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Summary - Medical Microbiology (5BBBMB28)
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Medical microbiologyExam: Choice of 2 essays out of 5
Two major divisions of cellular organisms: prokaryotes (no nucleus) and eukaryotes (nucleus)
Bacteria are prokaryotes; all other organisms are eukaryotes
Microbes can be seen by microscopy – electron using a beam of electrons (viruses) or light
microscope
Robert Hook – 1665 – Englishman, used a primitive compound (two magnifying lenses)
microscope, reported that life’s smallest units were little boxes – Cells, his work started the
process of the development of the Cell theory of life
Edward Jenner is credited with first vaccine – in epidemics of smallpox during the late 1700’s
he observed that milk maids didn’t get the disease, cattle had a similardisease – cowpox,
milk maids had cow pox lesions, but not small pox, he purposefully took scrapings from
cowpox blister and scraped a 8 year old volunteer. With the material – child got mild illness
but not small pox.
Robert Koch - Developed Koch’s postulates – important technique for determining the actual
microbial cause agent of a disease
Paul Ehrlich, German doctor, wanted to find a “magic bullet” an agent that would kill the
disease agent without hurting the patient
Alexander Fleming - Scottish physician and bacteriologist - 1928 Observed mold growing on a
bacteria culture, there was a ring of clearing around the mold where the bacteria didn’t
grow, the mold was later found to be a Penicillium species and the naturally secreted
chemical was called penicillin, an antibiotic
Bacterial pathogenesis
Gram positive bacteria have one thicker peptidoglycan membrane – lots of peptidoglycan – Gram
stain stains purple
Gram-negative bacteria have inner (PG) and outer membranes (LPS) – less peptidoglycan – Gram
stain stains pink
5 different groups of bacteria according to their shape
Spherical (cocci)>Streptococcus
Rod (bacilli)>Salmonella, Shigella
Spiral (spirilla)>H. Pylori
Comma (vibrios)>V. Cholera
Corkscrew (spirochaetes)>Treponema pallidum
They can exist as single cells, in pairs, chains or clusters.
Koch’s postulates:
,A microorganism is responsible for a disease if,
(1) The microorganism must be found in all individuals with the disease (but not healthy
individuals)
(2) The microorganism must be grown in culture from diseased individuals
(3) Inoculation of a healthy, susceptible individual with the cultured microorganism must
recapitulate the disease
(4) The microorganism must be re-isolated from the inoculated, diseased individual and
matched to the original microorganism.
Limits: healthy, asymptomatic carriers
Identify which bacteria responsible for someone’s disease by microscopy, symptoms, antibody
testing, isolate and grow , take tissue, secretion sample , gram staining gram positive(purple) or
negative (pink), many bacterial shapes behaviour of bacteria , genetics PCR sequencing
Strict pathogens need a host to replicate
- Always associated with disease
Opportunistic pathogens can replicate in the environment
- Did not evolve to be pathogens but can harm people with damaged dampened immune
system
- Can be commensal or present in the environment and cause infections in
immunocompromised individuals.
Pathogenesis – to transmit to a new host:
- Successful pathogens are good at evading host defences
- Damage host to make it better to replicate – damage leads to disease
- Virulence of an organism is measured by
o ID50 = 50% infectious dose
o LD50 = 50% lethal dose
- Lower dose of pathogen the more pathogenic the strain is (lower LD50) – fewer bacteria
needed to infect
- Different strains of the same species of bacteria can be more or less virulent (=have different
LD50)
- Shigella is known to have a lower LD50 than Salmonella and pathogenic E. coli.
Routes of entry:
- Oral –food borne agents – Salmonella, Shigella
- Respiratory – Diphtheria, M. Tb
- Cutaneous – Rickettsia (typhus transmitted by lice)
- Transcutaneous – Clostridium tetani, S. pyogenes
- Sexual – N. Gonorrhea, Chlamydia
Transmission can be direct or indirect.
,Bacteria is a pathogen due to virulence factors !!
- Toxins
- Adhesins
- Capsule
- Secretion systems
Virulence factors are often encoded on Pathogenicity islands (PAI):
- Clusters of genes acquired through horizontal gene transfer – can be on the chromosome or
on plasmids
- Present in pathogenic strains but absent in non-pathogenic strains of the same species
- PAIs carry genes encoding one or more virulence factors such as toxins, adhesins, invasins
and secretion systems and their effectors.
Virulence factors include…
Adherence factors
- Adhesion of bacteria to host cells initiates colonization
- Mediated by Adhesins including fimbriae
- Will determine the site of colonization
- For example Yersinia species infect different sites and cause very different diseases (plague or
gastroenteritis).
Toxins
- Alter the normal metabolism of host cells
- Responsible for major symptoms of bacterial infection
- Different types :
o Exotoxins = secreted proteins
, o Endotoxin=on the surface of bacteria
o Type III cytotoxins=effector proteins injected directly into host cells
- Toxins damage the host, kill cells, disrupt organelle functions
- Toxins are main determinant of disease, crucial for pathogenesis
- Exotoxin –
o 2 component complex
o B subunit binds to cell receptor and helps with complex internalization.
o A subunit exerts toxic effects on cell
o AB toxin mediated binding to host receptor, toxin is endocytosed by body cell to
induce cell death or modify receptors on surface of host cell to lead to secretion of
ions, water (cholera)
o Neurotoxins – such as botulinum toxin causes flaccid paralysis and tetanus toxin
causes muscle spasm/contraction
- Endotoxin –
o Lipopolysaccharide =component of Gram-ve bacteria cell wall
o Activates PRR (TLR4)> pro-inflammatory cytokines – TNF-a
o Gram negative infection leads to uncontrolled upregulation of inflammation, too
many cytokines
o At high concentration can trigger septic shock
Secretion systems:
- Enable the transport of toxins or effector proteins
- Type T3SS, T4SS, T6SS secretion systems inject effectors directly into human cells
- Mostly used by Gram-ve bacteria meaning that they need to cross both bacterial membranes
and the host cell membrane
- Effectors are exported or translocated into host cells and help bacteria:
o invade or block phagocytosis - take over cellular processes (trafficking etc) - inhibit
host responses and defenses
- Some secretion systems used to kill other bacteria – bacteria-bacteria warfare
o Competition to colonise the same tissue – inject toxin into the other bacteria to kill it
- Also used in pathogenesis as can target microbiome in gut
- Secretion systems don’t exist in gram positive bacteria (thick peptidoglycan wall makes it
impossible to exist) so ONLY in gram negative bacteria
Type III secretion system or injectisome:
- Conserved among many virulent Gram-ve bacteria
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