Decreasing Harmful Bacteria

research area

The intestinal microbiota comprises a range of bacteria: some beneficial, some neutral for health, and some that are pathogenic or harmful. In the latter case, this may be due to production of toxins, carcinogens or other substances that, over a period of time, may be associated with a negative effect on health.

In an early study by Dr Shirota, infants who were fed L. casei Shirota fermented milk for 14-days were shown to have decreased levels of Enterobacteriaceae and streptococci, compared to infants who were fed a placebo fermented milk drink (heat-treated to kill the L. casei Shirota) (Shirota et al. 1966).

Since then, several studies with L. casei Shirota have reported that its consumption has been associated with reduction of harmful bacterial species in the gut (Kato-Kataoka et al. 2016, Nagata et al. 2011, Nagata et al. 2016, Tsuji et al. 2014). 

Study: Decreasing harmful bacteria

Reduction of potentially harmful bacteria in the gut - Nagata et al. (2016) Ann Nutr Metab 68(1):51–59.

Method: A double-blind, placebo-controlled randomized trial of residents (n=72) and staff (n=20) at a facility for the aged, consumed either an L. casei Shirota fermented milk drink (4x1010 CFU) or placebo daily for 6 months. Faecal samples were collected from all subjects at baseline and at months 1, 3 and 6.

Results: In those who had consumed L. casei Shirota, faecal analysis identified a significant increase in bifidobacteria, and a significant decrease in Clostridium difficile, Clostridium perfringens, and Enterobacteriaceae in both residents and staff.  Additionally, the elderly residents who had consumed L. casei Shirota, had significantly lower levels of Staphylococcus, and Pseudomonas.  

Interestingly, residents who had consumed L. casei Shirota, had significantly lower incidence of diarrhea and constipation, and fewer days with a fever, compared to placebo (P < 0.05).

Reducing harmful substances

There is no consensus of ‘an ideal intestinal microbiota’, but it is generally considered healthier for it to be predominantly saccharolytic, resulting in the production of short chain fatty acids such as butyrate, acetate, and propionate. These metabolites increase gut motility, decrease pH, provide energy for the commensal bacteria and help absorb minerals. In contrast, proteolytic fermentation results in potentially toxic and carcinogenic metabolites including ammonia, phenols, indoles and amines.

Study: reducing harmful substances

Decrease in toxic bacterial metabolites in the gut (healthy adults) - De Preter et al. (2004) Brit J Nutr 92:439-446.

Method: Researchers in Belgium investigated the effects of L. casei Shirota on toxic fermentation metabolites (NH3 and p-cresol) in the gut. In this crossover study, healthy subjects (n=19) consumed either a L. casei Shirota fermented milk drink (6.5 x109 CFU), a prebiotic or the respective placebo twice a day for two weeks, with a two-week washout period in-between. A test meal was consumed at the end of weeks 2, 4, and 6, which contained stable isotope-labelled biomarkers (a 2H and a 15N marker) .  Urine samples were collected before each test meal, and for the following 48-hours after the meal to determine phenolic compounds, total nitrogen, and 15N. 

Results: The data indicated a reduction in production of the toxic fermentation metabolites NH3 (15N biomarker,  P=0.047) and p-cresol (2H4 biomarker, P=0.032) for the probiotic group, which was significantly different to the placebo group (P=0.016 and P=0.042, respectively).


De Preter 2004

Recommended Further Reading

Other studies have confirmed association of L. casei Shirota consumption with significant decrease of urinary p-cresol excretion (De Preter et al. 2007) and an in vitro study showed binding of aflatoxin by this strain (Hernandez-Mendoza et al. 2009).

Another in vitro model, assessing the ability of 14 Lactobacillus strains to bind to acrylamide, a potential carcinogen found in some foods, found L. casei Shirota had one of the highest binding abilities.  At concentrations of 5μg/ml, L. casei Shirota was able to remove up to 24.95% of acrylamide (Serrano‐Niño et al. 2014). Furthermore, in a dynamic model system simulating the gastrointestinal tract, researchers found that L. casei Shirota removed 65-73% of acrylamide in potato chips – a food with low acrylamide content. Repeating this experiment using food with originally high acrylamide content led to a 5-10% removal rate of acrylamide, which could be due to a possible saturation of binding sites of the bacteria after exposure to a certain acrylamide concentration (Rivas-Jimenez et al. 2016). 


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Increasing beneficial bacteria

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