Microbiology
Principles
Which of the following best describes a pathogen:
Answer:
A pathogen is an organism that is capable of causing disease.Pathogens and Pathogenicity
Microbiology / Principles / Mechanisms of Disease
Last Updated: 21st April 2019
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 |
