Interest in postbiotics is growing. Postbiotics are non-viable probiotic organisms or cellular components thereof that exert beneficial effects on health or well-being (Shortt 1999). The key distinction between a postbiotic and a probiotic is that a probiotic must be a live microbe when administered. A postbiotic can be dead cells or fragments thereof.
The uptick in interest in postbiotics stems from a the practical reality that when you don’t need to worry about keeping the microbe alive, manufacture, packaging, storage, transport and virtually all handling are greatly simplified. Additionally, in cases where administration of a probiotic might raise concerns of potential infectivity, dead microbes are a safer option. Further, there is a growing body of evidence describing effects that postbiotics have on human health.
A systematic review was published in 2017, which reviewed randomized, controlled human studies with any clinical endpoint where the intervention was a killed probiotic (Zorzela et al, 2017). Forty studies were included in the review. These 40 studies were heterogeneous with regard to endpoint (prevention or treatment of an array of diseases), microbe, study population (adults or pediatric). Most studies were assessed as having a low risk of bias. For about 75% of the studies that included a comparison to a live microbe, no difference in efficacy between the live and killed microbe was observed. Authors point out, though, that studies were likely not powered to detect a difference. In two treatment studies, killed probiotics were better than live. In one prevention study, live was better than killed. The review also looked for evidence of adverse effects of the killed microbes. Unfortunately, as is too often the case, most studies did a poor job of either collecting or reporting adverse events, so no conclusion could be made.
Surprising to me, however, was the number of studies that showed that killed cells performed better than placebo or live cells. About 40% of comparisons favored the postbiotic over the placebo or live microbe, supporting the idea that postbiotics may be a legitimate functional ingredient. Mechanistically, it seems evident that live microbes have a larger toolbox by which they can impact host physiology, but dead cells or cell fragments may act via interaction with immune or other host cells. Further research will enable fine-tuning the types of effects to be expected from postbiotics.
A paper from Colin Hill’s lab (Warda et al 2019) showing that a heat-killed fermentate of Lactobacillus fermentum and Lactobacillus delbrueckii plus culture medium had an impact on behavior of healthy male mice. The mice consuming this product as 5% of their diet had reduced resting corticosterone levels and decrease in locomotor activity in a stress test. (No comparison to the live microbes was made.) Mice are not people, but these results allow formation of an interesting hypothesis regarding the role that postbiotics might play in anxious behavior.