C.4 Microbial Growth and its control

If you remove all phosphorus (P) from a chemically defined medium, which specific cellular macromolecules will a bacterium be unable to synthesize?

The cell will be unable to produce RNA (20.5% dry weight), DNA (3.1%), and Lipids (9.1%) (specifically phospholipids), as phosphorus is a mandatory component of the sugar-phosphate backbone in nucleic acids and the polar heads of membrane lipids

What is the universal chemical composition of a microbial cell by dry weight?

Carbon (50%), Oxygen (20%), Nitrogen (14%), Hydrogen (8%), Phosphorus (3%), and Sulfur (1%). These six elements account for 96% of dry weight in all life forms.

 What is the rank of macromolecules by dry weight in a cell?

 

 Protein is highest (55%), followed by RNA (20.5%), Lipids (9.1%), Polysaccharides (5%), and DNA (3.1%)

What is needed for culturing micro organisms?

1. Carbon source (CO  or organic);

2. Energy source (light or chemicals);

3. Nitrogen source (NH 3,NO 3, or NO2);

4. Inorganic salts (P, S, K, Mg);

5. Trace elements

What is defined media?

A medium where the exact molecular formula and quantity of every ingredient are known. It is composed of pure biochemicals (salts, pure carbon/nitrogen sources).

• Purpose: Highly reproducible. Ideal for studying microbial metabolism, nutritional requirements, or performing standardized research.

What is complex media?

• A rich medium containing ingredients of natural origin (yeast extract, peptone, beef extract) whose exact chemical composition is unknown and varies batch-to-batch.

• Purpose: Provides a wide array of vitamins and nutrients to support the growth of fastidious (picky) or unknown bacteria.

What is selective media?

A medium designed to encourage the growth of specific microbes while suppressing unwanted ones. It contains inhibitory agents like dyes, antibiotics, or high salt concentrations.

• Purpose: Used to isolate specific organisms from mixed samples (e.g., separating pathogens from normal flora).

What is differential media?

• A medium that allows multiple types of microorganisms to grow, but uses indicators (usually pH indicators/dyes) to reveal specific biochemical or metabolic differences.

• Purpose: Identifies specific bacteria based on their appearance (color change or clearing).

Why does Escherichia coli reach a higher maximum temperature for growth when cultured in a complex medium compared to a defined medium?

High temperatures can denature specific heat-sensitive enzymes. In a complex medium, the "rich" ingredients (like yeast extract) provide pre-formed amino acids and vitamins that the cell no longer needs to synthesize itself, bypassing the need for those damaged biosynthetic pathways

You observe a culture in the lag phase. Why is the total biomass increasing while the cell count remains constant?

During the lag phase, cells are adapting to new conditions by synthesizing new proteins, RNA, and ribosomes to prepare for division. The cells are increasing in size and metabolic activity, but they have not yet reached the point of physical septum formation and separation

Why is iron bioavailable in anoxic environments?

Iron remains in the reduced Fe²⁺ form, which is soluble in water. Thus microorganisms can easily acquire it.

Why does iron become limiting in oxygenated environments?

Fe²⁺ is oxidized to Fe³⁺, which forms insoluble hydroxide precipitates:

These precipitates are not bioavailable.

What are siderophores?

High-affinity iron-chelating molecules secreted by microorganisms that bind Fe³⁺ and transport it into the cell.

Functions:

1. Bind insoluble Fe³⁺

2. Solubilize it

3. Deliver it to membrane receptors

Iron is limiting in hosts and environments, so microbes with efficient siderophores gain a major growth advantage.

Many pathogens use siderophores to steal iron from host proteins.

Why do microscopic counts require cell densities above ~10⁶ cells/mL?

At low densities there are too few cells per grid square, producing large counting errors and unreliable estimates.

Why does turbidity increase as microbial cell numbers increase?

Cells scatter incoming light, making the culture appear cloudy; higher cell numbers scatter more light, increasing optical density (OD).

Why must a standard curve be created when using turbidity to estimate cell numbers?

Because optical density measures light scattering, not cell number directly, so OD values must be correlated with actual cell counts.

Why do plate counts measure only viable cells?

Because colonies form only from cells capable of dividing, so dead or non-reproductive cells are not counted.

Why might a single colony not represent a single cell?

Cells may exist in clusters or chains, so one colony may originate from multiple cellls.

Why do enrichment cultures favor fast-growing microorganisms?

Because organisms with higher growth rates consume nutrients faster, allowing them to dominate the culture even if they were rare in the environment.

Why does pre-dilution help isolate slower-growing microbes in enrichment cultures?

Dilution reduces competition from fast growers, giving slower-growing but abundant organisms a chance to grow.

Why do microbial populations grow exponentially during log phase?

Because each cell divides into two cells, and each generation doubles the population size.

If a bacterium has a generation time of 20 minutes, how many cells will exist after 2 hours starting from 1 cell?

2 hours = 120 min
120 / 20 = 6 generations

Cells =
2⁶ = 64 cells

Why are cells most physiologically uniform during exponential phase?

Because all cells are growing and dividing at the same constant rate under stable conditions.

How do microorganisms maintain membrane fluidity at low temperatures?

By incorporating shorter and unsaturated fatty acids, which prevent tight lipid packing.

Why do thermophilic proteins contain more ionic bonds and hydrophobic interactions

These interactions stabilize protein structure, preventing unfolding at high temperatures.

Why are microscopic counts higher than plate counts?

Microscopic counts include all cells (live, dead, and dormant), while plate counts include only viable, culturable cells. Many microbes cannot grow under laboratory conditions. This leads to the great plate count anomaly.

Why is oxygen toxic to certain microorganisms?

toxic to obligate anaerobes; They do not produce enzymes like superoxide dismutase or catalase.
Oxygen leads to formation of reactive oxygen species (ROS) such as superoxide, hydrogen peroxide, and hydroxyl radicals. These oxidize DNA, proteins, and lipids, disrupting cellular function. Cells lacking detoxifying enzymes are therefore killed by oxygen.

Why do halophiles require high salt concentrations?

Halophilic proteins are adapted to function in high ionic strength environments and may lose structure at low salt. Cells maintain osmotic balance via compatible solutes or high intracellular ions. Low salt disrupts both protein stability and osmotic equilibrium.

How do compatible solutes protect cells?

Compatible solutes increase intracellular osmolarity without interfering with enzyme activity. They prevent water loss by balancing external solute concentration. They also stabilize proteins and cellular structures.

Why do cells enter stationary phase?

Growth slows due to nutrient depletion and accumulation of toxic metabolites. Cell division equals cell death, resulting in no net growth. Cells often activate stress responses and survival pathways.

Why is high salt dehydrating to cells?

Water moves out of the cell by osmosis toward the higher external solute concentration. This reduces cytoplasmic volume and disrupts cellular processes. Without adaptation, this leads to plasmolysis.

Why are halophilic proteins negatively charged?

Negatively charged amino acids bind water and stabilize proteins in high-salt environments. This prevents aggregation and maintains solubility.

Why do low temperatures inhibit microbial growth?

Membranes become rigid, reducing transport efficiency. Enzyme activity decreases due to reduced molecular motion. This slows metabolism and prevents growth.