Regular readers will know that infectious disease issues often land the top billing in microbial news. However, most of our microbial interactions do not negatively impact our health – in fact, some interactions even lead to health improvements. The microbes that lead to these beneficial interactions are called probiotics. These probiotic microbes can be found in many niches of the body, but are most commonly studied with respect the gut microbiome. There, probiotic effects can range from stimulating or regulating the immune system, to promoting healthy division of mammalian cells, to stimulation of nutrient absorption and digestion. Some of the best-studied probiotics are the Lactobacillus and Lactococcus bacteria*. Two papers published this week answer some fundamental questions about these bacteria as probiotics. These findings will affect both current and future utilization of probiotics.
How do probiotics work?
Evidence has stood for decades that certain Lactobacillus species benefit human health. Why they benefit human health is a lingering question in this field, and the subject of the first study. In their study published this week in mBio, first author Chunxu Gao and lead scientist James Versalovic describe the signal molecule, histamine, generated by L. reuteri that helps modulate gut immunity.
Lactobacillus reuteri is a natural part of the gut microbiome of people, as well as other mammalian and avian animals. It has been studied by conventional microbiology techniques since the 1960s, and is known to be prevalent in healthy human individuals. One of the model organisms of the gut microbiome, L. reuteri has been widely marketed as a probiotic species for more than 25 years commercially.
Gao and Versalovic wanted to test exactly how this bacterium functioned as a probiotic. “Our hypothesis was that microbe-derived histamine can suppress intestinal inflammation in a mammalian host,” says Versalovic. Gao elaborates, “in vitro studies showed that bacteria-derived histamine decreases pro-inflammatory cytokine TNF production in activated human monocyte, and this effect is due to H2 receptor signaling.” The scientists assessed their idea by inoculating mice with a histamine-generating bacterial strain and testing whether the histamine generated could suppress intestinal inflammation.
Histamine is normally associated with activating the immune system – think antihistamines used to dampen an allergic reaction. However, ‘the histamine type 2 receptor is very different from the histamine type 1 receptor, which is the target of antihistamines,” explains Versalovic. Because the intestine is enriched with type 2 receptor, an immune suppression signal, the research team expected more histamine, delivered in the right place, would lead to less inflammation. And they were right; the mice with L. reuteri has less intestinal inflammation than mice without it (see figure, right).
However, the observed suppressive effects depended not only on the bacteria, but also what the mice were eating. When L. reuteri-colonized mice were fed a histidine-free diet, they were not protected against inflammation. Thus any potential probiotic treatment must be complemented with the right diet. These results “make us think in new ways about how diet and nutrition, combined with the function of the microbiome and probiotics, will enable us to really advance applied microbiology and think about functional foods and human health in new ways,” Versalovic says.
While this current research uses an acute colitis model, the team is moving their findings into chronic colitis models. This will evaluate whether the effects of L. reuteri immune suppression can be seen over a time span of weeks, rather than days. Uncovering these mechanisms used by probiotic bacteria to benefit our health may lead to a more finely tuned application in the future. “In the future,” Versalovic says, “we hope to be able to administer specific probiotics to patients and perhaps healthy patients who are at risk for certain diseases, including inflammatory bowel disease.” Prescribing specific probiotic strains with the corresponding dietary supplement may be a new tactic for preventing or treating diseases in the not-so-distant future.
Can engineered probiotics help fight additional diseases?
Most research focuses on naturally-occurring bacterial strains as choices for probiotic, but what if a slight tweak could lead to an even greater impact of probiotic use? This is the question being addressed in new research published by first author Evelyn Durmaz and lead scientist Todd Klaenhammer in Applied and Environmental Microbiology this week.
The researchers introduced a gene into Lactococcus lactis*, another occasionally used probiotic strain of bacteria, and a strain important for generating foods such as cheese and yogurt. The gene introduced, cry5B, was taken from Bacillus thuringiensis, and was chosen based on its well-characterized ability to kill nematodes. Cleverly, the gene was placed under control of a nisin promoter, so that the L. lactis-produced antimicrobial peptide nicin must be present for the protein to be produced. This peptide can be used as a natural preservative and is therefore safe for inclusion in any bacterial cultures.
Several L. lactis strains were generated: one that accumulated Cry5B internally, and one with additional genes leading to a “leaky” membrane to allow release of large proteins, such as Cry5B. Most of the Cry5B in this latter strain was found in the culture supernatant, showing that the toxin could potentially reach its nematode target if cultured together. But secretion doesn’t matter if the exogenously expressed toxin isn’t effective.
To test its helminth-killing abilities, the Cry5B-accumulating L. lactis were lysed and then exposed to the model nematode Caenhorrabiditis elegans. Lysates from these bacteria were toxic to the worms, while L. lactis lysates without Cry5B were not. This was true with both the full-length and a truncated version of the Cry5B protein, showing that B. thuringiensis proteins produced by L. lactis can kill worms (see figure, right).
Over 1 billion people all over the world suffer from various nematode parasite infections, with the vast majority concentrated in Africa and Asia. These infections can have detrimental effects on development, nutrition, and pregnancies, as well as more direct negative health effects, including death. The recent Nobel prize and President Carter’s wish to eliminate the roundworm causing riverblindness have drawn attention to the needs of neglected tropical diseases. An engineered L. lactis strain may someday be added to the arsenal of antihelmitic drugs, with administration as easy as eating a serving of yogurt or fermented milk.
-- Julie Wolf
Correction: This blog originally referred to both bacteria as members of the Lactobacillus genus, when Lactococcus lactis is clearly in a separate genus. I apologize for the sloppy nomenclature - I guess I was too excited by the results to remember that Lactococcus species are necessary to make the yogurt or fermented milk referenced at the end of the blog! Thanks to Baltasar for pointing out this error!