Bacteria: General Information

General Bacteria Information

• Bacteria are the simplest prokaryotic microorganisms
• They lack a nuclear membrane
• Bacteria's composition includes DNA, RNA, and protein
• They were among the first living things
• Bacteria are present in soil, water, acidic hot springs, radioactive wastes, and deep portions of Earth's crust
• They also inhabit symbiotic and parasitic relationships with plants and animals
• Bacteria can flourish in manned spacecraft
• Their adaptability allows them to survive in extreme conditions
• An estimated 5 X 10^30 bacteria exist on Earth
• The biomass of bacteria exceeds that of all plants and animals combined
• Bacteria was discovered in 1676 by Antonio Van Leeuwenhoek
• Van Leeuwenhoek used a single-lens microscope of his own design
• The term "bacterium" was introduced in 1828 by Christian Gottfried Ehrenberg
• Bacteriology, the study of bacteria, was established by Pasteur
• Pasteur showed that bacteria are disease-causing organisms
• Typical bacterial cells are a few micrometers (0.5 – 5.0 µm) in length
• There are over 2,000 species of bacteria belonging to the kingdom Monera

Characteristics:

• Bacteria are single-celled organisms lacking organelles like chloroplasts and mitochondria
• They lack a true nucleus
• They possess double-stranded, continuous, and circular DNA located in a nucleoid without a nuclear membrane
• A cell membrane and a peptidoglycan cell wall are present
• They have a large surface area to volume ratio, giving them an advantage in nutrient-poor environments
• Bacteria grow in numbers, not size and reproduce through binary fission after reaching a certain size
• They divide in half
• Some exist as single cells, while others form colonies
• Most are heterotrophic (parasitic or saprophytic), but some are autotrophic
• Genetic recombination happens through conjugation, transformation, and transduction, true sexual reproduction is absent
• The plasma membrane invaginates to form mesosomes
• They do not have a natural death

Morphology:

• Bacteria are diverse in shape from spheres to rods and spirals
• Shape influences bacteria's ability to acquire nutrients, attach to surfaces, swim, and escape predators
• Most are spherical (cocci) or rod-shaped (bacilli)
• Some are comma-shaped (vibrio), spiral-shaped (spirilla), or tightly coiled (spirochetes)
• A small number of species have tetrahedral or cuboidal shapes (Arcula sp.)
• Some can elongate to form filaments
• Some species alter their shape/size in response to environmental conditions (e.g., Deinococcus sp, Helicobacter pylori, and Mycosplasma sp)
• Cocci can be modified into various arrangements
  • Staphylococcus/Micrococcus: Irregular clusters
  • Diplococcus: Paired cocci after division
  • Streptococcus: Chains due to divisions in the same axis
  • Sarcina: Cubical packets of 8 or multiples of 8
  • Tetrad: Groups of 4 cocci after division in 2 planes

Motility and Flagellar Arrangement:

• They can be motile or non-motile, moving by gliding
• Examples: Pseudomonas aerigunosa, Myxococcus xanthus, Oscillatoria sp., Nostoc pruniforme, and Cylindrospermum sp
• Motile bacteria may have whip-like flagella at one or both ends/all over the cell
• Atrichous refers to non-flagellated motile bacteria
• Types of flagellar arrangements
  • Monotrichous: Single polar flagellum (e.g., Xanthomonas sp.)
  • Lophotrichous: Several flagella at one or both ends (e.g., Pseudomonas fluorescens, Spirillum undula)
  • Peritrichous: Flagella over the entire surface (e.g., Erwinia sp., Escherichia coli, and Salmonella sp.)
  • Amphitrichous: Single/multiple flagella at both ends (e.g., Alcaligenes faecalis, Caulobacter cresentus, Rhodospirillum rubrum, and Idiomaria loihiensis)

Bacterial Cell Structure:

• Regions of a typical bacterial cell: Surface Appendages, Surface Adherents, Cell Wall, Plasma Membrane, and Cytoplasm

Surface Appendages (Flagellum, Fimbriae, and Pili):

• Flagellum (Flagella plural): Rigid structure for motility, about 20 nm in diameter and up to 20μm in length
• It begins in the cytoplasm and is made of flagellin
• A flagellum has a basal body, hook, and filament
• Basal body-anchored in cytoplasmic membrane and cell wall
• It is driven by the transfer of ions down an electrochemical gradient
• Fimbriae (Fimbriae plural): Fine protein filaments, 2-10 nm diameter and several μm long, shorter and stiffer than flagella
• Distributed on the cell wall surface
• They have adhesins for host tissue identification and substrate attachment and are essential for virulence
• Pili (Pili plural): Small tube-like surface projections, similar to fimbriae but slightly larger
• Sex pilus, a specialized pilus, transfers plasmid DNA during conjugation and can generate movement

Surface Adherents (Glycocalyx):

• This includes the slime layer and capsule, which vary in structural complexity
• They protect against engulfment by eukaryotic cells
• They act as antigens, aid in cell recognition/attachment, and serve as storage
• Slime Layer: A disorganized, non-fluid extracellular polymer loosely associated with the bacterium and easily washed off
• It protects against antibiotics and desiccation
• Capsule: Highly structured outermost layer composed of polysaccharides/disaccharides, sometimes polypeptides
• It is attached tightly to the bacterium with definite boundaries
• Thickness determines classification as macro-capsule (>2 µm) or micro-capsule (<2 µm)

Cell Wall:

• Material is peptidoglycan, including glycoproteins, lipopolysaccharides, and lipoproteins
• It accounts for 20% of the cell's dry weight
• It gives shape/firmness, resists mechanical/chemical injuries/lysozyme attacks
• Two types of cell walls exist
  • Gram-positive (thick)
  • Gram-negative (thin)
• Gram Staining Test: Developed in 1884 by Hans Christian Gram, it classifies bacteria based on cell wall reaction to Gram stain

Gram Staining Process:

• Bacterial suspension is prepared from a young culture and spread on a glass slide
• It is fixed by heating
• The slide is dipped in crystal violet, then dilute iodine
• Both groups of bacteria turn deep purple
• The prep is treated with alcohol/acetone for decolorization
• Gram-positive bacteria retain the stain, Gram-negative bacteria are decolorized
• Counterstaining with safranin/carbol-fuschin turns Gram-negative bacteria red while Gram-positive remain deep purple
• Gram-positive bacteria examples: Staphylococcus aureus, Streptococcus mutans, Bacillus subtilis, Lactobacillus, Mycobacterium, Mycoplasma, Nocardia, Gardnerella, etc
• Gram-negative bacteria examples: Escherichia coli, Pseudomonas aeruginosa, Neisseria gonorrhoeae, Chlamydia trachomatis, Yersinia pestis, etc.

Differences Between Gram-Positive and Gram-Negative Cell Walls:

• Gram-positive: Homogenous layer, Mucopeptide is a major component (85%), Teichoic acid is present, and much-rigid
• Gram-negative: 3-layered, Lipoprotein and lipopolysaccharides major component (88-99%), Teichoic acid absent, less-rigid
• Gram-positive: Thicker walls with traces of lipids, do not contain all the amino acids
• Gram-negative: Thin walls containing up to 20% lipids, contain all the amino acids, muramic acid is less content

Plasma Membrane:

• It is made of lipids
• It encloses the cell contents/acts as a barrier to hold nutrients and essential components
• Functions of the plasma membrane
  • Biosynthetic pathways
  • Permeability barrier for nutrient transport
  • Contains components for electron transport chain and oxidative phosphorylation in aerobic bacteria
  • Highly folded with light-gathering complexes in photosynthetic bacteria
  • Controls chemotaxis
  • Attachment sites for chromosomal and plasmid DNA

Cytoplasm:

• It contains non-membrane-bound organelles like nucleoid, cytoskeleton, ribosomes, chromtophores, volutine/metachromatic granules, gas vacuoles/vesicles, carboxysomes, inclusions, magnetosomes, and plasmids
• Carboxysomes are micro-compartments controlling bacterial metabolism and carbon fixation enzymes
• Nucleoid is the genetic material, a single circular chromosome with proteins and RNA
• Ribosomes form polyribosomes for protein production, different than eukaryotes
• Bacterial ribosomes have a 70S sedimentation rate
• Subunits have 30S and 50S rates
• Chromatophores: Intracellular membranes for photosynthesis in photosynthetic bacteria, containing bacterio-chlorophyll pigments and carotenoids
• Inclusions: Non-living components made of glycogen, polyphosphate, sulphur, polyhydroxyalkanoates, lipid droplets, crystals, and pigments; they store intracellular nutrients for future use
• Gas vesicles/vacuoles: Regulate buoyancy in water by allowing movement through water layers and different light/nutrient levels
• Volutin/metachromatic granules: Complex inorganic phosphate inclusions with metachromatic effects
• Magnetosomes: Micro-compartments in magnetotactic bacteria containing magnetic crystals
• Mesosomes: In-folded plasma membrane active in cell wall synthesis, extracellular substance secretion, genome replication, and cell division
• Plasmids: Non-essential pieces of DNA with similar role as nucleoids

Bacterial Physiological Processes:

• Bacteria exhibit an extremely wide variety of metabolic processes
• They take in materials as relatively small molecules through diffusion or channels in the plasma membrane
• Nutritional types:
  • Phototrophs: energy from sunlight
  • Litothrophs: energy from inorganic compounds
  • Organotrophs: energy from organic compounds
• Reproduction: Asexual reproduction through binary fission/budding
• Growth: Rapid under optimal conditions
• Growth is affected by temperature (0-85°C), pressure, moisture, and pH (5-9)

Classification/Diversity

• Describes the diversity of bacterial species by naming and grouping them based on similarities in cell structure, cellular metabolism, cell components (DNA, fatty acids, pigments, antigens, quinines), and Gram staining
• The classification is unreliable due to the limited distinctive structures in most bacteria
• Modern bacterial classification emphasizes molecular systematics, using genetic techniques
  • Guanine : cytosine ratio determination
  • Genome – genome hybridization
  • Sequencing genes with no extensive lateral gene transfer (e.g., rRNA gene)
• Major groups:
  • Phylum Proteobacteria: Gram-negative (purple bacteria and their relatives) (e.g., Escherichia, Salmonella, Vibrio, Nitrobacter, Nitrosomonas, Helicobacter)
  • Phylum Actinobacteria: Gram-positive (e.g., Mycobacterium, Nicardia, Rhodococcus, Corynebacterium)
  • Phylum Firmicutes: Mostly Gram-positive (e.g., Megasphaera, Pectinatus, Sclenomonas, Zymophilus)
  • Phylum Cyanobacteria: Also known as blue-green bacteria/Cyanophyta (e.g., Chroococcales, Pleurocapsales, Osillatoriales, Nostocales, Stigonematales)
  • Phylum/Class Chlamydales: Obligate intracellular pathogens, infect eukaryotic host cells are small (e.g., Chlamydia trachomatis, Chlamydia pneumonia, and Chlamydia psittaci)
  • Phylum Planctomycetes: Aquatic, Ovoid with a holdfast stalk for budding (e.g., Kolteria novifilia, Gemmata obiscuriglobus)
  • Phylum Verrucomicrobia: They exist in fresh water, soil and human faeces.(e.g., Verrucomicrobium spinosum, Chthoniobacter flavus)
  • Phylum Acidobacteria: Physiologically diverse, abundant in soil, Acidophilic (e.g., Acidobacterium capsulatum, Holophaga foetida, Geothrix fermentans, Acanthopleuribacter pedis, Bryobacter aggregatus)
  • Phylum Cytophaga: Gram-negative, rod-shaped; utilizes gliding for locomotion (e.g., Cytophaga columnaris, Cytophaga johnsonae, Cytophaga psychrophila)
  • Phylum Bacteroidetes: Gram-negative, rod-shaped, non-spore forming, anaerobic, Classes include Bacteroidia and Porphyromonas (e.g., Chlorobium chlorochromatii, Chlorobium llimicola, Chlorobium tepidium)
  • Phylum Chlorobi is obligately aerobic and photoautotrophic, includes the green sulphur bacteria
  • Phylum Spirochaetes: Double-membrane, long, spiral-shaped, Chemoheterotrophic, free-living, anaerobic, Families include Brachyspiraceae, Leptospiraceae and Spirochaetaceae. Animal pathogenic spirochaetes are Leptospira, Borrelia afzeli, Borrelia recurrentis, Treponema pallidium sub. sp. pallidium Syphilis, Treponema pallidium sub. sp. pertenue – yaws, Brachyspira pilosicoli and Brachyspira aalborgi – internal spirochetosis

Economic Importance:

• Decompose dead organic matter of plants/animals by secreting enzymes and is used in waste management/bioremediation
• Some maintain/increase soil fertility (ammonifying, nitrifying, nitrogen-fixing bacteria)
• Ammonifying examples: Bacillus ramosus and B. vulgaris
• Nitrifying examples: Nitrobacter spp. and Nitrosomanas spp.
• Nitrogen fixing examples: Bacillus radicula, Clostridium spp, Azotobacteer spp
• They play important roles in industries and their activities can’t be chemically duplicated
• (a) Alcohol production examples: Clostridium acetobutylycum in sugar solutions makes ethyl/butyl alcohols
• (b) Vinegar production examples: Acetobacter aceti acts on sugarcane juice to convert it to vinegar
• Preparation of antibiotics -Bacitracin: Bacillus subtilis -Streptomycin: Streptomyces griseus -Terramycin: Streptomyces rimosus -Aureomycin: Streptomyces spp
• They are bio-engineered for therapeutic proteins/growth factor production
• Serve as biological control for pests and plant diseases -Bacillus thuringiensis
• They cause plant/animal diseases -Diplocaccus pneumonia: pneumonia in humans -Xanthomonas axonopodis pv. Manihotis: bacterial blight of cassava
• They cause food spoilage: Souring of milk/rotting of meat/vegetables/fruits -Staphylococcus and Clostridium botulinum: Food poisoning when humans eat rotten food
• Some (denitrifying bacteria) destroy soil fertility by reducing nitrates and releasing nitrogen into the atmosphere -Bacillus denitrificans

Viruses and Bacteria Differences:

• Viruses are acellular; bacteria are cellular
• Viruses require a host to replicate; bacteria reproduce independently
• Viruses are mostly pathogenic; bacteria can be useful or pathogenic
• Viruses have DNA or RNA genetic material; bacteria have circular DNA
• Viruses are visible only under electron microscopes; bacteria are visible under light and electron microscopes
• Viral infections are untreatable; antibiotics treat bacteria
• Viruses are non-living; bacteria are living
• Viruses are non-motile; some bacteria are motile