[The effect of acidification on Salmonella enteritidis in a defined medium]

Effects of five acids on three highly virulent strains of Salmonella enteritidis in a defined medium were investigated at various inoculum sizes. The following minimal inhibition concentrations (MIC) were found for the acids: citric and tartaric-0.2%, pH 4.0 and 3.8, respectively; acetic and propionic-0.1%, pH 4.5 and 4.6, respectively; hydrochloric-0.05%, pH 3.6. Bactericidal effect was evident in the organic acids at the concentration 0.4% within the pH range 3.3-3.9, and in the hydrochloric acid at the concentration 0.025%, pH 2.5 (Fig. 2, Tab. I). As to the intensity of inhibition at MIC for Salmonella suspensions with cell densities 7 to 8 logs per ml, the acids were arranged into the following ascending order: citric < tartaric < propionic < acetic < hydrochloric. The respective order for the cell densities 6 to 7 log per ml was: propionic < tartaric < hydrochloric < citric < acetic. The mean difference between the initial and the final count of salmonellae was -1.3 to -1.8 log and -2.5 to -4.6 log for the weakest and the strongest acids, respectively (Tab. II). Differences not only between the acids, but also between the strains, depending on inoculum sizes, were found at the nearest-to-MIC lower concentrations (subMIC). The weakest effects at subMIC were found in propionic (0.05%; pH 5.0) and citric (0.1%; pH 4.5) acids. Tartaric acid (0.1%; pH 4.5) did not stop the growth of salmonellae at cell densities 1.7 to 2.6 log per ml, but, unlike the former two, inhibited strain II (food isolate) and strain III (egg isolate) at cell densities 0.5 and 0.6 log per ml, respectively. The strains survived the acidification with acetic acid at cell densities 4.6 to 4.9 log per ml, but the densities decreased by 1.3 to 1.7 log. Hydrochloric acid (0.025%; pH 4.6) inhibited strains II and III already at 2.3 to 3.0 log per ml, whereas strain I (human carrier isolate) was relatively acid-fast even at 1.7 log per ml. Limiting factors in the acid inhibition of salmonellae include not only the type, concentration and dissociation rate of the acid, but also the amount of the exposed Salmonella cells.