Can a non-pure culture be used for reliable rapid phenotypic diagnostic tests (e.g. catalase, oxidase & gram stain)?
"Slide Test Method
Obtain a pure culture of the organism to be tested. Using an inoculating needle or applicator stick, pick a well-isolated colony and transfer to a glass slide. add 1 or 2 drops of the Catalase Reagent to the smear." Manual of Liofilchem® - Catalase Reagent
"Test isolates should be in pure culture and 18-24 hours old" Manual of Oxichrome Reagent, Thermo Fisher Scientific
"A pure culture is required for identification tests. However, the tests can be applied directly to colonies removed from isolation plates if there are many suspect colonies that have been correctly isolated. In the latter case, the test must be repeated after reinoculation of the colonies tested, to obtain a pure culture. Identification tests are as follows: appearance and motility, if living: Gram staining, oxidase test and catalase test." Manual of Standards for Diagnostic Tests and Vaccines: Lists A and B Diseases of Mammals, Birds and Bees, Chapter 2.3.2 Bovine genital campylobacteriosis [Campylobacter fetus]
Is it a concidence that the author stated the usability of these tests with non pure cultures when the concerned organism, Campylobacter fetus, has a positive result in all of them?
What i mean, the author could be indirectly implying that these tests in such condition are useful only for pre-screening purposes (i.e an experimental positive result with an expected positive result should be interpreted that at least one organism from the tested growth is positive, where an experimental positive result with an expected negative result doesn't necessarily mean that our subject organism is not there)?
I want to understand correctly why a non-pure culture with discrete subject colonies can be bad for atleast some diagnostic procedures. Is it because "Where there is one rat, there are more"? In other words, a fast-growing thus apparent type of contaminant means that there could be more slow-growing thus inapparent types of contaminants, which can be within discrete colonies of the subject organism.
If this is the correct reason, then it sounds to be a problem for highly sensitive molecular methods (e.g. PCR) and culture-based tests (e.g. OF-glucose test & simmon's citrate test) where the number of contaminating cells will be magnified, but not for rapid phenotypic tests as those mentioned, which are relatively insensitive and on-spot.
If this wasn't just a theoretical concern that was commonly exaggerated, then this leaves Gram-negative selective media as MacConkey, not usable for such tests even in pure culture, as:
In obtaining a pure culture, it is important to realize that the selection of a single colony from a plate does not necessarily assure purity. This is especially true if selective media are used; live but non-growing contaminants may often be present in or near a colony and can be subcultured along with the chosen organism. Krieg, N.R. (2005). Identification of Procaryotes. In: Brenner, D.J., Krieg, N.R., Staley, J.T., Garrity, G.M. (eds) Bergey’s Manual® of Systematic Bacteriology. Springer, Boston, MA. https://doi.org/10.1007/0-387-28021-9_5