Microbiology
This was previously featured in an exam
This was previously featured in an exam
Principles
Question 155 of 180
A patient presenting to ED with sepsis is found to have Staphylococcus aureus bacteremia. Which of the following is a virulence factor of Staphylococcus aureus:
Answer:
Staphylococcus aureus expresses protein A which binds host immunoglobulin preventing opsonisation and complement activation.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 |
