6Differences Between Cell Types Prokaryotic CellEukaryotic CellSingle circular chromosomeMultiple linear chromosomesChromosome found in a cytoplasmic region called the nucleoid.Chromosomes found in a membrane-bound nucleus.No internal membranesSome infolded plasma membraneExtensive network of internal membranes Specialized areas called organelles
7Prokaryotes One circular chromosome, not membrane bound No histones No organellesPeptidoglycan cell wallsBinary fission
8Size and Shape of Prokaryotic Cells Average size: µm µmThree basic shapesBacillus,Coccus,Spirals (Vibrio, Spirillum, Spirochete)Most monomorphic, some pleomorphicVariations in cell arrangements (esp. for cocci)
13Cytoplasm and Internal Structures Location of most biochemical activitiesNucleoid: nuclear region containing DNA (up to 3500 genes). Most bacterial chromosomes are circularPlasmids: small, nonessential, circular DNA (5-100 genes); replicate independentlyRibosomes (70S vs. 80S)Inclusion bodies: granules containing nutrients, monomers, Fe compounds (magnetosomes)
15Closed Circular Chromosome Also Plasmids, which are smaller, circular pieces of DNA.Plasmids usually encode expendable functions, e.g., antibiotic resistance.
16Ribosomes: Sites of Translation On order of 10,000 per cell!
17Internal Structures: Cell Membrane Analogous to eukaryotic cell membrane:Phospholipid bilayer with proteins (Fluid mosaic model)Permeability barrier (selectively permeable)Diffusion, osmosis and transport systemsDifferent from eukaryotic cell membrane:Role in Energy transformation (electron transport chain for ATP production)Damage to the membrane by alcohols, quaternary ammonium (detergents), and polymyxin antibiotics causes leakage of cell contents.
22Cell Wall Rigid for shape & protection prevents osmotic lysis Consists of Peptidoglycan (murein) polymer of two disaccharide subunitsN-acetylglucosamine (NAG) andN-acetylmuramic acid (NAM)Linked by polypeptides with tetrapeptide side chain attached to NAMFully permeable to ions, aa, and sugars (Gram positive cell wall may regulate movement of cations)
23Gram + Cell Wall Gram – Cell Wall Thin peptidoglycan No teichoic acids Thick layer of peptidoglycanNegatively charged teichoic acid on surfaceThin peptidoglycanNo teichoic acidsOuter membrane
25Gram-negative Cell Wall Lipid A of LPS acts as endotoxin; O antigens for typing, e.g., E. coli O157:H7Gram neg. bacteria are less sensitive to medications because outer membrane acts as additional barrier.LPS layer = outer layer of outer membrane(protein rich gel-like fluid)
26Gram Stain Mechanism Crystal violet-iodine crystals form in cell. Gram-positiveAlcohol dehydrates peptidoglycanCV-I crystals do not leaveGram-negativeAlcohol dissolves outer membrane and leaves holes in peptidoglycan.CV-I washes out
27Atypical Bacteria: Mycoplasmas Lack cell wallsInstead, have cell membrane which incorporates cholesterol compounds (sterols), similar to eukaryotic cellsCannot be detected by typical light microscopyMycoplasmas cannot be detected by the naked eye or even by typical light microscopy. The morphology of mycoplasma colonies is often likened to a "fried-egg" because they form a dense central core, which penetrates downward into the agar, surrounded by a circular spreading area that is lighter in color.When Mycoplasma species were first cultured, they were thought to have been viruses because of their size. Mycoplasma pneumoniae is a very small bacterium, in the class Mollicutes. This class of organisms lack a peptidoglycan cell wall present on all other firmicute bacteria. Instead, it has a cell membrane which incorporates cholesterol compounds, similar to eukaryotic cells. Lacking a cell wall, these organisms are resistant to the effects of penicillins and other beta-lactam antibiotics, which act by disrupting the bacterial cell wall.M. pneumoniae has one of the smallest genomes known, with 816 kilobase pairs (kbs). Its genome and proteome has been fully characterized. It uses some unique genetic code, making its code more similar to mitochondria than to other bacteria. It lacks the cellular machinery for making many essential compounds. Because of this, it is an obligate parasite. No mycoplasma is found free-living. In this respect, mycoplasma is more similar to viruses than to bacteria.M. pneumoniae is spread through respiratory droplet transmission. Once attached to the mucosa of a host organism, M. pneumonia extracts nutrients, grows and reproduces by binary fission. Attachment sites include the upper and lower respiratory tract, causing pharyngitis, bronchitis and pneumonia. The infection caused by this bacterium is called atypical pneumonia because of its protracted course and lack of sputum production and wealth of extra-pulmonary symptoms. Chronic mycoplasma infections have been implicated in the pathogenesis of rheumatoid arthritis and other rheumatological diseases.Second generation macrolide antibiotics and second generation quinolones are effective treatments. Disease from mycoplasma is usually mild to moderate in severity.Mycoplasma pneumoniae lacks a cell wall which leads to osmotic instability. The combination of these unique characteristics creates a different scenario for treatment of a mycoplasmal infection than other bacteria. The lack of a cell wall prevents the utilization of a B-lactam antibiotic, such as penicillin and cycloserine, because they act specifically to disrupt the cell wall. The use of cholesterol in M. pneumoniae, however, allows for a different avenue for antibiotic therapies usually ineffective on bacteria, such as the use of polyenes.The absence of a cell wall is likely to facilitate a bacterium to host interaction through which compounds can be exchanged. This transfer can include not only the nutrients and supplementary amino acids, etc. that is necessary for the support of bacterial growth, but also toxic metabolic compounds. It is thought that this bacterial surface parasitism causes severe damage to the host cell, however, not one toxin has been identified as the culprit.This EM shows some typically pleomorph mycoplasmas, in this case M. hyorhinis
28Acid-fast Cell Walls Genus Mycobacterium and Nocardia mycolic acid (waxy lipid) covers thin peptidoglycan layerDo not stain well with Gram stain use acid-fast stain
29Damage to Cell Wall Lysozyme digests disaccharide in peptidoglycan. Penicillin inhibits peptide bridges in peptidoglycan.Protoplast is a wall-less cell.Spheroplast is a wall-less Gram-positive cell.L forms are wall-less cells that swell into irregular shapes.Protoplasts and spheroplasts are susceptible to osmotic lysis.
30External Structures located outside of cell wall Flagella Axial filamentsFimbriaePiliGlycocalyx
31GlycocalyxMany bacteria secrete external surface layer composed of sticky polysaccharide (EPS), polypeptide, or bothA capsule is neatly organizedA slime layer is unorganized and looseAllows cells to attach key to biofilmsPrevents phagocytosis virulence factorE.g.: B. anthracis, S. pneumoniae, S. mutans
32Flagellum – Flagella Anchored to wall and membrane Number and placement determines if atrichous, monotrichous, lophotrichous, amphitrichous, or peritrichousAllows for move toward or away from stimuli: Chemotaxis (phototaxis and magnetotaxis)Flagella proteins are H antigens (e.g., E. coli O157:H7)
33Flagellar Arrangements Polar FlagellumFlagellar Arrangementse.g., E. colialso “atrichous”
37EndosporesDormant, tough, non-reproductive structure; germination vegetative cellsSpore forming genera: __________Resistance to UV and radiation, desiccation, lysozyme, temperature, starvation, and chemical disinfectantsRelationship to diseaseSporulation: Endospore formationGermination: Return to vegetative state
38In response to starvation, B In response to starvation, B. subtilis differentiates into a dormant stress-resistant cell-type known as a spore. Cells initiate spore formation with an asymmetric cell division that generates a large cell (the mother cell) and a small cell (the prospective spore or forespore). Initially these two cells lie side-by-side but later in development the mother cell engulfs the forespore in a phagocytic-like process to produce a cell-within-a-cell. The mother then nurtures the forespore as it prepares for dormancy. Once the spore is fully mature, the mother cell lyses. We use this elegant morphological process to address basic questions in bacterial differentiation.