The Bacteria

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The Bacteria Powered By Docstoc
					The Bacteria

January 12th, 2010
Taxonomy
Bacterial taxonomy
           Bacterial taxonomy
• Genus
  – Escherichia
• Species
  – coli
• Within species
  –   Strains
  –   Serotypes
  –   Serovars
  –   Biovars
Bacterial Cell Structure
      Bacterial Cell Structure
• Single-celled organism
• Typical size ~1×3 microns
• Possess a cell wall
• Main component: peptidoglycan
  – Glycan (sugar) chains with amino acid
    crosslinks
  – Forms a thick, stable structure
      Bacterial Cell Structure
• Two main types of cell wall structure
• Differentiated using the Gram Stain
  – Gram positive
  – Gram negative
• Differences
  – Location of cell membrane
  – Thickness of peptidoglycan
  – Important for the way antibiotics work
      Bacterial Cell Structure
• Gram positive bacteria
• Thick outer cell wall consisting of
  peptidoglycan
• Cell membrane (lipid) on the inside
• Teichoic acids
• Negatively charged cell surface
      Bacterial Cell Structure
• Gram negative bacteria
• Thin layer of peptidoglycan
• Innermost layer: cytoplasmic membrane
• Next: periplasm
• Peptidoglycan
• Outer membrane
• Lipopolysaccharide (LPS)
  – Interacts with human immune system
Cell wall structure: Gram positive
       vs. Gram negative
   Principle of the Gram Stain
• Fix bacteria on a slide
• Add crystal violet
• Add Gram’s iodine
• Wash with alcohol
• Add counterstain
        Other cell structures
• Flagella
  – On outer cell wall
  – Contribute to motility
         Other cell structures
• Pili
  – Structures that extend from cell
  – Exchange of genetic material
Internal Cell Structures
         Internal structure
• Lack the internal organization of
  eukaryotic (animal, plant) cells
• No membrane bound organelles inside the
  cell
• Lack mitochondria
• Ribosomes are free within the cytoplasm
Internal structure
             Nucleic Acids
• Genetic material: DNA
  – Circular chromosomes
  – One copy of each gene
• Extrachromosomal elements
  – Plasmids
  – transposons
     Sharing genetic material
• Bacteria are promiscuous- they share
  genetic material all the time
• Between cells
• Across species
• They possess mobile genetic elements that
  move from cell to cell
                Plasmids
• Pieces of nucleic acid separate from the
  bacterial chromosome
• Double stranded DNA
• Circular or linear
• Free floating in the cytoplasm
• Replicate independently of host
  chromosome
                Plasmids
• Major pathway by which bacteria share
  genetic material
• Passed down to daughter cells during
  replication
• Also transferred from one bacterial cell to
  another
• This process is called conjugation
                 Plasmids
• Can carry a variety of different genes
• Ones of greatest public health significance:
  Antibiotic resistance
  – Resistance traits can be transferred from one
    bacterium to another
• Genes encoding for toxins
  – Important factors in bacterial virulence
             Transposons
• Pieces of DNA separate from the main
  bacterial chromosome
• Can carry multiple genes
• Have an enzyme, transposase, that allows
  them to insert themselves into the
  bacterial chromosome
       Bacterial Identification
• Traditional: Structural
• Morphology
  – Rods
  – Cocci
• Cell wall stains
  – Gram
  – Acid Fast


                        Acid Fast Stain
Bacterial Identification: Gram Stain




 Gram negative rod   Gram negative cocci




Gram positive rod     Gram positive cocci
      Bacterial Identification
• Traditional: Biochemical
  – Utilization of sugars
  – Utilization of amino acids
  – Utilization of other carbon sources
• Combination of all of these discriminates
  between genus and species
                                          Citrate test (citrate as a sole
                                          carbon source)
Methyl Red Test (acid production by
glucose utilization)

                                                            Indole test
                                                            (breakdown of
                                                            amino acid
                                                            tryptophan )


Triple sugar iron agar (acid production
by utilization of 3 sugars )
      Bacterial Identification
• New: nucleic acid methods
• PCR
• Sequencing of whole genomes
• In many cases, this is changing our
  understanding of bacterial taxonomy
• How many species in a genus?
Bacterial Growth and
    Metabolism
           Bacterial growth
• Bacteria are capable of using a wide
  variety of energy sources
• Utilization of a wide variety of carbon
  sources
• Some are photosynthetic
• Observing the effects of different types of
  metabolism is key to identification
   Bacteria in the environment
• Some types of bacteria only survive in
  association with humans
  – Treponema pallidum (syphillis)
• Many others can live free in the
  environment
• Use energy sources found in the
  environment
• Seek out these sources (chemotaxis)
   Bacteria in the environment
• Bacteria are hardy- they can live in a wide
  range of environmental conditions
• Wide temperature range (psychrophilic to
  thermophilic)
• Resistance to desiccation
• Spore forming
    Bacterial growth: Oxygen
• Obligate aerobes
  – Growth only in the presence of oxygen
  – Micrococcus species
• Microaerophiles
  – Grow best in reduced oxygen concentrations
    (lower than atmosphere)
  – Campylobacter
    Bacterial growth: Oxygen
• Facultative anaerobes
  – Growth with or without oxygen
  – Escherichia coli
• Aerotolerant anaerobes
  – Grow best in the absence of oxygen, but
    tolerate some present
  – Campylobacter
    Bacterial growth: Oxygen
• Obligate anaerobes
  – Growth only in the absence of oxygen
  – Oxygen is toxic
  – Clostridium spp (botulism, tetanus)
• Anaerobic metabolism
  – Use fermentation pathways (producing acids
    and alcohols)
  – Uses alternative electron acceptors (nitrate,
    sulfur compounds)
       Bacterial Pathogenesis
• Bacteria have a variety of ways they cause
  infection and disease
• Bacteria that cause disease: Pathogens
• Ability to cause disease: virulence
       Bacterial Pathogenesis
• Virulence factors
  – Extracellular proteins that aid in infection
  – Tissue breakdown
• Invasiveness
• Adherence
• Toxins
                 Toxins
• Usually proteins
• May be carried on plasmids
• Synthesized and released by the cell
• Often highly immunogenic (human
  immune system reacts strongly)
• This is useful for vaccine development
  – Anthrax
  – Diptheria
  – Tetanus
              Key Concepts
• Bacteria are single-celled organisms
• Bacteria are divided based on the
  structure of their cell wall
• Cell wall structure is important for
  – Identification
  – Action of antibiotics
             Key Concepts
• Bacteria have less organized internal
  structures than animal or plant cells
• Circular chromosomes
• Extrachromosomal genetic elements
• Bacteria can trade genetic material
  between cells, transferring traits like
  toxins and antibiotic resistance
            Key Concepts
• Bacteria can survive a wide range of
  environmental conditions
• Temperature, oxygen levels
• Use a wide variety of energy sources
  (carbons, amino acids)
• We can harness this to identify them

				
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