Prokaryotes

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PRIONS

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- Consist of protein only

- Small self replicating proteins without any nucleic acid

- Derived from 'normal' proteins associated with membranes of the central nervous system (CNS)

- Have different (abnormal) conformation from normal but the same amino acid composition

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DISEASES ASSOCIATED WITH PRIONS

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Associated with Creutzfeld Jakob disease (CJD) and 'Kuru' in humans, scrapie in sheep and mad cow disease (bovine spongiform encephalitis - BSE)

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PRIONS WHEN INGESTED

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- Accumulate in CNS membranes

- By an autocatalytic process they convert pre-existing normal forms of protein to their abnormal form (meaning the reproduce without making new copies of themselves)

- The abnormal forms cannot be destroyed by the body and build up leads to destruction of CNS tissue in brain

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VIRUSES

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- Not cells

- Static structures (have no metabolic activity of their own)

- Rely on host biosynthetic machinery for their protein synthesis

- All classes of organisms can be infected by viruses (even bacteria - bacteriophages)

- Enormous diversity (especially among bacteriophages)

- Not susceptible to antibiotics

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CHLAMYDIAE: LIFE CYCLE

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CHLAMYDIAE: KEY SPECIES IMPORTANT IN DISEASE

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- Chlamydia trachomatis (ocular (eye) infections, genito-urinary tract infections)

- Chlamydia psittaci (respiratory tract infections in humans, gut and systematic infections in birds)

- Chlamydia pneumonia (respiratory tract infections)

- Chlamydia abortus (leads to abortion in sheep, rare cases can cause abortion in women when exposed to sheep)

- Chlamydia caviae (infection in guinea pigs, model to study human disease)

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RICKETTSIAE

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- Gram-negative bacteria

- Small cells (0.7-2 micrometres)

- Pleomorphic (cell morphology vary from cocci to filaments)

- Slow growing obligate intracellular parasites

- Unlike chlamydiae rickettsia can survive outside cell but cannot replicate (lack the ability to generate many important metabolites and have a requirement for coenzyme A, NAD and ATP)

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RICKETTSIAE: KEY SPECIES INVOLVED IN DISEASE

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- Rickettsia rickettsii (causes spotted fever)

- Reckettsia typhi, R. prowazekii, R. Tsutsugamushi (typhus)

- Coxiella burnetii (Q-fever)

- Ehrlichia chafeensis (ehrlichiosis)

- Rickettsia felis (emerging human pathogen, typhus like disease)

- All expect coxiella burnetti (inhalation) are transmitted to humans via arthropod bite (fleas, ticks, lice and mites, no direct person to person spread)

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MYCOPLASMAS

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- Smallest prokaryotic cells capable of growing on cell-free media (free living)

- Fastidious in their nutritional requirements (need complex growth media, limited biosynthetic capability)

- Small genome size (0.1 - 1 micrometre a quarter of the size of E.coli genome)

- Generally parasitic organisms that inhabit plant and animal hosts (but unlike chlamydiae and rickettsiae can replicate independently of host cells)

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MYCOPLASMA STRUCTURE

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- Lack peptidoglycan in their cell walls (wall has no rigidity leading to pleomorphic cells, do not stain as gram-positive but are closely related to gram-positive bacteria)

- Some species contain cholesterol in their cell walls (usually found in mammalian cell membranes and is absent in other bacteria)

- Outer layer is triple-layered structure comprising of proteins and lipids

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THREE GENERA OF MYCOPLASMAS

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- Mycoplasma

- Ureaplasma

- Acholeplasma

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MYCOPLASMAS IN DISEASE

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- Mycoplasma pneumoniae (important cause of atypical pneumonia, can also cause genital infections - non-specific urethritis and other joint and inflammatory conditions)

- Other mycoplasmas are important pathogens of animals and birds

- Can be treated with some antibiotics (tetracycline or erythromycin) but not antibiotics acting on cell walls (beta-lactams)

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NON-ENVELOPED ANIMAL VIRUSES

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- Adenovirus

- Rhinovirus - common cold

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ENVELOPED ANIMAL VIRUSES

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- Herpes Simplex virus

- Varicella-Zoster virus

- Influenza A virus

- Hepatitis B

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BACTERIOPHAGE

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- T4 (dsDNA virus that infects Escherichia Coli - E.coli)

- Lambda

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VIRAL REPLICATION CYCLE

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- Virus' deliver their nucleic acid genome to host cells and 'hijack' the biochemical machinery (causing it to replicate the virus)

- Ultimately the host cell becomes filled with virus particles - virions

- Virions burst out the cell and disperse to infect new cells

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VIRAL REPLICATION CYCLE: ADENOVIRUS

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- Integrins are transmembrane receptors with alpha and beta subunits

- Adenovirus is a human infecting virus

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BACTERIAL CELL WALL

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- Structure of cell wall distinguishes two different categories of bacteria: gram-positive and gram-negative (distinguished by a staining method based on ability of cell wall to retain or lose the dye stain crystal violet - gram staining)

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GRAM STAINING METHOD

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- Hans Christian Gram (1884)

- Application of primary stain (crystal violet) to a heat-fixed smear of bacterial culture

- The addition of iodine which binds to crystal violet forming a larger complex helping it 'trap' into the cell

- Rapid decolourisation (alcohol or acetone) and counterstaining with safranin

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GRAM POSITIVE BACTERIA

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- Thick mesh-like cell wall, mostly peptidoglycan - PNG

- Stained purple by crystal violet

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GRAM NEGATIVE BACTERIA

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- Have a thinner PNG (peptidoglycan) layer

- Does not retain crystal violet

- Can be counter-stained pink by safranin

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CHLAMYDIAE

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- Small cells which weakly stain as gram-negative

- Contain lipopolysaccharide (LPS) in their cell walls

- Range between 0.3-1 micrometre depending on stage in life cycle

- Obligate intracellular parasite (a parasitic microorganism that cannot reproduce without entering a host cell) with poor metabolic capabilities

- Genome 1/4 size of E.coli

- Cell wall thought not to contain peptidoglycan although genes for peptidoglycan synthesis present in genome

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CHLAMYDIAE: ELEMENTARY BODY (EBs)

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- Infectious form

- Metabolically inactive

- Small (0.3 micrometre diameter)

- Resistant - capable of withstanding environmental conditions outside of host cell

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CHLAMIDYDIAE: RETICULATE BODY (RBs)

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- Replicative form

- Larger than elementary body (0.8-1.2 micrometre)

- Metabolically active

- Capable of replication (by binary fission)

- Fragile - cannot survive in external environment