Abstract
The paper shows that the phenomenological trends of both growth and decay of a microbial population in a given medium are easily reproducible with simple equations that allow gathering the experimental data (plate counts) related to different microbial species, in different mediums and even at different temperatures, in a single master plot. The guideline of the proposed approach is that microbes and surrounding medium form a system where they affect each other and that the so-called “growth curve” is just the phenomenological appearance of such interaction. The whole system (cells and medium) changes following a definite pathway described as the evolution of a “virtual” microbial population in planktonic conditions. The proposed equations come from the assumption of a duplication mechanism with a variable generation time for the growth and of an exponential-like decline with a linear increase of the rate for the decay. The intermediate phase between growth and decay is a time span during which growth and death counterbalance each other and age differences within the virtual cell population tend to level off. The proposed approach does not provide an a priori description of this phase but allows the fit of the whole evolution trend of a microbial culture whenever the experimental data are available. Deviations of such a trend concern microbes able to form spores, modify their metabolism, or express phenotypic heterogeneity, to counterbalance adverse medium conditions.
Highlights
IntroductionMicrobial populations undergo a number of changes that depend on the surrounding environment and the attained level of the population density itself. ese changes affect the biochemical activity within each single cell (synthesis of nucleic acids, number of active ribosomes, synthesis of proteins, uptake of external resources, etc.) and the exchange of signals between neighbor cells (quorum sensing). e overall result of such changes is the phenomenological evolution (“growth curve”) of the population density, N, that goes through different “phases,” which correspond to observed changes of the specific rate N_ /N (where N_ stands for dN/dt, t being the time)
Microbial populations undergo a number of changes that depend on the surrounding environment and the attained level of the population density itself. ese changes affect the biochemical activity within each single cell and the exchange of signals between neighbor cells. e overall result of such changes is the phenomenological evolution (“growth curve”) of the population density, N, that goes through different “phases,” which correspond to observed changes of the specific rate N_ /N
“Shifting cultures from a medium that affords a slow growth rate to one that leads to a higher rate results in a rapid acceleration of ribosome synthesis. e converse, going from fast to slow growth, imposes a long lag required for the synthesis of biosynthetic enzymes repressed in the rich medium
Summary
Microbial populations undergo a number of changes that depend on the surrounding environment and the attained level of the population density itself. ese changes affect the biochemical activity within each single cell (synthesis of nucleic acids, number of active ribosomes, synthesis of proteins, uptake of external resources, etc.) and the exchange of signals between neighbor cells (quorum sensing). e overall result of such changes is the phenomenological evolution (“growth curve”) of the population density, N, that goes through different “phases,” which correspond to observed changes of the specific rate N_ /N (where N_ stands for dN/dt, t being the time). Microbial populations undergo a number of changes that depend on the surrounding environment and the attained level of the population density itself. International Journal of Microbiology e adjustments of the cells are the result of coordinated processes at the macromolecular level that regulate the cytoplasm and membrane biochemical machinery through activation of some enzymes and repression of some others, following the so-called “passive control” regulation [5]. E converse, going from fast to slow growth, imposes a long lag required for the synthesis of biosynthetic enzymes repressed in the rich medium. Both patterns could be partly understood in terms of the partitioning of the transcriptional and translational apparatus between synthesis of the repressible biosynthetic enzyme systems and making the protein synthetic system.” [3]
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