Bacterial growth

Bacterial growth is process in which two clone daughter cells are produced by the cell division of one bacterium. Hence, local doubling of the bacterial population occurs. Both daughter cells from the division do not necessarily survive. However, if the number surviving exceeds unity on average, the bacterial population undergoes exponential growth. The measurement of an exponential bacterial growth curve in batch culture was traditionally a part of the training of all microbiologists; the basic means requires bacterial enumeration (cell counting) by direct and individual (microscopic, flow cytometry), direct and bulk (biomass), indirect and individual (colony counting), or indirect and bulk (most probable number, turbidity, nutrient uptake) methods. Models reconcile theory with the measurements.

In autecological studies, bacterial growth in batch culture can be modeled with four different phases: lag phase (A), exponential or log phase (B), stationary phase (C), and death phase (D).


 * 1) During lag phase, bacteria adapt themselves to growth conditions. It is the period where the individual bacteria are maturing and not yet able to divide.
 * 2) During the exponential phase, the number of new bacteria appearing per unit time is proportional to the present population.  This gives rise to the classic exponential growth curve, in which the logarithm of the population density rises linearly with time (see figure).  The actual rate of this growth (i.e. the slope of the line in the figure) depends upon the growth conditions, which affect the frequency of cell division events and the probability of both daughter cells surviving.  Exponential growth cannot continue indefinitely, however, because the medium is soon depleted of nutrients and enriched with wastes.
 * 3) During stationary phase, the growth rate slows as a result of nutrient depletion and accumulation of toxic products. This phase is reached as the bacteria begin to exhaust the resources that are available to them.
 * 4) At death phase bacteria run out of nutrients and die.

This basic batch culture growth model draws out and emphasizes aspects of bacterial growth which may differ from the growth of macrofauna. It emphasizes clonality, asexual binary division, the short development time relative to replication itself, the seemingly low death rate, the need to move from a dormant state to a reproductive state or to condition the media, and finally, the tendency of lab adapted strains to exhaust their nutrients.

Batch culture is the most common laboratory growth environment in which bacterial growth is studied, but it is only one of many. It is ideally spatially unstructured and temporally structured. Bacteria are presented with a single burst of nutrients and left to their own devices. In some experimental regimes, they are periodically removed to a new batch of nutrients. In the extreme case, this leads to the continual renewal of the nutrients. This is a chemostat aka continuous culture. It is ideally spatially unstructured and temporally unstructured, in an equilibrium state defined by the nutrient supply rate and the reaction of the bacteria. Related devices include turbidostats and auxostats.

Liquid is not the only laboratory environment for bacterial growth. Spatially structured environments such as biofilms or agar surfaces present additional complex growth models.

In reality, even in batch culture, the four phases are not well defined. The cells do not reproduce in synchrony without explicit and continual prompting (as in experiments with stalked bacteria ) and their logarithmic phase growth is often not ever a constant rate, but instead a slowly decaying rate, a constant stochastic response to pressures both to reproduce and to go dormant in the face of declining nutrient concentrations and increasing waste concentrations.

Bacterial growth can be suppressed with bacteriostats, without necessarily killing the bacteria. In a synecological, a true-to-nature situation, where more than one bacterial species is present, the growth of microbes is more dynamic and continual.