Entry 2
Probiotic science page
Stimulates NK cell activity via IL-12 signaling to reduce upper respiratory tract infection risk, supported by multiple RCTs.
Overview
LcS was isolated in 1930 by Minoru Shirota at Kyoto Imperial University. It is the founding strain of Yakult. Yakult first went on sale in 1935. LcS is one of the most widely consumed probiotic strains in the world. Human RCTs show evidence for immune modulation and for preventing upper respiratory tract infections \[1\]\[2\]\[3].
Mechanism
LcS boosts natural killer (NK) cells. These are immune cells that kill virus-infected cells. LcS triggers monocytes to release IL-12, an immune signal that activates NK cells [1]. When researchers blocked IL-12, the effect disappeared [1]. A crossover trial (each person tries both treatment and placebo) tested fermented milk with 4x10^10 live LcS for 3 weeks. NK cell activity rose in the body [1]. When NK cells were physically separated from the LcS-stimulated monocytes, NK activity dropped to intermediate levels. This confirms the monocyte-driven, IL-12-linked mechanism [1].
Health Benefits
An RCT tested 96 middle-aged office workers. Daily LcS-fermented milk for 12 weeks cut URTI rates (22.4% vs 53.2% placebo, P=0.002). It also shortened how long each episode lasted (RCT) [2]. A crossover RCT tested 30 healthy older volunteers. Four weeks of LcS raised NK activity. It shifted the cytokine profile toward anti-inflammatory, with higher IL-10/IL-12 (RCT) [3]. A double-blind RCT tested 72 male smokers. Three weeks of LcS raised NK killing activity and CD16+ cell counts (P<0.001) (RCT) [4]. A strain-specific systematic review with meta-analysis (pools results from multiple trials) found LcS helps overall IBS symptoms (SR/MA) [5]. Evidence level: RCT for immune/URTI endpoints. SR/MA for IBS.
Biology
Originally named Lactobacillus casei. It was reclassified as Lacticaseibacillus casei in 2020 [6]. It is Gram-positive. It grows with or without oxygen. It does not form spores. It is a rod. It produces lactic acid plus other byproducts. It was isolated from human gut microbes. Strain identifiers: YIT 9029, ATCC 27139.
Bibliography
Takeda K, Suzuki T, Shimada SI, Shida K, Nanno M, Okumura K. (2006). Interleukin-12 is involved in the enhancement of human natural killer cell activity by Lactobacillus casei Shirota. Clin Exp Immunol. 146(1):109-115. PMID: 16968405
Shida K, Sato T, Iizuka R, Hoshi R, Watanabe O, Igarashi T, Miyazaki K, Nanno M, Ishikawa F. (2017). Daily intake of fermented milk with Lactobacillus casei strain Shirota reduces the incidence and duration of upper respiratory tract infections in healthy middle-aged office workers. Eur J Nutr. 56(1):45-53. PMID: 26419583
Dong H, Rowland I, Thomas LV, Yaqoob P. (2013). Immunomodulatory effects of a probiotic drink containing Lactobacillus casei Shirota in healthy older volunteers. Eur J Nutr. 52(8):1853-1863. PMID: 23307112
Reale M, Boscolo P, Bellante V, Tarantelli C, Di Nicola M, Forcella L, Li Q, Morimoto K, Muraro R. (2012). Daily intake of Lactobacillus casei Shirota increases natural killer cell activity in smokers. Br J Nutr. 108(2):308-314. PMID: 22142891
Maslennikov R, Gosteeva E, Ananeva V, et al. (2026). Strain-Specific Systematic Review with Meta-Analysis of Probiotics Efficacy in the Treatment of Irritable Bowel Syndrome. J Clin Med. 15(3):1152. PMID: 41682832
Zheng J, Wittouck S, Salvetti E, et al. (2020). A taxonomic note on the genus Lactobacillus... Int J Syst Evol Microbiol. 70(4):2782-2858. PMID: 32293557
Available Products
Yakult Honsha Co., Ltd. (Tokyo, Japan) produces LcS. It is sold as the Yakult fermented milk drink. The bottle is about 65 mL. It contains about 6.5x10^9 CFU (colony-forming units, a live bacteria count) per bottle. Yakult is sold in over 40 countries.