Paolo G. Nuciforo
Overall, the work is of good quality. The topic is also clinically relevant, as being able to diagnose cancer from an analysis of a saliva sample using microbial markers would be a major breakthrough.
The concept is not entirely novel (particularly in the pancreas, where there are publications reporting microbial signatures associated with cancer). Perhaps the most novel aspect of this publication is the MF index [from ‘mouth to faeces’], derived from the intra-patient analysis of saliva and faecal samples. This index is based on the presence of bacterial sequences common to both types of sample (indicating potential transmissibility) rather than on overall relative abundance (as most studies do). The results regarding its diagnostic utility in colorectal cancer are promising (sensitivity is higher than that of the faecal occult blood test) but require independent validation, as the cohort in which the faecal occult blood test was examined is very small. The results for stomach cancer are less robust when validated in independent cohorts.
However, there are many limitations. The first and most important is that the analytical methodology (16S gene sequencing) does not allow for the precise classification of bacteria at the species and strain level. In other words, although the authors identify the same bacterial sequences in the two samples (saliva and faeces from the same patient) — and thereby generate the MF index — this does not necessarily indicate that the same bacterial strain has ‘travelled’ from the mouth to the gut. To determine this, more in-depth sequencing would be required. Secondly, the results do not allow for the establishment of causality (Do these bacteria that travel from the mouth to the gut have any effect once they are there? Do they help the tumour to grow?), and therefore do not allow for the evaluation of possible therapeutic interventions that might reduce the mouth-to-gut flow of these bacteria or promote that of other, more beneficial ones. And, clearly, a prospective clinical trial is needed to validate the MF index as a diagnostic test.