The bacteria noted above were identified as potential candidates for this "biopreservation" because they're lactic acid bacteria (LAB), which compete for nutrients with pathogenic organisms and produce antimicrobial compounds, while remaining safe to eat for humans. The study looked at a number of potentially pathogenic organisms that might be found in meat, including E. coli, Salmonella, Yersinia enterocolitica, and Listeria monocytogenes. The results were generally positive: whether alone, or in combination, the Lactobacillus strains were successful in limiting the growth of the pathogenic bacteria. With a couple of exceptions, combining the strains was much more successful at reducing bacterial growth than applying them individually.
The second part of the study performed by the researchers was to determine whether the inhibition was due to the Lactobacillus cells producing a compound that prevented the growth of pathogens. Since they had already shown that the presence of the cells themselves were effective, they simply removed the cells from the growth medium by way of centrifuge, and applied the supernatant to the growth plates. The supernatant produced no noticeable reduction in the growth of the pathogens. Based on the lack of success of the supernatant inhibiting pathogenic growth, the authors posit that the production of organic acids by the Lactobacillus species is responsible for preventing other bacteria from growing.
It's worth noting that while the study did show that the two Lactobacillus species had an inhibitory effect on the growth of potentially pathogenic organisms, it was very much an in situ experiment. There is still a lot of work to be done in determining how to actually inoculate pieces of meat with this type of organism to provide a net food safety benefit. However, this is an exciting step forward in preventing pathogens from multiplying in meat: biopreservation is natural, safe to consume, adds no chemicals, and does not alter the taste or texture of the product - all things that consumers want.
Source: Tirolini, E., Cattaneo, P., Ripamonti, B., Agazzi, A., Stella, S., & Bersani, C. (2014). In vitro evaluation of Lactobacillus animalis SB310, Lactobacillus paracasei subsp. paracasei SB137 and their mixtures as potential bioprotective agents for raw meat. Food Control, 41, 63-68