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Time Completed: 01:42:54

Final Score 86%

154
26

Questions

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Microbiology

Principles

Question 74 of 180

Which of the following best describes a pathogen:

Answer:

A pathogen is an organism that is capable of causing disease.

Pathogens and Pathogenicity

Definitions

Humans encounter bacteria, viruses and parasites that do not cause disease. An infection occurs when microorganisms cause disease.

Important definitions:

  • A pathogen is an organism that is capable of causing disease.
  • A commensal is an organism that is part of the normal flora.
  • Colonisation is a normal state and is not pathological.
  • Pathogenicity is the ability to cause disease.
  • Virulence is the ability to cause severe disease.
  • Infection describes a microbe-induced state of disease.

Types of Pathogen

Obligate pathogens cannot survive long outside of the human body and must cause disease in order to be transmitted.

Conditional pathogens may cause disease only if certain conditions are met. For example, Bacteroides fragilis is a normal gut commensal, but if it invades the peritoneal cavity, it will cause severe infection, or Neisseria meningitidis, a normal nasopharynx commensal, may cause meningitis by direct or haematogenous spread.

Opportunistic pathogens usually cause infection only when the host defences are compromised. For example, Pneumocystis jiroveci may cause lung infection in a host who has severely compromised T-cell immunity as seen in HIV infection.

Pathogen Examples
Obligate HIV, Mycobacterium tuberculosis, Treponema pallidum, Neisseria gonorrhoeae
Conditional Bacteroides fragilis, Neisseria meningitidis, Staphylococcus aureus
Opportunistic Candida albicans, Pneumocystis jiroveci, Pseudomonas aeruginosa

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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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