Microbiology
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Principles
Question 36 of 180
A 21 year old girl presents to ED with abdominal cramps and watery diarrhoea. Her boyfriend has recently returned from travelling abroad and was diagnosed with giardiasis. She is concerned that she may have also developed the infection. How does Giardia lamblia attach to host cells:
Answer:
Giardia lamblia attaches to gut mucosa via a specialised sucking disc.Mechanisms of Attachment, Invasion and Persistence
Microbiology / Principles / Mechanisms of Disease
Last Updated: 21st April 2019
Mechanisms of Attachment
Microorganisms must attach themselves to host tissues to colonise them and each organism has a different strategy.
For example:
| Microorganism | Mechanism of Attachment |
|---|---|
| Neisseria gonorrhoeae | Adheres to the genital mucosa using fimbriae |
| Influenza | Attaches by its haemagglutinin antigen |
| Giardia lamblia | Attaches to gut mucosa via a specialised sucking disc |
| HIV | Binds strongly to CD4 antigen |
| Plasmodium falciparum | Causes red cell protein expression facilitating cerebral malaria |
Mechanisms of Immune Evasion
Once past natural barriers, to survive in the human host, microorganisms must overcome the host immune defences.
For example:
| Microorganism | Mechanism of Evading Host Defence |
|---|---|
| Neisseria meningitidis | Secretes an IgA protease that degrades host immunoglobulin |
| Staphylococcus aureus | Expresses protein A, which binds host immunoglobulin, preventing opsonisation and complement activation |
| Streptococcus pneumoniae | Has a polysaccharide capsule which inhibits phagocytosis by neutrophils |
| Vibrio cholerae | Motile by virtue of its flagellum, increasing its virulence |
| Gram-negative organisms | Lipopolysaccharide coat makes them resistant to the effect of complement |
| Trypanosoma spp. | Alter surface antigens to evade antibodies |
Mechanisms of Damaging Host
In addition to these mechanisms, some microorganisms have the ability to generate damaging exotoxins:
- Vibrio cholerae (causing cholera) produces exotoxin which binds to gut epithelial cells, activating adenylyl cyclase and resulting in sodium and chloride efflux from the cell, ultimately causing profuse watery diarrhoea.
- Clostridium tetani (causing tetanus) produces the exotoxin tetanospasmin which has the ability to disrupt neurological signalling and integrity causing its neurotoxic effects.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
