Most bacteria and some yeasts divide by a process of binary fission whereby the cell enlarges or elongates, then forms a cross wall (septum) that separates the cell into two more or less equal compartments each containing a copy of the genetic material.
CULTIVATION METHODS
Most
bacteria and some yeasts divide by a process of binary fission whereby the cell
enlarges or elongates, then forms a cross wall (septum) that separates the cell
into two more or less equal compartments each containing a copy of the genetic
material. Septum formation is often followed by constriction such that the
connection between the two cell compartments is progressively reduced (see
Figure 2.1a) until finally it is broken and the daughter cells separate. In
bacteria this pattern of division may take place every 25–30 minutes under
optimal conditions of laboratory cultivation, although growth at infection
sites in the body is normally much slower owing to the effects of the immune
system and scarcity of essential nutrients, particularly iron. Growth continues
until one or more nutrients is exhausted, or toxic metabolites (often organic
acids) accumulate and inhibit enzyme systems. Starting from a single cell many
bacteria can achieve concentrations of the order of 109 cells ml−1
or more following overnight incubation in common liquid media. At
concentrations below about 10 7 cells ml−1 culture media
are clear, but the liquid becomes progressively more cloudy (turbid) as the
concentration increases above this value; turbidity is, therefore, an indirect
means of monitoring culture growth. Some bacteria produce chains of cells, and
some produce elongated cells (filaments) that may exhibit branching to create a
tangled mass resembling a mould mycelium (Figure 2.1d). Many yeasts divide by
budding (see section 1.2.3 and Figure 2.1b) but they, too, would normally grow
in liquid media to produce a turbid culture. Moulds, however, grow by extension
and branching of hyphae to produce a mycelium (Figure 2.1c) or, in agitated
liquid cultures, pellet growth may arise.
When
growing on solid media in Petri dishes (often referred to as ‘plates’) individual
bacterial cells can give rise to colonies following overnight incubation under
optimal conditions. A colony is simply a collection of cells arising by
multiplication of a single original cell or a small cluster of them (called a
colony forming unit or CFU). The term ‘colony’ does not, strictly speaking,
imply any particular number of cells, but it is usually taken to mean a number
sufficiently large to be visible by eye. Thus, macroscopic bacterial colonies
usually comprise hundreds of thousands, millions or tens of millions of cells
in an area on a Petri dish that is typically 1–10 mm in diameter (Figure 2.1
e). Colony size is limited by nutrient availability and/or waste product
accumulation in just the same way as cell concentration in liquid media. Colonies
vary between bacterial species, and their shapes, sizes, opacities, surface
markings and pigmentation may all be characteristic of the species in question,
so these properties may be an aid in identification procedures .
Anaerobic
organisms may be grown on Petri dishes provided that they are incubated in an
anaerobic jar. Such jars are usually made of rigid plastic with airtight lids,
and Petri dishes are placed in them together with a low-temperature catalyst.
The catalyst, consisting of palladium coated pellets or wire, causes the oxygen
inside the jar to be combined with hydrogen that is generated by the addition
of water to sodium borohydride; this is usually contained in a foil sachet that
is also placed in the jar; alternatively, oxygen may be removed by combination
with ascorbic acid. After its removal, an anaerobic atmosphere is achieved and
this is monitored by an oxidation– reduction (redox) indicator; resazurin is
frequently used as a solution soaking a fabric strip.
Yeast
colonies often look similar to those of bacteria, although they may be larger
and more frequently coloured. The appearance of moulds growing on solid
microbiological media is similar to their appearance when growing on common
foods. The mould colony consists of a mycelium that may be loosely or densely
entangled depending on the species, often with the central area (the oldest,
most mature region of the colony) showing pigmentation associated with spore production
(Figure 2.1f). The periphery of the colony is that part which is actively
growing and it is usually nonpigmented.
Related Topics
TH 2019 - 2024 pharmacy180.com; Developed by Therithal info.