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Making bacteria visible

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For a microbiologist, viewing bacteria is rarely a problem. When I look at bacteria in the lab they are samples that I have grown specially, in aseptic conditions to stop any other bacteria getting in. They will all be the same species of bacteria, grown in media that favours them exclusively and is designed to make them easy to see under a microscope.

For medical microbiologists and pathologists it’s a whole different story. They are attempting to view bacteria within a sick patient, which means that as well as infectious bacteria the slide will be covered in human cells, sometimes tumor cells, random bits of blood platelets, random non-infectious bacteria and a whole load of other small blobs which are hard to differentiate between.

It's the small purple line on the right hand side about half way up the slide.

Can you spot the bacteria? Image from wikimedia commons

This means that it’s often hard to spot the start of a bacterial infection. It’s only when there are large enough numbers of a bacteria that they can be viewed with most imaging methods. This is where a new type of imaging probe can come in handy. By attaching an imaging probe (i.e a very visible molecule) onto a bacterial sugar called maltodextrin, very small amounts of bacteria can be viewed, at far lower concentrations than other imaging techniques.

Maltodextrin is special because although bacterial cells use it as a source of sugar, human cells are not able to internalise it. This means that it picks up only bacterial cells with high specificity. It also has a clever trick for avoiding all of the normal bacteria found within a human body. The ‘normal’ (or commensal) bacteria that live in your gut are surrounded by mucas from the intestinal wall. Large glucose oligomers (such as maltodextrin) are unable to get through this, so the gut bacteria won’t pick up the signal.

*Fear* my dodgy paint skills

The Maltohexaose molecule with the imagine probe attached. Maltohexaose is the helical form of maltodextrin. Modified from fig. 1 of the reference.

By disguising the imaging probe as food, this technique allows a large concentration of the probe to build up inside the bacteria. This produces far more visible entities than conventional probes which merely attach to the bacterial cell surface. Maltodextrin is  internalised in both aerobic and anaerobic conditions, and by a wide variety of bacteria including Gram negative and Gram positive species. It is also taken up by biofilms, which are large aggregates of bacteria all working together.

This kind of research is really useful within a clinical setting, where the ability to detect small quantities of bacteria in an infection can provide a swift diagnosis for treatment. In the case of biofilms it is vital to identify an infection as quickly as possible because biofilms are notoriously difficult to treat once they have become properly established. Techniques like this can provide useful and simple ways to correctly identify bacterial infections.

Ref 1 = Ning X, Lee S, Wang Z, Kim D, Stubblefield B, Gilbert E, & Murthy N (2011). Maltodextrin-based imaging probes detect bacteria in vivo with high sensitivity and specificity. Nature materials, 10 (8), 602-7 PMID: 21765397

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Lucas 5:57 am 07/29/2011

    That’s a clever solution! Although I guess the probe is not good for differentiating between pathogenic and commensal bacteria right?

    PS: I suspect the pink sausage-shaped cells of being the bacteria in the picture!

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  2. 2. S.E. Gould (labrat) 4:46 am 07/30/2011

    The differentiation between pathogenic and commensal cells is done via location I believe. Commensal cells tend to be locked behind mucus barriers where the probe doesn’t go.

    And yes! It’s the little pink line on the right hand side. There are remarkably few pictures of rally ‘confusing’ slides out there, this was the best I could find :p

    Link to this

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