Relationship between nucleic acid ratios and growth in Listeria monocytogenes

Listeria monocytogenes is a pathogen whose distribution in a range of foods tuffs requires the development of methods for sensitive and rapid detection . Molecular biological methods usually rely on specific detection of L. mon ocytogenes rDNA directly amplified by the application of PCR to DNA extract s. Information on the metabolic status of L. monocytogenes populations woul d be valuable and can, in theory, be provided by quantitative detection of rRNA itself. Both fluorometry and oligonucleotide probe assays were applied to L. monocytogenes cultures to quantify RNA and DNA and produced more mea ningful data than previous estimates for bacteria based on eukaryotic nucle ic acid standards. In batch culture, the RNA-DNA ratio was found to be grea test at the end of exponential growth, after which RNA became degraded in a ccordance with the rapid decrease in viability. When the pH of the medium w as controlled at neutrality, culture viability was dramatically extended an d although RNA was degraded, intact DNA was maintained for the duration of the experiment. Ribosome numbers per cell were estimated to decrease from a bout 25000 observed during mid-exponential growth to about 600 during stati onary phase, under pH-controlled conditions. Like Escherichia coli, therefo re, L. monocytogenes loses viability and rRNA rapidly once exponential grow th has ceased in batch culture. However, much improved survival of a cultur able L. monocytogenes population when pH is controlled has clear implicatio ns for the persistence of this species in buffered environments such as dai ry products.