ABSTRACT

https://www.niso.org/standards/z39-96/ns/oasis-exchange/table"> Types of microbial growth Prokaryotes can use an enormous range of compounds as sources of carbon and energy. The few anthropogenic compounds they have yet to evolve to metabolize are known as xenobiotics. The mode of growth can be used as a system of classification. History Microorganisms have been cultured in the laboratory for more than 120 years and those who have made the greatest contributions to our understanding have included Kluyver, Monod, Novick, Szilard, and Pirt. Growth of bacteria in liquid culture When prokaryotes are grown in their planktonic form as free-living organisms in a liquid medium and not attached to any surface, they will divide in a regular manner until one growth factor runs out. The limiting growth factor may be its carbon source, energy source, or one of any number of micronutrients. Microbial growth can be measured conveniently by optical density (OD), a colligative property. If OD is plotted against time, the plot will be exponential during cell growth. Thus a plot of log OD against time results in a straight line with a slope equal to the specific growth rate. The growth rate can also be expressed as doubling time. Growth phases of a planktonic culture In a closed system to which no nutrients are added (batch culture), the growth curve of optical density and time can be divided into lag, log (or exponential), stationary (carbonlimited), and death phases. The phases between these main types are known as interphases and represent a switchover in the organism’s metabolism from one mode to another. Yield The maximum amount of biomass per mole of a particular growth substrate is known as the yield. This can also be expressed as YATP, the yield according to energy generated. Interpretation of growth curves When a microorganism is presented with more than one growth substrate, it will preferentially metabolize the one with the best energy yield first, before using the remaining substrates (diauxic growth). The growth curve will always follow the pattern of a high growth rate followed by a lower one. Primary and secondary metabolism A compound that is formed during the log phase of microbial growth is known as a primary metabolite. Those compounds that are formed at the very late stages of growth or during stationary phase are known as secondary metabolites. Secondary metabolites have a nonessential role in growth, are dependent on growth conditions for production, and may be produced as one of a family of similar compounds. Related topics (C2) Prokaryotic diversity (C6) The major prokaryotic groups (C9) Cell division (D2) Batch culture in the laboratory (D3) Large-scale and continuous culture 70