Lecture notes
MEDICAL MICROBIOLOGY
- Module
- Biomedical Sciences (BB2716)
- Institution
- Brunel University (BU)
MEDICAL MICROBIOLOGY NOTES
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BB2716 | MEDICAL MICROBIOLOGYDisease Epidemiology
Deals with one population
(eg human)
Risk → case
Identifies causes
Infectious disease epidemiology
2 or more populations
A case is a risk factor
The cause often known
Infectious diseases: caused by an infectious agent
Communicable diseases: directly or indirectly from an infected person
Transmissible diseases: through unnatural routes – from an infected person
The 3 different modes of transmission are:
- Contact transmission, direct and indirect
- Vehicle transmission: airborne, waterborne, foodborne
- Vector transmission: mechanical, biological
Pathogens that can cross the placenta are:
- Protozoan
- Bacteria
- DNA viruses
- RNA viruses
When exposed to infectious agents:
At transmission...
- Index is the first case identified
- Primary is the case that brings the infection into a population
- Secondary is infected by a primary case
- Tertiary is infected by a secondary case
Disease is the result of forces within a dynamic system consisting of the agent, host
and environment.
, - Infectivity (ability to infect) = (number infected / number susceptible) x 100
- Pathogenicity (ability to cause disease) = (number with clinical disease /
number infected) x 100
- Virulence (ability to cause death) = (number of deaths / number with disease)
x 100
Kock’s postulates
Endemic : Transmission occur, but the number of cases remains constant within a
certain area
Epidemic: A rapid increase in the numbers of cases in a certain area
Pandemic: When epidemics spread and occur at several continents
Staphylococci (MRSA) are facultative, non-sporulating, non-motile, gram-positive
cocci. Their cell division occurs in 3 planes however their daughter cells do not fully
separate so form clusters. There are 27 species but the 3 most important ones are:
- Staphylococcus aureus; important human pathogen, common in hight salt
environments and on skin and nose. Transmission is person to person and
asymptomatic carriage sites are the nares (nostrils or nasal passages),
perineum, rectum and the pharynx. Can cause repeated, brief skin
colonization
- Staphylococcus epidermidis; normal skin flora, usually not pathogenic but
can cause disease under special circumstances
- Staphylococcus saprophyticus; UTIs in young females
, Risk factors that would make a person more susceptible to Staphylococcus
infections are:
- Having skin disease, losing a barrier to infection
- Trauma, expose binding sites for bacteria to attach
- Viral respiratory tract infection (influenza), expose binding sites and
decreased clearance
- Foreign body implantation, eg catheter, pacemeaker
- Liver disease
- Neoplasia (benign growth)
- Diabetes
- Renal failure
- Leukocyte and immunoglobulin defects
- Elevated Serum IgE levels
- Narcotics addiction
- Broad spectrum antibiotic therapy, kills bacteria indiscriminately so can kill the
bacteria beneficial to us
Virulence factors: attributes that allow bacteria to cause infection in humans.
Virulence factors associated with Staphylococcus aureus…
Kill phagocytes
Kill leukocytes
Staphylococcus aureus as 2 patterns of disease:
1. Invasion with tissue destruction
Types of tissue destruction: Superficial folliculitis (hair on the body gets
infected with Staphylococcus and you get a 1mm red pustule with areas of
sweat and abrasion), Deep folliculitis (cause infection in deeper layers),
Furuncle (there is a 1cm tender red papule which has a single draining point
commonly found in areas of sweat perforation, starts off as folliculitis), multiple
, furuncles joint together form a Carbuncle (several cm red plaque with several
draining points, firm and extends into subcutaneous fat )
Superficial infection of
Severity of infection depends on the layer of skin infected. skin usually caused by
Staphylococcus aureus
or streptococcus
pyogenes
Often misdiagnosed as
a cold sore
Acute spreading
infection, involving
both skin and
subcutaneous tissues
Prior deep trauma on
the skin would result in
2. Toxin mediated this
Foods most associated with Staphylococcal food poisoning are custard filled bakery
good, canned food, potato salad and ice cream. Symptoms are:
- 2-6 hours incubation period
- Enterotoxin stimulates intestinal peristalsis and CNS
Luckily symptoms only last roughly 8 hours
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