Ovotransferrin Plays a Major Role in the Strong Bactericidal Effect of Egg White against the Bacillus cereus Group

Abstract

Bacillus cereus group bacteria are opportunistically pathogenic spore-forming microorganisms well known in the sector of pasteurized food products because of their involvement in spoilage events. In the sector of egg product processing, these bacteria may lead to important economic losses. It seemed then relevant to study their behavior in egg white, a widely used egg product usually recognized as developing different levels of antimicrobial activities depending on the environmental conditions. A strong bactericidal effect (decrease in the bacterial population of 6.1 ± 0.2 log CFU/ml) was observed for 68 B. cereus group isolates, independently incubated at 30°C in egg white at pH 9.3 (natural egg white pH). To determine which components could explain such a strong bactericidal effect, an experimental strategy was carried out, based on egg white fractionation by ultrafiltration and by anion-exchange liquid chromatography. The role of the protein fraction was thus demonstrated, and subsequent nano-liquid chromatography–tandem mass spectrometry analyses allowed identification of ovotransferrin as a major protein involved. The strong bactericidal effect was confirmed in the presence of commercial ovotransferrin. Such a bactericidal effect (i.e., a decrease in the bacterial population through cell death) had never been described because ovotransferrin is known for its bacteriostatic effect (i.e., inhibition of growth) due to its ability to chelate iron. Surprisingly, the addition of iron did not reverse the bactericidal effect of ovotransferrin under alkaline conditions (pH 9.3), whereas it completely reversed this effect at pH 7.3. Ovotransferrin was shown to provoke a perturbation of the electrochemical potential of the cytoplasmic membrane. A membrane disturbance mechanism could, hence, be involved, leading to the lysis of B. cereus group bacteria incubated in egg white.