6: Bacteria - surface structures (2023)

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    layers outside the cell wall

    What have we learned so far regarding cell layers? All cells have a cell membrane. Most bacteria have a cell wall. But there are a few extra layers that bacteria may or may not have. These, if present, would be found both outside the cell membrane and the cell wall.


    A bacteriumHaircutis a polysaccharide layer that completely envelops the cell. It's well-organized and densely packed, which explains its resistance to discoloration under a microscope. The capsule provides protection against a variety of different threats to the cell, such as: B. desiccation, hydrophobic toxic materials (e.g. detergents) and bacterial viruses. The capsule may enhance the ability of bacterial pathogens to cause disease and provide protection against phagocytosis (swallowed by white blood cells known as phagocytes). Finally, it can help with attachment to surfaces.

    slime layer

    A bacteriumslime layerresembles the capsule in that it is typically composed of polysaccharides and completely surrounds the cell. It also offers protection from various threats such as dehydration and antibiotics. It can also allow adhesion to surfaces. How is it different from the capsule? A mucus layer is a loose, disorganized layer that easily detaches from the cell that made it, unlike a capsule that integrates tightly around the bacterial cell wall.

    6: Bacteria - surface structures (2)

    (Video) Bacterial structure cell surface structures

    S shift

    Some bacteria have a highly organized layer of secreted proteins or glycoproteins that self-assemble into a matrix on the outer part of the cell wall. This regularly structuredS shiftis anchored in the cell wall, but is not officially considered part of the cell wall of bacteria. S-Layers play very important roles for the bacteria that have them, especially in the areas of growth and survival and cell integrity.

    S-layers help maintain the overall rigidity of the cell wall and surface layers, as well as cell shape, which are important for reproduction. S layers protect the cell from ionic/pH changes, osmotic stress, harmful enzymes, bacterial viruses and predatory bacteria. They can provide cell adhesion to other cells or surfaces. For pathogenic bacteria, they can provide protection against phagocytosis.

    Structures outside the cell wall

    Bacteria can also have structures outside the cell wall, often attached to the cell wall and/or cell membrane. The building blocks for these structures are typically made inside the cell and then excreted past the cell membrane and cell wall to be assembled on the outside of the cell.

    Fransen (sing. Fransen)

    Fimbrienare thin threadlike appendages extending from the cell, often in tens or hundreds. they consist ofyour batteryProteins and are used by the cell to attach to surfaces. They can be particularly important for pathogenic bacteria, which use them to attach to host tissues.

    Pili (sing. pilin)

    Relatedare very similar to fimbriae (some textbooks use the terms interchangeably) in that they are thin thread-like appendages that extend from the cell and are made of pilin proteins. Pili can also be used to attach to surfaces and host cells, such asNeisseria gonorrhoeCells that use their pili to grab onto sperm cells to get to the nearest human host. So, why would some researchers bother to distinguish between fimbriae and pili?

    Pili are typically longer than fimbriae, with only 1-2 present on each cell, but that hardly seems enough to distinguish the two structures from one another. It really boils down to the fact that some specific pili are involved in functions beyond attachment. ThePili conjugativeparticipate in the process known asconjugation, which allows the transfer of a small piece of DNA from a donor cell to a recipient cell. TheType-IV-Piliplay a role in an unusual type of agility known astwitch mobility, where a pilus attaches to a solid surface and then contracts, pulling the bacterium forward in a jerky motion.

    (Video) Bacterial Structure and Functions

    Flagella (sing. Flagellum)

    Bacterial motility is typically provided by structures known asscourges. The bacterial flagellum differs in composition, structure, and function from the eukaryotic flagellum, which functions as a flexible, whip-like tail using microtubules. The bacterial flagellum is rigid in nature and functions more like a boat's propeller.

    The bacterial flagellum consists of three main components:

    1. DieFilament- a long thin appendage extending from the cell surface. The filament consists of the proteinFlagellinand is hollow. Flagellin proteins are transcribed in the cell's cytoplasm and then transported across the cell membrane and cell wall. A bacterial flagellar filament grows from its tip (as opposed to the hair on your head), adding more and more units of flagellin to increase the length until it is the right size. The flagellin units are guided into place by a protein cap.
    2. DieHook– This is a curved coupler that attaches the suture to the flagellar motor.
    3. DieMotor- a rotary engine spanning both the cell membrane and the cell wall, with additional components for the gram-negative outer membrane. The engine consists of two components: thebasal body, which provides the rotation, and theStator, which provides the torque required for rotation. The basal body consists of a central shaft surrounded by protein rings, two in gram-positive bacteria and four in gram-negative bacteria. The stator consists ofAgainst proteinssurrounding the ring(s) embedded in the cell membrane.

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    Flagellum base diagram. By LadyofHats (Own work) [Public Domain], Via Wikipedia Commons

    bacterial movement

    Bacterial motility typically involves the use of flagella, although there are some other possibilities (e.g. using type IV pili for twitch motility). But certainly the most common type of bacterial movementBaden, which is achieved with the use of a flagellum or flagella.


    The rotation of the flagellar basal body occurs due to the proton driving force, where protons accumulating on the outside of the cell membrane are driven through pores in the Mot proteins and interact with charges in the ring proteins as they pass through the membrane. The interaction causes the basal body to rotate and the filament extending from the cell to rotate. The rotation can occur at 200–1000 rpm and lead to speeds of 60 cell lengths/second (for comparison: a cheetah moves with a maximum speed of 25 body lengths/second).

    Rotation can occur in oneclockwise (CW)or acounterclockwise (CCW)direction, with different results to the cell. A bacterium moves forward, called "run' when there is a CCW rotation, and a random realignment reported as 'overthrow” when there is CW rotation.

    corkscrew motility

    Some spiral shaped bacteria known as thespirochetes, use acorkscrew motilitydue to their unusual morphology and flagellar conformation. These gram-negative bacteria have specialized flagella that attach to one end of the cell, extend back through the periplasm, and then attach to the other end of the cell. If thoseEndoflagellatwisting twists the entire cell, resulting in a bending motion that's particularly effective for burrowing through viscous fluids.

    Gliding mobility

    Gliding mobilityis exactly what it sounds like, a slower and more graceful movement than the other shapes discussed so far. Lubricity is exhibited by certain filamentous or bacillus bacteria and does not require the use of flagella. It requires cells to be in contact with a solid surface, although more than one mechanism has been identified. Some cells rely on the mucus drive, with secreted mucus propelling the cell forward, while other cells rely on surface layer proteins to pull the cell forward.

    (Video) Microbiology - Bacteria (Structure)

    (Video) Bacteria (Updated)


    Now that we've covered the basics of the bacterial flagella motor and the mechanics of bacterial swimming, let's combine the two topics we want to talk aboutChemotaxisor any other kind of taxes (just not my taxes). Chemotaxis refers to the movement of an organism toward or away from a chemical. you can have it toophototaxis, where an organism responds to light. In chemotaxis, a beneficial substance (such as a nutrient) is referred to as anattract, while a substance with a harmful effect on the cell (e.g. a toxin) is denoted as adismissive. In the absence of an attractant or an antidote, a cell becomes attached to a "aimless walk' where it alternates between falling and running and ends up getting nowhere. In the presence of a gradient of some sort, the cell's movements become biased, which over time causes the bacterium to move toward an attractant and away from any antibodies. How does this happen?

    First, let's explain how a bacterium knows which direction to go. Bacteria rely on protein receptors embedded in their membrane, calledchemoreceptorsthat bind specific molecules. Binding typically results in methylation or phosphorylation of the chemoreceptor, triggering an elaborate protein pathway that eventually affects flagellar motor rotation. The bacteria intervenetemporal perception, where they compare the concentration of a substance with the concentration measured just a few seconds (or microseconds) before. In this way they collect information about the orientation of the concentration gradient of the substance. As a bacterium approaches the higher concentrations of an attractant, the runs (dictated by the clockwise flagellar rotation) lengthen while the tumbling (dictated by the clockwise flagellar rotation) decreases. There will still be times when the bacterium goes the wrong way away from an attractant, as the tumbling leads to random reorientation of the cell, but it won't go the wrong way for very long. The resulting "biased random walk” allows the cell to rapidly move up the gradient of an attractant (or move down the gradient of an antigen).

    6: Bacteria - surface structures (4)

    Bacterial Movement. By Brudersohn (Own work (Original text: self-created)) [CC BY-SA 2.0 de], via Wikimedia Commons


    Capsule, mucous layer, S layer, fimbriae/fimbriae, pilin, pili/pilus, conjugative pili, conjugation, pili type IV, twitch motility, flagella/flagellum, filament, flagellin, hook, motor, basal body, stator, mot proteins, Swimming, Clockwise (CW), Counterclockwise (CCW), Running, Tumbling, Spirochetes, Corkscrew Motility, Endoflagella, Sliding Motility, Chemotaxis, Phototaxis, Attractant, Repellant, Random Walk, Chemoreceptors, Temporal Sensing, Biased Random Walk.

    Essential questions/goals

    1. What is the composition and function of capsules and mucus layers? When will they be produced? How do capsules or slime layers increase the chances of bacteria surviving in different environments?
    2. What are fimbriae and pili; What are their compositions and functions? How big are bacterial flagella and how can they be arranged on a bacterial cell? How common are flagella in bacteria?
    3. What is the basic composition of a bacterial flagellum and how does it differ from flagella found in eukaryotes? How do bacterial flagella grow and how are proteins transported across the membrane? How do they keep you moving? How does movement differ from eukaryotic flagella?
    4. How are bacterial flagella attached to the body? How do the two inner rings cause movement and what drives the movement? What is the purpose of the 2 outer rings in the basal body of Grambacteria? What do gram+ have instead?
    5. How do endoflagella differ from flagella and what type of bacteria are they found in? Where do they work better than flagella?
    6. What is chemotaxis? How does the direction of rotation of the flagella affect the locomotion of a bacterium? What do we know about the mechanism of chemotaxis in terms of membrane-binding proteins and chemotactic mediators? How long do chemotaxis stimuli last and why is this important to the phenomenon?

    Exploration Questions (OPTIONAL)

    1. How could chemotaxis in microbes be used to address pollution problems?
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