E. coli can survive in a dormant state.

Lactic and acetic acids are widely used as food preservatives to inhibit microbial growth and extend a product's shelf life. However, little is known about whether these acids, at the commonly used concentrations, can induce a viable but non-culturable (VBNC) state in E. coli O157:H7.

Some believe that the VBNC is a dormant state in which bacterial cells are metabolically active but cannot be cultured by known laboratory techniques. Bacteria supposedly enter this state in response to harsh environmental conditions, such as a temperature change, high salinity or nutrient deprivation. The question surrounding the VBNC concept involves whether bacterial cells can be resuscitated out of the dormant state and can start dividing in the external environment, which in the case of human enteric bacteria may constitute a public health concern.

Scientists at the University of Missouri investigated whether E. coli O157:H7 can adapt to acidic challenges by transforming it to the VBNC state. They found that E. coli O157:H7 can survive, especially in the VBNC state, for a long period of time in acidic environments. This raises great concerns regarding the microbiological analysis of acidified foods.

In their work, the researchers harvested E. coli O157:H7 cells from an overnight culture, washed them with sterilized peptone water and inoculated them into lactic acid or acetic acid at about 106 CFU per ml. They tested three acid concentrations--1%, 0.1% and 0.01%. With samples held at 5 C, the culturability of the cells was assayed at several intervals up to 14 days after inoculation on tryptic soy agar (TSA) and MacConkey sorbital agar (MSA). The viability of the cells was monitored under a fluorescence microscope.

During incubation, the bacterial counts on non-selective TSA were markedly higher than the corresponding counts on selective MSA, suggesting that the cells were injured by the acids. E. coli O157:H7 survived well in both acids at the 0.01% concentration. The cells lost culturability on the first and seventh day in 1% and 0.1% lactic acid, respectively.

The viability assay disclosed that E. coli O157:H7 entered the VBNC state in 0.1% lactic acid, but not in 1% lactic acid. In 1% and 0.1% acetic acid, the cells became VBNC on the second day and maintained this state up to day 14. Enrichment using tryptic soy broth did not resuscitate the VBNC cells.

Further information. Azlin Mustapha, Department of Food Science, University of Missouri, 256 William C. Stringer Wing, Eckles Hall, Columbia, MO 65211; phone: [phone]; email: [e-mail].