A 72 year-old male smoker arrives by ambulance to the accident emergency department of the hospital. He was discharged from hospital 10 days ago following treatment with oral Tetracycline, 500 mg four times a day for 5 days for an acute exacerbation of chronic bronchitis. In the past 24 hours he has developed a fever and a painful, productive cough. In the last few hours his breathing has become increasingly laboured and he is experiencing chest pain. He uses a Terbutaline inhaler (two puffs when required) for his bronchitis but otherwise has no other relevant medical history.
Clinical examination excludes cardiovascular causes for the chest pain. Using a stethoscope the doctor identifies bilateral consolidation of the lower lungs. The doctor makes a provisional diagnosis of bronchopneumonia and obtains a sputum culture for microbial assessment. The doctor initiates therapy with intravenous Cefuroxime (750 mg every 8 hours) and Erythromycin (1 g every 6 hours) with a view to switching to oral therapy with these agents within 48 hours if the patient responds to therapy and continuing for 10 days.
Aetiology of Bronchopneumonia
Bronchopneumonia or bronchial pneumonia is an acute bacterial pneumonia (inflammation of the lung parenchyma - alveoli) where there are areas of patchy, exudative consolidation that are scattered throughout the lower parts of both lungs. This is in contrast to lobar pneumonia where the consolidation is localised to a large portion of an entire lobe of a lung1. The symptoms of Bronchopneumonia include a cough that produces blood-stained sputum as well as tightening chest pain and bacteraemia (bacteria present in the blood). Fever and an elevated white blood cell count are also associated with the condition2.
Although pneumonia can be classified in anatomical terms (Bronchial or lobar), the range of organisms which can cause pneumonia is so wide that it is much more practical to classify the pneumonia by its place of origin. The usual classification for pneumonia is community-acquired or hospital-acquired3. Bronchopneumonia is usually hospital-acquired and the result of opportunistic gram-negative bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa (which accounts for approximately 60% of cases of hospital-acquired pneumonia)4.
Bronchopneumonia is usually the result of a pre-existing condition such as chronic Bronchitis or Bronchiolitis where the defence mechanisms in the lungs are already greatly impaired and there is increased susceptibility to opportunistic microorganisms. Factors such as old age (associated with a decline in immunity) and smoking (causes the cilia to become paralysed and unable to remove pathogens from the respiratory tract) also increase the probability of Bronchopneumonia5.
Review of Doctor's Presumptive Diagnosis
The sputum sample obtained from the patient was grown on three different agar plates: Mannitol Salt Agar (MSA), MacConkey Agar and Cetrimide Agar.
Mannitol Salt Agar is a selective, differential medium used for the primary isolation and identification of Staphylococci as it inhibits most organisms except Staphylococci6. No visible microorganisms were present on the plate leading to the conclusion that the pathogen was not Staphylococci.
MacConkey Agar is a selective, differential medium used for the isolation and differentiation of lactose fermenting and non-lactose fermenting gram-negative bacteria. The crystal violet and bile salts inhibit the growth of gram-positive organisms. Lactose fermenting bacteria produce a deep red colour while non-lactose fermenting bacteria remain colourless7. In the plate, colourless colonies were present about 0.2-0.5 mm in size. The colonies aggregated into very smooth patterns with no gaps or clumps between them and they produced a damp, fruity odour. It was concluded that the pathogen was a non-lactose fermenting, gram-negative bacteria.
Cetrimide Agar is a selective medium used for the isolation and identification of Pseudomonas aeruginosa since it inhibits bacterial growth except for Pseudomonas Aeruginosa. Cetrimide also enhances Pseudomonas pigment production such as fluorescein and pyocyanin8. Blue/green colonies were visible about 0.2-0.5 mm in size. The colonies aggregated into very smooth patterns with no gaps or clumps between them along with the damp, fruity odour recognisable from the MacConkey agar previously. Based on the results from the three agar plates it was concluded that the pathogen was likely to be the non-lactose fermenting, gram-negative bacteria - Pseudomonas aeruginosa. However, other tests were carried out to ensure that this was the case.
A sample of the sputum was gram stained and fixed onto a glass slide. Gram-negative bacteria produce a violet or deep blue colour when stained, while gram-positive bacteria produce a pink-red colour. Microscopic examination revealed pink-red, rod shaped colonies which were packed together in clusters. This agreed with the earlier results that the pathogen was a gram-negative bacteria and it was now recognised to be rod-shaped. Literature has published many pictures of Pseudomonas aeruginosa when viewed under a light microscope and they match the characteristics of the pathogen in this case (see Figure 1).
N Figure 1. Gram stain of Pseudomonas Aeruginosa cells n n © Kenneth Todar, Ph.D (University of Wisconson)9
However, an oxidase test was carried out on a sample of the sputum to definitely confirm that the pathogen was Pseudomonas aeruginosa. The test is useful in distinguishing Enterobactericiae (produces a negative result) and Pseudomonas (produces a positive result). A positive test is indicated by the presence of an intense deep blue colour within seconds10. The result of the test was positive confirming that the pathogen was Pseudomonas aeruginosa. There is also a decision tree, printed by the University of Strathclyde, used for the identification of unknown microorganisms which confirms that all the results of the tests carried out lead to the diagnosis of Pseudomonas aeruginosa (see Appendix 1).
A series of broth macrodilutions were carried out on the sputum sample to determine the sensitivity of the pathogen (i.e. Pseudomonas aeruginosa) against certain antibiotics. The results of the broth macrodilutions are below (Figure 2).
Antibiotic
Bottle 1
Bottle 2
Bottle 3
Bottle 4
Bottle 5
Erythromycin
20 (+)
10 (+)
5 (+)
2.5 (+)
1.25 (+)
Amoxicillin
20 (-)
10 (+)
5 (+)
2.5 (+)
1.25 (+)
Ticarcillin
64 (-)
32 (-)
16 (-)
8 (-)
4 (+)
Cefuroxime
64 (-)
32 (-)
16 (-)
8 (+)
4 (+)
Amikacin
32 (-)
16 (-)
8 (-)
4 (-)
2 (+)
Figure 2. Activity of certain antibiotics against Pseudomonas Aeruginosa determined using broth macrodilutions b Note: Figures in the table are concentrations of the antibiotic expressed as µg/ml. B (+) represents bacterial growth, (-) represents no growth.
Using the results of the broth macrodilutions, the Minimum Inhibitory Concentration (MIC) of each antibiotic was calculated. The Minimum Inhibitory Concentration is defined as the lowest concentration of antibiotic that is required to inhibit bacterial growth11. The Minimum Inhibitory Concentrations of the five antibiotics used in the broth macrodilutions are listed below and using the Greater Glasgow Health District Profile on Antibiotic Sensitivity Interpretation (see Appendix 2), the sensitivity of Pseudomonas Aeruginosa to these antibiotics was determined (Figure 3).
Antibiotic
Minimum Inhibitory Concentration (µg/ml)
Sensitive / Resistant
Erythromycin
>20
Resistant
Amoxicillin
20
Resistant
Ticarcillin
8
Sensitive
Cefuroxime
16
Resistant
Amikacin
4
Sensitive
Figure 3.Minimum Inhibitory Concentrations of certain antibiotics and the sensitivity of Pseudomonas Aeruginosa to these antibiotics
It was concluded that the Pseudomonas Aeruginosa was sensitive to Ticarcillin and Amikacin and was resistant to Erythromycin, Amoxicillin and Cefuroxime.
The results above are consistent with most literature which claims that Pseudomonas aeruginosa is very resistant to second-generation Cephalosporins (such as Cefuroxime), narrow spectrum antibiotics (like Amoxicillin) and Erythromycin (macrolide)12.
As said before, the link between Pseudomonas aeruginosa and Bronchopneumonia is strong and this pathogen is usually the most likely cause of hospital-acquired pneumonia. The test results confirm without any doubt that the pathogen responsible for the respiratory tract infection was Pseudomonas aeruginosa. The patient's symptoms (fever, painful and productive cough, chest pain) and the fact that the consolidation in his lungs is bilateral (on both lungs), rather than specific to one lobe (lobar pneumonia), indicate Bronchopneumonia. In light of all this information, it was confirmed that the doctor's presumptive diagnosis of Bronchopneumonia was correct.
Review of Doctor's Antimicrobial Therapy
The British National Formulary states that Cefuroxime used in combination with Erythromycin is a recommended course of treatment of high-severity community acquired pneumonia as well as treating for atypical pathogens (e.g. Legionella pneumophila in Legionnaires disease)13. This treatment would be effective against most gram-positive organisms that typically cause community-acquired pneumonia such as Staphylococcus aureus, as well as some gram-negative organisms such as Haemophilia Influenzae14.
However, considering that the presumptive diagnosis (which was correct) was Bronchopneumonia and was very likely to be caused by gram-negative bacteria from his stay at the hospital ten days ago, his initial treatment plan was incorrect. The treatment should have been aimed at tackling the most likely causing organism15, in this case the gram-negative Pseudomonas Aeruginosa. Cefuroxime and erythromycin usually have really poor activity against Pseudomonas Aeruginosa16 which is highlighted by the sensitivity tests which proved that the pathogen was resistant to these antibiotics. As a result of this new information, the initial therapy should be changed and the doctor's initial treatment following the presumptive diagnosis should be criticised.
Since, the bacteria are sensitive to Ticarcillin and Amikacin then these antibiotics should be involved in the recommended treatment of choice. Amikacin is an aminoglycoside which, like all aminoglycosides, has potential toxic side effects (e.g. nephrotoxicity and ototoxicity) which are really common. Gentamicin is normally the aminoglycoside of choice in the UK and Amikacin is only recommended for the treatment of Gentamicin-resistant organisms17. Therefore, Amikacin should only be considered as a 'last-resort' treatment for the patient.
As a result, the antibiotic of choice should be Ticarcillin, which is a very broad-spectrum penicillin that is effective against most gram-positive and gram-negative bacteria, especially Pseudomonas Aeruginosa18. It is only available in combination with Clavulanic Acid as an intravenous infusion and the recommended dose is 3.2g every 6-8 hours for at least seven days19. The initial treatment for the 72 year old male should be 3.2g Ticarcillin with Clavulanic Acid given intravenously every 6 hours for at least seven days.
The subsequent treatment plan should also be changed. The recommended treatment plan should be to see if the patient responds to the initial therapy after 24 hours. If the patient has not responded then the addition of Amikacin must be considered13. If the patient does respond then he should be switched to an oral antibiotic after seven days. More sensitivity tests should be carried out on the pathogen with more antibiotics, especially ones that can be taken orally, that are usually recognised as having good bactericidal activity against Pseudomonas Aeruginosa and other gram-negative bacteria (e.g. Ciprofloxacin)20. Local sensitivity patterns in the hospital should also be observed and the Pseudomonas Aeruginosa that caused the infection in the first place should be isolated and disinfected.
Appendix
Appendix 1
Decision tree for the identification of unknown microorganisms21 Note: Yellow line indicates results of the tests
Appendix 2
Greater Glasgow Health District Profile on Antibiotic Sensitivity Interpretation
MIC's (µg/ml)
Antibiotics
Sensitive
Intermediate
Resistant
Amikacin
≤ 6
>6 - <10
>10
Cefuroxime
≤ 5
>5 - <10
>10
Ticarcillin
≤ 10
>10 - <20
>20
Amoxicillin
≤ 2
>2 - <12
>12
Erythromycin
≤ 0.5
>0.5 - <3
>3
