Probiotics have been consumed safely via food for thousands of years…
ProVen to be safe
A large number of clinical trials have been undertaken using probiotics for a wide range of diseases, with few associated adverse events. As a result, probiotics are generally assumed to be safe, although few systematic safety studies have been done with probiotic organisms, especially in vulnerable populations. 
Most of the rare cases of infection with lactobacilli have occurred in patients with severe underlying conditions, although populations identified as potentially vulnerable include those with immune compromise, premature infants, patients with short bowel syndrome, those with central venous catheters and patients with cardiac valve disease. 
Probiotics generally regarded as safe
Whilst there are some theoretical adverse risks relating to the use of probiotics in humans, Lactobacilli, bifidobacteria and lactococci strains of probiotics are generally regarded as safe.
Experts recommend that strains used for probiotics should be selected from the commensal flora of humans and should not carry intrinsic resistance to antibiotics as this would prevent treatment of a rare probiotic infection. They also state that responsible probiotics manufacturers have procedures for surveying adverse events caused by existing strains and for screening new strains. 
Safety of ProVen Probiotics
The Lab4 consortia of bacteria used in ProVen Probiotics were developed more than 15 years ago. Since then they have been studied in 10 clinical trials with a variety of populations, including pregnant women and newborn babies.
As part of the Swansea Baby Trial with pregnant mothers and their newborn infants, the safety of the probiotics was tracked and shown to be safe.
The Lab4 bacteria are also used in a wide range of products sold in very large numbers worldwide and are monitored for reporting of adverse events.
The gut microbiota interacts with the immune system and provides signals to promote the growth of immune cells and the normal development of immune function. Improper training of the immune system by the microbiota is known to result in immune-mediated diseases, including susceptibility to influenza, retrovirus transmission and more serious diseases such as colon cancer and auto-immune demyelination. 
 Doron S & Snydman DR 2015 Risk and Safety of Probiotics Clin Infect Dis 60 (Suppl 2)
 Snydman DR 2008 The Safety of Probiotics Clin Infect Dis 46 (Suppl 2)
 Borriello SP et al 2003 Safety of Probiotics That Contain Lactobacilli or Bifidobacteria Clin Infect Dis 36:775-80