Microbial Growth & Control of Microbial Growth: Study Notes

Microbial Growth

Definition and Characteristics

Microbial growth refers to the increase in the number of cells, not the size of individual cells. Microbes grow by accumulating and forming colonies, which are groups of hundreds or thousands of bacterial cells.

• Biofilms: Some bacteria form biofilms, which are structured communities of cells attached to surfaces. Example: Serratia marcescens forms red-pigmented colonies and is found in soil, water, and can cause hospital-acquired infections.

• Opportunistic Pathogens: Certain bacteria, such as Serratia, can cause disease in immunocompromised patients.

Bacterial Reproduction

Methods of Reproduction

Bacteria reproduce by several methods, each contributing to population growth and genetic diversity.

• Binary Fission: The most common method. DNA is copied and the cell splits into two identical cells.

• Budding: Some species form a small outgrowth that enlarges and separates from the parent cell.

• Asexual Spores: Filamentous bacteria (e.g., actinomycetes) produce chains of spores externally.

• Fragments: Filamentous species separate into fragments, which grow into new cells.

Binary Fission Process

• Cell elongates and DNA is replicated.

• Plasma membrane and cell wall begin to divide.

• Cross wall forms, separating the two DNA copies.

• Cells separate, resulting in two identical daughter cells.

Streptomyces and Asexual Spores

Characteristics and Importance

• Actinobacteria: Highly pleomorphic, includes pathogenic species such as Mycobacterium.

• Actinomycetes: Includes Streptomyces, Frankia, Actinomyces, and Nocardia. Known for branching filamentous growth.

• Streptomyces: Gram-positive, abundant in soil, and produce commercial antibiotics.

• Grow as extended, branching filaments and form reproductive asexual spores at the end of aerial filaments.

• Produce extracellular enzymes to utilize soil nutrients and geosmin (gives soil its characteristic odor).

Bacterial Growth Curve

Phases of Growth

Bacterial populations follow a predictable growth curve in a closed system:

• Lag Phase: Period of little or no cell division; cells are metabolically active, synthesizing enzymes and molecules.

• Log (Exponential) Phase: Rapid cell division and population increase; generation time is constant.

• Stationary Phase: Growth rate slows; number of deaths balances new cells; population stabilizes due to limited nutrients and waste accumulation.

• Death Phase: Death rate exceeds new cell formation; population declines logarithmically.

Generation Time

• The time required for a cell to divide and its population to double.

• Most bacteria: 1 to 3 hours; some require more than 24 hours.

• Uncontrolled binary fission can lead to massive populations in short time (e.g., E. coli can reach 1 billion cells in 10 hours).

Calculation Formula

Number of cells after n generations: $N = N_0 imes 2^n$ Where $N_0$ is the initial number of cells and $n$ is the number of generations.

Microbial Growth Controls

Physical and Chemical Methods

• Physical: Temperature, pH, osmotic pressure.

• Chemical: Antimicrobials (chemicals, toxins, oils).

First Microbial Control Practices

• Pasteur: Used heat to destroy microorganisms, leading to modern sterilization techniques.

• Aseptic Techniques: Developed by Ignaz Semmelweis and Joseph Lister; include hand washing and aseptic surgery to prevent contamination.

Healthcare-Associated Infections (HAIs)

• Infections acquired in healthcare settings (e.g., UTIs, pneumonias, bloodstream, surgical site infections).

• Often caused by normal microbiota introduced via medical procedures.

Metabolic Classification of Organisms

Energy and Carbon Sources

• Chemotrophs: Use chemical compounds for energy.

• Phototrophs: Use light for energy.

• Chemoheterotrophs: Use organic compounds for both energy and carbon; most animals, fungi, protozoa, bacteria.

• Chemoautotrophs: Use CO2 as carbon source and oxidize inorganic compounds for energy.

• Photoheterotrophs: Use light for energy and organic compounds for carbon.

• Photoautotrophs: Use light for energy and CO2 for carbon; includes plants, algae, cyanobacteria.

Antimicrobial Chemotherapy

Principles and Types

• Antimicrobial Chemotherapy: Use of medications to kill or inhibit microbial growth.

• Selective Toxicity: Drugs must target microbes without harming the host.

• Antibiotics: Naturally produced by bacteria and fungi.

• Synthetic Drugs: Chemically synthesized in laboratories.

Important Discoveries

• Quinine: Extracted from cinchona bark; used to treat malaria caused by Plasmodium species.

• Penicillin: Discovered by Alexander Fleming; inhibits bacterial growth via antibiosis.

Quinoline Derivatives (Antimalarial Activity)

• Examples: Chloroquine, hydroxychloroquine, quinine, quinidine, primaquine, mefloquine, halofantrine.

• Mode of action: Accumulate in erythrocytes, bind DNA, interfere with replication, produce toxic complexes, raise pH in vesicles, destroy erythrocytic stage of parasite.

Other Treatments for Protozoans

• Metronidazole: Interferes with DNA synthesis; treats giardiasis, amebic dysentery, anaerobic bacterial infections.

• Tinidazole: Treats giardiasis, amebiasis, trichomoniasis.

• Nitazoxanide: Treats cryptosporidial diarrhea.

• Miltefosine: Repurposed for Leishmaniasis and amebic encephalitis.

Other Microorganisms Producing Antimicrobial Substances

Sources and Examples

• Most antibiotics are produced by Streptomyces species (filamentous soil bacteria).

• Some are produced by endospore-forming bacteria (Bacillus), and molds (Penicillium, Cephalosporium).

Microorganism	Antibiotic
Streptomyces griseus	Streptomycin
Bacillus polymyxa	Polymyxin
Penicillium chrysogenum	Penicillin
Cephalosporium acremonium	Cephalosporin
Additional info: Table inferred from slide and textbook context.

Physical Methods to Control Microbial Growth

Moist Heat Methods

• Pasteurization: 63°C for 30 min or 72°C for 15 sec; kills pathogens and spoilage organisms.

• Boiling: 100°C for 10 min; kills vegetative cells but not endospores.

• Autoclaving: 121°C, 15 psi, 15 min; kills endospores, sterilizes media and equipment.

Dry Heat Methods

• Direct Flaming: Used for sterilizing inoculating loops.

• Incineration: Disposes of contaminated materials.

• Hot Air Sterilization: Oven at 170°C for 2 hours.

Filtration

• Removes microbes from liquids and gases using membranes with small pores.

• HEPA Filters: Remove particles >0.3 μm; used in hospital rooms.

Other Methods

• Low Temperature: Slows microbial growth; pathogenic bacteria usually do not grow.

• Desiccation: Microbes cannot grow but remain viable (e.g., freeze-drying).

• Radiation: Gamma rays, UV light; damages DNA and kills microbes.

Chemical Methods of Microbial Control

Types of Disinfectants

• Phenol and Phenolics: Disrupt membranes; used in surgical procedures.

• Bisphenol: Hexachlorophene; prescription lotion for surgery.

• Essential Oils: Extracted from plants; antimicrobial properties.

• Halogens: Iodine and chlorine; impair protein synthesis and membrane function.

• Alcohols: Denature proteins, disrupt membranes.

• Heavy Metals: Oligodynamic action; antimicrobial effect.

• Surfactants: Decrease surface tension; soaps and detergents.

Summary Table: Bacterial Growth Curve Phases

Phase	Description
Lag	Little/no division, intense metabolic activity
Log	Exponential growth, rapid division
Stationary	Equilibrium, deaths balance new cells
Death	Decline, deaths exceed new cells

References

• Tortora, G. J., et al. (2021) Microbiology: An Introduction, 13th Edition. Pearson Education Inc.