Lecture 30 - How and why infections happen Flashcards
where do infections come from
endogenous or communicable
Normal microbial flora
vary according to site
Upper airways – saliva 10^8 -10^9 bacteria per ml
GI tract – faeces 10^11 -10^12 bacteria per gram
Endogenous infection
Normal flora: Skin Gut Upper airways Genital tract Normal gut flora gets into wrong place Urinary tract infection e.g. cystitis
Cystisis
Infection of lower urinary tract Lower abdominal pain Urgency Dysuria Frequency
What bacteria is the most common cause of cystisis
gut flora
eg. eschericia coli
e coli - gram negative
where do communicable infections come from
person to person
non - human - animals/birds/insects
-environment
How do we get infections endogenously
migration
perforation
blood
How do we get infections exogenously
contact - direct and indirect injuries - trauma and bites airborne oral - food/water blood-borne sex mother- baby
Endogenous infection - what is migration
Bowel flora - E.coli contaminates perineum -gains access to urethra causes local infection spreads to bladder and beyong UTI
Endogenous infection - what is perforation
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
Endogenous infection - what is blood spread
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
Exogenous infection: direct contact
Impetigo
superficial skin infection due to staphylococci and/or streptococci
spreads rapidly from person to person
Exogenous infection: indirect contact
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
Injuries – dirty wound
Tetanus (Lockjaw) Clostridium tetani Bacterium present in soil Contaminates wounds Releases toxin causing muscle spasm Prevented by vaccination
Bites
malaria
- parasite infection
- life-cycle
- mosquitoes and humans
- present in large areas of tropics
- causes severe febrile illness affecting all systems
Airborne infection examples
influenza virus
- coughing and sneezing
- droplets formed containing infectious viruses
- inhaled by others
oral route - food bourne
Food prepared with poor hygiene E.g. not washing hands after going to toilet Contaminate food with harmful bacteria Result: food poisoning! Vomiting, diarrhoea
Blood -borne infection
hep B -liver infection -some viruses spill into blood -transmission by blood exposure -transfusion sharing of needles -body piercing
Sexual transmission
Chlamydia - risks increased by
unprotected sex, new partners
multiple partners
partners with high risk
Mother to baby transmission ‘vertical’
During pregnancy e.g. rubella
At time of birth e.g. herpes
Breast milk e.g. HIV
infections transmitted in more than one way?
HIV Blood borne e.g injection drug use Vertical perinatal breast feeding Sexual
Which infections require more than one step?
eg staph aureus - soft tissue infection
step 1: colonisation of skin - joins skin flora
step 2: penetration of skin - spreads and damages
Factors affecting infection
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
Ebola infection
direct infection and damage/destruction of cells
Virulence factors
allow invasion of host tissues eg streptolysin O
-Lyses cells - cytolysin
produces by certain streptococci - group A
`Toxins
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
Cholera toxin mechansim
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
Antibiotic resistance
Micro-organisms readily mutate e.g. HIV, staphylococci Frequently mutate to escape antibiotic use More antibiotic use = more mutations Failure of antibiotic treatment
Antibiotic resistance has been observed in
antibiotic resistance
host patient factors in infection?
environment
barriers to infection
genetics
Environmental factors affecting infection
Geography Climate Poverty Availability of health care e.g. vaccination Public Health infrastructure Sanitation Distribution of other infection hosts insect or other vectors
What is dengue fever
virus infection
transmitted by aedes mosquito
fever and rash and muscle pain
sever form: bleeding, shock and multi-organ failure
What are barriers to infection
Skin & mucous membranes Stomach acid Native bacteria Immune system Genetics Behaviour
Skin as a barrier
cuts - wound infection
abrasions - conjunctivitis
Stomach acid
Hydrochloric acid - pH 2
Very few organisms survive this
what do drugs for stomach ulcers do to to stomach acid
increase pH
What are patients taking drugs for stomach ulcers more prone to
food poisoning
E.g. Campylobacter from poorly cooked chicken
Normal gut bacteria
1012/g faeces
Prevent colonisation by pathogenic (disease-causing) bacteria
What does antibiotic treatment do to gut flora
Antibiotic treatment destroys the normal ‘friendly’ bacteria
allows pathogen to reproduce and cause disease
eg. diarrhoea due to clostridium difficile
Treatments for cancer cause
Chemotherapy damages the immune system
Diseases due to overactive immune systems ‘autoimmune’
Asthma, types of arthritis,
bowel inflammation ’colitis’
Usually treated by drugs which make the immune system less active
Which drugs make immune system less active
corticosteroids
What happens in HIV?
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’
Inherited immunodefieciency
Major problems rare Life threatening infections Unusual infections Minor differences common Hepatitis C virus
When can genetic defects protect against infection
eg. sickle cell disease - abnormal red cells
- heterozygotes are protected against malaria
What factors affect Behaviour and infection risk
Occupation Travel Recreation Sex Drugs Contacts Friends and pets
