9. Microbial Growth and Biosynthesis
9.3 Media Used for Bacterial Growth
Learning Objectives
- Name the major macroelements and explain why they are required at such high levels relative to other elements
- Explain, with an example, how microbes can often use more than 1 macronutrient as a source of a particular major macroelement
- Identify and describe culture media for the growth of bacteria, including examples of all-purpose media, enriched, selective, differential and defined
- Differentiate between enriched media and enrichment medium
- Explain, with examples, the concept of growth factors, and why this may or may not be a requirement.
The study of microorganisms is greatly facilitated if we are able to culture them, that is, to keep reproducing populations alive under laboratory conditions. Culturing many microorganisms is challenging because of highly specific nutritional and environmental requirements, and the diversity of these requirements among different species. Another problem is the “patchiness” of the physicochemical conditions of an environment – that is conditions may change over microscopic distances. So although the golden rule for laboratory cultivation is to try to replicate the conditions from the natural environment, this is not always effective and cultivation conditions are always selective.
Nutrient requirements
The majority of dry weight of cells comes from organic molecules. Organic molecules contain, minimally, carbon and hydrogen, however oxygen is typically also present. In addition to requiring a source of carbon, hydrogen and oxygen to build these molecules, cells need a source of three other elements at relatively high levels: nitrogen, phosphorus, and sulphur. In particular, cells need nitrogen for the formation of proteins, nucleic acids, and a few other cell components. Phosphorous is also a crucial component of nucleic acids; it is also required for the synthesis of phospholipids, and adenosine triphosphate or ATP. Sulphur is necessary for a few amino acids, as well as several vitamins. Required at slightly lower levels are cations: potassium (K+), which is needed for enzymes, and magnesium ions (Mg2+), which is used to stabilize ribosomes and membranes, iron (Fe2+) which is needed for electron transport systems, as well as for enzymes and calcium (Ca2+). Collectively these elements are referred to as the macronutrients. These can be further differentiated into major macronutrients (C, H, O, N, P, S) which have traditionally been added to microbiological growth media at g/L amounts, and minor macronutrients (K+, Mg2+ and Fe), traditionally added at mg/L amounts. Micronutrients (e.g.cobalt, copper, manganese, zinc) are also required, but at ~1000-fold lower levels. These are present as contaminants of glassware and water and are typically structural components of enzymes or cofactors.
Copiotrophs vs oligotrophs
The traditional g/L amounts of major macronutrients support the growth of many bacteria and archaea