9 Prokaryotes inside and out Flashcards
Classification - The Gram Stain
Allows you to determine the two major classes - Gram +ve or Gram -ve
- Heat with crystal violet
- Add iodine solution
- Decolourise with alcohol (Gram +ve stain purple)
- Stain with safranin (gram -ve stain red)
Key Features of a Prokaryote Cell
- No membrane-bound nucleus
- Single circular DNA genome
- Plasmid DNA free in cytoplasm
- Enzymes and ribosomes free in cytoplasm
- No mitochondria, Golgi or ER
- Rigid, complex cell wall with appendages
- Survival by adaptation to environment
Streptococcus pneumoniae
- Gram +ve
- Encapsulated coccus
- Causes respiratory infections
- SEM x3750
Escherichia coli
- Gram -ve
- Flagella and fimbriae
- Enteric, urinary and nosocomal infections
- SEM x3515
Clostridium difficile
- Gram +ve rods
- Anaerobic
- Forms part of normal gut microflora
- Causes inflammation (colitis) and abdominal disease
- SEM x1200
Escherichia coli 0157:H7
- Gram -ve rods
- Anaerobic
- Causal agent of haemorragic disease due to toxin secretion
- SEM x9335
Helicobacter pylori
- Gram -ve spirals
- Moves using flagella
- Associated with chronic superficial gastritis
- SEM x2200
Mycobacterium tuberculosis
- Gram +ve rods
- Moves using flagella
- Able to reside inside tissues due to hydrophobic cell wall
- SEM x3000
Staphylococcus aureus
- Gram +ve cocci
- MRSA
- Causes a variety of infections and disease
- SEM x13335
Prokaryote DNA - Nucleoid
- Circular, supercoiled, double stranded
- Associated with histone-like proteins
- Generally only one chromosome
- E.coli chromosome 4.6 x 106 base pairs
Prokaryote DNA - Plasmids
- Small loops of extrachromosomal DNA
- Not essential for survival a priori
- May contain genes that are expressed in response to certain situations
- May be used to transfer genes between cells, e.g. resistance to antibiotics
- Can be incorporated into the chromosome
- Single DNA strand transferred between cells by conjugation using ‘rolling circle’ replication
plasmids used to transfer one bacterium to another
Ribosomes and Translation
- Free in cytoplasm, composed of two large subunits
- Small 30S subunit binds to mRNA
- Large 50S subunit binds, completes 70S ribosome
Plasma (Cytoplasmic) Membrane
- Phospholipid bilayer, embedded proteins
- Contains cytoplasm, regulates transport
- Three types of active transport in bacteria:
- PMF-driven transport
- ABC transport (ATP-Binding Cassette) • Group translocation e.g. glucose 6-P
- Site of assembly of cell wall components
Peptidoglycan
- Highly cross-linked polymer composed of carbohydrates and amino acids
- Differences between Gram +ve and Gram –ve bacteria
- Rigid, prevents osmotic lysis
Gram +ve Peptidoglycan
Staphylococcus aureus
Gram –ve Peptidoglycan
Escherichia coli
Gram +ve Cell Wall
Draw
Gram -ve Cell Wall
Draw
Periplasmic Space
only present in gram -ve bacteria
- Aqueous gel-like region between two membranes of Gram –ve bacteria
- Contains enzymes associated with nutrient acquisition and transport
- Also contains defence enzymes used to resist antibiotics, e.g. b-lactamase
Outer Membrane and Capsule
- Porins allow passive diffusion of molecules into periplasmic space
- Braun’s lipoprotein
- Lipopolysaccharides
- Proteinaceous capsule, slime layer
Lipopolysaccharide LPS
- Lipid A embedded in membrane
- Core polysaccharide, contains KDO
- Antigenic, varies between bacteria
- Net negative charge over cell surface of Gram –ve bacteria
- May allow Gram –ve bacteria to evade immune response
Flagella, Fimbriae and Pili
- Flagella used for motility
- Fimbriae and pili used for cell adhesion
- Sex Pili important for movement of DNA between cells by conjugation
- Tremendous diversity
Bacteria and Disease
- Few bacteria cause disease in humans
- Commensal versus pathogenic bacteria
- To cause disease the organism must:
- Encounter and gain access to host • Evade defence mechanisms
- Colonise and maintain colony
- Carriage and spread of infection
- Asymptomatic vs disease
Inhibition of cell wall synthesis:
Penicillins, cephalosporins, bacitracin, vancomycin
Inhibition of protein synthesis:
Chloramphenicol, erythromycin, tetracyclines, streptomycin
Inhibition of nucleic acid replication and transcription:
Quinolones, rifampin
Inhibition of synthesis of essential metabolites:
Sulfanilamide, trimethoprim
Injury to plasma membrane:
Polymyxin B