In a recent study (Kamono et al. 2018) heat-killed Lactococcus lactis strain Plasma (100 billion heat killed cells per daily dose) was administered to athletes to determine the impact on certain markers of immune function and on symptoms associated with influenza, upper respiratory tract infections, and fatigue.
The study had a small number of subjects (51 split between active and placebo groups) and the study spanned only 13 days of administration. The subjects were young, physically fit males undergoing daily high intensity exercise. The exercise regime was not controlled, but the overall intensity was equivalent between the 2 groups.
The severity of symptoms was tracked based on subjective questionnaires. In short, the heat-killed L. lactis group showed lower fatigue and symptom scores for sneeze/running nose, and the placebo group had a higher number of cumulative days with any upper respiratory tract symptom. Modest, but positive, results.
The impact of probiotics (not heat-killed cells) on the incidence or duration of upper respiratory tract infections has been evaluated in several studies. In fact, meta-analyses of these clinical studies conclude that there is an overall benefit to probiotics (Hao et al. 2015 and King et al. 2014). Although evidence on efficacy of heat-killed bacteria is limited, there is a growing interest in assessing the beneficial effects of dead microbes.
The term “abiotic” has emerged to describe non-viable probiotic organisms or their cellular components or metabolites that exert beneficial effects on health. Keep in mind that dead bacteria – despite potential for benefit – are not probiotics. But still, studies evaluating their potential benefits are on the rise. The potential mechanisms of action of dead microbes are more limited than those for live microbes, but immune effects of bacterial components are well-documented. Further, the technological challenges of maintaining probiotic viability in products over the course of shelf-life would not be a concern.
We’re likely to see more studies on health benefits of dead bacteria.