The ability of four Listeria monocytogenes strains to initiate growth in brain heart infusion broth adjusted to various pH values with either acetic, lactic, citric or hydrochloric acid was investigated. Acetic acid was the most effective inhibitor tested, since in broth adjusted with this acid a higher minimum pH was required for growth of the various strains at both 4 and 30°C, as compared with broth adjusted with the other acidulants. The minimum pH value required for the initiation of growth of L. monocytogenes ranged from 5·0 to 5·7 at 4°C, and from 4·3 to 5·2 at 30°C, depending upon the acidulant used.

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Toxoid was prepared by treating exotoxin A of Pseudomonas aeruginosa with formalin. Coincubation of exotoxin A for two to four weeks at 37 degrees C with 0.5% formalin and 10(-3) M lysine followed by one to two weeks of storage in the absence of these reagents reduced cytotoxicity, preserved antigenicity, and minimized subsequent reversion of the toxoid. Formalin enhanced the adenosine diphosphate (ADP)-ribosyl transferase activity of the toxin while decreasing its toxicity, whereas formalin plus lysine reduced both. Although no antigenic changes were detected by immunodiffusion analysis, analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed minor alterations in the toxin structure. Toxoid induced high titers of antibody to exotoxin A in the sera of mice and rabbits. Antiserum to toxoid neutralized mouse lethality, cytotoxicity, and ADP-ribosyl transferase activity of untreated exotoxin A. Toxoid-immunized mice were resistant to large doses of exotoxin A administered iv.

High-molecular-weight polysaccharides from the extracellular slime of Pseudomonas aeruginosa were evaluated as immunogens in Pseudomonas burn infections in mice. Immunization with immunotype 1 or 2 polysaccharides induced a strong immunotype-specific and weak cross-reactive antibody response but protected mice against burn infections caused by either immunotype. Passive protection was provided by rabbit antiserum to immunotype 1 polysaccharide against burn infection by the homologous organism. Pseudomonas high-molecular-weight polysaccharides are potentially effective vaccines in burn infections.