Based on studies of both humans and mice, ketogenic diets dramatically reduced the common probiotic Bifidobacteria. However, the studies also found that the ketogenic diet reduces the gut microbial content of phyla Actinobacteria, Bacteroidetes, and Firmicutes, and 19 different bacterial genera. The studies were led by Peter Turnbaugh, Ph.D., a University of California San Francisco associate professor of microbiology and immunology, and a member of the UCSF Benioff Center for Microbiome Medicine.
With the lower level of carb intake, based on studies of mice, a ketogenic diet may lead to some of the effects of ketosis quite quickly. Ketosis is not necessarily good for you as acidic byproducts can build up in our bodies called ketoacidosis.
See Medical Express article here, https://medicalxpress.com/news/2020-05-ketogenic-diets-gut-microbiome-humans.amp . The article was first reported in the Cell, https://www.cell.com/cell/pdf/S0092-8674(20)30490-6.pdf