4 Microbial Growth and Control

What microbial growth means

Microbial growth usually means an increase in the number of cells in a population. Whether growth occurs, and how quickly, depends on nutrients, environmental conditions, and the microorganism’s ability to reproduce in those conditions.

Nutrients microbes need

Microbes need water, an energy source, a carbon source, and chemical building blocks to make new cells. The major elements are carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, often grouped as CHONPS. Carbon forms the backbone of cellular molecules; nitrogen is needed for proteins and nucleic acids; phosphorus is part of DNA, RNA, and phospholipids; and sulfur occurs in some amino acids and coenzymes. Minerals such as potassium, magnesium, and iron support cell structures and enzyme activity.

Some microbes also need growth factors, such as vitamins or amino acids that they cannot make themselves. Nutritional requirements differ among species, so a medium that supports one microorganism may not support another.

Takeaway: Successful growth requires both basic resources and any species-specific nutrients.

Environmental conditions that shape growth

Microorganisms grow within a range of conditions, with a minimum, optimum, and maximum for many environmental variables. Growth is generally strongest near the optimum and restricted near the limits.

• Temperature: Cold slows many cellular reactions, while excessive heat damages proteins and nucleic acids. Psychrophiles prefer cold conditions, mesophiles prefer moderate temperatures, and thermophiles prefer high temperatures. Many microbes associated with humans are mesophiles.

• pH: Most bacteria grow best near neutral pH. Acidophiles prefer acidic conditions, while alkaliphiles prefer basic conditions. Extreme pH can disrupt proteins and other cell components.

• Oxygen: Obligate aerobes require oxygen; obligate anaerobes are harmed by it. Facultative anaerobes can grow with or without oxygen, while microaerophiles need oxygen at lower-than-atmospheric levels. These differences affect how microbes are cultivated.

• Water and solutes: Low available water or high salt or sugar can draw water out of cells and restrict growth. Halophiles require high salt, whereas halotolerant microbes can withstand it. Drying, salting, and sugaring help preserve foods by reducing available water.

• Other conditions: Pressure matters to barophiles, and light matters to photosynthetic microbes.

Takeaway: Growth conditions must match the microorganism’s tolerances and preferences.

Growth phases and population change

A batch culture is a closed system in which nutrients are not replenished and waste is not regularly removed. Its population commonly passes through four phases:

1. Lag: Cells adjust to the environment and make needed molecules, so cell numbers change little.

2. Log (exponential): Cells divide rapidly at a relatively steady rate. The generation time is the time required for the population to double.

3. Stationary: Nutrients become limited and wastes accumulate; new cells form at roughly the rate that cells die.

4. Death (decline): Cell death exceeds cell formation, so the number of viable cells falls.

During exponential growth, the population can be estimated as:

Here, $N_0$ is the starting number of cells and $n$ is the number of generations. For example, a starting population of $100$ cells after three doublings is $100 \times 2^3 = 800$ cells.

By contrast, a continuous culture system, such as a chemostat, adds fresh medium and removes culture at a controlled rate. This can help maintain growth conditions over time.

Takeaway: Batch populations change as resources are used and waste accumulates; continuous systems replenish and remove material.

Cultivating microorganisms

Cultivation means growing microorganisms under laboratory conditions. A culture medium supplies nutrients and may be a liquid broth or a solid or semisolid preparation, often made solid with agar. Both the medium and incubation conditions need to suit the organism. Aseptic technique helps prevent unwanted microbes from contaminating cultures and spreading between samples.

Media differ according to their purpose:

• Defined media have a known chemical composition. Complex media contain ingredients such as extracts whose exact composition varies.

• Enriched media provide extra nutrients for fastidious organisms with specific requirements.

• Selective media inhibit some microbes while allowing others to grow. Differential media distinguish organisms through visible reactions.

• MacConkey agar is an example that selects for many Gram-negative bacteria and differentiates lactose fermenters, which form pink colonies, from nonfermenters.

• Enrichment cultures use conditions that favor a desired organism when it is present among other microbes.

Takeaway: Choose the medium, incubation conditions, and handling method to fit the organism and the purpose of cultivation.

Measuring microbial growth

Growth measurements can estimate total cells, viable cells, or culture density. Each method answers a different question, so the method and its limitations matter.

• Direct microscopic counts estimate the number of cells in a measured volume, but may include dead cells and debris.

• Viable plate counts estimate organisms able to form colonies under the chosen conditions. Results are reported as colony-forming units per milliliter (CFU/mL). A basic calculation is:

  $$\mathrm{CFU/mL} = \frac{\text{colonies counted} \times \text{dilution factor}}{\text{volume plated (mL)}}$$

  A colony may arise from a clump rather than a single cell, and some living cells may not grow on the selected medium.

• Turbidity (optical density) provides a quick, indirect estimate of cell density in a cloudy liquid culture. It does not distinguish living from dead cells by itself, but can be calibrated against viable counts.

• Most probable number (MPN) is a statistical estimate often used when microbes are too sparse to count reliably on plates, such as in some water or food samples.

Takeaway: Interpret a growth estimate in light of what the method counts and what it may miss.

Controlling microbial growth

Control methods may kill microbes, described as *-cidal, or inhibit their multiplication, described as *-static. The outcome depends on the microorganism and treatment conditions.

• Sterilization destroys or removes all forms of microbial life, including bacterial endospores. Pressurized steam is commonly used for heat-stable materials; filtration can remove microbes from heat-sensitive liquids.

• Disinfection reduces or inactivates many microbes on inanimate objects, but may not destroy endospores. Antisepsis applies antimicrobial agents to living tissue. Sanitation reduces microbial numbers to public-health standards.

• Heat can sterilize equipment or reduce pathogens in food. Pasteurization lowers pathogens and spoilage microbes but does not make food sterile.

• Cold generally slows growth rather than reliably sterilizing an item. Drying and adding salt or sugar restrict available water.

• Radiation can damage microbial genetic material, while filtration physically removes microbes from air or liquids. Chemical agents, including disinfectants and antiseptics, should be chosen for the surface or tissue being treated.

Takeaway: Control terms describe different outcomes and settings; reducing microbial growth is not always the same as eliminating all microbial life.