A matter of bacterial life and death

Abstract

Over 50 years ago, standard microbiological methods were established for determining whether bacterial cells were dead or alive. Recently there has been a flurry of reports suggesting that bacteria may exist in an eclipsed state, escaping detection by standard methods. Whether there really is such a state is of more than academic interest, considering the implications for public health. The ensuing debate has been unusually energetic for the normally cultured community of microbiologists.

Fig. 1. Cell division of the bacterium Enterobacter aerogenes. A microbiologist contemplating a single cell (A) cannot tell whether it is dead or alive. Confirmation of culturability, namely ability to yield a population discernible to the observer (B and C), is proof that the single cell in (A) was alive. The viability status of the progeny cells (D) in turn cannot be determined unless they exhibit the ability to grow and divide.

Fig. 2. The mixed culture recovery (MCR) test. The flask at the top contains non-culturable cells (blue circles) of the test bacterium, suspended in a nutrient medium appropriate for their growth. A smaller number of culturable cells (orange circles) are introduced, and the cultures allowed to grow. If the culturable cells resuscitate the non-culturable cells, both cell types increase in number (lower right flask). If growth is limited to the culturable cells, only that cell type increases in number (lower left flask). The key to the MCR test is that the two cell types are easily discriminated.

Fig. 3. Regrowth versus resuscitation. A mixture of culturable (dark blue) and non-culturable (light blue) cells are subjected to a proposed resuscitation technique. If the only response is the growth of the culturable cells, then regrowth has occurred (right). If there is conversion of non-culturable cells into culturable cells without any change in cell numbers due to regrowth (left), then true resuscitation has occurred.

Edward V. Bourneuf & Gregg Bogosian