The relatively large surface area of microorganisms exposed to the environment where they live enables them to take up and assimilate nutrients readily and a to multiply at impressive rates. An important parameter is the specific growth rate, wich is often expressed as the mean doubling time, defined as the time required by the microbial population to douyble its cellular protein content. The doubling time varies widely depending on the microbial species, the nature of the substrate and the degree of adaption to the substrate.

The growth of a bacterial culture can be determined by measuring the increase of turbidity in the medium as optical density or OD. The most common way to do this is to compare the absorbance of the culture to inoculated medium by shining light with a wavelength of 600 nm through the culture. The more growth occurs, the more turbid the culture will become and more light will be absorbed. Using optical density gives indirect measurement of bacterial growth. It doesn’t tell anything about how many living cells are in the culture. This becomes especially important in stationary phase. Dead cells still absorb light. To determine the actual number of live bacteria the broth is diluted and plated on appropriate media and incubated. In theory, colonies arising on a plate originate from one single bacterium and give therefore an accurate number of the live cells in the culture at that time point.

Procedure:

The kinetics of selected bacterial growth, it is used 10 mL of 24 h culture of the strains and inoculated into 60 mL of nutrient broth, the following conditions: 35 °C and 100 rpm agitation. Samples were read every 30 minutes in a Spectronic 20D+ visible spectrophotometer at 600 nm, substituting readings transmittance (% T) in the following equation:

A = 2 — logM(%T)

Where A, is the absorbance.

The different phases of growth kinetics can be observed by plotting the log (% T) versus time [1].