Out of curiosity the Microbiologist then decided to check if the patient had been in before. This threw up something unexpected. The patient had had multiple attendances to the haematology ward as well as the outpatient clinic. Pulling up the clinic letters showed that the patient actually had chronic lymphoblastic leukaemia (CLL) and would therefore be immunosuppressed. This changed the situation; the blood culture could be an opportunistic pathogen and not a contaminant at all, muttering to himself, “chronic lymphoblastic anaemia is significant info for the specimen request card, when will they learn!” the Microbiologist decided to phone out the result straight away.
Still cursing healthcare staff who don’t fill out request forms with adequate clinical details, the Microbiologist bleeped the Haematology Registrar to find out more…
It turned out that the patient had presented to the Emergency Department with a fever and shortness of breath. The fever settled and the blood tests showed the patient wasn’t neutropaenic. The blood cultures had been taken before the blood test results came back. A dose of Piptazobactam had also been given in case the patient had febrile neutropaenia but this had been stopped when the neutrophil count was shown to be in the normal range. “As the patient was feeling better they were allowed to go home”, the Registrar said a little sheepishly knowing something was amiss if the Microbiologist was “showing concern”.
“Mmmmm…” said the Microbiologist to himself, “a blood culture positive on day 3 with a Gram-positive bacillus in the aerobic bottle only”… “Mmmmm…” he said again “this could be Nocardia”. The Microbiologist suggested getting the patient back in for a review whilst the laboratory tried to identify the microorganism. By the time the patient had been reassessed the organism had a name, Nocardia farcinica.
Nocardia spp. are a group of aerobic Gram-positive bacilli which do not take up Gram’s stain very well and so appear as long beaded Gram-positive bacilli. They have some mycolic acid in their cell wall, as a result they can be stained using a modified Ziehl-Neelsen (ZN) stain. The modified ZN stain uses 0.5-1% acid-alcohol as a decolouriser rather than the normal 3% used for Mycobacterium spp.
Nocardia spp. live ubiquitously in soil, water and organic material within the environment. They are found worldwide.
On culture Nocardia spp. are slow growing, taking 2-3 days to produce typical colonies which look a bit like small fungi with hyphae. In fact early Microbiologists thought they were fungi and hence named all of the similar organisms to Nocardia spp. the Actinomycetes (a mycete is a fungus). They are bacteria though.
There are over 80 species within the Nocardia genus but the most common causing human disease include:
- Nocardia asteroides
- Nocardia farcinica
- Nocardia nova
- Nocardia camea
- Nocardia braziliensis
- Nocardia otitidiscaviarum
- Nocardia cyriacigeorgica
- Nocardia abscessus
How does infection with Nocardia spp. present?
The most common presentation of infection with Nocardia spp., known as nocardiosis, is with pneumonia, lung abscesses and lung cavitation. This is because the bacteria are most commonly acquired by inhalation. People with a normal immune system usually clear the bacteria but immunosuppression, especially defective cell mediated immunity, predisposes to this opportunistic pathogen. The most common immune problems include prolonged steroid use, haematological cancer, chemotherapy, organ transplantation and Human Immunodeficiency Virus infection.
Occasional infections occur in skin and soft tissue following contamination of wounds e.g. soil inoculation of a thorn injury or an animal or insect bite. In this case cellulitis or superficial abscesses are the most common presentations.
With pulmonary infections, infection spreads from the lungs via the blood stream to any body organ, but especially the brain where the bacteria cause abscesses; this is more common with N. farcinica than other species. Untreated pulmonary infections sometimes erode through the tissues of the thorax to discharge onto the chest wall through sinuses.
Nocardiosis is diagnosed through the culture of Nocardia spp. from normally sterile sites. Identification of the bacterium used to be a bit of a nightmare and was based upon colonial appearance on agar, Gram appearance, modified ZN and unreliable biochemical methods. Nowadays MaldiTOF and PCR are excellent and reliable ways of identifying this bacterium.
If someone is suspected of having nocardiosis then further imaging is required, especially of the thorax. It is worth talking directly to the Radiologists to find the best modality for investigating. In my experience CT is often useful but I’m not a Radiologist. Confirmation of infection usually requires sticking a needle into the suspected site and aspirating pus for further laboratory testing; again this is often conducted by Radiologists.
Any immunocompromised patient with cutaneous or pulmonary infection should also have a brain scan to look for dissemination.
How is nocardiosis treated?
The treatment of nocardiosis can be difficult. It is important to test the individual bacteria for antimicrobial sensitivity as they are not always predictable. There is no national or international standard method for doing this but both Etests and broth dilution are suitable.
The species of bacterium is also important as some are more resistant than others e.g. N. farcinica is usually resistant to 3rd generation cephalosporins such as Ceftriaxone as well as Imipenem. Source control is also important, including drainage of abscesses and debridement of infected tissue wherever possible.
Trimethoprim-Sulfamethoxazole (Septrin) is the usual first line treatment with other agents added as required. Empirical first line treatments include:
Mild/moderate pulmonary infection
PO Trimethoprim-Sulfamethoxazole 5mg/kg BD (TDS if immunocompromised)
Severe pulmonary or disseminated infection (initial treatment)
IV Trimethoprim-Sulfamethoxazole 5mg/kg TDS
IV Amikacin 7.5mg/kg BD
IV Imipenem 500mg QDS
PO Trimethoprim-Sulfamethoxazole 5mg/kg BD
Severe pulmonary or disseminated infection
2 of the following:
PO Trimethoprim-Sulfamethoxazole 5mg/kg BD
PO Minocycline 100mg BD
PO Co-amoxiclav 875mg BD (note this is an unusual dose)
- Pulmonary 6-12 months
- Immunocompromised or disseminated ≥ 12 months
- For severe infection or disseminated disease IV antimicrobials should be continued for at least 6 weeks at which stage the patient can be converted to orals
- Cutaneous 3-6 months
Once sensitivities are known patients on three agent regimens can be converted to two agent regimens.
If there is no improvement within 2 weeks of starting treatment then the patient should be reviewed, further source control undertaken and consideration given to changing the antibiotics to alternative agents. Other antibiotics shown to have activity against some Nocardia spp. include: Co-amoxiclav, Ceftriaxone, Ciprofloxacin, Clarithromycin, Linezolid, Minocycline, Doxycycline and Tobramycin.
It is important to keep a close eye on these patients to look for disease progression as well as treatment toxicity. The most important toxicity is bone marrow suppression from the prolonged Trimethoprim-Sulfamethoxazole treatment. At least fortnightly full blood count, Urea and electrolytes and liver function tests should be checked. In addition further imaging is often required; some experts repeat imaging at 1, 3, 6 and 12 months of treatment and then 6 and 12 months after treatment to ensure there is no relapse.
With early diagnosis and targeted antimicrobial treatment the long term survival is 90%.
So our patient was brought back into hospital and reassessed. A CT scan of their chest showed a number of cavities however the scan of their head was normal. They were started on IV antimicrobials and by two weeks had started to improve. The patient was advised it was going to be a long course of treatment, the Microbiologist returned to the list of blood cultures to see if he could “intervene”, or more accurately “meddle”, with any other patient and their unsuspecting Registrar!