Nutrition and Growth - Factors Affecting the Growth of Bacteria

NUTRITION AND GROWTH

Bacteria vary considerably in their requirements for nutrients and in their ability to synthesize for themselves various vitamins and growth factors. Clearly the major elemental requirements for growth will match closely the elemental composition of the bacteria themselves. In this fashion there is a need for the provision of carbon, nitrogen, water, phosphorus, potassium and sulphur with a minor requirement for trace elements such as magnesium, calcium, iron, etc. The most independent classes of bacteria are able to derive much of their nutrition from simple inorganic forms of these elements. These organisms are called chemolithotrophs and can even utilize atmospheric carbon dioxide and nitrogen as sources of carbon and nitrogen. Indeed, such bacteria are, in addition to the green plants and algae, a major source of organic molecules and so they are more beneficial than problematic to humans. The majority of bacteria require a fixed carbon source, usually in the form of a sugar, but this may also be obtained from complex organic molecules such as benzene, paraffin waxes and proteins.

Nitrogen can generally be obtained from ammonium ions but is also available by deamination of amino acids, which can thus provide both carbon and nitrogen sources simultaneously. Many classes of bacteria are auxotrophic and can grow on simple sugars together with ammonium ions, a source of potassium and trace elements. Such bacteria can synthesize for themselves all the amino acids and ancillary factors required for growth and division. These bacteria, e.g. pseudomonads and Achromobacter species, are generally free-living environmental strains but they can sometimes cause infections in immunocompromised people. In the laboratory they can be grown in simple salts media with few, if any, complex supplements. The rate of growth of such organisms depends not only on temperature and pH but also on the nature of the carbon and nitrogen sources. Thus, a faster rate of growth is often obtained when glucose or succinate is the carbon source rather than lactose or glycerol, and when amino acids are provided as sources of nitrogen rather than ammonium salts. If faced with a choice of carbon and nitrogen sources then the bacteria will adapt their physiology to the preferred substrate and only when this is depleted will they turn their attention to the less preferred substrate. Growth in liquid cultures with dual provision of substrate such as this is often characterized by a second lag phase during the logarithmic growth period while this adaptation takes place. This is called diauxic growth (Figure 3.11).

As the association between bacteria and higher life forms becomes closer then more and more preformed bio-synthetic building blocks become available without the need to synthesize them from their basic elements. Thus a pathogenic organism growing in soft tissues will have available to it glucose and metal ions from the blood and a whole plethora of amino acids, bases, vitamins, etc. from lysed tissue cells. While most bacteria will utilize these when they are available, a number of bacteria that have become specialized pathogens have lost their ability to synthesize many of these chemicals and so cannot grow in situations where the chemicals are not provided in the medium. Consequently many pathogens require complex growth media if they are to be cultured in vitro.

All that has been discussed earlier about the physico-chemical and nutritional constraints on bacterial growth has been based on laboratory studies. By definition, the only bacteria that we can describe in this way are those that can be cultured artificially. It cannot be overstated that a majority of bacterial species and genera cannot be cultured in the laboratory. In the past the presence of such non-culturable bacteria has been attributed to moribund cells. With the advent of modern molecular tools, however, it has now been realized that these organisms are viable. By amplifying their DNA and sequence mapping, the genetic relationship of such bacteria to the culturable ones can be demonstrated and whole new families of hitherto unrecognized bacteria are being identified. It is possible that in the future many disease states currently thought to have no microbiological involvement could be identified as being of bacterial origin. A recent example of this has been the association of Helicobacter pylori with gastric ulcers and gastric cancers.