VetBact

Introduction

Bacteria multiply by binary fission (two identical daughter cells are formed upon the cell division). If the availability of nutrient is constant, and the physico-chemical properties of the culture medium does not change, they divide by the same speed all the time. This means that growth is exponential. The exponential growth phase (log phase) can not continue indefinitely, because the culture medium will become depleted of nutrients and the pH of the medium usually changes. When a bacterial culture is initiated in a liquid medium, it takes some time before the bacteria will start to grow, especially from clinical specimens or from hypothermic cultures and this phase of cultivation is known as the lag phase.

Generations time

The time required for the number of bacteria to double during cultivation is called generation time or doubling time and it can vary greatly for different bacteria. Escherichia coli, which is cultivated under optimum conditions, has a generation time of 20 min, whereas Mycobacterium avium subsp. paratuberculosis has a generation time of about 24 hours. This means in practice that if you spread these two bacteria on appropriate culture plates, you can easily see colonies of E. coli after one day, while colonies of M. avium subsp. paratuberculosis cannot be observed until after at least 3 months!

Growth curve

Bacterial growth can be described by a so-called growth curve with four different phases: lag phase, log phase, stationary phase and death phase (= decline phase) (labelled A, B, C and D, respectively, in the figure). Note that the scale of the y axis is logaritmic. During the stationary phase bacteria die at about the same rate as they are formed during cell divisions, and in the death phase, bacteria die faster than they regenerate.

Construction of a growth curve

When a growth curve is constructed, you most often want to be able to follow the growth of bacteria in real time to know when to make e.g. addition of a substance, or when cultivation should be interrupted. The fastest and easiest method is to regularly collect samples from the culture and determine light scattering by spectrophotometry. With a spectrophotometer you normally determine absorbance at different wavelengths, but in a suspension (e.g. of bacteria) light scattering (OD = optical density) can also be measured. Light scattering is proportional to the number of bacteria per ml, and by means of a standard curve, one can determine the number of bacteria in absolute terms.

Updated: 2017-10-17.