Medical Fun Facts Podcast

MFF0049: Sputum microbiology

If I’ve timed this correctly, tonight’s show is dropping on Monday 20 March 2017. I’ve just returned from a meeting in Sydney.

Tonight, I want to mention some points about sputum and how we think about it in microbiology.

So, what is sputum? Sputum is the material (or phlegm) that is coughed or expectorated from the lower respiratory tract, that’s mainly the bronchi. While true sputum reflects the lower respiratory tract, to get it out, it must pass through the upper respiratory tract, which means sputum is prone to contamination with material from the throat and mouth, especially the bacteria in the mouth and sometimes the nose if the patient also has rhinorrhoea (a runny nose).

Sputum isn’t only useful for microbiology but also cytopathology. Cytopathologists look for malignant cells in sputum, it’s never good news when malignant cells are found.

There are three phases to sputum examination in microbiology. Firstly, we look at the specimen with the naked eye. If it looks like a juicy oyster with little or no mucus, it’s good to continue for microscopy. If it’s mostly saliva, a note will be made before the next stage. If the specimen contains food particles then it’s rejected.

Sputum can be submitted for routine microbiology as well as for evidence of acid-fast bacilli and the presence of mycobacteria.

You may wonder how could we reject a specimen from a patient who is unwell? Well, part of being a pathologist and medical laboratory scientist is a deep desire to do the very best for each patient. We can only do that if we evaluate each specimen and referral and act based on quality improvement principles. There is no point processing a poor specimen. A poor-quality specimen will often grow normal oral flora. If we report those bacteria, the referring medical practitioner may gain the incorrect impression that the authorising pathologist considers those bacteria as clinically significant. That may lead to inappropriate use of antimicrobials which will have no bearing on the patient’s illness and may contribute to further antimicrobial resistance.

Apart from how the specimen looks when we hold the container up to the light, we also assess the suitability of the specimen for culture by microscopic examination using a Gram’s stain. The Gram’s stain is one of my favourite tests in microbiology and a test that can reveal a wealth of information. Assuming a sputum looks good, a smear is made on a glass slide in a biological safety cabinet class II. The smear needs to dry and then it’s fixed by warming the slide. I won’t go into the details of the Gram’s stain here, I’ll leave that to another show. Suffice to say when looking at the slide down a microscope, if I see a lot of epithelial cells it indicates the specimen is mostly saliva and not sputum. Good sputum will have pus cells (polymorphonuclear cells) and an abundance of bacteria. It’s possible to diagnose some respiratory infections from a Gram’s stain, e.g., pulmonary Melioidosis has a typical Gram’s stain appearance. In a patient with cavities in their chest x-ray, the Gram’s stain can reveal staphylococcal pneumonia or an anaerobic infection associated with lung abscesses. As I mentioned in episode 21, ghosting in a Gram’s stain may suggest a mycobacterial infection.

While the Gram’s stain is being prepared, various agar plates will be inoculated. The typical respiratory pathogens routinely sought include pneumococcihæmophili, moraxellæ, staphylococci, pseudomonads, and some streptococci. Depending on the scenario a sputum will typically be set up on blood agar, chocolate agar and MacConkey agar. In big teaching hospitals, some patients at risk of being colonised with multi-resistant bacteria will also have screening agar set up to select for certain resistant bacteria.

After overnight incubation, the plates are examined and assessed in the context of the clinical information as well as the Gram’s stain results. If a respiratory pathogen grows in pure culture, or heavy growth as the predominant flora, and if pus cells are seen in moderate to high numbers in the Gram’s stain along with corresponding bacteria, the result is clear and the bacterium will be selected for antimicrobial susceptibility testing.

In my opinion, I’d reject most sputum specimens because they aren’t really sputum and quite often what we see and grow doesn’t have a good clinicopathological correlation.

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