Chapter 6 Microbial Growth: Requirements, Culture, and Measurement

Microbial Growth

Introduction

Microbial growth refers to the increase in the number of microbial cells, not cell size. Understanding the requirements and methods for microbial growth is essential for microbiology students, as it underpins laboratory techniques, food safety, and infection control.

The Requirements for Growth

Physical Requirements

• Temperature: Microbes are classified based on their preferred temperature ranges.

• pH: Most bacteria grow best between pH 6.5 and 7.5; molds and yeasts prefer pH 5–6.

• Osmotic Pressure: The concentration of solutes affects water movement and cell viability.

Temperature Groups of Microbes

• Psychrophiles: Cold-loving; optimal growth at ~15°C; found in deep ocean and polar regions.

• Psychrotrophs: Grow at 0°C, optimal 20–30°C; cause food spoilage in refrigerators.

• Mesophiles: Moderate-temperature-loving; optimal at 25–40°C; includes most pathogens and normal microbiota.

• Thermophiles: Heat-loving; optimal at 50–60°C; found in hot springs and compost.

• Hyperthermophiles: Optimal growth >80°C; found in extreme environments.

pH

• Most bacteria: pH 6.5–7.5

• Molds and yeasts: pH 5–6

• Acidophiles: Grow in acidic environments.

• Buffers are often added to media to maintain pH stability.

Osmotic Pressure

• Hypertonic environments cause plasmolysis (cell shrinkage) due to water loss.

• Extreme/obligate halophiles: Require high salt (up to 30% NaCl).

• Facultative halophiles: Tolerate high salt (2–10% NaCl).

Chemical Requirements

• Carbon: Backbone of organic molecules; chemoheterotrophs use organic carbon, autotrophs use CO2.

• Nitrogen: Needed for proteins, DNA, ATP; obtained from protein decomposition, ammonium, nitrates, or nitrogen fixation.

• Sulfur: Used in amino acids, thiamine, biotin; sourced from protein decomposition or inorganic sulfates.

• Phosphorus: Used in DNA, RNA, ATP, and membranes; supplied as phosphate ions.

• Trace Elements: Inorganic elements (e.g., Fe, Cu, Zn) required in small amounts as enzyme cofactors.

• Oxygen: Microbes are classified by their oxygen requirements (see table below).

• Organic Growth Factors: Compounds like vitamins, amino acids, purines, and pyrimidines obtained from the environment.

Oxygen Requirements

Type	Oxygen Requirement	Growth Pattern	Key Enzymes
Obligate Aerobes	Require O2	Growth at top of tube	SOD, catalase
Facultative Anaerobes	Grow with or without O2	Best growth at top, but throughout	SOD, catalase
Obligate Anaerobes	Cannot tolerate O2	Growth at bottom	None
Aerotolerant Anaerobes	Tolerate O2, do not use it	Even growth	SOD only
Microaerophiles	Require low O2	Growth in middle	Produce less SOD/catalase

Toxic Forms of Oxygen and Detoxification

• Singlet oxygen (O2*): Highly reactive.

• Superoxide radicals (O2-): Removed by superoxide dismutase (SOD).

• Peroxide anion (O22-): Removed by catalase and peroxidase.

• Hydroxyl radical (OH•): Most reactive, damaging to cells.

Key reactions:

• SOD: $2O_2^- + 2H^+ \rightarrow O_2 + H_2O_2$

• Catalase: $2H_2O_2 \rightarrow 2H_2O + O_2$

• Peroxidase: $H_2O_2 + 2H^+ \rightarrow 2H_2O$

Biofilms

Formation and Importance

• Biofilms are microbial communities that form slime or hydrogels adhering to surfaces.

• Bacteria communicate via quorum sensing and secrete inducers to attract others.

• Biofilms share nutrients and provide protection from environmental threats.

• They are found in natural and artificial environments (digestive system, sewage, catheters).

• Biofilms are up to 1000x more resistant to microbicides and are involved in 70% of infections.

Culture Media and Techniques

Types of Culture Media

• Chemically Defined Media: Exact chemical composition is known; used for fastidious organisms.

• Complex Media: Contains extracts/digests of yeasts, meat, or plants; composition varies.

• Agar: Solidifying agent, not metabolized by microbes; liquefies at 100°C, solidifies at ~40°C.

Special Media Types

• Reducing Media: For anaerobes; contains chemicals to remove O2.

• Selective Media: Suppress unwanted microbes, encourage desired ones (e.g., bismuth sulfite agar for Salmonella Typhi).

• Differential Media: Distinguish colonies of different microbes (e.g., blood agar).

• Enrichment Culture: Increases numbers of desired microbes to detectable levels.

Table: Types of Culture Media and Their Purposes

Type	Purpose
Chemically Defined	Growth of chemoautotrophs, photoautotrophs; assays
Complex	Growth of most chemoheterotrophs
Reducing	Growth of obligate anaerobes
Selective	Suppress unwanted, encourage desired microbes
Differential	Differentiation of colonies
Enrichment	Increase numbers of desired microbes

Biosafety Levels

• BSL-1: Basic teaching labs; no special precautions.

• BSL-2: Lab coat, gloves, eye protection.

• BSL-3: For highly infectious airborne pathogens; biosafety cabinets, negative pressure.

• BSL-4: Sealed, negative pressure; "hot zone"; workers wear "space suits"; only four in the U.S.

Obtaining and Preserving Pure Cultures

Pure Cultures and Isolation

• Pure Culture: Contains only one species or strain.

• Colony: Population of cells from a single cell or group; called colony-forming unit (CFU).

• Streak Plate Method: Used to isolate pure cultures.

Preservation Methods

• Deep-freezing: Storage at -50°C to -95°C.

• Lyophilization (freeze-drying): Frozen (-54°C to -72°C) and dehydrated in a vacuum.

The Growth of Bacterial Cultures

Bacterial Division

• Growth is an increase in cell number, not size.

• Binary Fission: Main method of bacterial cell division; each division doubles the cell number.

• Other methods: Budding, conidiospores, fragmentation.

Generation Time

• Time required for a cell to divide (20 min to 24 hr).

• Number of cells doubles each generation.

• Total number of cells: $N = N_0 \times 2^n$ (where $N_0$ is the initial number, $n$ is the number of generations).

• Growth curves are plotted logarithmically.

Phases of Growth

• Lag Phase: Little/no cell division; cells prepare for growth.

• Log (Exponential) Phase: Rapid cell division; constant generation time.

• Stationary Phase: Growth rate slows; deaths balance new cells; nutrients deplete, wastes accumulate.

• Death Phase: Deaths exceed new cells; population declines logarithmically.

Measuring Microbial Growth

Direct Methods

• Plate Count: Counts colonies (30–300 CFUs) after serial dilution; pour plate or spread plate methods.

• Filtration: Solution filtered, filter placed on nutrient medium, colonies counted.

• Most Probable Number (MPN): Statistical estimation using multiple tube tests.

• Direct Microscopic Count: Uses Petroff-Hausser cell counter; calculates average number per field.

Indirect Methods

• Turbidity: Measures cloudiness with a spectrophotometer.

• Metabolic Activity: Amount of metabolic product proportional to cell number.

• Dry Weight: Bacteria filtered, dried, and weighed; used for filamentous organisms.