In vitro antibacterial effect of yogurt onEscherichia coli

Abstract

We investigated the bactericidal and bacteriostatic effects of yogurt on three strains ofEscherichia coli: human toxigenic (078∶H11), rabbit pathogenic (RDEC-1) and rabbit nonpathogenic [015∶K14(L)∶H4]. Approximately 10⁶ organisms were incubated in yogurt, milk, broth, and modifications of these materials. Aliquots were removed at various intervals and plated on MacConkey's agar for enumeration ofE. coli. Yogurt was bactericidal (at least 5 log₁₀ reduction in bacterial counts) to all three strains ofE. coli with <10 CFU/ml remaining by 9 hr. In contrast, all three strains replicated rapidly in milk and broth, reaching maximum concentrations by 9 hr. TheE. coli strains survived and multiplied in milk acidified to the same pH as the yogurt. Yogurt (native pH 4.1–4.4) in which the pH was brought up to and maintained at pH 5.5 or pH 7 for 8 hr was not bactericidal toE. coli Heat-treated yogurt and the filtered supernatant of centrifuged yogurt (both containing no yogurt bacteria) were only bacteriostatic. We conclude that both live yogurt bacteria and a pH near 4.5 are necessary for the bactericidal activity of yogurt. The possibility that yogurt ingestion could protect against infection via other foods contaminated with pathogenicE. coli merits further in vivo investigation.