Microbiology Review Chapter 4 a Survey of Prokaryotic Cells and Microorganisms

Learning Objectives

By the end of this section, y'all will be able to:

• Explain the distinguishing characteristics of prokaryotic cells
• Describe common cell morphologies and cellular arrangements typical of prokaryotic cells and explicate how cells maintain their morphology
• Describe internal and external structures of prokaryotic cells in terms of their physical structure, chemical structure, and function
• Compare the distinguishing characteristics of bacterial and archaeal cells

Cell theory states that the cell is the fundamental unit of life. Even so, cells vary significantly in size, shape, structure, and function. At the simplest level of construction, all cells possess a few fundamental components. These include cytoplasm (a gel-like substance composed of water and dissolved chemicals needed for growth), which is independent within a plasma membrane (too called a cell membrane or cytoplasmic membrane); one or more than chromosomes, which contain the genetic blueprints of the cell; and ribosomes, organelles used for the production of proteins.

Beyond these basic components, cells tin can vary greatly between organisms, and even inside the same multicellular organism. The two largest categories of cells—prokaryotic cells and eukaryotic cells—are defined past major differences in several jail cell structures. Prokaryotic cells lack a nucleus surrounded past a complex nuclear membrane and generally have a single, circular chromosome located in a nucleoid. Eukaryotic cells have a nucleus surrounded by a complex nuclear membrane that contains multiple, rod-shaped chromosomes.^xvi

All plant cells and animal cells are eukaryotic. Some microorganisms are composed of prokaryotic cells, whereas others are composed of eukaryotic cells. Prokaryotic microorganisms are classified within the domains Archaea and Bacteria, whereas eukaryotic organisms are classified within the domain Eukarya.

The structures inside a cell are analogous to the organs inside a human torso, with unique structures suited to specific functions. Some of the structures establish in prokaryotic cells are like to those constitute in some eukaryotic cells; others are unique to prokaryotes. Although there are some exceptions, eukaryotic cells tend to be larger than prokaryotic cells. The insufficiently larger size of eukaryotic cells dictates the demand to compartmentalize various chemic processes inside different areas of the cell, using complex membrane-bound organelles. In contrast, prokaryotic cells generally lack membrane-spring organelles; nonetheless, they often contain inclusions that compartmentalize their cytoplasm. Effigy 3.12 illustrates structures typically associated with prokaryotic cells. These structures are described in more detail in the next section.

Effigy 3.12 A typical prokaryotic cell contains a prison cell membrane, chromosomal DNA that is concentrated in a nucleoid, ribosomes, and a prison cell wall. Some prokaryotic cells may besides possess flagella, pili, fimbriae, and capsules.

Common Cell Morphologies and Arrangements

Individual cells of a particular prokaryotic organism are typically similar in shape, or cell morphology. Although thousands of prokaryotic organisms have been identified, only a handful of cell morphologies are ordinarily seen microscopically. Figure 3.xiii names and illustrates cell morphologies ordinarily found in prokaryotic cells. In addition to cellular shape, prokaryotic cells of the aforementioned species may grouping together in certain distinctive arrangements depending on the plane of jail cell sectionalisation. Some mutual arrangements are shown in Figure 3.14.

Figure 3.thirteen (credit "Coccus" micrograph: modification of work past Janice Haney Carr, Centers for Disease Control and Prevention; credit "Coccobacillus" micrograph: modification of work by Janice Carr, Centers for Illness Control and Prevention; credit "Spirochete" micrograph: modification of work by Centers for Disease Control and Prevention)

Figure 3.14

In almost prokaryotic cells, morphology is maintained by the cell wall in combination with cytoskeletal elements. The cell wall is a construction found in most prokaryotes and some eukaryotes; it envelopes the cell membrane, protecting the cell from changes in osmotic pressure level (Figure 3.15). Osmotic pressure occurs because of differences in the concentration of solutes on opposing sides of a semipermeable membrane. H2o is able to laissez passer through a semipermeable membrane, only solutes (dissolved molecules like salts, sugars, and other compounds) cannot. When the concentration of solutes is greater on 1 side of the membrane, water diffuses beyond the membrane from the side with the lower concentration (more water) to the side with the higher concentration (less water) until the concentrations on both sides become equal. This diffusion of water is called osmosis, and information technology can cause extreme osmotic pressure on a cell when its external environment changes.

The external environment of a cell tin be described as an isotonic, hypertonic, or hypotonic medium. In an isotonic medium, the solute concentrations inside and outside the cell are approximately equal, and so there is no cyberspace movement of h2o across the jail cell membrane. In a hypertonic medium, the solute concentration exterior the jail cell exceeds that within the cell, so water diffuses out of the cell and into the external medium. In a hypotonic medium, the solute concentration inside the cell exceeds that outside of the cell, then h2o will move by osmosis into the prison cell. This causes the cell to swell and potentially lyse, or flare-up.

The caste to which a particular prison cell is able to withstand changes in osmotic force per unit area is called tonicity. Cells that have a prison cell wall are better able to withstand subtle changes in osmotic force per unit area and maintain their shape. In hypertonic environments, cells that lack a cell wall tin can become dehydrated, causing crenation, or shriveling of the jail cell; the plasma membrane contracts and appears scalloped or notched (Effigy iii.15). By dissimilarity, cells that possess a cell wall undergo plasmolysis rather than crenation. In plasmolysis, the plasma membrane contracts and detaches from the cell wall, and at that place is a decrease in interior volume, only the jail cell wall remains intact, thus allowing the cell to maintain some shape and integrity for a period of time (Effigy 3.sixteen). Likewise, cells that lack a cell wall are more prone to lysis in hypotonic environments. The presence of a cell wall allows the jail cell to maintain its shape and integrity for a longer time before lysing (Figure 3.16).

Figure 3.15 In cells that lack a jail cell wall, changes in osmotic force per unit area can lead to crenation in hypertonic environments or prison cell lysis in hypotonic environments.

Figure 3.xvi In prokaryotic cells, the cell wall provides some protection against changes in osmotic pressure, assuasive information technology to maintain its shape longer. The jail cell membrane is typically fastened to the cell wall in an isotonic medium (left). In a hypertonic medium, the cell membrane detaches from the cell wall and contracts (plasmolysis) equally water leaves the cell. In a hypotonic medium (right), the jail cell wall prevents the cell membrane from expanding to the bespeak of bursting, although lysis will eventually occur if likewise much water is absorbed.

Check Your Understanding

• Explain the difference betwixt cell morphology and arrangement.
• What advantages do jail cell walls provide prokaryotic cells?

The Nucleoid

All cellular life has a DNA genome organized into one or more chromosomes. Prokaryotic chromosomes are typically circular, haploid (unpaired), and not leap by a complex nuclear membrane. Prokaryotic DNA and Dna-associated proteins are concentrated within the nucleoid region of the cell (Figure 3.17). In general, prokaryotic DNA interacts with nucleoid-associated proteins (NAPs) that assist in the system and packaging of the chromosome. In bacteria, NAPs function like to histones, which are the DNA-organizing proteins plant in eukaryotic cells. In archaea, the nucleoid is organized past either NAPs or histone-like DNA organizing proteins.

Figure 3.17 The nucleoid region (the expanse enclosed by the light-green dashed line) is a condensed area of Deoxyribonucleic acid found within prokaryotic cells. Because of the density of the area, it does non readily stain and appears lighter in color when viewed with a transmission electron microscope.

Plasmids

Prokaryotic cells may also contain extrachromosomal Deoxyribonucleic acid, or Deoxyribonucleic acid that is not part of the chromosome. This extrachromosomal Deoxyribonucleic acid is found in plasmid south, which are minor, circular, double-stranded Dna molecules. Cells that accept plasmids often have hundreds of them within a unmarried prison cell. Plasmids are more usually found in leaner; nonetheless, plasmids accept been plant in archaea and eukaryotic organisms. Plasmids often carry genes that confer advantageous traits such as antibiotic resistance; thus, they are of import to the survival of the organism. We will hash out plasmids in more than detail in Mechanisms of Microbial Genetics.

Ribosomes

All cellular life synthesizes proteins, and organisms in all iii domains of life possess ribosomes, structures responsible for protein synthesis. However, ribosomes in each of the 3 domains are structurally dissimilar. Ribosomes, themselves, are constructed from proteins, along with ribosomal RNA (rRNA). Prokaryotic ribosomes are found in the cytoplasm. They are chosen 70S ribosome south because they accept a size of 70S (Figure 3.18), whereas eukaryotic cytoplasmic ribosomes take a size of 80S. (The South stands for Svedberg unit, a measure of sedimentation in an ultracentrifuge, which is based on size, shape, and surface qualities of the construction being analyzed). Although they are the same size, bacterial and archaeal ribosomes have different proteins and rRNA molecules, and the archaeal versions are more similar to their eukaryotic counterparts than to those plant in leaner.

Figure 3.18 Prokaryotic ribosomes (70S) are composed of two subunits: the 30S (small subunit) and the 50S (big subunit), each of which are composed of protein and rRNA components.

Inclusions

Every bit unmarried-celled organisms living in unstable environments, some prokaryotic cells have the ability to store excess nutrients within cytoplasmic structures chosen inclusions. Storing nutrients in a polymerized form is advantageous because it reduces the buildup of osmotic pressure that occurs as a cell accumulates solutes. Various types of inclusions store glycogen and starches, which contain carbon that cells tin access for energy. Volutin granules, as well called metachromatic granules considering of their staining characteristics, are inclusions that store polymerized inorganic phosphate that can be used in metabolism and assist in the formation of biofilms. Microbes known to contain volutin granules include the archaea Methanosarcina, the bacterium Corynebacterium diphtheriae , and the unicellular eukaryotic alga Chlamydomonas. Sulfur granules, another type of inclusion, are found in sulfur bacteria of the genus Thiobacillus; these granules store elemental sulfur, which the bacteria apply for metabolism.

Occasionally, certain types of inclusions are surrounded past a phospholipid monolayer embedded with protein. Polyhydroxybutyrate (PHB), which can be produced by species of Bacillus and Pseudomonas, is an example of an inclusion that displays this type of monolayer structure. Industrially, PHB has likewise been used every bit a source of biodegradable polymers for bioplastics. Several different types of inclusions are shown in Figure three.19.

Figure iii.19 Prokaryotic cells may accept diverse types of inclusions. (a) A manual electron micrograph of polyhydroxybutryrate lipid droplets. (b) A lite micrograph of volutin granules. (c) A phase-dissimilarity micrograph of sulfur granules. (d) A transmission electron micrograph of magnetosomes. (e) A transmission electron micrograph of gas vacuoles. (credit b, c, d: modification of work by American Society for Microbiology)

Some prokaryotic cells take other types of inclusions that serve purposes other than nutrient storage. For instance, some prokaryotic cells produce gas vacuoles, accumulations of minor, poly peptide-lined vesicles of gas. These gas vacuoles allow the prokaryotic cells that synthesize them to alter their buoyancy and then that they tin adjust their location in the water column. Magnetotactic leaner, such as Magnetospirillum magnetotacticum , contain magnetosomes, which are inclusions of magnetic iron oxide or atomic number 26 sulfide surrounded past a lipid layer. These allow cells to align along a magnetic field, aiding their motion (Effigy 3.19). Cyanobacteria such every bit Anabaena cylindrica and bacteria such equally Halothiobacillus neapolitanus produce carboxysome inclusions. Carboxysomes are composed of outer shells of thousands of protein subunits. Their interior is filled with ribulose-1,v-bisphosphate carboxylase/oxygenase (RuBisCO) and carbonic anhydrase. Both of these compounds are used for carbon metabolism. Some prokaryotic cells as well possess carboxysomes that sequester functionally related enzymes in one location. These structures are considered proto-organelles considering they compartmentalize of import compounds or chemical reactions, much similar many eukaryotic organelles.

Endospores

Bacterial cells are by and large observed as vegetative cells, but some genera of leaner accept the ability to class endospores, structures that essentially protect the bacterial genome in a dormant state when ecology atmospheric condition are unfavorable. Endospores (not to be confused with the reproductive spores formed past fungi) allow some bacterial cells to survive long periods without food or h2o, likewise equally exposure to chemicals, farthermost temperatures, and even radiation. Table 3.1 compares the characteristics of vegetative cells and endospores.

Characteristics of Vegetative Cells versus Endospores
Vegetative Cells	Endospores
Sensitive to farthermost temperatures and radiation	Resistant to extreme temperatures and radiation
Gram-positive	Practice not absorb Gram stain, merely special endospore stains (run into Staining Microscopic Specimens)
Normal water content and enzymatic action	Dehydrated; no metabolic activity
Capable of active growth and metabolism	Dormant; no growth or metabolic activity

Table 3.one

The process by which vegetative cells transform into endospores is called sporulation, and information technology generally begins when nutrients become depleted or environmental conditions go otherwise unfavorable (Figure 3.20). The process begins with the formation of a septum in the vegetative bacterial cell. The septum divides the prison cell asymmetrically, separating a Deoxyribonucleic acid forespore from the female parent cell. The forespore, which volition form the cadre of the endospore, is substantially a re-create of the cell'southward chromosomes, and is separated from the female parent jail cell past a 2nd membrane. A cortex gradually forms effectually the forespore by laying downward layers of calcium and dipicolinic acrid between membranes. A poly peptide spore coat so forms around the cortex while the DNA of the mother prison cell disintegrates. Further maturation of the endospore occurs with the formation of an outermost exosporium. The endospore is released upon disintegration of the female parent jail cell, completing sporulation.

Effigy 3.20 (a) Sporulation begins following asymmetric cell division. The forespore becomes surrounded past a double layer of membrane, a cortex, and a protein spore coat, before being released as a mature endospore upon disintegration of the mother cell. (b) An electron micrograph of a Carboxydothermus hydrogenoformans endospore. (c) These Bacillus spp. cells are undergoing sporulation. The endospores have been visualized using Malachite Greenish spore stain. (credit b: modification of work past Jonathan Eisen)

Endospores of sure species have been shown to persist in a fallow land for extended periods of time, upwards to thousands of years.¹⁷ However, when living conditions meliorate, endospores undergo germination, reentering a vegetative state. After germination, the cell becomes metabolically active again and is able to carry out all of its normal functions, including growth and cell partition.

Not all bacteria have the ability to grade endospores; however, there are a number of clinically significant endospore-forming gram-positive bacteria of the genera Bacillus and Clostridium. These include B. anthracis, the causative agent of anthrax, which produces endospores capable of survive for many decades¹⁸; C. tetani (causes tetanus); C. difficile (causes pseudomembranous colitis); C. perfringens (causes gas gangrene); and C. botulinum (causes botulism). Pathogens such equally these are particularly hard to gainsay because their endospores are so hard to kill. Special sterilization methods for endospore-forming bacteria are discussed in Command of Microbial Growth.

Check Your Understanding

• What is an inclusion?
• What is the part of an endospore?

Plasma Membrane

Structures that enclose the cytoplasm and internal structures of the prison cell are known collectively equally the jail cell envelope. In prokaryotic cells, the structures of the cell envelope vary depending on the blazon of cell and organism. Most (but not all) prokaryotic cells have a jail cell wall, but the makeup of this cell wall varies. All cells (prokaryotic and eukaryotic) take a plasma membrane (likewise called cytoplasmic membrane or cell membrane) that exhibits selective permeability, assuasive some molecules to enter or leave the cell while restricting the passage of others.

The structure of the plasma membrane is ofttimes described in terms of the fluid mosaic model, which refers to the power of membrane components to move fluidly within the aeroplane of the membrane, besides as the mosaic-like composition of the components, which include a diverse array of lipid and poly peptide components (Figure 3.21). The plasma membrane structure of nearly bacterial and eukaryotic cell types is a bilayer composed mainly of phospholipids formed with ester linkages and proteins. These phospholipids and proteins accept the ability to move laterally within the aeroplane of the membranes as well as between the two phospholipid layers.

Effigy 3.21 The bacterial plasma membrane is a phospholipid bilayer with a variety of embedded proteins that perform various functions for the prison cell. Annotation the presence of glycoproteins and glycolipids, whose carbohydrate components extend out from the surface of the prison cell. The abundance and arrangement of these proteins and lipids can vary profoundly between species.

Archaeal membranes are fundamentally different from bacterial and eukaryotic membranes in a few pregnant means. First, archaeal membrane phospholipids are formed with ether linkages, in contrast to the ester linkages institute in bacterial or eukaryotic cell membranes. Second, archaeal phospholipids take branched bondage, whereas those of bacterial and eukaryotic cells are directly chained. Finally, although some archaeal membranes tin exist formed of bilayers like those found in bacteria and eukaryotes, other archaeal plasma membranes are lipid monolayers.

Proteins on the jail cell's surface are important for a variety of functions, including cell-to-cell communication, and sensing ecology conditions and pathogenic virulence factors. Membrane proteins and phospholipids may have carbohydrates (sugars) associated with them and are called glycoproteins or glycolipids, respectively. These glycoprotein and glycolipid complexes extend out from the surface of the cell, allowing the cell to interact with the external surround (Figure 3.21). Glycoproteins and glycolipids in the plasma membrane can vary considerably in chemic composition amongst archaea, bacteria, and eukaryotes, assuasive scientists to use them to characterize unique species.

Plasma membranes from dissimilar cells types as well contain unique phospholipids, which incorporate fat acids. As described in Using Biochemistry to Place Microorganisms, phospholipid-derived fat acid assay (PLFA) profiles tin can be used to identify unique types of cells based on differences in fatty acids. Archaea, leaner, and eukaryotes each take a unique PFLA profile.

Membrane Transport Mechanisms

I of the most important functions of the plasma membrane is to control the send of molecules into and out of the jail cell. Internal atmospheric condition must be maintained within a certain range despite whatsoever changes in the external surroundings. The ship of substances across the plasma membrane allows cells to do and then.

Cells utilise diverse modes of ship across the plasma membrane. For example, molecules moving from a higher concentration to a lower concentration with the concentration gradient are transported by elementary diffusion, also known as passive ship (Figure 3.22). Some small molecules, like carbon dioxide, may cross the membrane bilayer straight by simple diffusion. Even so, charged molecules, also as large molecules, need the help of carriers or channels in the membrane. These structures ferry molecules across the membrane, a procedure known as facilitated improvidence (Effigy 3.23).

Active send occurs when cells motility molecules across their membrane against concentration gradients (Figure 3.24). A major deviation between passive and active transport is that active transport requires adenosine triphosphate (ATP) or other forms of energy to motion molecules "uphill." Therefore, active transport structures are frequently called "pumps."

Figure three.22 Simple diffusion downwardly a concentration slope directly across the phospholipid bilayer. (credit: modification of piece of work by Mariana Ruiz Villareal)

Figure 3.23 Facilitated diffusion downwardly a concentration slope through a membrane protein. (credit: modification of work past Mariana Ruiz Villareal)

Effigy 3.24 Active transport confronting a concentration gradient via a membrane pump that requires energy. (credit: modification of work by Mariana Ruiz Villareal)

Group translocation also transports substances into bacterial cells. In this case, as a molecule moves into a cell against its concentration gradient, it is chemically modified so that it does not require transport against an unfavorable concentration gradient. A mutual example of this is the bacterial phosphotransferase system, a series of carriers that phosphorylates (i.e., adds phosphate ions to) glucose or other sugars upon entry into cells. Since the phosphorylation of sugars is required during the early stages of sugar metabolism, the phosphotransferase arrangement is considered to be an energy neutral organisation.

Photosynthetic Membrane Structures

Some prokaryotic cells, namely cyanobacteria and photosynthetic bacteria, take membrane structures that enable them to perform photosynthesis. These structures consist of an infolding of the plasma membrane that encloses photosynthetic pigments such as dark-green chlorophylls and bacteriochlorophylls. In blue-green alga, these membrane structures are chosen thylakoids; in photosynthetic bacteria, they are called chromatophores, lamellae, or chlorosomes.

Cell Wall

The primary office of the prison cell wall is to protect the prison cell from harsh conditions in the outside surround. When nowadays, in that location are notable similarities and differences amongst the prison cell walls of archaea, leaner, and eukaryotes.

The major component of bacterial cell walls is called peptidoglycan (or murein); it is only establish in bacteria. Structurally, peptidoglycan resembles a layer of meshwork or fabric (Effigy 3.25). Each layer is composed of long chains of alternating molecules of North-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). The construction of the long bondage has pregnant 2-dimensional tensile force due to the formation of peptide bridges that connect NAG and NAM within each peptidoglycan layer. In gram-negative bacteria, tetrapeptide chains extending from each NAM unit are directly cross-linked, whereas in gram-positive leaner, these tetrapeptide bondage are linked by pentaglycine cross-bridges. Peptidoglycan subunits are made inside of the bacterial cell and and so exported and assembled in layers, giving the jail cell its shape.

Since peptidoglycan is unique to bacteria, many antibiotic drugs are designed to interfere with peptidoglycan synthesis, weakening the cell wall and making bacterial cells more than susceptible to the effects of osmotic force per unit area (see Mechanisms of Antibacterial Drugs). In addition, certain cells of the human being immune arrangement are able "recognize" bacterial pathogens by detecting peptidoglycan on the surface of a bacterial cell; these cells then engulf and destroy the bacterial cell, using enzymes such every bit lysozyme, which breaks down and digests the peptidoglycan in their cell walls (see Pathogen Recognition and Phagocytosis).

Figure 3.25 Peptidoglycan is composed of polymers of alternating NAM and NAG subunits, which are cross-linked by peptide bridges linking NAM subunits from various glycan chains. This provides the jail cell wall with tensile strength in two dimensions.

The Gram staining protocol (run into Staining Microscopic Specimens) is used to differentiate two common types of cell wall structures (Effigy 3.26). Gram-positive cells have a cell wall consisting of many layers of peptidoglycan totaling 30–100 nm in thickness. These peptidoglycan layers are commonly embedded with teichoic acids (TAs), carbohydrate chains that extend through and beyond the peptidoglycan layer.¹⁹ TA is thought to stabilize peptidoglycan by increasing its rigidity. TA also plays a role in the ability of pathogenic gram-positive leaner such every bit Streptococcus to bind to sure proteins on the surface of host cells, enhancing their ability to crusade infection. In addition to peptidoglycan and TAs, leaner of the family Mycobacteriaceae have an external layer of waxy mycolic acids in their jail cell wall; as described in Staining Microscopic Specimens, these leaner are referred to as acid-fast, since acid-fast stains must be used to penetrate the mycolic acid layer for purposes of microscopy (Figure 3.27).

Effigy 3.26 Leaner incorporate two mutual prison cell wall structural types. Gram-positive cell walls are structurally uncomplicated, containing a thick layer of peptidoglycan with embedded teichoic acrid external to the plasma membrane.²⁰ Gram-negative jail cell walls are structurally more complex, containing a thin layer of peptidoglycan and an outer membrane containing lipopolysaccharide. (credit: modification of work by "Franciscosp2"/Wikimedia Eatables)

Figure 3.27 (a) Some gram-positive bacteria, including members of the Mycobacteriaceae, produce waxy mycolic acids found exterior to their structurally-singled-out peptidoglycan. (b) The acid-fast staining protocol detects the presence of cell walls that are rich in mycolic acid. Acrid-fast cells are stained scarlet by carbolfuschin. (credit a: modification of work past "Franciscosp2"/Wikimedia Commons; credit b: modification of piece of work by Centers for Disease Command and Prevention)

Gram-negative cells accept a much thinner layer of peptidoglycan (no more than than virtually 4 nm thick²¹) than gram-positive cells, and the overall structure of their jail cell envelope is more complex. In gram-negative cells, a gel-like matrix occupies the periplasmic space between the cell wall and the plasma membrane, and at that place is a second lipid bilayer chosen the outer membrane, which is external to the peptidoglycan layer (Effigy 3.26). This outer membrane is attached to the peptidoglycan by murein lipoprotein. The outer leaflet of the outer membrane contains the molecule lipopolysaccharide (LPS), which functions as an endotoxin in infections involving gram-negative bacteria, contributing to symptoms such as fever, hemorrhaging, and septic shock. Each LPS molecule is composed of Lipid A, a cadre polysaccharide, and an O side chain that is composed of sugar-similar molecules that comprise the external face of the LPS (Figure three.28). The composition of the O side chain varies between different species and strains of bacteria. Parts of the O side chain called antigens tin can be detected using serological or immunological tests to place specific pathogenic strains similar Escherichia coli O157:H7, a deadly strain of bacteria that causes bloody diarrhea and kidney failure.

Effigy 3.28 The outer membrane of a gram-negative bacterial cell contains lipopolysaccharide (LPS), a toxin composed of Lipid A embedded in the outer membrane, a core polysaccharide, and the O side chain.

Archaeal cell wall structure differs from that of bacteria in several meaning ways. First, archaeal cell walls do not contain peptidoglycan; instead, they contain a similar polymer called pseudopeptidoglycan (pseudomurein) in which NAM is replaced with a different subunit. Other archaea may have a layer of glycoproteins or polysaccharides that serves as the cell wall instead of pseudopeptidoglycan. Last, equally is the case with some bacterial species, there are a few archaea that appear to lack cell walls entirely.

Glycocalyces and S-Layers

Although nearly prokaryotic cells have cell walls, some may have additional jail cell envelope structures outside to the cell wall, such as glycocalyces and Southward-layers. A glycocalyx is a carbohydrate coat, of which there are two important types: capsules and slime layers. A capsule is an organized layer located outside of the prison cell wall and ordinarily equanimous of polysaccharides or proteins (Effigy 3.29). A slime layer is a less tightly organized layer that is only loosely attached to the cell wall and can exist more than easily washed off. Slime layers may be equanimous of polysaccharides, glycoproteins, or glycolipids.

Glycocalyces allows cells to adhere to surfaces, aiding in the germination of biofilms (colonies of microbes that form in layers on surfaces). In nature, most microbes live in mixed communities inside biofilms, partly because the biofilm affords them some level of protection. Biofilms generally hold water like a sponge, preventing desiccation. They also protect cells from predation and hinder the action of antibiotics and disinfectants. All of these properties are advantageous to the microbes living in a biofilm, but they present challenges in a clinical setting, where the goal is frequently to eliminate microbes.

Effigy 3.29 (a) Capsules are a type of glycocalyx equanimous of an organized layer of polysaccharides. (b) A capsule stain of Pseudomonas aeruginosa , a bacterial pathogen capable of causing many unlike types of infections in humans. (credit b: modification of piece of work by American Society for Microbiology)

The ability to produce a capsule tin can contribute to a microbe's pathogenicity (ability to crusade illness) because the capsule can make it more hard for phagocytic cells (such as white claret cells) to engulf and kill the microorganism. Streptococcus pneumoniae , for example, produces a capsule that is well known to help in this bacterium's pathogenicity. As explained in Staining Microscopic specimens, capsules are difficult to stain for microscopy; negative staining techniques are typically used.

An S-layer is another type of cell envelope structure; it is composed of a mixture of structural proteins and glycoproteins. In leaner, South-layers are found outside the cell wall, simply in some archaea, the S-layer serves as the prison cell wall. The exact function of S-layers is not entirely understood, and they are hard to study; but available evidence suggests that they may play a diverseness of functions in different prokaryotic cells, such equally helping the cell withstand osmotic pressure and, for certain pathogens, interacting with the host immune organization.

Clinical Focus

Function 3

Afterwards diagnosing Barbara with pneumonia, the PA writes her a prescription for amoxicillin, a commonly-prescribed type of penicillin derivative. More than than a week after, despite taking the full course as directed, Barbara all the same feels weak and is not fully recovered, although she is yet able to get through her daily activities. She returns to the health centre for a follow-upward visit.

Many types of leaner, fungi, and viruses can cause pneumonia. Amoxicillin targets the peptidoglycan of bacterial jail cell walls. Since the amoxicillin has not resolved Barbara's symptoms, the PA concludes that the causative amanuensis probably lacks peptidoglycan, meaning that the pathogen could exist a virus, a fungus, or a bacterium that lacks peptidoglycan. Another possibility is that the pathogen is a bacterium containing peptidoglycan but has adult resistance to amoxicillin.

• How tin can the PA definitively identify the crusade of Barbara'south pneumonia?
• What form of treatment should the PA prescribe, given that the amoxicillin was ineffective?

Leap to the side by side Clinical Focus box. Go back to the previous Clinical Focus box.

Filamentous Appendages

Many bacterial cells have poly peptide appendages embedded within their cell envelopes that extend outward, allowing interaction with the surroundings. These appendages can adhere to other surfaces, transfer DNA, or provide movement. Filamentous appendages include fimbriae, pili, and flagella.

Fimbriae and Pili

Fimbriae and pili are structurally similar and, because differentiation betwixt the two is problematic, these terms are often used interchangeably.^{22 23} The term fimbriae commonly refers to short bristle-like proteins projecting from the cell surface by the hundreds. Fimbriae enable a cell to attach to surfaces and to other cells. For pathogenic leaner, adherence to host cells is of import for colonization, infectivity, and virulence. Adherence to surfaces is too of import in biofilm formation.

The term pili (singular: hair) commonly refers to longer, less numerous protein appendages that assistance in attachment to surfaces (Figure three.30). A specific blazon of pilus, called the F pilus or sexual practice pilus, is important in the transfer of Dna between bacterial cells, which occurs between members of the same generation when two cells physically transfer or exchange parts of their respective genomes (see How Asexual Prokaryotes Achieve Genetic Diversity).

Effigy iii.thirty Bacteria may produce two different types of protein appendages that aid in surface attachment. Fimbriae typically are more numerous and shorter, whereas pili (shown here) are longer and less numerous per cell. (credit: modification of work past American Society for Microbiology)

Micro Connections

Group A Strep

Before the construction and part of the various components of the bacterial prison cell envelope were well understood, scientists were already using cell envelope characteristics to classify leaner. In 1933, Rebecca Lancefield proposed a method for serotyping various β-hemolytic strains of Streptococcus species using an agglutination assay, a technique using the clumping of bacteria to discover specific cell-surface antigens. In doing so, Lancefield discovered that one grouping of S. pyogenes, found in Group A, was associated with a variety of human diseases. She adamant that various strains of Group A strep could be distinguished from each other based on variations in specific jail cell surface proteins that she named 1000 proteins.

Today, more 80 different strains of Grouping A strep take been identified based on M proteins. Diverse strains of Group A strep are associated with a wide variety of homo infections, including streptococcal pharyngitis (strep throat), impetigo, toxic shock syndrome, scarlet fever, rheumatic fever, and necrotizing fasciitis. The M poly peptide is an important virulence factor for Group A strep, helping these strains evade the immune system. Changes in M proteins announced to alter the infectivity of a particular strain of Grouping A strep.

Flagella

Flagella are structures used past cells to move in aqueous environments. Bacterial flagella act like propellers. They are stiff screw filaments equanimous of flagellin protein subunits that extend outward from the cell and spin in solution. The basal trunk is the motor for the flagellum and is embedded in the plasma membrane (Figure 3.31). A hook region connects the basal body to the filament. Gram-positive and gram-negative bacteria accept different basal trunk configurations due to differences in cell wall construction.

Different types of motile bacteria exhibit different arrangements of flagella (Figure 3.32). A bacterium with a singular flagellum, typically located at one stop of the cell (polar), is said to have a monotrichous flagellum. An example of a monotrichously flagellated bacterial pathogen is Vibrio cholerae, the gram-negative bacterium that causes cholera. Cells with amphitrichous flagella take a flagellum or tufts of flagella at each end. An example is Spirillum minor , the cause of spirillary (Asian) rat-bite fever or sodoku. Cells with lophotrichous flagella have a tuft at 1 finish of the jail cell. The gram-negative bacillus Pseudomonas aeruginosa , an opportunistic pathogen known for causing many infections, including "swimmer's ear" and burn down wound infections, has lophotrichous flagella. Flagella that cover the entire surface of a bacterial cell are chosen peritrichous flagella. The gram-negative bacterium East. coli shows a peritrichous organisation of flagella.

Figure 3.31 The basic construction of a bacterial flagellum consists of a basal torso, hook, and filament. The basal body limerick and arrangement differ betwixt gram-positive and gram-negative bacteria. (credit: modification of work by "LadyofHats"/Mariana Ruiz Villareal)

Effigy 3.32 Flagellated leaner may showroom multiple arrangements of their flagella. Mutual arrangements include monotrichous, amphitrichous, lophotrichous, or peritrichous.

Directional movement depends on the configuration of the flagella. Bacteria tin can move in response to a variety of environmental signals, including light (phototaxis), magnetic fields (magnetotaxis) using magnetosomes, and, well-nigh commonly, chemical gradients (chemotaxis). Purposeful move toward a chemical attractant, like a food source, or abroad from a repellent, similar a poisonous chemical, is achieved by increasing the length of runs and decreasing the length of tumbles. When running, flagella rotate in a counterclockwise direction, allowing the bacterial cell to movement forward. In a peritrichous bacterium, the flagella are all bundled together in a very streamlined style (Effigy 3.33), assuasive for efficient move. When tumbling, flagella are splayed out while rotating in a clockwise direction, creating a looping motion and preventing meaningful forward movement merely reorienting the jail cell toward the direction of the attractant. When an attractant exists, runs and tumbles yet occur; all the same, the length of runs is longer, while the length of the tumbles is reduced, allowing overall movement toward the higher concentration of the attractant. When no chemic gradient exists, the lengths of runs and tumbles are more equal, and overall movement is more random (Figure 3.34).

Effigy 3.33 Bacteria achieve directional movement by changing the rotation of their flagella. In a cell with peritrichous flagella, the flagella parcel when they rotate in a counterclockwise direction, resulting in a run. All the same, when the flagella rotate in a clockwise direction, the flagella are no longer arranged, resulting in tumbles.

Figure three.34 Without a chemical gradient, flagellar rotation cycles between counterclockwise (run) and clockwise (tumble) with no overall directional movement. However, when a chemical gradient of an attractant exists, the length of runs is extended, while the length of tumbles is decreased. This leads to chemotaxis: an overall directional motility toward the higher concentration of the attractant.

Check Your Understanding

• What is the peptidoglycan layer and how does it differ between gram-positive and gram-negative bacteria?
• Compare and contrast monotrichous, amphitrichous, lophotrichous, and peritrichous flagella.

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Source: https://openstax.org/books/microbiology/pages/3-3-unique-characteristics-of-prokaryotic-cells

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