• Fimbriae are protein filaments (2-10 nm diameter, up to several μm length) that are shorter and stiffer than flagella.
• Distributed on the cell wall surface, resembling fine hairs under an electron microscope
• Have adhesins for host tissue identification.
• Help bacteria attach to substrates and contribute to the virulence of pathogenic bacteria.
• Pili are tube-like projections on the cell wall surface, similar to fimbriae but larger
• Specialized pili (sex pili) transfer plasmid DNA during conjugation and can generate movement.
• Surface Adherents or Glycocalyx: Include the slime layer and capsule
• Vary structurally and protect against engulfment by eukaryotic cells.
• Act as antigens and aid in cell recognition and surface attachment; can also serve as a storage product
• Slime Layer: A disorganized, non-fluid extracellular polymer that is loosely associated with the bacterium, protecting against environmental threats like antibiotics and desiccation.
• Capsules: The outermost bacterial cell layer, made of polysaccharides, disaccharides, or polypeptides, tightly attached with defined boundaries.
• Cell Wall: Made of peptidoglycan, comprising 20% of the cell's dry weight
• Cell Wall: Gives shape, firmness, and resistance to mechanical/chemical injuries, including lysozyme. Can be thick (Gram-positive) or thin (Gram-negative).

Gram Staining

• Gram staining distinguishes bacteria based on cell wall composition
• Gram-positive bacteria retain the stain after alcohol treatment
• Gram-negative bacteria do not.
• Gram staining technique developed by Hans Christian Gram in 1884 involves applying a bacterial suspension to a glass slide, fixing it with gentle heating, and successively dipping it in crystal violet and dilute iodine solutions.
• The preparation is treated with alcohol or acetone for decolorization.
• Gram-positive bacteria retain the stain's color, while gram-negative bacteria become decolorized.
• Counterstaining with safranin or carbol-fuschin turns Gram-negative bacteria red and Gram-positive bacteria deep purple.

Composition

• Gram-positive bacteria have a homologous cell wall layer; Gram-negative have 3 layers.
• Gram-positive walls consist of 85% mucopeptide, while Gram-negative walls contain 88-99% lipoprotein/lipopolysaccharides
• Gram-positive bacteria have teichoic acid, which is absent in Gram-negative bacteria
• Gram-positive bacteria are much-rigid, while Gram-negative bacteria are less-rigid
• Gram-positive cell walls are thicker with only traces of lipids while Gram-negative walls are thinner and may contain up to 20% lipids
• Gram-positive cell walls do not contain all the amino acids, but Gram-negative bacteria do
• Gram-positive bacteria have muramic acid plenty while Gram-negative have less.

Cell and Plasma Membrane

• Composed of lipids.
• Encloses cell contents, serving as a barrier for nutrients and components.
• Involved in biosynthetic pathways, nutrient transport via enzyme permeases, electron transport and oxidative phosphorylation in aerobic bacteria, light-gathering complexes in photosynthetic bacteria, chemotaxis, and attachment sites for chromosomal/plasmid DNA.

Cytoplasm

• Lacks membrane-bound organelles; contains nucleoid, cytoskeleton, ribosomes, chromtophores, volutine/metachromatic granules, gas vacuoles/vesicles, carboxysomes, inclusions, magnetosomes, and plasmids.
• Carboxysomes are micro-compartments to controls bacterial metabolism, containing carbon fixation enzymes.
• The nucleoid contains genetic material, including proteins and RNA.
• Ribosomes, grouped in chains called polyribosomes produce proteins.
• Ribosomal sedimentation rate is 70S.
• Subunits are 30S and 50S.
• Chromatophores facilitate photosynthesis using bacterio-chlorophyll pigments and carotenoids.
• Inclusions are non-living made of glycogen, polyphosphate, sulfur or polyhydroxyalkanoates, lipid droplets, crystals and pigments which are granules for nutrient storage.
• Gas vesicles and vacuoles regulate the buoyancy of the bacterial cells in water.
• Volutin or metachromatic granules are cytoplasmic inclusions of complex inorganic phosphate and show metachromatic effects.
• Magnetosomes are compartments containing magnetic crystals.
• Mesosomes are involved in cell wall synthesis, secretion of extracellular substances, genome replication, and cell division.
• Plasmids are non-essential DNA pieces that act as nucleoids.

Physiological Processes

• Bacteria exhibit metabolic diversity using small molecules that enter the cell via diffusion or channels
• Nutritional types include phototrophs (sunlight), lithotrophs (inorganic compounds), and organotrophs (organic compounds).
• Asexual reproduction occurs through binary fission or budding.
• Growth is affected by temperature (0-85°C), pressure, moisture, and pH (5-9).

Classification and Diversity

• Diversity among bacteria is categorized by structure, metabolism, cell components, and Gram staining.
• Modern classification utilizes guanine-cytosine ratios, genome hybridization & rRNA sequencing.
• Major bacterial groups include Proteobacteria (Gram-negative bacteria belonging to genera like Escherichia/Salmonella), Actinobacteria (Gram-positive bacteria belonging to genera like Mycobacterium), Firmicutes (Gram-positive bacteria belonging to genera like Megasphaera), Cyanobacteria (blue-green algae like Chroococcales), Chlamydales (obligate intracellular pathogens that infect eukaryotic host cells like Chlamydia trachomatis), Planctomycetes (aquatic bacteria like Kolteria novifilia), Verrucomicrobia (bacteria isolated from water, soil and human feces like Verrucomicrobium spinosum), Acidobacteria (acidophilic bacteria such as Acidobacterium capsulatum), Cytophaga (gram-negative, rods using locomotion such as Cytophaga columnaris), Bacteroidetes (gram-negative, non forming spore such as Chlorobium Chlorochromatii), Cholorbi (green sulphur bacteria), Spirochaetes (double membrane spiral shaped bacteria such as Leptospira).

Economic Importance

• Decomposition of organic matter via enzyme secretion is useful in waste management and bioremediation.
• Impact soil fertility - some maintain, others increase soil fertility
• eg. Ammonifying bacteria (Bacillus ramosus & B. vulgaris), nitrifying bacteria (Nitrobacter/Nitrosomanas spp.), and nitrogen-fixing bacteria (Bacillus radicula)
• Play a role in various industries given bacterial activities cannot be chemically prepared; alcohol preparation using Clostridium acetobutylycum and vinegar production using Acetobacter aceti on sugarcane
• Involved in the creation of antibiotics: Bacillus subtilis produces bacitracin, Streptomyces griseus produces streptomycin, and Streptomyces rimosus produces terramycin
• Can be bio-engineered for therapeutic protein and growth factor production
• Used to control pests and diseases biologically; Bacteria thuringiensis
• Under certain proportions cause plant and animal diseases (Diplocaccus pneumonia & Xanthomonas axonopodis pv. manihotis)
• Can cause food decaying (Staphylococcus & Clostridium botulinum) via milk souring, meat/fruit/vegetable rotting.
• Denitrifying destroy soil fertitility via reducing the soil back in the atmosphere. (Bacillus denitricans).

Virus vs Bacteria

• Viruses do not have cells, while bacteria have cells.
• Unlike bacteria, viruses require a host to replicate.
• Viruses are often pathogenic
• Genetic material includes DNA or RNA, while bacteria is circular DNA.
• Viruses are visible with an Electron microscope.
• Unlike bacteria, viral Infections cannot be treated with antibiotics
• Viruses are non-living
• Unlike Bacteria, Viruses are non-motile.