Introduction To Infection (UTI)

Question 1

What is a symbiotic relationship?

Answer 1

Close and often long-term interaction between 2 different species. Can be:
Mutualistic
Commensal
Parasitic

Question 2

What is a commensal relationship?

Answer 2

Symbiotic relationship between 2 different species where one derives some benefit and the other is unaffected

Question 3

What is a mutualistic relationship?

Answer 3

Symbiotic relationship between 2 different species which is beneficial to both

Question 4

What is colonisation?

Answer 4

When a microbe grows on or in another organism without causing any disease

Question 5

What is infection?

Answer 5

The invasion and multiplication of microbes in an area of the body where they are not normally present which usually leads to disease (may be asymptomatic, sub-clinical or cause varying degrees of symptoms and be clinically apparent)

Question 6

Explain commensal host-microbe interactions within the body.

Answer 6

Normal commensal bacteria (‘flora’) that occupy a majority of available body areas such as skin, mouth, upper airways, lower airways, GI tract and genital tract (urinary tract is sterile)
Probably mutualistic as prevent pathogenic bacteria occupying these areas
Antibiotic treatments can eliminate normal flora making infections more likely e.g. C.Dif diarrhoea (another reason to limit antibiotic use and consider use of friendly bacteria as treatment e.g. faecal microbiota transplant)

Question 7

What are the host risk factors for infection?

Answer 7

Extremes of age
Stress and starvation
Compromised barrier to infection (physical/anatomical e.g. skin or biochemical/physiological e.g. stomach acid)
Immunocompromised host (primary immunodeficiency from birth, secondary immunodeficiency e.g. HIV or immunosuppression e.g. iatrogenic)

Question 8

What are the 3 components of barrier immunity?

Answer 8

Physical - skin, mucus, respiratory cilia and commensal organisms
Biochemical - sebaceous secretions in skin, lysozyme in tears, spermine in sperm and gastric acidity
Mucosal - nasopharynx, respiratory tract, alimentary tract and GUT

Question 9

What are the 8 stages of bacterial pathogenesis?

Answer 9

1. Access - adherence/invasion
2. Adherence - stick/bind to surface
3. Invasion - penetration of barriers
4. Multiplication - replicate
5. Evasion - evade immune system
6. Resistance - resist treatment
7. Damage - direct or indirect (via immune system)
8. Transmission - released to infect other hosts

Question 10

How do you prove the pathogen-disease link?

Answer 10

Find evidence of microbe and correlate this to clinical features of the patient to check whether the diagnosis is plausible

Question 11

What could you do for a patient who’s sample has not been cultured yet but they are in a lot of pain/very ill?

Answer 11

Empirical anti-microbial treatment i.e. best guess of antibiotic based on clinical features

Question 12

How do you diagnose an infection? What investigations could you do?

Answer 12

Clinical features from history and exam may be sufficient but will remain important even if a microbiological cause is found

Investigations:
Markers of inflammation in blood/body fluids
Microscopy, culture and sensitivity for bacteria
Nucleic acid (DNA/RNA) detecting using PCR for viruses
Antibody detection (serology) for viruses and unusual pathogens (used for pathogens we don’t have better tests for e.g. HIV as its indirect)
Antigen detection of microbial component for unusual pathogens
Imaging e.g. X-rays, US, CT and MRI

Question 13

How does the urinary tract become colonised commonly?

Answer 13

Foreign body in there e.g. catheter or kidney stone but this may not cause infection/harm although it can progress to an infection

Question 14

Why do women get more UTIs than men?

Answer 14

Shorter urethras

Womens urethra is nearer to the anus and most pathogens are from the colon

Question 15

What are the host risk factors for UTIs?

Answer 15

Extremes of age
Post-menopausal woman
Compromised barrier to infection - physical/anatomical: shorter urethra (sexually active/post-menopausal), malformations (PKD, renal/ureteric malformations, strictures), internal obstructions (stones/tumors), bladder outflow obstruction (pregnancy, prostate enlargement) & iatrogenic (urinary catheters, operations, post-operative changes)
Immunocompromised host e.g. diabetes

Question 16

What are the 8 stages of bacterial pathogenesis in UTI?

Answer 16

1. Access - most bacteria commensals in colon
2. Adherence - using pili (fimbriae) & adhesion molecules
3. Invasion - haemolysin (breaks down cells) increases invasive potential
4. Multiplication - colonisation may precede infection
5. Evasion - few immune cells in urinary tract
6. Resistance - many bacteria causing UTIs have multi-drug resistance
7. Damage - urethritis, cystitis, pyelonephritis, nephritis & septicaemia
8. Transmission - easily passed out in urine (limits infection risk)

Question 17

What would a generalised culture be a good idea to diagnosis UTIs?

Answer 17

Because there is such a huge number of organisms involved in causing them and there is usually little correlation between organism and type of UTI

Question 18

How do you diagnosis a UTI?

Answer 18

Symptoms = dysuria, frequency/urgency of micturition, haematuria, opaque/malodorous urine & lower abdominal/loin pain
Risk factors: age, compromised barriers to infection, immunocompromised
Signs: Lower abdominal/loin tenderness, fever or septic shock
Investigations - urinalysis (dipstick) for WBCs, blood, nitrite, protein, mid-stream specimen of urine for m+c+s, bloods (FBC, U&E, CRP), blood cultures (for bacteria) & imaging (USS, CT urogram)

Question 19

Why is it important to get a mid stream urine specimen?

Answer 19

To obtain an uncontaminated urine sample because the first part of urine will be contaminated with epithelial cells sloughed off superficially and the end will have remnants of the bladder constituents in it

Question 20

What would you expect to find in a dipstick urinalysis interpretation?

Answer 20

Leukocytes/WBCs high
Blood 
Nitrites sometimes raised 
Raised protein
Should check if patient is diabetic with glucose

Question 21

What is involved in the microbiological diagnosis of UTIs?

Answer 21

MSSU - cells can be counted and characterized
Microscopy - WBCs > 100/μl suggests infection, RBCs suggest haemorrhage or infection, epithelial cells suggest contaminated sample, casts suggest renal disease
Culture - CFUs > 100 μl suggests infection
Sensitivities - tested with a range of appropriate antibiotics

Question 22

How is urine cultured?

Answer 22

Plated onto agar
Using a 1 μl sampling loop
Cultured over night (why empirical treatment is useful)
Once it has grown you can look under microscope to some degree of certainty

Question 23

How do sensitivity tests work?

Answer 23

Antibiotics discs are coated in antibiotic and there will be a zone of inhibition that is clear around the disc if the antibiotic has inhibited the growth of the bacteria - the smaller the zone the more ineffective the drug is for that bacteria -> helps determine the best drug to use

Question 24

What does a low epithelial cell count show in a m+c+s?

Answer 24

That the sample is clean and not contaminated so we can have confidence in its results