• P.K. Peterson

In Praise of Lactobacillus: My Favorite Germ

“The majority of diseases begin in the digestive system where ‘good’ bacteria are no more able to control ‘bad’ bacteria."

- Elie Metchnikoff, father of cellular immunology

“If you don’t like bacteria, you’re living on the wrong planet.”

- Stuart Brand, American writer, editor, Whole Earth Catalog

Four months ago, one of my granddaughters asked, “Grandpa, what is your favorite germ?” Given the quality of the contenders, I found the question difficult to answer. I finally opted for the Gram-positive, rod-shaped bacterium Lactobacillus. In this Germ Gem post, I briefly explain my thought process in choosing my favorite germ and the reasons why Lactobacillus ended up on top.

The contenders. I considered Wolbachia and Thermus acquaticus, two other members of the same domain of life—Bacteria—as contenders. (An article in 2016 in the online magazine Inverse, “Ranking the best bacteria on Earth,” doesn’t even mention Lactobacillus—a shocking omission—but ranks Wolbachia sixth “because they make butterfly sex exciting”).

To my knowledge, Wolbachia has never caused a single infection in humans. Yet it is the most successful infectious agent in the world. It is estimated to infect about two-thirds of the 5 million species of arthropods (insects, spiders, and crustaceans) on our planet and, as I highlighted in my January 20, 2021 Germ Gem post, “Microbes That Stop Malaria,” is a major foe of mosquitoes. Wolbachia infects not only Anopheles mosquitoes (the ones that carry the parasite that causes malaria), but also mosquitoes belonging to the Aedes genus. Promising studies are ongoing of Wolbachia released into tropical environments to control A. aegypti and A. albopictus, the vectors of several devastating arboviral infections, including dengue, chikungunya, Zika, and yellow fever.

Thermus acquaticus is an extremophile. In 1966, Professor Thomas D. Brock isolated this bacterium from a hot spring in Yellowstone Park. To make a long and very interesting story short, Dr. Brock’s discovery of this bacterium led to the development of the Polymerase Chain Reaction (PCR), one of the most widely used techniques in microbiology and biomedical research. (In 1993, two biochemists, Kary Mullis and Michael Smith shared the Nobel Prize in Chemistry for developing the PCR.) PCR is also the gold-standard test for COVID-19.

I considered only one germ belonging to the Eukarya domain of life potentially worthy of my “favorite germ” status—the fungus, Penicillium. Experts argue about what species of this mold (notatum, chrysogenum, or rubens) accidentally floated onto a culture plate in Alexander Fleming’s laboratory a century ago. All, however, agree that Fleming’s discovery of penicillin, an antibiotic produced by this fungus, merited not only his 1945 Nobel Prize for Physiology or Medicine but also recognition of penicillin as the first “miracle drug.”

The first microbe that came to my mind as a “favorite germ” candidate is a virus—actually an enormous group of viruses—called bacteriophages. As I mentioned in my June 10, 2020 Germ Gems post, “Viruses That Eat Bacteria,” only 128 of the estimated 100 million viral species on Earth are human pathogens. A vast majority of the non-pathogenic viruses are called bacteriophages—germs we couldn’t live without. They provide us the essential service of destroying bacteria (for example, they cull about 40% of the bacteria in oceans every day). As such, they are enemies of our enemies. They are even used to treat patients with infections caused by antibiotic-resistant bacteria. Despite all the incredible things that bacteriophages do, given their sheer number, it was too overwhelming for me to single out one as my favorite germ.

Why Lactobacillus? According to an article in the April 15, 2020 issue of ISAPP Science News, “New names for important probiotic Lactobacillus species,” Lactobacillus is the fifth most important category of living organisms to have influenced our planet throughout its evolutionary history. As their name suggests, bacteria belonging to the genus Lactobacillus produce lactic acid. They also derive their energy from the fermentation of lactose and other sugars to lactate. Lactobacilli are an exceedingly rare cause of human infections, but when they do cause infection, treatment with penicillin or a related antibiotic is highly effective.

Currently, the consensus is that the genus Lactobacillus contains 44 different species. For my purposes—to identify my favorite germ—I picked only two, L. acidophilus and L. bulgaricus.

Several features of the bacterium L. acidophilus weighed in my decision to declare Lactobacillus my favorite germ. As most people know, L. acidophilus is commonly used in probiotic supplements (a probiotic is defined most simply as “living microorganisms that are good for you, especially for your digestive system”). In my May 12, 2021 Germ Gems post, “Probiotics: Health Benefits or Hype?,” I discussed many of the perceived health benefits and adverse effects of probiotics. Whether probiotics that contain L. acidophilus are any more or less beneficial than probiotics that harbor other species is unknown. One thing that is clearly established is its benefit to people working in the probiotic industry as the global market for probiotics captured close to $50 billion in 2018.

L. acidophilus also gets credit for its importance in fermentation, the process by which a substance breaks down into a simpler substance, such as occurs in creating beer, wine, bread, kimchi, yogurt and other foods. L. acidophilus is found naturally in a number of fermented foods including sauerkraut, miso, and tempeh, and it is added to foods like cheese and yogurt. The role of lactobacilli in fermentation of food and milk is of huge benefit to industry.

Another valuable feature of L. acidophilus is its role in vaginal health. Lactobacillus is the most frequently isolated microbe from the healthy human vagina, where it helps treat and prevent vaginal infections. It is also a member of the gastrointestinal microbiome, and its work in that ecosystem may be even more important for human health.

The second species of the Lactobacillus genus that shares my “favorite germ” designation is L. bulgaricus. It too is found in probiotics, participates in the process of fermentation, and is a member of the gut microbiome. What makes L. bulgaricus special for me is its link to one of my major heroes of medical science, Elie Metchnikoff—the father of the field of cellular immunology.

During a holiday in Messina, Italy in 1882, Metchnikoff had the brilliant idea that cells, later called phagocytes (“cells that eat”), provide defense of our body against foreign invaders, namely microbial pathogens. In 1908, he was awarded the Nobel Prize for Physiology or Medicine for his contributions to the field of immunity.

By the time Metchnikoff garnered the Nobel Prize, he had established his research program at the Pasteur Institute in Paris. It was there that Metchnikoff founded the field of probiotics postulating that host-friendly “good bacteria,” such as lactobacilli, in the gut could promote health and longevity. He was one of the earliest researchers in the field of ageing and became known as the “father of gerontology,” a disciplinary term that he coined.

In 1905, Stamen Grigorov, a Bulgarian, isolated L. bulgaricus from a Bulgarian yogurt sample. At the time, it was thought that Bulgarians lived an excedingly long life. Metchnikoff researched the relationship between the longevity of the Bulgarians and their consumption of yogurt. He advocated the consumption of lactic acid-producing bacteria to promote health and longevity. (To date, there are no data to support that Bulgarians live longer than most other people and there hasn’t been a run on the market for Bulgarian yogurt.)

Given the rich history of interest in the anti-ageing effects of Lactobacillus, it should come as no surprise that researchers continue both to pursue the basic mechanisms underlying the putative benefits and to carry out randomized clinical trials of the impacts of Lactobacillus on the ageing brain, skin, and other organ systems. (See, “Microbiota Targeted Interventions of Probiotic Lactobacillus as an Anti-Ageing Approach: A Review” in the November 2021 issue of the journal Antioxidants).

So why did I choose Lactobacillus as my favorite germ? The answer boils down to this: not only are lactobacilli a major constituent of probiotics and pivotal contributors to the process of fermentation, but they are also chief among the “good bacteria” in the healthy gut and vagina. Plus, lactobacilli captured the attention and imagination of Elie Metchnikoff, a Ukrainian born zoologist who laid the groundwork for human microbiome biology and was recognized as the father of cellular immunology, probiotics, and gerontology. Simply put, if they were good enough for Elie Metchnikoff, lactobacilli are certainly deserving of the top spot in my list of “favorite germs.”

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.


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