Shigella species

Hello, everyone. My name is Dr. Sarah Fouch, and within this session, we will be considering Shigella species, the clinical conditions that they are associated with, and the virulence factors that make them successful pathogens.

We have four species of Shigella. We have Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei. Now, when isolating organisms in the laboratory, we have to be able to differentiate different species, so how would we differentiate an E. coli with a Shigella? One of the quick and easy methods we can use is to culture this organism on a media that contains lactose, because Shigella is a non-lactose-fermenting organism. A good example of a media that we could use would be MacConkey. MacConkey contains lactose. It also contains an indicator. If we isolate E. coli, E. coli can ferment the lactose, which means that there's a pH change and the indicator would be activated, so the organism would appear as big pink colonies. If we have Shigella present, Shigella is unable to ferment the lactose. The indicator will not be activated because there's no change in pH, so the Shigella will appear as orange-coloured colonies because that's the colour of the media. We can also perform biochemical tests. However, Shigella are what we call biochemically inert, which means they have very little or no reactions, so they do not produce any gas when they break down carbohydrates. We can use ONPG to identify Shigella sonnei, and this means that Shigella sonnei can decarboxylate ornithine. This enables us to distinguish between late lactose fermenters, so those organisms that will ferment lactose at a later point in time, in comparison to non-lactose fermenters. It also helps us to identify beta-galactosidase activity, so these organisms can hydrolyse beta-galactosidase and monosaccharides. We can also use mannitol to be able to differentiate between different types of Shigella, because Shigella dysenteriae is non-mannitol fermenting. If we have a media that contains mannitol, we would be able to differentiate Shigella dysenteriae. Again, we can also serotype Shigella, as we've discussed in many of the other recordings when we think about serotyping, and there are 45 antigen-based serotypes. These are antigens that will be expressed on the bacterial cell wall. We wouldn't routinely do this in clinical practice, because if we know that the patient has a Shigella infection, we would then be thinking if we wanted to treat them or not. But if medics consider that it's clinically necessary to serotype the organism, then we are able to do this.