The results provided by our cross-sectional data set should be further confirmed in a follow-up study and, if possible, with a larger sample and an intervention trial to see, if the response to training is actually genotype-dependent.
Laboratory studies which artificially exposed a research sample to an intervention were also excluded.
Since this study is a validation study, it is neither reasonable nor necessary to separate the study sample into an intervention and a control group nor to evaluate the groups for imbalances.
To further evaluate the applicability of the method, a subset of urine samples from an intervention study that involved patients on a diet high in flavonoid content, and with supplementation of isoflavones was analysed.
In the field of infectious diseases, planning the required sample size for an intervention trial raises special issues.
Strengths of the study included a large practice and patient sample size and an intervention based on published trials and delivered at an intensity feasible in primary care.
The limitations are familiar to feasibility studies: a relatively small sample size and an intervention that needed additional refinement, specifically with input from the target population.
The real-time PCR conditions were optimised and the specificities of the assays were confirmed by analysing DNA from related bacterial strains and from faecal samples originating from an intervention study.
We could not use the complete sample at follow-up (N = 582) for the longitudinal analyses, since half of the sample took part in an intervention to reduce gender violence.
The prospective obtainment of biological samples from subjects involves an intervention with human subjects that differ in kind from the interaction that occurs with qualitative research designs (e.g., use of questionnaires, interviews, and focus group discussions), and hence, investigators might hold different perceptions regarding the need for certain ethical safeguards.
