Organic acids, like formic acids, have to ways in inhibiting the growth of gram-negative bacteria, like Salmonella, E.coli, Campylobacter etc. One is via a pH-decrease of the environment in which the bacteria strives. Bacteria have a certain pH-optima in which they can grow. For Salmonella the minimum pH in which it can still grow may be around 4.5; but the optimum is certainly nearer to neutral. So even if you don't reach pH-levels below 4.5 you are able to increase the generation time (in other words make the Salmonella less quick in reproducing) of the bacteria. The second way of inhibiting the bacteria is by penetrating its cell membrane (works only fro gram-negative bacteria). The undisscociated part of the acid is able to penetrate this membrane and can release H+ ions into cytoplasm of the cell. This will lead to a counter-reaction of the cell (pumping H+ ions out with the use of energy. This energy expenditure will weaken the bacteria and may lead ultimatively to cell death. Formic acid in particulra has shown in several MIC-studies that it is among the strongest acids to stop growth of for instance Salmonella!