Functional constipation affects 5-20% of the population. Intestinal microflora has a significant impact on intestinal motility. Probiotics can improve intestinal motility. Based on a summary of 11 randomized, double-blind studies with a total of 464 subjects, it has been concluded that probiotics can increase intestinal transit time.  Bifidobacterium lactis stran bacteria are the most effective in this regard.
In a 2-week study involving 100 subjects (average age – 44), one group received a dose of B.lactis (17.2 billion CFU, n=33), one received a low dose (1.8 billion CFU, n=33), and one was a placebo group (n=34). After 2 weeks the overall intestinal transit time, i.e. the time it takes for the intestines to empty, had significantly decreased in the two B.lactis groups, while remaining unchanged for the placebo group. 
Probiotics speed up intestinal transit because they reduce the formation of methanogens, which can hinder intestinal transit by producing methane, which, in turn, blocks intestinal motor function or motility. It has been found that people with slow intestinal transit have more methane producing bacteria in the intestines as compared to healthy individuals. Furthermore, probiotics promote fermentation processes in the intestines thus boosting the peristalsis, i.e. the frequency of contraction, of the large intestine thus speeding up transit time. 
- Miller LE, Ouwehand AC.Probiotic supplementation decreases intestinal transit time: meta-analysis of randomized controlled trials. World J Gastroenterol. 2013 Aug 7;19(29):4718-25.
- Waller PA, Gopal PK, Leyer GJ, Ouwehand AC, Reifer C, Stewart ME, Miller LE. Dose-response effect of Bifidobacterium lactis HN019 on whole gut transit time and functional gastrointestinal symptoms in adults. Scand J Gastroenterol. 2011 Sep;46(9):1057-64.
- Chang Hwan Choi and Sae Kyung Chang. Alteration of Gut Microbiota and Efficacy of Probiotics in Functional Constipation. J Neurogastroenterol Motil. 2015 Jan; 21(1): 4–7.