BACTERIAL MORPHOLOGY - PROKARYOTES
PORTIONS / DETAILSTHE NUCLEOID / · Have no true nucleus, nuclear membrane, & mitotic apparatus.
· Have nucleoid:
§ Package of DNA in e cytoplasm.
§ Consists of a single continuous circular molecule ranging in size from 0.58 to almost 10 million base pairs.
§ E no. of nucleoid & chromosomes depend on e growth condition.
§ Rapidly growing bacteria have > nucleoid per cell than slowly growing one.
§ It is Feulgen-positive: indicates e presence of DNA (negatively charged).
· Nuclear region is filled with DNA fibrils. / EXCEPTIONS
1. Planctomycetes: have nucleoid surrounded by a nuclear envelope with 2 membranes.
2. Vibrio cholerae & Brucella melintesis: have 2 dissimilar chromosomes.
3. Borrelia burgdorferi & Streptomyces coelicolor: have linear chromosome.
CYTOPLASMIC STRUCTURES / · Lack of autonomous plastids such as mitochondria & chloroplasts.
· E electron transport enzymes r localized in e cytoplasmic membrane.
· In photosynthetic bacteria: e photosynthetic pigments r located in intracytoplasmic membrane systems such as lamellae, chromatophores, or chlorosomes.
· In cynobacteria: have thylakoids & e major accessory pigments used for light harvesting r found on e surface of e thylakoid membranes.
· Storage of reserve material:
§ Is in the form of insoluble granules which appear as refractile bodies in the cytoplasm
§ Function in e storage of energy or as a reservoir of structural building block.
§ Most r bounded by a thin nonunit mebrane consisting lipids.
· Types of inclusion bodies of storage materials:
1. Poly-β-hydroxybutyric acid (PHB):
§ A lipid-like compund consisting of chains of β-hydroxybutyric acid units connected through ester linkages.
§ Produced when e source of nitrogen, sulfur, or phosporus is limited while carbon is excess.
2. Gylogen:
§ Formed when carbon is excess.
§ Used s carbon source when protein & nucleic aci synthes r resumed – same as PHB.
3. Sulfur:
§ Prokaryotes oxidize reduced sulfur compounds such as hydrogen sulfite & thiosulfate to produce intracellular granules of elemental sulfur.
§ When e reduced sulfur is limiting, e granuled sulfur is oxidized to sulfate & e granule will slowly disappear.
4. Phosphate:
§ Inorganic phosphate is accumulated in e form of granules of polyphosphate.
§ E granules can be used as sources of phosphate for nucleic acid & phospholipid synthesis to suport growth.
§ E granules r also termed as volutin granules or metachromatic granules because they stain red with a blue dye.
5. CO2 fixation:
§ Autotrophic bacteria contain polyhedral bodies surrounded by a potein shell (carboxysomes).
§ E bodies contain e key enzyme of CO2 fixation, ribulosebisphosphate carboxylase.
6. Magnetosomes:
§ Intracellular crystal particles of iron magnetite that allow certain aquatic bacteria to exhibit magnetotaxis.
§ Surrounded by a nonunit membrane containing phospholipids, proteins, & glycoproteins.
7. Gas vesicles:
§ Provide buoyancy for aquatic bacteria.
§ E mebrane is a 2 nm thick layer of protein, impermeable to water & solutes but permeable to gas.
8. Cytoskeletal protein:
§ Play cytoskeletal roles.
§ Actin homologs (MreB & Mbl) help to determine ell shape, segregate chromosomes, & localize protein with e cell.
§ Nonactin homologs (FtsZ) & bacterial cytoskeletal proteins (SecY & MinD) help to determine cell shape, regulate cell division, & segregate chromosomes.
THE CELL ENVELOPE / · Consists of capsule, cell wall, cell membrane, flagella, & pili.
· Function to protect bacteria from hostile environments such as extreme osmolarity, harsh chemical, & antibiotic.
THE CELL MEMBRANE / STRUCTURE / · Also called cytoplasmic membrane which is visible in electrom micrograph as thin section.
· Composed of phospholipids & 200 kinds of proteins which make up 70% of the membrane mass.
· Have no sterols except mycoplasma which incorprate sterols into their membrane when growing in the sterol-containing media.
· At least 50% of e cytoplasmic membrane must be in semifluid state for cell growth to occur. At low temperature, this is done by incorpration of unsatured fatty acid into e phospholipids of e membrane.
FUNCTIONS / 1. Permeability & Transport:
· Forma a hydrophobic barrier impermeable to most hydrophilic molecules which can only be transported through:
a. Passive transport: relies on diffusion, uses no energy, & operates only when e solute is at higher concentration outside than inside e cell. The examples are simple diffusion, facilitated diffusion, & channel protein.
b. Active transport: 2 types depending on e source of enegy employed:
§ Ion-coupled transport: moves a molecule across e cell membrane across e cell membrane at e expense of a previously established ion gradient such as proton-motive or sodium-motive force. There r 3 basic types i.e. uniport, symport, & antiport.
§ ABC transport: employs ATP directly to transport solutes into e cell. It is facilitated by specific binding proteins.
c. Group translocation: e substrate is phosphorylated during e process, no concentration gradient is involved & it allows bacteria to utilize their energy sources by couling transport with metabolism.
d. Special transport process: some bacteria secrete siderophores (compound that chelate Fe & promotes its transport as a soluble complex) such as hydroxamic acid.
2. Electron Transport & Oxidative Phosphorylation:
· E cell membrane has a functional analog with e mitochondrial membrane.
· It contains cytochromes & other enzymes & e component of respiratory chain for oxidative phosphorylation.
3. Excretion of Hydrolytic Exoenzymes & Pathogenicity Protein:
· E cell membrane excretes hydrolytic enzymes that degrade e polymers to subunits small enough to enetrate e cell membrane.
· Bacteria secrete them directly into e external medium or into e periplasmic space b/w e peptidoglycan layer & eouter membrane of e cell wall (gram –ve).
4. Biosynthetic Function:
· E cell membrane is e site of e carrier lipid on which e subunits of e cell wall r assembled.
· E enzymes for cell wall biosynthesis & phospholipid synthesis are located here too.
5. Chemotactic Systems:
· Attractants & repellents bind to specific receptors in e bacterial membrane.
THE CELL WALL
· E functions:
a. Gives osmotic protection.
b. Helps in cell division.
c. Serves as a primer for its own biosynthesis.
d. Site of antigenic determinants.
· Exception to mycoplasma which r cell wall-lacking bacteria containing no peptidoglycan. / PEPTIDOGLYCAN LAYER / · A complex polymer consists of:
1. A backbone of alternating N –acetylglucosamine & N –acetylmuramic acid.
§ Same in all species.
2. A set of identical tetrapeptide side chain attached to e N –acetylmuramic acid.
§ Vary from species to species.
§ In all specis, there r features in common. Most hae L-alanine at position 1, D-glutamate at position 2, & D-alanine at position 4. Position 3 is e most variable one – Gram –ve bacteria have diamnopimelic acid (DAP) while Gram +ve bacteria have L-lysine.
3. A set of identical peptide cross-bridges.
§ Vary from species to species.
§ In many Gram –ve cell wall, e cross bridge consists of a direct peptide linkage b/w DAP amino group of 1 side chain & e carboxyl group of e terminal D-alanine on e 2nd side chain.
· In Gram +ve bacteria, there r 40 sheets of peptidoglycan & on 1 or 2 sheets in Gram –ve bacteria.
GRAM-POSITIVE BACTERIA / 1. Techoic & Teichuronic Acids:
· Consists of glycerophosphate or ribitol phospate residues which r connected by phosphodiester linkages & usually have other sugars & D-alanine attached (to position 2 or 3 of glycerol or position 3 or 4 of ribitol).
· R –vely charged & partially responsible for e –ve charge of e cell surface.
· 2 types i.e. wall techoic acid (WTA) which is covalently linked to peptidoglycan & membrane techoic acid or lipotechoic acid (LTA) whish is associated with lipids.
· Lie b/w e cytoplasmic membrane & e peptidoglycan layer, extending through pores in e peptidglycan layer.
· E repeat unit may be glycerols (joined by 1,3 or 1,2-linkages), ribitol (joined by 1,5-linkages), or complex consists of both joined to glucose, galactose, orN-acetylglucosamine.
· Functions:
a. Make up a polyanionic network or matrix with peptidoglycan which provides functions relating to elsticity, porosity, tensile, strenghth, & electrostatic properties of e envelope.
b. Constitue e major surface Ag of those Gram +ve species.
c. Associated (LTA) with e M protein molecule forming microfibril that facilitate e attachment of S pyogenes to animal cells.
· Teichuronic acids: r similar polymer but e repeat units include sugar acids & r synthesized in place of techoic acids when phosphate is limiting.
2. Polysaccharides:
· Consists of sugars such as mannose, arabinose, rhamnose, & glucosamines as well as acidic sugars such as glucuronic acid & mannuronic acid.
· Exist as subunits of polysaccharides in e cell wall.
GRAM-NEGATIVE BACTERIA / 1. Outer Membrane:
· Bilayered in structure with inner leaflet resembles cell membrane while e outer leaflet contains LPS.
· Functions:
a. Protection from deleterious substances such as bile salts by excluding hydrophbic molecules.
b. Has channel protein called porins that permit passive diffusion of low MW hydrophilic compounds.
c. Has a relatively high antibiotic resistance of Gram –ve bacteria.
d. Participates in anchoring to peptidoglycan layer through OmpA protein.
e. Acts as sex pilus receptor in F-mediated bacterial conjugation through OmpA protein.
f. Contains proteins involved in e transport of specific molecules such as Vit. B12.
2. Lipopolysaccharide (LPS):
· Consists of a complex glycolipid called Lipid A, attached to a polysaccharide & a terminal series of repeat units.
· Lipid A consists of phosphorylated glucosamine disaccharide units to which r attached long chains fatty acids (β-Hydroxymyristic is unique to e lipid).
· E polysaccharide core has LPS & includes ketodeoxyoctanoic acid (KDO) & a heptose.
· E repeat unit composed of linear trisaccharides & others & known as O Ag which functions to cover e bacterial surface & exclude hydrophobic compounds through its hydrophilic carbohydrate chain.
· Functions:
a. E –vely charged LPS molecules r noncovalently cross-bridged by divalent cations to stabilize e membrane & provide a barrier to hydrophobic molecules.
b. Has an important virulance factor (lipooligosaccharides- e sialylation of its N-acetyllactosamine causes molecular mimicry in e host).
3. Lipoprotein:
· Cross-link e outer membrane & peptidoglycan layers.
· Contains 57 aa, representing repeats of a 15-aa-sequence.
· E lipid component consists of diglyceride thiother linked to a terminal cysteine which s noncovalently inserted in e outer membrane.
· Function: to stabilize e outer membrane & anchor it to peptidoglycan layers.
4. Perisplasmic Space:
· Space b/w e inner & outer membrane which contains peptidoglycan layers & a gel-like solution of proteins.
· ~20-40% of cell volume.
· E periplasmic proteins:
· Contains a high concentration of D-glucose which r variously substituted with glycerol phosphate & phosphatidylethanolamine residues or O-succinyl esters.
· These r called membrane-derived oligosaccharides which control osmoregulation.
ACID-FAST CELL WALL / · Composed of:
1. Peptidoglycan.
2. An external asymmetrical lipid bilayer.
3. E inner leaflet which contains mycolic acids (contain waxes) lnked to an arabinoglycan.
4. E outer leaflet which contains other extractable lipids.
· Advantages:
1. E highly ordered lipid embeded with proteins allows slow passage of nutrients & drugs by forming water-filled pores.
2. E hydrophobic structure renders e bacteria resistant to many harsh chemicals like strong acid.
3. Its permeability to hydrophilic molecules slows e bacterial grothw rate.
· Known as Acid-Fast because when introduced to a dye by heating or treatment with detergent, e dye can’t be removed by dilute HCL.
· E.g.: mycobacteria.
CRYSTALLINE SURFACE LAYER / · Many both Gram +ve & -ve bacteria posses 2D crystalline subunit-type lyer lattice of protein or glycoprotein molecules (S-layer) as outermost component.
· S-layer:
1. Composed of a single kind protein molecule which capable of self-assembly.
2. Resistant to proteolytic enzymes & protein-denaturing agents.
3. Protects e cell from wall-degrading enzymes & from e invasion of predatory bacerium.
4. Maintains e cell shape.
5. May involve in cell adhesion to host epidermal surface.
CAPSULE & GLYCOCALYX / · Glycocalyx:
1. Definition: e polysaccharide-containing material lyingoutside e cell.
2. Formed from sucrose.
3. Functions: resistant to desiccation & correlates dental carries with sucrose consumption in human.
· Capsule:
1. Definition: a condensed, well-defined layer closely surrounding e cell that excludes particles.
2. Functions in e:
- Invasiveness of pathogeinc bacteria.
- Protection from phagocytosis.
- Adherence to bacteria to surface of their environment.
· Slime layer: glycocalyx which is loosely associated with e cell & does not exclude particles.
FLAGELLA / STRUCTURE / · Thread-like appendages composed of protein, 12 – 30 nm in diameter.
· Organ of locomotion, 3 types of arrangement:
1. Peritrichous: flagella all around e body of e bacteria, e.g. Salmonella typhi.
2. Amphitrichous: a single flagellum at each pole.
3. Lophotrichous: bunch of flagella at one or both ends, e.g. Spirilium minus.
4. Monotrichous: 1 flagellum at 1 end, e.g. V. cholerae.
· Building blocks: thousands molecules of a protein subunit called flagellin.
· E flagellum is formed by e aggregration of subunits to form a helical structure
· R highly antigenic: H Ag.
· Structure for attachment: a complex composed of a hook (short curved structure acts as universal joint b/w motor & flagellum) & basal body (bears a set of rings, i.e. one pair in Gram +ve & two pairs in Gram –ve).
MOTILITY / · R semirigid helical rotors to which e cell imparts a spining movement.
· Flagellar motor & its components r located in e cell envelope.
PILI (FIMBRIAE) / · R rigid surfce appendages.
· Shorter & finer than flagella.
· Composed of structural protein subunits termed pilins.
· Minor protein called adhesins r located at e tips of e pili & r responsible for attachment properties.
· 2 types:
1. Ordinary pili: adherence of symbiotic & pathognic bacteria to host cells.
2. Sex pili: attachment of donor & recipients cell in bacterial conjugation.
· Pilin molecules r arranged helically to form a straight cylinder that does not rotate & lacks a complete basal body.
· Pili grow from inside of e cell outward.
ENDOSPORE / OVERVIEW
SPORULATION
PROPERTIES
GERMINATION
THE STAPHYLOCOCCI