The Bacteria January 12th, 2010

Taxonomy

Bacterial taxonomy

Bacterial taxonomy Genus Species Within species Escherichia coli 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 Extrachromosomal elements 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