Microbiology, Lec 14.

Microbial GROWTH and Reproduction

Read Chapter 6

We now understand how microbes get energy and how they make macromolecules - let's now see how they put it all together and grow. Remember microbes are so small that one microbial cell can't make a difference (except as inoculum). Indeed, in all processes that we will discuss, it will take millions of cells per cc to cause any sort of effect. For example, one cell of the most virulent microbe on the planet cannot hurt you unless it grows. The study of microbial growth is very important to studies of microbial pathology and ecology.

Let's consider the growth of single-celled prokaryote. The growth of an individual bacterial cell is extremely hard to study because they are so small. Therefore, when we talk about growth we are also referring to reproduction - i.e. the increase in the number of individuals. Bacteria basically clone themselves when they reproduce. Genetic exchange (lec. 15) is not part of the process but may occur during the life of an individual microbe.

In many species, one cell splits into two daughter cells of equal size and genetic makeup. This process is called:

Binary Fission (Fig. 12.20).

Other species can reproduce by budding off of of smaller cells:

Hyphomicrobium (Fig. 22.4, 22.5).

Some fungi (Eucarya) like Saccharomyces cerevisiae (brewer's yeast) also bud (see Figure 25.2a)

Filamentous bacteria and fungi grow via extension of the hyphal tip (thus growth is a way to move as well!). In fungi this growth is supplied by transport vesicles that carry building blocks from non-growing portions of the fungus. In some cases fungi can "grow" into a new environment without taking in new energy sources. They do this by digesting older parts of their hyphae and transporting the nutrients to the growing tips.
Fungi and many bacteria (e.g. Actinomycetes) also reproduce via spores which is essentially hyphal growth followed by fragmentation (see Fig. 25.7)
Draw Actinomycete..(see figures 24.13 - 24.16).

Explain the difference between reproduction and growth in microorganisms

Population Growth

From here on we will be discussing the growth of microbial populations (Fig. / draw fungal branching....). The simplest concept for understanding exponential growth is the generation (or doubling) time (G), i.e. the time it takes for the mass of a population to double

(Figs. 6.1 - 6.3).

We can describe the exponential growth of a microbial population using the following equation:

Nt = No x 2-to-the nth power

where No is the initial cell number, Nt is the final cell number and n is the number of generations.

Taking the log of both sides and solving for n we get:

n = (log Nt - log No) / log 2

Example: start with 20,000,000 cells per ml (= No). After 2 hours there are 320,000,000 cells per ml (= Nt).

What is the generation time?

plugging in numbers we get n = (8.505 - 7.301) / 0.301 = 4 generations

Generation time = G = 2 hours / 4 generations = 0.5 hrs.

Generation times for some typical bacteria - Table 6.2
Note pathogens

The great potential for microbial growth can be demonstrated by considering that one E. coli cell (G = 0.333 hr. or 20 minutes) would grow to be a mass 2000 x that of earth if it could grow unchecked for 48 hours!

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So why don't bacteria take over the world???

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Growth curve....

Before we leave the microbial growth curve I want to talk about a few metabolic processes that relate to the growth curve (e.g. Figure 6.4) or actually I want to say a little more about the non-growth parts of that curve....

Secondary Metabolism

The last couple of lectures have concentrated on how organisms transform energy and utilize that energy to build cellular material. In contrast, secondary metabolic processes are metabolic pathways that are not involved in growth of cells (or energy harvesting) and often take place when cells have stopped growing. A good example is the production of antibiotics by bacteria and fungi. Figures 44.7 - 44.9 show the difference between primary and secondary metabolic products (metabolites).

Idiophase = Stationary Phase

Show also figure of production of Bacitracin by Bacillus licheniformis just prior to endospore formation.

Can you think of an ecological explanation for why antibiotic production precedes endospore production in Bacillus licheniformis?

Lag Phase

Back to the first part of the growth curve......

The lag phase usually occurs when bacteria are introduced into a new environment they usually need to synthesize new enzymes in order to utilize substrates in that environment. Lag phases are even longer if bacteria have to come out of dormancy at the beginning of the growth curve. Many bacteria have shortened lag phases (and faster growth rates) if they are supplied with metabolic intermediates, vitamins, amino acids etc.....

Many heterotrophic bacteria also require CO2 in order to initiate growth in an oligotrophic environment (or in minimal media in the lab). These bacteria can use Anaplerotic Reactions to prime the TCA (Krebs) cycle so that it can turn (see page 201 - 203). Anaplerotic reactions carboxylate pyruvate or PEP to form oxaloacetate.

Some high GC Gram + Bacteria (e.g. Arthrobacter) do the following:

Pyruvate + CO2 + ATP + H2O ----> Oxaloacetate + ADP + Pi

Some Gram - Proteoacteria (e.g. Escherichia, Salmonella) do the following:

PEP + CO2 ----> Oxaloacetate + Pi

Any questions about physiology and growth?