It was found that antagonistic activity toward S. aureus of all 14 LAB strains were lower than this capability against Salmonella sp. S. aureus is Gram-positive bacteria which have a thick mesh-like cell wall made ofpeptidoglycan(50e90% of cell wall), while Salmonella sp. is Gram-negative bacteria with a thinner layer (10% of cell wall). Gram-negative bacteria also have an additional outer membrane which contains lipids, and is separated from the cell wall by the periplasmic space. The antibacterial action of lactic acid is largely, but not totally, assigned to its ability in the undissociated form to penetrate the cytoplasmic membrane. This results in reduced intracellular pH and disruption of the transmembrane proton motive force (Ray & Sandine, 1992). The relative efficacy of lactic acid against Gram-negative bacteria is not unexpected. This is because the small water-soluble molecule lactic acid gains access to the periplasm through the water-filled porin proteins of the outer membrane (OM), as reviewed by Nikaido (1996).