Acid Resistance of Twelve Strains of Enterobacter sakazakii, and the Impact of Habituating the Cells to an Acidic Environment

TLDR: While there was diversity observed among the strains, in general the stationary-phase acid resistances of several of the strains were enhanced, at least transitorily, by growth in the acidogenic medium.

Abstract: The association of powdered infant formula with cases of severe Enterobacter sakazakii infections in immunocompromised and premature neonates has led to a need to learn about the basic behavior of this emerging pathogen in food systems and the environment. The current study examines the microorganism's stationary-phase acid resistance using 12 strains that had been previously used to characterize its thermal resistance. Acid resistance was determined by initially culturing the isolates for 18 h in brain heart infusion broth (BHI) at 36 °C, transferring the cells to tryptic soy broth (TSB) adjusted to pH 3.0 and 3.5, and determining E. sakazakii survival over the course of 5 h incubation at 36 °C. At pH 3.5, 10 of the 12 strains showed less than a 1 log cycle decline over the 5-h incubation period, with the most acid sensitive strain showing an approximate 3.5 log cycle decline. At pH 3.0, the decline over the 5-h incubation period ranged from 4.9 to >6.3 log cycles; however, substantial diversity was evident when the 1-h/pH 3.0 results were compared. The effect of habituating the cells to a moderately acidic environment was determined by growing the strains in TSB with 0% (nonacidogenic) and 1% glucose (acidogenic), transferring the cells to acidified (pH 3.0) BHI, and determining E. sakazakii survival over the course of 5 h of incubation at 36 °C. While there was diversity observed among the strains, in general the stationary-phase acid resistances of several of the strains were enhanced, at least transitorily, by growth in the acidogenic medium. No apparent correlation between the stationary-phase relative acid resistances of the strains based on the 1-h/pH 3.0 acid inactivation values and the previously reported thermal D-values was observed.