Classification of Infectious Agents. Dr W. D. Colby

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1 Classification of Infectious Agents Dr W. D. Colby

2 Nonliving Infectious Agents PRIONS: abnormally configured self-replicating protein templates VIRUSES: nucleic acid (DNA or RNA) genes packaged in protein coats which subverts host cells to replicate virus particles

3 Living Things: Cells Eukaryotes: Animals, Plants, Fungi, Algae & Protozoa Prokaryotes: Archaea & bacteria NOTE: viruses and prions are not living things

4 Eukarya Defining characteristic is a nuclear membrane Multiple chromosomes DS DNA stands Subcellular organelles (e.g. chloroplasts, mitochondria) Larger ribosomes 80S, membrane bound (ER) Larger cells, µm

5 Prokaryotes Archaea and bacteria No nuclear membrane (nucleoid) Single circular DS DNA chromosome No subcellular organelles Smaller ribosomes (70S), free in cytoplasm Smaller cell size (0.2-3 µm diameter)

6 Bacteria of Medical Importance: Classification Cell wall (none, rigid or flexible) Filamentous vs simple unicellular Obligate intracellular vs free-living Aerobic vs anaerobic Gram-positive vs Gram-negative Shape (rods vs cocci) Glucose fermention vs nonfermenter

7 Cell Walls Osmotic barriers, found in prokaryotes and plants NONE: Mycoplasma, Ureaplasma FLEXIBLE: Spirochaetes RIGID: most bacteria

8 Simple vs Higher Bacteria FILAMENTOUS: Mycobacterium, Actinomyces, Nocardia SIMPLE, UNICELLULAR: most bacteria

9 Obligate Intracellular vs Free Living OBLIGATE INTRACELLULAR: Chlamydia, Coxiella, Ehrlichia, Rickettsia FREE LIVING: most bacteria

10 Aerobic vs Anaerobic AEROBIC: able to tolerate significant concentrations of ambient oxygen ANAEROBIC: poisoned by significant concentrations of ambient oxygen

11 Gram stain Hans Christian Gram, a Danish physician, devised a differential stain in 1884 which classified bacteria into two groups depending on whether they were decolourized by alcohol after staining with gentian violet and iodine. Gram positive retained stain

12 Gram stain, cont. Gram-positive: simple cell wall with thick layer of peptidoglycan outside of the cell membrane. Contains teichoic acid. Gram-negative: complex structure with inner and outer membranes and thin peptidoglycan layer

13 Shapes of Bacteria Cocci (chains vs clusters) Rods (bacilli) Short rods (coccobacilli) Curved Spiral Branched (filamentous)

14 N. gonorrhoeae: Urethral Swab Gram Stain

15 Haemophilus ducreyi

16 Spirillum minus

17 Glucose Fermentation FERMENTATION: the anaerobic metabolism of sugars Used to classify Gram-negative rods NONFERMENTERS: more difficult as a group to identify in the laboratory

18 Structure-Function Relationships in Bacteria Dr W.D. Colby

19 Size Size limited by lack of guts Eukaryotic cell volume 8000 X that of prokaryotic cells Prokaryotic cell surface to volume ratio is 20 X that of eukaryotic cells High surface to volume ratio greater metabolic activity

20 Capsules & Slimes Hydrophobic extracellular polysaccharides or protein polymers Capsules have organized structure, resist phagocytosis Slime layers: large amounts of amorphous polymers. AKA biofilm, glycocalyx. Provides shelter.

21 Cell Walls Osmotic barrier: internal pressure 3-5 ATM (Gram neg), 5-20 ATM (Gram pos) Gram positive cell wall is a matrix of peptidoglycan and some associated polymers, esp. teichoic acid Gram negative cell wall is a thin layer of peptidoglycan anchored to the outer membrane by murein lipoprotein

22 The Gram negative cell envelope

23 Cell Walls of Mycobacteria Acid fast Similar to Gram positives, but contains more lipid (60%), including a unique class of long chain ß-hydroxylated fatty acids called mycolic acids

24 Peptidoglycan Alternating subunits of monosaccharides N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), cross linked at NAM by peptide chains

25 Cytoplamic Membrane very similar to that of eukayotic cells: phospholipid bilayer metabolically active invaginations called mesosomes

26 Pili AKA fimbriae mediate adhesion to other cells specialized sex pili mediate conjugation

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28 Flagella Large, whiplike structures responsible for cell motility Hollow tube and basal body, composed of flagellin Require cell energy to move Only motile bacteria have them Carry certain antigens polar vs peritrichous

29 Spores Repackaged copies of bacterial DNA in a highly impermeable, 6- layered envelope. Does not divide. 2 cell membranes with 2 layers of peptidoglycan in between, keratin coat, lipoprotein exosporium Germinates to vegetative bacteria after activation Bacillus and Clostridium only