Lecture 30 - How and why infections happen

Question 1

where do infections come from

Answer 1

endogenous or communicable

Question 2

Normal microbial flora

Answer 2

vary according to site
Upper airways – saliva 10^8 -10^9 bacteria per ml
GI tract – faeces 10^11 -10^12 bacteria per gram

Question 3

Endogenous infection

Answer 3

Normal flora:
Skin
Gut
Upper airways
Genital tract
Normal gut flora gets into wrong place
Urinary tract infection e.g. cystitis

Question 4

Cystisis

Answer 4

Infection of lower urinary tract
Lower abdominal pain
Urgency 
Dysuria
Frequency

Question 5

What bacteria is the most common cause of cystisis

Answer 5

gut flora
eg. eschericia coli
e coli - gram negative

Question 6

where do communicable infections come from

Answer 6

person to person
non - human - animals/birds/insects
-environment

Question 7

How do we get infections endogenously

Answer 7

migration
perforation
blood

Question 8

How do we get infections exogenously

Answer 8

contact - direct and indirect
injuries - trauma and bites
airborne 
oral - food/water
blood-borne
sex
mother- baby

Question 9

Endogenous infection - what is migration

Answer 9

Bowel flora
- E.coli contaminates perineum
-gains access to urethra
causes local infection 
spreads to bladder and beyong
UTI

Question 10

Endogenous infection - what is perforation

Answer 10

Diseases of bowel especially colon e.g.
Cancer
Diverticular disease
perforation leads to contamination of abdominal cavity by faecal flora
-severe life-threatening infection results - faecal peritonitis

Question 11

Endogenous infection - what is blood spread

Answer 11

dental work - may allow mouth flora to enter blood stream
circulation of organisms allows them to reach distant sites eg. heart valve
-invasion can occur especially if valve tissue is abnormal - eg defect
causes inflammation and structural damage

Question 12

Exogenous infection: direct contact

Answer 12

Impetigo
superficial skin infection due to staphylococci and/or streptococci
spreads rapidly from person to person

Question 13

Exogenous infection: indirect contact

Answer 13

Micro-organisms can be transmitted indirectly via hands, equipment, furniture etc
Major route of health-care associated infections

• methicillin-resistant staphylococcus aureus (MRSA)
• Clostridium difficile
• Norovirus gastroenteritis

Question 14

Injuries – dirty wound

Answer 14

Tetanus (Lockjaw)
Clostridium tetani
Bacterium present in soil
Contaminates wounds
Releases toxin causing muscle spasm
Prevented by vaccination

Question 15

Bites

Answer 15

malaria

• parasite infection
• life-cycle
• mosquitoes and humans
• present in large areas of tropics
• causes severe febrile illness affecting all systems

Question 16

Airborne infection examples

Answer 16

influenza virus

• coughing and sneezing
• droplets formed containing infectious viruses
• inhaled by others

Question 17

oral route - food bourne

Answer 17

Food prepared with poor hygiene
E.g. not washing hands after going to toilet
Contaminate food with harmful bacteria
Result: food poisoning!
Vomiting, diarrhoea

Question 18

Blood -borne infection

Answer 18

hep B
-liver infection 
-some viruses spill into blood
-transmission by blood exposure 
-transfusion
sharing of needles
-body piercing

Question 19

Sexual transmission

Answer 19

Chlamydia - risks increased by
unprotected sex, new partners
multiple partners
partners with high risk

Question 20

Mother to baby transmission ‘vertical’

Answer 20

During pregnancy e.g. rubella
At time of birth e.g. herpes
Breast milk e.g. HIV

Question 21

infections transmitted in more than one way?

Answer 21

HIV
Blood borne e.g injection drug use
Vertical
perinatal
breast feeding
Sexual

Question 22

Which infections require more than one step?

Answer 22

eg staph aureus - soft tissue infection
step 1: colonisation of skin - joins skin flora
step 2: penetration of skin - spreads and damages

Question 23

Factors affecting infection

Answer 23

infectious dose

• minimum number of organisms required to produce disease
• direct infection of cells/tissues
• virulence factors and toxins
• invasion and disease
• resistance to antibiotics

Question 24

Ebola infection

Answer 24

direct infection and damage/destruction of cells

Question 25

Virulence factors

Answer 25

allow invasion of host tissues eg streptolysin O
-Lyses cells - cytolysin
produces by certain streptococci - group A

Question 26

`Toxins

Answer 26

Cholera 
severe watery diarrhoea - rice water
-bacteria produce toxin 
- binds to gut lining cells
massive loss of fluid and electrolytes
Causes severe dehydration, kidney failure and death

Question 27

Cholera toxin mechansim

Answer 27

Cholera toxin enters cells of gut lumen
Activates adenyl cyclase increasing cAMP
Reduces Na+ absorption
Increases Cl- secretion
Water and other electrolytes drawn into bowel lumen = diarrhoea

Question 28

Antibiotic resistance

Answer 28

Micro-organisms readily mutate e.g.
HIV, staphylococci
Frequently mutate to escape antibiotic use
More antibiotic use = more mutations
Failure of antibiotic treatment

Question 29

Antibiotic resistance has been observed in

Answer 29

antibiotic resistance

Question 30

host patient factors in infection?

Answer 30

environment
barriers to infection
genetics

Question 31

Environmental factors affecting infection

Answer 31

Geography
Climate
Poverty
Availability of health care e.g. vaccination
Public Health infrastructure
Sanitation	
Distribution of other infection hosts
	insect or other vectors

Question 32

What is dengue fever

Answer 32

virus infection
transmitted by aedes mosquito
fever and rash and muscle pain
sever form: bleeding, shock and multi-organ failure

Question 33

What are barriers to infection

Answer 33

Skin & mucous membranes
Stomach acid
Native bacteria
Immune system
Genetics
Behaviour

Question 34

Skin as a barrier

Answer 34

cuts - wound infection

abrasions - conjunctivitis

Question 35

Stomach acid

Answer 35

Hydrochloric acid - pH 2

Very few organisms survive this

Question 36

what do drugs for stomach ulcers do to to stomach acid

Answer 36

increase pH

Question 37

What are patients taking drugs for stomach ulcers more prone to

Answer 37

food poisoning

E.g. Campylobacter from poorly cooked chicken

Question 38

Normal gut bacteria

Answer 38

1012/g faeces

Prevent colonisation by pathogenic (disease-causing) bacteria

Question 39

What does antibiotic treatment do to gut flora

Answer 39

Antibiotic treatment destroys the normal ‘friendly’ bacteria
allows pathogen to reproduce and cause disease
eg. diarrhoea due to clostridium difficile

Question 40

Treatments for cancer cause

Answer 40

Chemotherapy damages the immune system

Question 41

Diseases due to overactive immune systems ‘autoimmune’

Answer 41

Asthma, types of arthritis,
bowel inflammation ’colitis’
Usually treated by drugs which make the immune system less active

Question 42

Which drugs make immune system less active

Answer 42

corticosteroids

Question 43

What happens in HIV?

Answer 43

Infects white cells which control immune system
Makes most infections commoner & more severe
Causes infections by micro-organisms which don’t usually cause harm
‘opportunistic infection’

Question 44

Inherited immunodefieciency

Answer 44

Major problems rare
Life threatening infections
Unusual infections
Minor differences common
Hepatitis C virus

Question 45

When can genetic defects protect against infection

Answer 45

eg. sickle cell disease - abnormal red cells

- heterozygotes are protected against malaria

Question 46

What factors affect Behaviour and infection risk

Answer 46

Occupation
Travel
Recreation
Sex
Drugs
Contacts
Friends and pets