Very quickly, within 24 hours of initiating a high-fat/low-fiber or low-fat/high-fiber diet the microbiome composition changed detectably (Wu GD, et al. 2011).
The animal based diet supports a link between dietary fat and the growth of microorganisms capable of triggering inflammatory bowel disease.
Just by making changes to the diet without adding any bacterial products, you can change your bacterial flora, and it does not take very long.
Short term consumption of diets composed entirely of animal or plant products alters the microbial community very differently (David LA, et al. 2014).
Does that mean that probiotics are useless?
No, probiotics can be very helpful if you have to take an antibiotic.
David LA1, Maurice CF2, Carmody RN2, Gootenberg DB2, Button JE2, Wolfe BE2, Ling AV3, Devlin AS4, Varma Y4, Fischbach MA4, Biddinger SB3, Dutton RJ2, Turnbaugh PJ2. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014 Jan 23;505(7484):559-63. doi: 10.1038/nature12820. Epub 2013 Dec 11.
Wu GD1, Chen J, Hoffmann C, Bittinger K, Chen YY, Keilbaugh SA, Bewtra M, Knights D, Walters WA, Knight R, Sinha R, Gilroy E, Gupta K, Baldassano R, Nessel L, Li H, Bushman FD, Lewis JD. Linking long-term dietary patterns with gut microbial enterotypes. Science. 2011 Oct 7;334(6052):105-8. doi: 10.1126/science.1208344. Epub 2011 Sep 1.
The probiotic yeast Saccharomyces boulardii has shown to exert an anti-inflammatory effect on intestinal epithelial cells (Sougioultzis S, et al. 2006). Saccharomyces has shown to significantly reduce the frequency and duration of acute diarrhea in children (Billoo AG, et al. 2006. Kurugol Z, et al. 2005). There is evidence that Saccharomyces reduces the risk of antibiotic-associated diarrhoea in children (Kotowska M, et al. 2005).