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Time Completed: 01:07:02

Final Score 54%

98
82

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Microbiology

Principles

Question 86 of 180

Which of the following microbes expresses protein A:

Answer:

Staphylococcus aureus expresses protein A, which binds host immunoglobulin, preventing opsonisation and complement activation.

Mechanisms of Attachment, Invasion and Persistence

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

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