Microbiology
Infections
Question 35 of 77
A 45 year old man presented with fever and upper back pain. MRI imaging demonstrated inflammation of the T8 - T9 vertebral bodies consistent with osteomyelitis. A bone marrow biopsy, when cultured, produced colonies of Gram-positive cocci. Catalase and coagulase tests were positive. What is the most likely pathogen:
Answer:
Staphylococcus aureus infection is the most common cause of osteomyelitis, and this is consistent with the findings of Gram-positive cocci colonies with positive catalase and coagulase tests.Osteomyelitis
Microbiology / Infections
Last Updated: 20th April 2022
Infective Agents
- Staphylococcus aureus (most common)
- Streptococcus pyogenes
- Haemophilus influenzae
- Escherichia coli
- Salmonella spp. (in sickle cell disease)
- Mycobacterium tuberculosis
Pathogenesis
- Haematogenous spread - distant infection causes bacteraemia and subsequent osteomyelitis
- Adjacent joint infection - septic arthritis can spread from the joint to the bone
- Direct infection - trauma, surgery or a deep ulcer can infect adjacent bone
Clinical Disease
- Systemic features: fever, malaise
- Localised pain over bone
- Erythema, swelling and pus drainage through sinuses
- Pathological fractures
Diagnosis
- Clinical diagnosis
- X-ray - changes only visible after 7 - 10 days, bone destruction and joint effusions
- MRI - bone destruction, effusions and joint damage
- Swab pus for M, C & S
- Blood cultures
Treatment
- Immobilise bone
- Antibiotic therapy
- Flucloxacillin first line (clindamycin if penicillin allergic)
- Vancomycin (or teicoplanin) if MRSA suspected
- Consider adding fusidic acid or rifampicin for initial 2 weeks
- Suggested duration of treatment 6 weeks for acute infection
- Surgical therapy
- Drainage and excision of the sequestrum
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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 |
