Microbiology
Principles
Which of the following is NOT an anatomical barrier to infection:
Answer:
Anatomical barriers to infection include:- tight junctions between cells of the skin and mucosal membranes
- the flushing action of tears, saliva and urine
- the mucociliary escalator in the respiratory tract (together with the actions of coughing and sneezing)
- the acidic pH of gastric and vaginal secretions
- the acidic pH of the skin (maintained by lactic acid and fatty acids in sebum)
- enzymes such as lysozyme found in saliva, sweat and tears
- pepsin present in the stomach
- biological commensal flora formed on the skin and the respiratory, gastrointestinal and genitourinary tracts which protect the host by competing with pathogenic bacteria for nutrients and attachment sites and by producing antibacterial substances
Barriers to Infection
Microbiology / Principles / Natural and Innate Immunity
Last Updated: 8th November 2021
Anatomical Barriers
Anatomical barriers to infection include:
- tight junctions between cells of the skin and mucosal membranes
- the flushing action of tears, saliva and urine
- the mucociliary escalator in the respiratory tract (together with the actions of coughing and sneezing)
- the acidic pH of gastric and vaginal secretions
- the acidic pH of the skin (maintained by lactic acid and fatty acids in sebum)
- enzymes such as lysozyme found in saliva, sweat and tears
- pepsin present in the stomach
- biological commensal flora formed on the skin and the respiratory, gastrointestinal and genitourinary tracts which protect the host by competing with pathogenic bacteria for nutrients and attachment sites and by producing antibacterial substances
Skin as a Barrier
The skin provides a physical barrier to infection, with secreted sebum and fatty acids inhibiting bacterial growth.
Some pathogens can penetrate intact skin e.g. Leptospira and Treponema, or are introduced via the bite of a vector e.g. in malaria or dengue fever.
Some organisms colonise mucosal surfaces and use this route to gain access to the body e.g. Streptococcus pneumoniae.
If skin integrity is broken e.g. by intravenous cannulation, blood-borne viruses such as hepatitis B and HIV can be transmitted.
Diseases of the skin such as eczema, psoriasis or burns, permit colonisation and invasion by pathogens e.g. Staphylococcus epidermidis.
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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 |
