The laboratory detection of verotoxin/shigatoxin producing E. coli (sometimes called enterohaemorrhagic E. coli) has caused much grief for diagnostic microbiology laboratories over the decades. It is a relatively nasty infection, and it can cause bloody diarrhoea in a good proportion of patients. In a small minority it can cause severe complications such as Haemolytic Uraemic Syndrome (HUS) or Thrombotic Thrombocytopenic Purpura (TTP).
Diagnosis was initially centred on the culture of E. coli 0157 which can produce verotoxin/shiga toxin.
SMAC (Sorbitol MacConkey) agar plates were all the rage in the 1990s, taking advantage of the fact that E. coli 0157 does not ferment sorbitol.
How lucky is that?
These were soon replaced by the more selective CTSMAC (Cefixime Tellurite Sorbitol MacConkey), with the cefixime and tellurite inhibiting other annoying non-sorbitol fermenters such as Proteus Spp.
The only problem is that E. coli 0157 is not the only E. coli serotype that can produce verotoxins. Lots of other E. coli serotypes are capable of doing this as well, e.g. 0111, 026, 045, 0145, etc.. As time passed, and our understanding of the infection improved, it became very apparent that a very significant proportion of VTEC induced diarrhoea was actually not due to E. coli 0157.
Nevertheless, CTSMAC plates were now entrenched in laboratories. And it was better than nothing.
As the years passed, alternative methods came onto the scene.
ELISAs used for “direct” VTEC toxin detection in stool were employed in some labs in the early 2000s. At least they detected non-0157 associated disease, but sensitivity remained an issue when used directly on samples. They were not widely adopted by diagnostic laboratories.
Chromagar plates have also been developed to pick up the main VTEC serotypes. A little pricey however, and still need follow-up work for confirmation.
Then came PCR, and more recently multi-plex PCR, not only detecting (the toxins of) VTEC, but all the other common gastrointestinal pathogens as well.
In the molecular age, CTSMAC plates are starting to look a bit dated. What was seen as modern methodology a generation ago no longer cuts the mustard.
As we move through this transition period for VTEC detection there is a real mish-mash of different VTEC methodologies used in laboratories worldwide. I don’t think this messy situation will last. In a decade or so I suspect 90% or more of microbiology laboratories will be using molecular methods for VTEC detection (and everything else stool related).
However at the moment, there are still plenty of CTSMAC plates being manufactured worldiwde. We still (guiltily) use them at our lab, as we continue to work out how to afford molecular testing for enteric pathogens…
But now they are used in the knowledge that they will clearly not pick up all VTEC strains in the patient samples, or anywhere close.
CTSMAC plates are getting old, and I for one can’t wait to see the back of them…
Note that the Infectious Diseases Society of America has just brought out updated guidelines on Infectious Diarrhoea, including quite a bit of detail on VTEC/STEC. Apart from the incorrect spelling of diarrhoea, they are very good!
I will add them to the guidelines section of this website also.