One of the best courses I took in university was a wildlife biology ecology course. OBVIOUSLY, the course material is not relevant to my current job, but the professor spent a better course of a month drilling into us that there are always two sides to every story and not to read and believe everything you see in print. He did not care if it was a blog (admittedly those weren’t around in my university days), a newsletter article or a peer reviewed journal article. We were taught to look at what was presented critically and determine if both sides of the story were being presented equally or if the story, article or study was being presented in a manner that was a little less balanced. As a result of this course and a couple of great mentors, I admit I may be a bit jaded when reading articles and studies. I generally don’t take things at face value and generally drive people insane with my sometimes never-ending stream of questions. Much to the chagrin of my colleagues, I’m the self proclaimed “Ya, but..." girl.
However, there are times when this irritatingly inquisitive approach comes in handy. Case in point was with a recent question as to the efficacy of a product against an arm’s length list of Salmonella strains where I was asked to answer yes or no to whether Product A was effective against the following:
- Salmonella enterica – pullorum
- Salmonella enteritidis
- Salmonella schottmuelleri
- Salmonella typhi
- Salmonella typhimurium
As we have discussed in several previous blogs, the number of kill claims is not what is relevant. Microorganisms, particularly bacteria that have numerous strains, should be considered as a single pathogen – if you kill one, you kill them all!
In the case with Salmonella spp, in order to obtain a disinfectant claim in Canada or the US Salmonella enterica is one of the three surrogate organisms used to ensure a product is considered a hospital disinfectant with broad-spectrum efficacy against vegetative bacteria. My response to this question was as expected - If the product kills S. enterica it would be effective against all strains of Salmonella.
Upon giving that response however, I received an email citing a study conducted in 2002 that concluded “variations in susceptibility to disinfectants has been observed between Salmonella strains” so I did what anyone respectable person would do. I found the reference, reviewed it and called someone far smarter than I (a world renowned microbiologist) to comment! The response I received back nearly made me snort my coffee out my nose and took me back to my third year Wildlife Biology Ecology course.
The response I got back was “The paper is not even worth the paper it is written on, it was a good example of how one can make a mockery of the peer-review process and should not be given any further consideration”. This respected microbiologist then went on to agree with my stance that if you kill one strain you kill them all. But why did this microbiologist draw such strong conclusions? For this person it was easy. First, the study stated that the disinfectant test method it used was “close” to how the chosen products were used in the field. Seems reasonable, BUT the test method, based on a Master’s thesis by the first author, was a carrier test using pieces of stainless steel and the test disinfectant was sprayed on the carriers with dried bacterial inocula, which is not a test method accepted by Health Canada or the EPA for disinfectant product registration. Further, among its numerous weaknesses with the test method were: (a) no added soil load, (b) no mentioned neutralizer, (c) no quantitation of viable bacteria - the results were based on the presence or absence of turbidity in the inoculated tubes, (d) the fact that at the end of the contact time, the excess disinfectant was simply drained off into a filter paper - this has major implications for the degree of dislodgement of the bacteria from the carrier surface depending on the detergent activity of the formulation under test, (e) the inclusion of 3% hydrogen peroxide (unformulated) in the testing as a ‘disinfectant’, (f) the requirement of a complete kill as the product efficacy criterion, (g) no indication of the number of viable bacteria on the carriers after the inoculum drying process, and (h) the dipping of the entire carrier into the recovery medium, which could have resulted in turbidity (therefore, failure of product) from the growth of one single bacterium surviving the disinfectant treatment.
In a nutshell, there were numerous holes in the method used and while interesting, the test method used was not reflective of the requirements for manufacturers to register products, and therefore the results could be considered suspect. The conclusion of both the microbiologist and me on using representative strains of pathogens to draw conclusions on the overall field effectiveness of its products is valid in general, and also endorsed by regulatory agencies.
I hope the next time you read a study you’ll consider if both sides of the story are being told, particularly when reading a study on the efficacy of products you consider, and validate if the test method used is one that will be accepted by regulatory bodies for registration. If it’s not, the study is interesting, but not necessarily one that you can apply to your practice.