Bacterial Growth on Different Media

BACTERIAL GROWTH ON DIFFERENT MEDIA

Growth on solid surfaces

If microorganisms are immobilized on a solid surface from which they can derive nutrients and remain moist, cell division will cause the daughter cells to form a localized colony. In spite of the small size of the individual organisms, colonies are easily visible to the naked eye. Indeed, microbial growth can often be seen on the tonsils of an infected individual or as colonies on discarded or badly stored foods. In the laboratory solidified growth media are deployed to separate different types of bacteria and also as an aid to enumerating viable cell numbers. These media comprise a nutrient soup (broth) that has been solidified by the addition of agar (see Chapter 2). Agar melts and dissolves in boiling water but will not re-solidify until the temperature is below 45 °C. Agar media are used in the laboratory either poured as a thin layer into a covered dish (Petri dish or plate) or contained within a small, capped bottle (slant). If suspensions of different species of bacteria are spread on to the surface of a nutrient agar plate then each individual cell will produce a single visible colony. These may be counted to obtain an estimate of the original number of cells. Different species will produce colonies of slightly different appearance, enabling judgements to be made as to the population diversity. The colour, size, shape and texture of colonies of different species of bacteria vary considerably and form a useful diagnostic aid to identification. Transfer of single colonies from the plate to a slant enables pure cultures of each organism to be maintained, cultured and identified.

Growth in liquids

When growing on a solid surface the size of the resultant colony is governed by the local availability of nutrients. These must diffuse through the colony. Eventually growth ceases when the rate of consumption of nutrients exceeds the rate of supply. When grown in liquids the bacteria, being of colloidal dimensions and sometimes highly motile, are dispersed evenly through the fluid. Nutrients are therefore equally available to all cells. When considering growth of bacterial populations in liquids it is necessary to consider whether the environment is closed or open with respect to the acquisition of fresh nutrient. Closed systems are typified by batch culture in closed glass flasks. In these waste products of metabolism are retained and all the available nutrients are present at the beginning of growth. Open systems, on the other hand, have a continual supply of fresh nutrients and removal of waste products.

A) Liquid batch culture (closed)

Figure 3.8 shows the pattern of population growth obtained when a small sample of bacteria is placed within a suitable liquid growth medium held in a glass vessel. As the increase in cell numbers is exponential (1, 2, 4, 8, 16, etc.) then during active growth a logarithmic plot of cell number against time gives a straight line (B). This period is often referred to as the logarithmic growth phase, during which the generation or doubling time may be calculated from the slope of the line. However, the exponential phase is preceded by a lag period (A), during which time the inoculum adapts its physiology to that required for growth on the available nutrients. As growth proceeds nutrients are consumed and waste materials accumulate. This has the effect of reducing the rate of growth (late logarithmic phase) towards an eventual halt (stationary phase, C). Starvation during the stationary phase will eventually lead to the death of some of the cells and adaptation to a dormant state in others (decline phase, D). Patterns of growth such as this occur within inadequately preserved pharmaceutical products, in water storage tanks and in industrial fermentations.

B)  Growth in open culture

Except under circumstances of feast–famine, growth of bacteria in association with humans and in our environment is subject to a gradual but continuous provision of nutrients and a dilution of waste products. Under such circumstances the rate of growth of bacteria is governed by the rate of supply of nutrients and the population size. Accordingly, bacteria in our gastrointestinal tracts receive a more or less continuous supply of food and excess bacteria are voided with the faeces (indeed, bacteria make up >90% of the dry mass of faeces). In many situations the bacteria become immobilized, as a biofilm, upon a surface and extract nutrients from the bulk fluid phase.