Microbiology
Infections
Question 55 of 180
A 69 year old presents to ED with a fever, a cough productive of thick green sputum and shortness of breath, 48 hours after being discharged from hospital following a prolonged ICU stay for treatment of sepsis. X-ray demonstrates a left lower lobe pneumonia. Which of the following pathogens is the most likely infectious agent:
Answer:
Nosocomial infections are defined as those occurring within 48 hours of hospital admission, 3 days of discharge or 30 days of an operation, therefore the patient most likely has hospital acquired pneumonia. The most common causes of hospital acquired pneumonia are Gram-negative bacilli (Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa (especially in ICU, ventilated patients, or those with structural lung disease), Enterobacter spp.), Staphylococcus aureus and anaerobes.Pneumonia
Microbiology / Infections
Last Updated: 21st May 2025
Infective Agents
- Community acquired pneumonia
- Streptococcus pneumoniae (most common)
- Haemophilus influenzae
- Staphylococcus aureus
- Hospital acquired pneumonia (develops at least 24 hours after hospital admission)
- Gram-negative bacilli (Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Enterobacter spp.)
- Staphylococcus aureus
- Anaerobes
- Atypical pneumonia
- Mycoplasma pneumoniae
- Legionella pneumophila
- Chlamydophila pneumoniae
- Chlamydophila psittaci
Clinical Features
- Symptoms
- Systemic features: fever, myalgia, arthralgia, rigors, headache
- Cough (+/- productive)
- Dyspnoea
- Pleuritic chest pain
- Signs
- Dull percussion note over infected area
- Reduced chest expansion on affected side
- Bronchial breathing
- Crackles
- Increased tactile fremitus and vocal resonance
- Tachypnoea and tachycardia
Specific Pneumonias
- Mycoplasma pneumoniae
- More common in younger patients (school age and young adults)
- Outbreaks occur approximately every 4 years in the UK
- Flu-like illness followed by dry cough associated with extrapulmonary complications such as erythema multiforme and SJS, autoimmune haemolytic anaemia, pericarditis and myocarditis, meningoencephalitis
- Legionella pneumophila
- Outbreaks among patients staying at institutions with contaminated water tanks
- Flu-like illness before developing a dry cough and shortness of breath, associated with renal failure, gastrointestinal upset, confusion and hyponatraemia
- Chlamydia psittaci (psittacosis)
- Linked with exposure to infected birds
- Flu-like illness, dry cough, high temperature, photophobia and neck stiffness
- Klebsiella pneumoniae
- Associated with older patients, diabetes mellitus and alcoholism
- Sudden onset flu-like illness, high fever and productive cough with blood-tinged sputum (red currant jelly sputum)
Diagnosis
- Chest x-ray – consolidation +/- pleural effusion
- Sputum for M, C & S
- Urinary antigens for L. pneumophila and M. pneumoniae
- Blood cultures
- Bloods (FBC, U&Es, LFTs, CRP)
- Serology for atypical organisms
Assessment of CAP
In adults, severity is assessed by clinical judgement guided by mortality risk score (CURB65):
- Confusion (abbreviated mental test, AMT score ≤ 8)
- Urea (> 7 mmol/L)
- Respiratory rate (≥ 30/min)
- Blood pressure (< 90 systolic or ≤ 60 diastolic)
- 65 (age ≥ 65 years)
Patients are stratified for risk of death as follows:
- 0 or 1: low risk (less than 3% mortality risk)
- 2: intermediate risk (3‑15% mortality risk)
- 3 to 5: high risk (more than 15% mortality risk).
Treatment of CAP
Use clinical judgement in conjunction with the CURB65 score to guide the management of community‑acquired pneumonia, as follows:
- consider home‑based care for patients with a CURB65 score of 0 or 1
- consider hospital‑based care for patients with a CURB65 score of 2 or more
- consider intensive care assessment for patients with a CURB65 score of 3 or more.
| Infection | First Choice Antibiotic |
|---|---|
| Low-severity community acquired pneumonia |
|
| Moderate-severity community acquired pneumonia |
|
| High-severity community acquired pneumonia |
|
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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 |
