The human microbiota is a source of probiotics capable of modulating the host immune system. In this
study, we collected fecal samples from 100 healthy infants and isolated lactic acid bacteria which were
screened for immune modulating effects on tumor necrosis factor a (TNF-a) production. Cell-free culture
supernatants from 26 isolates were able to decrease TNF-a production in vitro and three of the isolates
were selected as candidate probiotics (MSMC39-1, MSMC39-3, MSMC57-1). These isolates were identified
using 16S ribosomal DNA sequencing as Lactobacillus paracasei, Lactobacillus casei, and Weissella
confusa respectively. All three isolates were acid tolerant and bile tolerant to pH 3.0 and 4% bile
respectively. Preparations of cell-free culture supernatants were processed and tested, and revealed that
cell-free culture supernatants of isolates L. paracasei MSMC39-1, L. casei MSMC39-3, and W. confusa
MSMC57-1 decreased the production of TNF-a significantly and were heat resistant. Only L. paracasei
MSMC39-1 supernatant was proteinase-K sensitive. The effects of viable bacteria, heat-killed bacteria,
and sonicated bacteria were compared. The heat-killed preparations of isolate W. confusa MSMC57-1
decreased the production of TNF-a. Sonicated cell preparations did not significantly alter TNF-a production.
For isolates L. paracasei MSMC39-1 and L. casei MSMC39-3, this suggests that a substance in the
cell-free culture supernatant may be responsible for in vitro cytokine modulation.