Bacterial surfaces

Bacterial Morphology and Structure

Overview of Bacterial Morphology

  • It might be expected that because bacterial cells are small and relatively simple, they would be uniform in shape and size. This is not the case, as the microbial world offers considerable variety in terms of morphology.

  • However, the two most common shapes are cocci and rods (figure 3.1).

Common Shapes

  • Cocci (s., coccus): Roughly spherical cells.

    • Can exist singly or can be associated in characteristic arrangements that can be useful in their identification.

    • Diplococci (s., diplococcus): arise when cocci divide and remain together to form pairs.

    • Long chains of cocci result when cells adhere after repeated divisions in one plane; this pattern is seen in the genera Streptococcus, Enterococcus, and Lactococcus.

    • Members of the genus Staphylococcus divide in random planes to generate irregular, grapelike clusters (figure 3.1a).

    • Divisions in two or three planes can produce symmetrical groupings of cocci.

      • Bacteria in the genus Micrococcus often divide in two planes to form square groups of four cells called tetrads.

      • In the genus Sarcina, cocci divide in three planes, producing cubical packets of eight cells.

  • Rods (s., bacillus): An example is Legionella pneumophila (figure 3.1b).

    • Rods, sometimes called bacilli, differ considerably in their length-to-width ratio, the coccobacilli being so short and wide that they resemble cocci.

    • The shape of the rod’s end often varies between species and may be flat, rounded, football-shaped, or bifurcated.

    • Although many rods occur singly, some remain together after division to form pairs or chains.

Less Common Cell Shapes

  • Vibrios: comma-shaped.

  • Spirilla: rigid, spiral-shaped cells.

  • Spirochetes: flexible, spiral-shaped bacteria (figure 3.2a,b).

  • Other bacteria are pleomorphic, being variable in shape and lacking a single, characteristic form (see figure 7.10).

    • Phylum Spirochaetota (section 20.6); Family Vibrionaceae includes aquatic bioluminescent bacteria and pathogens (section 21.2).

Multicellular Characteristics

  • Some bacteria can be thought of as multicellular.

    • Many actinobacteria form long filaments called hyphae (figure 3.2c).

    • The hyphae form a network called a mycelium, similar to eukaryotic filamentous fungi.

    • Many cyanobacteria, a group of photosynthetic bacteria, are also filamentous.

      • Being filamentous allows some degree of differentiation among cells in the filament.

    • For instance, some filamentous cyanobacteria form specialized cells within the filament, called heterocysts, that carry out nitrogen fixation (see figure 20.8a).

    • Myxobacteria are also morphologically complex. These bacteria sometimes aggregate to form structures called fruiting bodies (figure 3.2d).

Size Range and Variability

  • Escherichia coli (E. coli) is an excellent example of an average-sized bacterial cell.

    • This rod-shaped bacterium is 1.1 to 1.5 μm wide by 2.0 to 6.0 μm long.

    • However, the size range of bacterial cells extends far beyond this average (figure 3.3).

    • Near the small end of the size continuum are members of the genus Mycoplasma (0.3 μm in diameter).

    • Parasitic bacteria in the size range of 0.2–0.4 μm can only survive when growing attached to their hosts, which provide nutrients to the symbionts (figure 3.4a).

    • At the other end of the continuum are bacteria such as some spirochetes, which can reach 500 μm in length.

    • Some bacteria are huge by bacterial standards. For instance, Epulopiscium fishelsoni grows as large as 600 by 80 μm, a little smaller than a printed hyphen and clearly larger than the eukaryote Paramecium (figure 3.4b).

    • An even larger bacterium, Thiomargarita namibiensis, lives in ocean sediment (see figure 21.22).

    • Thus a few bacteria are much larger than the average eukaryotic cell (typical plant and animal cells are around 10 to 50 μm in diameter).

Size and Shape Determinants

  • The variety of sizes and shapes exhibited by bacteria raises a fundamental question: What causes a bacterial species to have a particular size and shape?

    • For many years, it was thought that microbes had to be small to increase the surface area-to-volume ratio (S/V ratio; figure 3.5).

    • As this ratio increases, the uptake of nutrients and the diffusion of other molecules within the cell become more efficient, which facilitates rapid growth.

    • Shape affects the S/V ratio. A rod with the same volume as a coccus has a higher S/V ratio than does the coccus, meaning a rod can have greater nutrient flux across its plasma membrane.

    • However, the discovery of E. fishelsoni demonstrates that bacteria can be very large.

    • For bacteria to be large, they must have other characteristics that maximize their S/V ratio, or their size must be beneficial in some way.

      • For instance, E. fishelsoni has a highly convoluted plasma membrane, which increases its S/V ratio.

    • Additionally, large cells are less likely to be eaten by predatory protists. Cells that are filamentous, have stalks, or are oddly shaped are also less susceptible to predation.

    • Predators come in all sizes (section 27.4).

Cell Organization

  • Structures often observed in bacterial cells are summarized and illustrated in figure 3.6.

  • No single bacterium possesses all these structures at all times. Some are found only in certain cells under specific conditions or in specific phases of the life cycle.

Common Features of Bacterial Cell Structure

  • Bacterial cells are surrounded by several layers, collectively called the cell envelope.

    • The most common cell envelope layers are the plasma membrane, cell wall, and capsule or slime layer.

    • The innermost layer of the cell envelope is the plasma membrane, which surrounds the cytoplasm.

    • Most bacteria have a chemically complex cell wall, which covers the plasma membrane.

    • Many bacteria surround the cell wall with a capsule or slime layer.

    • Because most bacteria do not contain internal membrane-bound organelles, their interior appears morphologically simple.

    • The genetic material is localized in a discrete region called the nucleoid, which is not separated from the surrounding cytoplasm by a membrane.

    • Ribosomes and larger masses called inclusions are scattered throughout the cytoplasm.

    • Filamentous structures termed pili (s. pilus) may protrude from the surface, facilitating gene transfer or attachment to surfaces.

    • Many bacteria use flagella for locomotion.

Plasma Membrane

  • The cell envelope is defined as the plasma membrane and all the surrounding layers external to it.

  • The cell envelopes of many bacteria consist of the plasma membrane, cell wall, and at least one additional layer (e.g., capsule or slime layer).

  • The plasma membrane is the most important because it encompasses the cytoplasm and defines the cell.

    • If it is removed or damaged, the cell’s contents spill into the environment and the cell dies.

    • The plasma membrane is also responsible for much of the cell’s relationship with the outside world.

Nutrient Uptake Mechanisms

  • All plasma membranes function as barriers. Yet they must also allow the movement of nutrients into the cell.

  • If a microbe does not obtain nutrients from its environment, it will quickly exhaust its supply of amino acids, nucleotides, and other molecules needed to survive.

  • The energy source is used to generate the cell’s major energy currency: ATP (adenosine triphosphate).

    • Thus, obtaining energy and nutrient sources is one of the most important jobs of an organism, primarily a function of the plasma membrane.

Nutrient Types

  • The six macronutrients are required in relatively large amounts (table 3.1).

    • They are found in organic molecules such as proteins, lipids, nucleic acids, and carbohydrates.

    • Other macronutrients exist as cations and generally contribute to the activity and stability of molecules and cell structures like enzymes and ribosomes.

    • Micronutrients, or trace elements, are required in small amounts (often obtained as contaminants in water, glassware, and growth media).

    • Some microbes are unable to synthesize certain organic molecules needed for survival. These are called growth factors, which must be obtained from the environment (table 3.1).

Common Features of Nutrient Uptake by Bacteria

  • Bacteria can only take in dissolved molecules.

    • Uptake mechanisms are specific; necessary substances are acquired.

    • Nutrients can be transported into the cell even when their concentration inside the cell is higher than outside.

    • Bacteria often live in nutrient-poor habitats, requiring multiple transport mechanisms: passive diffusion, facilitated diffusion, primary and secondary active transport, and group translocation.

Transport Mechanisms

Passive Diffusion

  • Passive diffusion is the process by which molecules move from a region of higher concentration to one of lower concentration (figure 3.10).

    • The rate of diffusion depends on the size of the concentration gradient.

    • A large concentration gradient is needed for effective nutrient uptake.

    • Most substances cannot freely diffuse into a cell; water and some gases easily cross the plasma membrane.

Facilitated Diffusion

  • During facilitated diffusion, substances move across the plasma membrane with the assistance of transport proteins that are either channels or carriers.

    • Channels form pores in membranes through which substances can pass.

    • The rate is generally greater than that of passive diffusion (figure 3.10).

    • When a transporter is a carrier, the diffusion rate reaches a plateau at saturation (figure 3.11).

    • Although facilitated diffusion involves transport proteins, it is still diffusion since it relies on concentration gradients and uses no energy.

Active Transport

  • Active transport moves solute molecules against a concentration gradient using energy.

    • There are three types of active transport: primary active transport, secondary active transport, and group translocation.

    • Primary Active Transport: Mediated by carriers that use ATP hydrolysis to move substances without modification.

    • Secondary Active Transport: Uses ion gradients to transport substances without modifying them (figure 3.12).

Group Translocation

  • Group translocation involves chemically modifying a molecule during import.

    • The best-known group translocation system is the phosphoenolpyruvate: sugar phosphotransferase system (PTS).

    • PTS imports and phosphorylates sugars using phosphoenolpyruvate (PEP) as the phosphate donor.

    • In E. coli and Salmonella, the PTS consists of two enzymes and a heat-stable protein (HPr) (figure 3.14).

Iron Uptake

  • Almost all microorganisms require iron for critical cellular processes (table 3.1).

    • Many bacteria secrete siderophores, small organic molecules that bind ferric iron to supply it to the cell (figure 3.15).

Bacterial Cell Wall Structure

Importance of Cell Walls

  • Essential for maintaining cell shape and protecting against osmotic lysis and toxic substances.

    • Most bacteria have cell walls; those lacking them have other structures that provide support.

Overview of Cell Wall Structure

  • Gram-positive bacteria: consist of a thick peptidoglycan layer (20-80 nm) (figure 3.16).

  • Gram-negative bacteria: have a thin peptidoglycan layer and an outer membrane.

  • Periplasmic Space: Space between the plasma membrane and outer membrane in Gram-negative bacteria.

Peptidoglycan Structure

  • Common in nearly all bacterial cell walls, forming a mesh-like structure often called the peptidoglycan sacculus.

    • Each subunit contains N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) and amino acids (figure 3.17).

  • The peptidoglycan sacculus forms by linking sugars to form strands; these strands are cross-linked (figure 3.18).

Variants of Peptidoglycan

  • Differences occur among Gram-positive bacteria (figure 3.20).

Gram-Positive Cell Walls

  • Composed mainly of peptidoglycan and large amounts of teichoic acids (figure 3.21).

    • Teichoic acids anchor the wall to the plasma membrane and protect from harmful substances.

Gram-Negative Cell Walls

  • More complex than Gram-positive, containing a thin peptidoglycan layer and an outer membrane.

    • Contains lipopolysaccharides (LPS) which provide protection and contribute to the negative charge (figure 3.24).

Transport in Gram-Negative Cells

  • Solute transport requires crossing both the outer and plasma membranes.

    • Porins allow passage of nutrients and hydrophilic molecules smaller than about 600 daltons (figure 3.25).

Cell Walls and Osmotic Protection

  • Protect cells from osmotic stress; without the peptidoglycan layer, cells could burst in hypotonic solutions.

    • Lysozyme and penicillin can disrupt cell walls, leading to cell lysis in hypotonic environments.

Bacteria that Lack Cell Walls

  • Some bacteria, such as mycoplasmas, are defined by their lack of a cell wall and are osmotically sensitive (see figure 22.21).

    • These bacteria often grow in dilute media or terrestrial environments due to their plasma membranes being more resistant to osmotic pressure.

Extracellular Vesicles (EVs)

  • Small membrane-bound particles (20–400 nm) observed in various bacterial cells (figure 3.27).

    • EVs form when membranes bud out and release from cells and can influence cell interactions, toxin transfer, and genetic material exchange.

Capsules and Slime Layers

  • Capsules: Well-organized polysaccharide layers providing protection against phagocytosis (figure 3.28).

  • Slime Layers: Diffuse polysaccharide layers not as tightly organized as capsules, affecting motility.

S-Layers

  • Ordered surface coverings made of protein or glycoprotein (figure 3.29).

    • Act to protect against stress and help maintain shape and rigidity.

Cytoplasm and Internal Structures

  • The cytoplasm includes cytosol, ribosomes, and inclusions; it contributes to various cellular functions.

    • Macromolecular crowding affects diffusion processes within the cytoplasm.

Bacterial Cytoskeleton

  • Composed of protein filaments, critical for maintaining shape, cell division, and DNA partitioning (table 3.2).

Bacterial Ribosomes

  • Sites of protein synthesis, numerous in bacterial cells (10,000 to 20,000) (figure 3.34).

    • Bacterial ribosomes are 70S, made of a 50S and a 30S subunit.

Nucleoid

  • The nucleoid contains the cell's chromosome, occupying about 20% of the cell volume (figure 3.35a).

    • Most bacteria have circular chromosomes; some are linear or polyploid.

Plasmids

  • Small, double-stranded DNA molecules that can exist independently of chromosomes; essential for various functions and advantages.

Bacterial Motility

  • Appendages for attachment or movement contribute to protection and horizontal gene transfer.

Bacterial Pili and Fimbriae

  • Fine, hairlike appendages used for attachment (fimbriae or pili) (figure 3.37).

    • Certain structures designated as sex pili are involved in conjugation.

Bacterial Flagella

  • Threadlike appendages enabling motility (figure 3.39).

    • Flagella are slender, rigid structures extending from the cell.

Flagellar Movement

  • Swimming involves filament rotation; two types of movement: a smooth swimming movement (run) and a tumbling movement (tumble).

Swarming

  • Group behavior with coordinated movement across moist surfaces to colonize.

Spirochete Motility

  • Undulatory movement due to periplasmic flagella within the cell wall (figure 3.43).

Twitching and Gliding Motility

  • Twitching: Involves type IV pili extending and retracting for movement.

    • Gliding: Smooth motion requiring no appendages.

Chemotaxis

  • Movement toward attractants or away from repellents; detected by chemoreceptors.

    • E. coli movements can be tracked, revealing run-tumble patterns influenced by concentration changes.

Endospores

  • Dormant cells formed within a mother cell produced by certain bacteria in the phylum Firmicutes (figure 3.45).

    • Important to distinguish bacterial endospores from spores formed by fungi and plants.

    • Bacteria that produce endospores create a single endospore per cell that later germinates to form a single cell.

    • Endospore formation is a response to nutrient depletion, representing a dormant state of the organism.

Importance of Endospores

  • Endospores are extraordinarily resistant to environmental stresses, including heat, UV radiation, and desiccation.

    • Some endospores remain viable for around 250 million years.

    • Certain species, like Clostridium botulinum (causes botulism), pose significant health risks due to their endospores.

Sporulation: Making Endospores

  • Sporulation commences when growth slows due to nutrient limitation.

    • It allows the bacterium to produce a dormant cell that can persist until nutrients become available.

  • Bacteria cycle between vegetative growth and survival as an endospore.

    • Vegetative growth is the continuous cycle of growth; sporulation is a complex process occurring over several hours.

    • Mature endospores have specific locations in the mother cell (referred to as the sporangium) based on the species (figure 3.46).

Stages of Sporulation (Bacillus subtilis)

  • Sporulation consists of registered stages that include chromosome replication, cell division, and endospore formation (figure 3.47).

    • Stage I: Chromosome replicates; Stage II: Division septum forms unequal cells (forespore and mother cell).

    • Stage III: Mother cell membrane engulfs forespore, creating an inner and outer membrane.

    • Stage IV: Peptidoglycan synthesized between membranes.

    • Stage V: Coat proteins are synthesized and deposited.

    • Stage VI: Continued dehydration changes the forespore's resistance properties.

    • Stage VII: Mother cell lyses to release the mature endospore.

Endospore Resistance

  • Endospores tolerate heat, radiation, and damaging chemicals due to protective enzyme and DNA layers.

    • The dense protein coat shields endospores from chemicals, and the inner membrane is impermeable to damaging substances.

    • The endospore core has low water content, high amounts of calcium dipicolinate (Ca-DPA), and a lower pH, contributing to its resilience.

    • The SASPs coating the DNA stabilize it and help protect against UV light.

Endospore to Vegetative Cell

  • The transformation of dormant endospores into active vegetative cells is complex, occurring in three stages: activation, germination, and outgrowth.

    • Activation: Prepares endospores for germination via treatments like brief heating.

    • Germination: The endospore breaks dormancy when germinant receptors detect small molecules (sugars, amino acids).

      • Triggers Ca-DPA complex release and water uptake, breaking down cortex peptidoglycan for expansion and hydration.

    • Water levels reach those typical of vegetative cells, reactivating core enzymes for synthesis and outgrowth