Medical Fun Facts Podcast

MFF0042: Microscopy, culture and susceptibility

If I’ve timed this correctly, tonight’s show is dropping on Thursday 23 February 2017.

Now that I’ve finished a series, that is an alphabet series, rather than starting at A and repeating the process, my friend Kaitlyn suggested I do a series on diagnostics. [In case you didn’t know, it was Kaitlyn who inspired Medical Fun Facts.]

Tonight, I’m starting with something dead easy. What does micro, culture and sensitivity or MCS mean when you see it on a request form?

In microbiology medical testing laboratories (accredited to ISO 15189), the bulk of specimens are for microscopy and culture, and if bacteria grow the referring doctor would like to know what antimicrobials the bacterium is resistant to as well as what it may be susceptible to. Sensitivity is an older word we used to use but which didn’t quite represent what we were trying to achieve in the laboratory. These days, we’re not just interested in susceptibility but also resistance. I like to think about ART or antimicrobial resistance testing as opposed to AST, i.e., antimicrobial susceptibility testing. Knowing what a bacterium is resistant to in many ways is more important.

The microscopy we do can vary. For example, for liquid specimens like urine or cerebrospinal fluid, we examine it neat under the microscope and look for cells, red blood corpuscles as well as parasites. When using a ×10 eyepiece and an ×40 objective lens we get 400× magnification and we can see bacteria, often we see bacilli moving because they have flagellæ. For more tenacious specimens like sputum or pus, we’ll do my favourite test in clinical microbiology, viz., the Gram’s stain. We examine the slide at 1000× magnification (with oil immersion) and we can better describe the morphology of the bacteria fixed to the slide.

Most specimens are cultured by applying some of the specimen to the surface of agar plates and for some we also inoculate a broth. Some broths are for enrichment of growth, while others are to inhibit commensal bacteria and permit pathogens to grow. For example, the Selenite broth used for faeces is commonly mistakenly called an enrichment broth. The beauty of Selenite is that it inhibits most enteric bacteria but permits Salmonella to flourish.

After overnight incubation, most pathogens will be apparent on solid phase agar, and we can use various pieces of technology to identify the bacteria to a species level and then undertake antimicrobial resistance testing. I’ll describe some of that technology in future shows in this series.

Now I often hear people use the term ‘speciate’ when they are trying to identify a bacterium. That is an incorrect use of the term. Speciate means to designate a new species. What people are really doing is identifying the bacterium to a species level. In public health, we go beyond that and we characterise the bacterium to determine if the bacterium is part of an outbreak.

For example, in Canberra, we currently (February 2017) have an outbreak of salmonellosis associated with a couple of restaurants. It’s important to make sure the Salmonella isolated from the premises is the same as the Salmonella isolated from patients. There are a few ways to do this. In the old days, we did phage typing. We currently rely on molecular microbiological techniques and soon we will employ whole genome sequencing. Microbiology is changing rapidly and it’s very exciting.

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