# Antimicrobial Activity of Cell-Free Supernatants Produced by Strains of Lactic Acid Bacteria Against Staphylococcus aureus

## TL;DR

This study shows that cell-free supernatants from certain lactic acid bacteria can effectively kill Staphylococcus aureus, offering a natural alternative to antibiotics for preventing infections like cow mastitis.

## Contribution

The study identifies specific antimicrobial components in CFSs from four lactic acid bacteria strains and their effectiveness against S. aureus.

## Key findings

- CFSs from S. thermophilus, B. infantis, L. plantarum, and L. rhamnosus showed strong antibacterial activity against S. aureus.
- The CFSs disrupted cell membranes and reduced biofilm formation and ATP content in S. aureus.
- Organic acids and proteins were the main antimicrobial substances in the CFSs of L. plantarum and L. rhamnosus.

## Abstract

S. aureus infection is a significant player in causing negative impacts such as cow mastitis, and it is necessary to seek alternatives to antibiotics for intervention. Cell-free supernatants (CFSs) produced by probiotics have emerged as novel antimicrobial candidates attracting significant interest due to their dual functionality in safety and multiple beneficial health effects. This study demonstrates that the CFSs produced from the culture of S. thermophilus, B. infantis, L. plantarum, and L. rhamnosus strains have antibacterial activity against S. aureus BNCC 186335 cultured in vitro. Therefore, there is a possibility that these four CFSs could be used as natural antibacterial agents to prevent the occurrence of mastitis in dairy cows. This work further improves our understanding of how probiotics and their metabolites resist pathogenic bacteria and expands the scope of application of CFSs.

Bacterial zoonoses pose a serious threat to the development of animal husbandry, food safety, and public health. Staphylococcus aureus (S. aureus) is a major infectious and food-borne pathogen worldwide, and there was an urgent need to develop relevant methodologies for the control of bacterial infections. This study aimed to evaluate the effectiveness of cell-free supernatants (CFSs) produced by selected strains of Lactiplantibacillus plantarum (L. plantarum), Lacticaseibacillus rhamnosus (L. rhamnosus), Streptococcus thermophilus (S. thermophilus), and Bifidobacterium longum subspecies infantis (B. infantis) to inhibit in vitrogrown S. aureus BNCC 186335. CFSs of S. thermophilus, B. infantis, L. plantarum, and L. rhamnosus not only showed good antibacterial activity against S. aureus but also have strong stability and tolerance, which could destroy the integrity of cell membrane, lead to changes in cell morphology, and then strongly and rapidly kill bacteria. Notably, the primary antimicrobial substances in the CFSs of L. plantarum and L. rhamnosus were organic acids and protein components, whereas the main antimicrobial substances in the CFSs of S. thermophilus and B. infantis were organic acids. Meanwhile, four CFSs achieved substantial removal of biofilms and inhibited decreased ATP content. These findings suggest that the CFSs of S. thermophilus, B. infantis, L. plantarum, and L. rhamnosus may have potential applications as biological preservatives to control the contamination of S. aureus in the food industry and animal husbandry.