1. The Bacteria
January 12th, 2010

2. Taxonomy

3. Bacterial taxonomy

4. Bacterial taxonomy Genus Species Within species Escherichia coli
Strains
Serotypes
Serovars
Biovars

5. Bacterial Cell Structure

6. 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

7. 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

8. 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

9. 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

10. Cell wall structure: Gram positive vs. Gram negative

11. Principle of the Gram Stain
Fix bacteria on a slide
Add crystal violet
Add Gram’s iodine
Wash with alcohol
Add counterstain

12. Other cell structures Flagella On outer cell wall
Contribute to motility

13. Other cell structures Pili Structures that extend from cell
Exchange of genetic material

14. Internal Cell Structures

15. 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

16. Internal structure

17. Nucleic Acids Genetic material: DNA Extrachromosomal elements
Circular chromosomes
One copy of each gene
Extrachromosomal elements
Plasmids
transposons

18. 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

19. 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

20. 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

21. 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

22. 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

23. Bacterial Identification
Traditional: Structural
Morphology
Rods
Cocci
Cell wall stains
Gram
Acid Fast
Acid Fast Stain

24. Bacterial Identification: Gram Stain
Gram negative rod
Gram negative cocci
Gram positive rod
Gram positive cocci

25. 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

26. 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 )

27. 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?

28. Bacterial Growth and Metabolism

29. 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

30. 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)

31. 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

32. 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

33. 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

34. 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)

35. Bacterial Pathogenesis
Bacteria have a variety of ways they cause infection and disease
Bacteria that cause disease: Pathogens
Ability to cause disease: virulence

36. Bacterial Pathogenesis
Virulence factors
Extracellular proteins that aid in infection
Tissue breakdown
Invasiveness
Adherence
Toxins

37. 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

38. 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

39. 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

40. 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