Aims: To determine inactivation of Listeria monocytogenes by the two lactic acid isomers. Methods and Results: The survival of four strains with varying sensitivity to acid was determined following treatment with L L-or D D-lactic acid at 100 mmol l )1 (pH 3AE7) or HCl at pH 3AE37. There was some, but not complete, similarity in the relative sensitivity of the four strains to the two types of acid. All strains were most sensitive to D D-lactic acid, which gave 0AE6-2AE2 log units greater reduction than L L-lactic acid midway in the inactivation curves. Even very low concentrations of the two isomers had an immediate effect on pH i which was identical for the two isomers. Conclusions:The results show that L. monocytogenes is more sensitive to D D-than to L L-lactic acid; however, this difference is less than the strain variation in L L-lactic acid sensitivity. Significance and Impact of the Study: This work has implications for the application of lactic acid for food preservation as well as for the understanding of the antibacterial mechanisms of weak organic acids.

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Zusammenfassung Aus unseren Versuchen ziehen wir folgende Schlüsse: PNH‐Erythrozyten hämolysieren, wie von Pillemer und Mitarb. mitgeteilt wurde, nur in Gegenwart von Properdin. Aus normalen menschlichen Erythrozyten läßt sich ein polysaccharidhaltiges Substrat herstellen, das, ähnlich wie Zymosan, Properdin zu absorbieren vermag. Dabei wird der Komplementtiter auchbei Inkubationstemperaturen von 37°C nicht wesentlich beeinträchtigt. Die Vermutung liegt nahe, daß das Properdinsystem auch für die normale Hämolyse von Bedeutung ist. Es grbt offenbar physiologische Hemmfaktoren des Properdinsystems. Summary As reported by Pillemer et al., PNH erythrocytes are hemolyzed only in the presence of properdin. A polysaccharide substrate can be prepared from normal human erythrocytes which, like zymosan, is able to absorb properdin. This absorption does not fix complement appreciably, even when it is carried out at 37°C. It is suggested that the properdin system is also important for normal hemolysis. The existence of physiological inhibitors of the properdin system has been demonstrated. Résumé Nous tirons de nos expériences les conclusions suivantes: Les érythrocytes PNH sont hémolysées, comme l'ont montré Pillemer et ses collaborateurs, seulement en présence de properdine. On peut préparer à partir d'érythrocytes hurnains normaux un substrat riche en polysaccharide qui, comme le zymosane, est capable d'absorber la properdine. Cette réaction, même A 37°, ne fixe pas le complement en quantités appréciables. Il est donc vraisemblable que la properdine est importante pour l'hémolyse normale. Il est évident qu'il existe des inhibiteurs physiologiques de la properdine.