Week 5 Magic Bullets
5.1 PATHOGENESIS OF BACTERIAL INFECTIONS
1. Identify the key differences between prokaryotic and eukaryotic organisms
Eukaryotes
Algae, protozoa, fungi, plants and animals
>5μm
Diploid genome, nuclear membrane
Ribosome 80S (60S + 40S)
Cytoplasmic organelles: mitochondria, golgi body,
ER
Cytoplasmic membrane: sterols
Cell wall absent/chitin
Sexual/asexual reproduction
Respiration via mitochondria
Prokaryotes
Bacteria
<5μm
Haploid, circular double-stranded DNA
Ribosome 70S (50S + 30S)
NA
Cytoplasmic membrane: no sterols
Cell wall: peptidoglycan, proteins & lipids (complex)
Asexual reproduction (binary fission)
Respiration at cytoplasmic membrane
2. List different categories of infectious agents and outline their differences.
Viruses: Obligate intracellular parasites that lack compliment enzymes necessary for their replication and
therefore rely on their host cells’ metabolic machinery for replication. Viruses consist of a nucleic acid genome
surrounded by a protein coat (capsid) that is sometimes encased in a lipid membrane. Viruses may contain a
nucleic acid genome of DNA or RNA, but not both. For human purposes they are divided into two main groups,
the RNA viruses and the DNA viruses.
Bacteria: Bacteria are prokaryotes, meaning they have a cell membrane but lack membrane-bound nuclei and
other membrane-enclosed organelles. Most bacteria are bound by a peptidoglycan cell wall (a polymer of long
sugar chains linked by peptide bridges). There are two forms of cell wall structures, a thick wall that retains
crystal-violet stain (gram-positive bacteria) or a think wall sandwiched between two phospholipid bilayer
membranes (gram-negative bacteria). Bacteria are classified by their gram stain (positive or negative), shape
(spherical ones and cocci; rod-shaped ones and bacilli) and need for oxygen (aerobic or anaerobic). Many
bacteria have flagella that enable movement and some possess pili, another kind of surface projection that can
attach bacteria to hots cells. Most bacteria synthesize their own DNA, RNA and proteins, but they depend on
their host for favourable growth conditions.
Other infectious agents include prions, fungi, protozoa and helminths. The table below gives examples.
Taxonomic
Size
Site of propagation
Examples
Disease
Prions
30-50kD
Intracellular
Prion protein
Creutzfeld-Jacob disease
Viruses
Bacteria
20-300nm
Fungi
2-200 μm
Protozoa
1-50μm
Helminths
3mm-10m
Obligate intracellular
Obligate intracellular
Extracellular
Facultative intracellular
Extracellular
Facultative intracellular
Extracellular
Facultative intracellular
Obligate intracellular
Extracellular
Intracellular
Poliovirus
Chlamydia trachomatis
Streptococcus pneumonia
Mycobacterium tuberculosis
Candida albicans
Histoplasma capsulatum
Trypanosoma gambiense
Trypansoma cruzi
Leishmania donovani
Wucherria bancrofti
Trichinella spiralis
Poliomyelitis
Trachoma, urethritis
Pneumonia
Tuberculosis
Thrush
Histoplasmosis
Sleeping sickness
Chagas disease
Kala-azar
Filariasis
Trichinosis
0.2-15 μm
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3. Describe the structure and characteristics of bacteria.
Nuceloid: site of double stranded circular DNA. In the cell it is condensed and looped into a coiled state.
Essentially haploid organisms with only one allele of each gene per cell. Nucleoid lies within the cytoplasm.
Ribosomes: Slightly smaller than those of eukaryote cells. Ribosomes are microscopic ‘factories’ found in all
cells including bacteria. They are responsible for protein synthesis. Bacterial ribosomes are never bound to
other organelles as they sometimes are (bound to the ER) in eukaryotes, but are freestanding structures
distributed throughout the cytoplasm. The differences between bacterial ribosomes and eukaryotic ribosomes
mean that some antibiotics will inhibit the functioning of bacterial ribosomes, but not a eukaryote's, thus killing
bacteria but not the eukaryotic organism they are effecting.
Cell wall: Supports the weak cytoplasmic membrane against the high osmotic pressures (survival). The
chemical composition differs considerably between the different bacteria species but the main component is
peptidoglycan. The thickness of this peptidoglycan layer is what is used to distinguish between the thick gram
positive (20-80nm) and the thinner gram negative cell walls (5-10nm).
Cell membrane: is a phospholipid bilayer allowing selective permeability. A key feature differentiating
prokaryotic cytoplasmic membranes to eukaryotic cell membranes is their multifunctional nature. Protein
secretion, packaging and processing, electron transport and oxidative phosphorylation, all must be performed
by the cytoplasmic membranes in prokaryotes (unlike eukaryotic cells). The membrane is therefore extremely
protein rich allowing very little space for phospholipids.
Capsule: some species of bacteria have a third protective covering. This capsule is made up of polysaccharides
(complex carbohydrates). The capsule keeps the bacteria from drying out and protects from phagocytosis. The
capsule is a major virulence factor in the major disease-causing bacteria, such as E. coli and s. pneumoniae.
Non-encapsulated mutants of these organisms are avirulent, i.e. they don't cause disease.
Slime layers: Like the capsule, the slime layer protects the bacteria from environmental damages such as
antibiotics. The slime layer also allows bacteria adherence.
Periplasmic space: Site of nutrient processing. This is a very active region, between plasma and cell wall and is
only present in gram-negative bacteria.
Inclusion bodies: Are nuclear or cytoplasmic aggregates of sustainable substances, usually proteins.
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Vacuole: gas vacuoles allow for buoyancy in aquatic environments.
Flagella: Motile bacteria possess filamentous appendages known as flagella, allowing locomotion. They are 2-3
times the length of the bacterial cell. The flagella rotates and pulls bacteria forward.
Pili (fimbriae): Filamentous appendages much more numerous than flagella and much shorter in length. They
are important in securing adhesion between the bacteria and host cell, although they are not the only way
bacteria adhere to host cells. Involved in bacterial mating and DNA transfer.
Endospores: survival in hostile environment; very resilient, can exist in dormant state for long periods and
germinate later.
CHARACTERISTICS
a) Metabolism
Growth phases: lag phase, exponential growth phase, stationary phase (nutrient deficit), death phase.
Reproduction (binary Fission): uncoiling of DNA, replication of DNA, cell elongation, septum formation and
separation. Results in two daughter cells, identical to each other and parent cell.
b) Temperature and Growth
Hyperthermophiles: 65-105°
Thermophiles: 40-80°
Mesophiles: 15-45° (grow at body temperature, almost all human pathogens)
Psychrotrophs: 2-35° (food microorganisms that can survive in fridge- food spoilage)
Psychrophiles: 5-18°
c) Sources of carbon, energy and hydrogen/electrons
Carbon sources

Autotrophs: CO2 sole or principle C source.

Heterotrophs: reduced, preformed organics (sugars, amino acids).
Energy Sources

Phototrophs: light

Chemotrophs: oxidation of organic/inorganic compounds
Hydrogen or Electron Sources

Lithotrophs: reduced inorganic molecules

Organotrophs: organic molecules
d) Antigens:
Initial division on basis of haemolysis on blood agar.
 α-haemolysis: 1-3mm greenish zone of incomplete haemolysis.
 β-haemolysis: zone of clearing/complete lysis without a market colour change.
 γ: no haemolysis.
e) Genetic characteristics:
 Ribosomal RNA: 16S (most eukaryote size is 18S) found in all bacteria.
 Genes: some common to all bacteria, others pathogen specific.
 Molecular diagnostics permits detection and identification without culture (quicker, can be used even
if pathogen can’t be grown, can use very small amounts).
 Information encoded in: chromosomes, plasmids and transpoons.
-Constitutive genes: expressed all the time
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
-Inducible genes: expressed when needed.
Regulation of gene expression ensures adaptation to environment and avoidance of
overproduction/waste.
Genetic diversity achieved by: mutation, recombination, gene transfer/exchange (important in virulence
(degree of pathogenicity) and antimicrobial resistance).
Mutations:
Spontaneous or external physical chemical factors. Involve point mutations (single
nucleotide change), deletions or insertions. Consequences may be lethal, phenotypic
change, no change, selective advantage, repair, spontaneous loss over Subsequent
replication cycles.
Recombination:
Process by which new genetic material is inserted into the genome via conjugation (direct
cell to cell contact mediated by fertility plasmids), transformation (naked DNA from
environment) or transduction (from a bacteriophage).
Plasmids:
Small circular DNA molecules that are not part of the bacterium’s chromosome. They have
their own replication origins.
Transpoons:
Segments of DNA that can move about chromosome within single organism or between
different organisms. Differ from plasmids in that they are unable to reproduce
independently.
4. Identify, and discuss the functions of, the structural components of bacteria which are involved in
the pathogenesis of infection.
Adhesions: specialized structures on the cell surface of bacterium that bind to complementary receptor sites
on host cell surfaces. Allow adherence with high specificity for certain tissues. Two types of adhesion: cell
recognition by the bacterial fimbriae and non-fimbrial adhesions regulated primarily by a large range of surface
proteins.
Capsule: well organised polysaccharide layer outside cell wall. Many bacterial pathogens require a capsule to
avoid phagocytosis and production of an extracellular capsule is the most common mechanism by which this is
achieved. Resist phagocytosis by reducing interactions with complement and specific antibodies.
Glycocalyx: network of polysaccharides extending from bacterial surface, aids in attachment to tissue.
S-layer: structured protein/glycoprotein layer. Protects against ion and pH fluctuations, osmotic stress and
enzymes. Helps maintain shape and rigidity and may promote cell adhesion to tissues.
Once inside cell, bacteria are lysed and the bacteria are released into the cell cytoplasm, multiplying rapidly
with inhibition of the host cell protein synthesis.
5. Describe the various mechanisms by which bacteria and viruses cause disease.
Whether it is viral or bacterial, for a disease process to be harmful to humans some form of interaction
between the infecting agent and the cell must occur.
Week 5 Magic Bullets
BACTERIA
1. Bacteria maintain a reservoir (before and
after infection).
2. Transport of pathogen to host: direct
contact (coughing, sneezing, body contact);
indirect contact (via soil, water and food).
3. Attachment and colonisation by pathogenestablishment of site of reproduction.
Depends on ability to compete with host
for nutrition.
4. Invasion of pathogen: entry into host cells
and tissue
-Production of lytic substances,
alter host tissue by attacking
ground substance, basement
membranes, degrading
carbohydrate-protein complexes
between cells or on cell surface;
disrupting cell surface.
-Passive entry: breaks, lesions in
mucous membranes, wounds,
burns.
-Once under mucous membrane,
pathogen can penetrate deeper
tissues and spread.
5. Growth and multiplication: must
find appropriate environment (pH,
temp, nutrients) will depend on
body site.
6. Leaving the host: most employ
passive mechanisms of escape
(faeces, urine, saliva).
VIRUSES
1. Entry: via body surfaces (skin, respiratory,
GIS, urogenital, conjunctiva)
Other (needle stick, blood transfusion,
insect vector).
2. Replication: at site of entry or spread then
replicate.
3. Viral spread: commonly bloodstream and
lymphatics. Sometimes via nerves.
4. Tropisms: specificities for cell, tissue or
organ.
5. Cell injury and clinical illness:
-Destruction of virus: infected cells in
target tissue and alterations in host
physiology are responsible for disease.
-Lytic infections: virus multiplies and kills
host cells immediately; new virions
released.
-Persistent infections; virus lives in host
cells and releases virions over a long period
with little damage to host cell.
-Latent infections: virus resides in cell but
produces no virions; activated later and
lytic infection occurs.
-Virus transforms cell into a cancer cell (eg
HPV).
6. Host immune response.
7. Recovery- host will either succumb to virus
or recover.
8. Virus shedding- shedding of virus back into
environment; stage where host is
infectious and can spread the virus.