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Sometimes they make food ferment, but used in the right way, these bacteria can also be a preservative

Lactic acid bacteria produce antibiotic-like compounds that kill off potentially harmful contaminants

Lauren Gandy

Biochemistry, Microbiology, and Chemical Biology

Rensselaer Polytechnic Institute

You probably don't often think about lactic acid bacteria, but you eat them nearly every day. They're responsible for the distinctive taste of sauerkraut, salami, yogurt, and other fermented foods. The sour “fermented” taste arises as lactic acid bacteria (LAB) break down sugars like glucose and sucrose into lactic acid. Since prehistory, LAB have been used for different fermentation processes. Not only do we get new and unique flavors from it, but the microbes act as a bio-preservative to keep food fresh.

Scientists long thought that this phenomenon was tied to the production of lactic acid, as the compound lowered the pH to where other microbes could not grow. This was a desirable quality for fermented foods that rely on the lactic acid for flavor, but what about ready-to-eat (RTE) sliced meats, which are not meant to taste sour?

Your sliced deli ham and salami can contain harmful bacteria like Listeria monocytogenes, even after preventative methods like sodium nitrites and high pressure processing have been applied. The European Union found that as high as 3% of randomly selected RTE meat products (an assortment of beef, pork, turkey, and broiler) contained L. monocytogenes in 2017. And in January of this year, the CDC put out a food safety alert for pork products contaminated with Listeria from Long Phung Food Products.

Clearly, we need more measures against food-borne pathogens to guarantee food safety: are LAB the answer? Could they be used to preserve RTE meats without creating that “fermented” flavor?

Small Lactobacillus bacteria under a microscope, seen near a skin cell.

Protective cultures are designed to be “biostatic,” meaning that they should prevent the growth of other bacteria, but not kill them, at the concentrations that they are used. The cultures also should not affect the “sensory” properties, such as color, texture, and, most importantly, taste, of the food products they are applied to. Chr. Hansen, a Danish biotech company, has cultivated specific strains of LAB (Lactobacillus curvatus and Lactobacillus sakei) and demonstrated that under refrigerated conditions, Listeria did not grow in vacuum-sealed RTE meat products when Lactobacillus was also present. The company filed a patent for the use of these strains of LAB as a food preservativein 2008.

Now, over ten years later, researchers at Shangdong University’s Lab of Beef Processing and Control wanted to see how the product stood the test of time. Focusing on locally obtained, vacuum-sealed Chinese beef, Yimin Zhang and her colleagues have evaluated how well Chr. Hansen’s SafePro® protective culture protected their meat.

Zhang's team monitored microbe activity over a 38-day period after applying the product to freshly-butchered meat. First, they treated the meat with either the protective L. sakei or L. curvatus strains, keeping some untreated for comparison later. They found that the protective cultures had the desired bacteriostatic effect not only on Listeria, but also on other food spoilage species like E. coli, Salmonella, and related bacteria. Their paper also documented the dynamics of the microbial community during storage of the vacuum-sealed beef, illustrating how early certain food-borne pathogens colonized the meat before their growth was stopped. Most importantly, the protective Lactobacillus strains caused no difference in color or freshness, and minimal difference in pH — though I did wonder about the taste.

How does LAB create bacteriostatic growth inhibition? At first, scientists thought it only had to do with the production of organic acids (things like the lactic acid found in milk and acetic acid, like in vinegar) lowering the pH. But the present study, like others, showed that inoculation of a product with LAB does not guarantee a significant drop in it's pH; over the 38 days of the experiment, the pH of the meat samples dropped from 5.5 to 5.3, a small change.

A more likely answer is the bacteriostatic peptides that the SafePro® strains produce, called bacteriocins. These molecules create holes in bacterial membranes very quickly, stopping potentially harmful bacteria in their tracks – and the bacteriocins themselves degrade just as quickly. Importantly, bacteriocins are also colorless, odorless, and tasteless, making them ideal as invisible protections in food.

A large concern, however, is the potential for bacteriocin resistance. Since bacteriocins function in antibiotic-like ways, it stands to reason that exposed bacteria, if some survive, could develop resistance mechanisms. Some strains of Listeria already have. However, the resistant pathogens were found to have severe reduction in growth that scientists link to an increased energy expenditure to develop that resistance, meaning that the mutation to counteract the killing compound hobbled the bacterium (in the same that a deer's giant antlers are good for fighting but also get tangled in tree branches). This is just one instance though, and the potential resistance mechanisms that foodborne pathogens could develop remains a grave concern as we look for alternative bacterial-control measures beyond antibiotics. In addition to biochemical means, another possible bacteriostatic effect could derive from gene regulation. Recently, researchers in Italy showed that other LAB strains have been shown to make the bacterium Staphylococcus aureus (which can cause dangerous staph infections) less infectious.

Overall, the bio-preservative future of lactic acid bacteria looks promising. Companies like Chr. Hansen in Denmark and Sacco System in Italy are already genetically modifying and cultivating LAB strains to seamlessly slip into cheeses, RTE meats, and more. But in places with the highest meat consumption, like the US, work still needs to be done to bring these tools to market.

Comment Peer Commentary

We ask other scientists from our Consortium to respond to articles with commentary from their expert perspective.

Allison J Matthews

Microbiology

Tufts University

What a creative method of food sterilization! I love that they challenged an old dogma as to why the presence of these Lactobacilli acted as a preservative. Thanks for sharing, Lauren!

I have two questions:

  1. Is it known if these bacteriocins from the lunch meats have any impact on the gut microbiome?
  2. Are the species of Lactobacillus used here similar to those used in probiotics? Do Lactobacilli used in probiotics also produce bacteriocins?

Lauren Gandy responds:

Thank you for the great questions Allison!

For the first, there isn’t a large body of literature on the impact of exogenous bacteriocins to the gut microbiome. There is speculation that due to the number of proteases in your gut (trypsin, chymotrypsin and more) that bacteriocins are sensitive to, the introduced bacteriocins will have minimal impact to human health and overall bacterial composition. In addition, many of the same bacteria in your gut actually produce bacteriocins naturally. Introducing a slightly greater concentration of natural bacteriocins, that are digested rapidly by proteases floating in your GI tract, most likely will not affect your microbiome - although the difference between gut microbiomes in different populations, where these species are not native, might tell a different story.

Moreover, many naturally-eaten LAB-containing foods (like yogurts and saukreat) are known to impact your gut microbiome, but only transiently. Even though we now know they produce bacteriocin during food fermentation, the overall impact to gut microbiomes most likely derives more from the consumed bacteria rather than the small amount of antimicrobial bacteriocin they produce.

For your second question: These Lactobacillus strains (L. sakei and L. curvatus) are currently sold as probiotic supplements, though they are not the most common Lactobacillus probiotic species. L. sakei is a probiotic nasal supplement and L. curvatus has been investigated  as a lipid metabolism probiotic. Chr. Hansen has not patented their L. sakei and L. curvatus strains as probiotic, but this does not mean they do not have probiotic properties.

And yes! Many probiotic Lactobacillus strains produce bacteriocins - for example, researchers recently found that the L. acidophilus strain in probiotic supplement Gynoflor made by Medinova naturally produces bacteriocins. There may be some exceptions (as there always are) but for the most part it looks like probiotic Lactobacillus strains naturally produce bacteriocins that we may only now understand contribute to their probiotic ability.

Rebecca Dzombak

Biogeochemistry

University of Michigan

Thanks Lauren! I have a sort of tangential question. Would similar preservation concerns and techniques be applied to lab-grown meats too (in the future), or would the focus of contamination be different because lab-grown meat wasn’t originally sourced from an animal?

Curious to hear your thoughts!

Lauren Gandy responds:

Yes! I love that as we gain a greater understanding of bacteria, we realize how much they do without us even knowing. The probiotic supplement Gynoflor I mentioned in response to Allison’s questions is actually a very well-established vaginal microbe probiotic and treatment, in the market for over 14 years, and just now we are beginning to understand their antimicrobial mechanisms via bacteriocins (research paper published in 2018). I can’t wait to see how we can harness the amazing natural power of microbes as we develop a deeper understanding of how and why they work when they do.

For Rebecca’s really intriguing question: Since foodborne pathogens can be introduced at any levels of the meat preparation/storage process for RTE foods, I would think that similar preservation concerns would apply to lab-grown meats after growth i.e. for packaging and month-long storage in grocery stores. It looks like the process is very well-monitored to avoid any unwanted bacterial contamination at least.

However, one of the things the main study showed is how the exogenous LABs interacted with the endogenous LABs on the locally-sourced beef, and though the exogenous LABs were dominant for a month period, the endogenous bacterial population also shifted in response to the environment (transport and fridge storage). How they shifted, like what metabolites they produced and what food source they ate, influences the RTE meat during storage.

I’m not sure what introducing LABs to an essentially (or minimally) non-inhabited meat product would do; most likely the LABs would become the dominant microbial community, thereby preventing foodborne pathogens like Listeria but perhaps influencing flavor, as there is nothing on the surface to compete with them. Definitely as we understand more how the lab-grown meat deteriorates under normal storage conditions, scientists can adjust what their biopreservative strategy will be.

Lauren Sara McKee

Microbiology, Biochemistry, and Agricultural Science

KTH Royal Institute of Technology

Great article, Lauren! As Allison already commented, it would be cool to know if these are the same Lactobacillus species that are used as probiotics. If so, they could be protecting us from pathogens inside our bodies as well.

This story reminded me that Lactobacilli are also the dominant bacteria in the human vaginal microbiome, particularly in women of reproductive age. This report discusses some of the reasons why we might have this very particular community, and they specifically give the examples of inhibiting growth of harmful bacteria, and promoting gene expression and DNA repair. How fascinating that the same mechanisms have relevance in food protection - microbes are amazing.

Molly Sargen

Bacteriology

Harvard University

It’s always exciting to hear about new applications for microbes! However, adding microbes to foods requires a careful consideration of other effects the species might have on the product. With greater knowledge of the bacteriocins’ targets, it might be possible to provide the bacteriocins without the LABs and avoid the effects LABs. It will be interesting to see what directions scientists pursue following these findings. 

Lauren Gandy responds:

Hi Molly,

Good point! Yes, there has definitely been a focus on isolating and purifying just the bacteriocin - as seen from nisin from Lactobacillus lactis sold as Nisaplin and pediocin PA1 from Pediococcus acidilactici sold as Microgard. However, these are the only bacteriocin preservative products approved to date. Much of the challenge with bacteriocins, such as pH and temperature stability, GRAS designation, high specific activity and broad spectrum application, are the same ones seen with using whole bacteriocin-producing microbial culture as preservatives.

But using the whole culture of bacteriocin-producing LAB over purifying the antimicrobial peptide has some advantages; for example, whereas the pure bacteriocins can be absorbed into the food and degraded, exogenous LABs will survive on the food surface and continue to produce bacteriocins for a longer period of time. In addition, the LABs investigated all have a GRAS (generally recognized as safe) designation, and therefore have been shown to not have significant side effects upon consumption, whereas certain bacteriocins may require much more extensive characterization and formulation to reach that level.

It will definitely be critical for a long-term switch from chemical to microbial preservatives to see how industry chooses to integrate either bacteriocin-pure-production or LAB-whole-culture into long-term production and handles challenges for scale-up.