Microbiology
Pathogens
Regarding Staphylococcus aureus, which of the following statements is CORRECT:
Answer:
Some strains of Staphylococcus aureus produce food poisoning-causing enterotoxin. Staphylococcus aureus is characterised by its production of coagulase which converts plasma fibrinogen into fibrin, and hence clots plasma. Asymptomatic carriage is very common and spread usually occurs via direct contact. It is not the most common cause of community acquired pneumonia.Staphylococcus Aureus
Microbiology / Pathogens / Streptococci and Staphylococci
Last Updated: 21st April 2019
| Microorganism | Staphylococcus Aureus |
|---|---|
| Gram stain | Gram positive |
| Shape | Cocci (clusters) |
| Catalase | Positive |
| Coagulase | Positive |
| Oxygen requirements | Facultative anaerobe |
| Additional features | Produces exotoxin |
| Reservoir | Skin, nasopharynx and fomites |
| Common diseases | Skin & joint infections, device related infection, infective endocarditis, toxic shock syndrome, scalded skin syndrome, toxic epidermal necrolysis, food poisoning |
Staphylococcus aureus is a catalase-positive, coagulase-positive, beta-haemolytic, Gram positive coccus found in grape-like clusters.
Transmission
Asymptomatic carriage of S. aureus is found in up to 40% of healthy people, in the nose, skin, in particular the axilla and perineum, in the gastrointestinal tract and the vagina. Organisms may spread and cause endogenous infection where there is lowered host resistance.
Exogenous spread is via direct skin contact or indirect contact by clothing or other fomites or by respiratory droplet transmission.
Clinical Disease
Staphylococcus aureus causes a wide range of infectious diseases:
- Primary skin infection (e.g. impetigo, paronychia, abscess, cellulitis)
- Secondary skin infections (e.g. in eczema, surgical wounds, intravenous devices)
- Pneumonia (may follow influenza or measles, typically cavitating)
- Endocarditis (most common cause of acute IE, rapid and destructive, associated with intravenous drug use or colonisation of intravenous devices)
- Osteomyelitis
- Septic arthritis
- Septicaemia
Exotoxin-mediated syndromes include:
- Scalded skin syndrome
- Toxic shock syndrome
- Food poisoning
Methicillin Resistant Staphylococcus Aureus (MRSA) is increasingly prevalent and causes the same range of infections as susceptible isolates. Hospital acquired MRSA is associated with antibiotic use, prolonged hospitalisation, surgical site infection, intensive care, haemodialysis and device related infections.
Treatment
Flucloxacillin is usually the antibiotic of choice to treat methicillin-susceptible strains and vancomycin or teicoplanin the agents of choice to treat MRSA.
Prevention
Patients colonised with MRSA should be isolated in a side room with barrier nursing. Carriage may be eliminated by using topical chlorhexidine body wash and mupirocin nasal ointment.
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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 |
