Control of Microbial Growth

Chapter 7

Chapter 7 - Learning Objectives

The germ theory of disease states that microorganisms, specifically pathogens, are the cause of many diseases. For example, Pasteur's experiments with rabies and anthrax demonstrated how germs can be responsible for illnesses.

Sterilization is the process of killing all microorganisms, including spores. It is typically used in medical settings for surgical instruments and laboratory equipment, ensuring they are free from all pathogens. An example is the autoclaving of surgical instruments.

Commercial sterilization refers to the process that kills all pathogens and most spores but may not destroy all organisms. It is commonly used in food preservation, particularly in canned goods, to prevent foodborne illnesses such as botulism. The canning process heats food to a high temperature to achieve this.

Disinfection is the elimination of most pathogens on surfaces. Disinfectants, such as bleach, are used in bathrooms and hospitals to reduce the risk of infections. For example, wiping down medical equipment with an alcohol-based solution is a form of disinfection.

Antisepsis involves applying antiseptic agents to body surfaces to prevent infection. Hand sanitizers, which contain alcohol, are commonly used in healthcare settings before surgeries. Another example is using iodine solutions to clean the skin before an incision.

Decontamination is the process of making something safe by removing or neutralizing hazardous substances. This is often used in laboratory settings after handling biohazardous materials. For example, decontaminating surfaces with disinfectants after experiments is crucial.

De-germing refers to the reduction of microbial numbers on skin or surfaces. Washing hands with soap and water to remove pathogens is a common practice before food preparation or medical procedures.

Sanitation involves reducing microbial counts to safe levels, often used in food service and public health contexts. For example, sanitizing kitchen surfaces ensures compliance with health regulations.

The suffix "-cide" indicates that a chemical kills organisms, such as in pesticide, which is used to eliminate pests in agriculture or hospitals.

The suffix "-static" indicates that a substance inhibits growth but does not kill, as in bacteriostatic, which prevents bacteria from multiplying without necessarily killing them.

Fomites are inanimate objects or materials that can carry infection, such as doorknobs, utensils, or medical equipment. They can harbor pathogens and facilitate their transmission.

Factors that influence microbial agent effectiveness include time, temperature, pH, concentration of the agent, organic matter presence, and the nature of the microorganisms being targeted. For example, higher temperatures may increase disinfectant effectiveness.

The four main ways that microbial control agents kill or inhibit microbes are:   - Altering cell membrane permeability (e.g., detergents disrupt membrane integrity).   - Disrupting cell walls (e.g., penicillin inhibits wall synthesis).   - Damaging proteins (e.g., heat denatures proteins essential for cell function).   - Damaging nucleic acids (e.g., radiation causes DNA breaks).

Physical methods of microbial control discussed in class include:   - Heat (both moist and dry)   - Filtration   - Radiation   - Desiccation

Heat controls microorganism growth by denaturing proteins and disrupting cell membranes. For instance, boiling water kills most bacteria and viruses after several minutes.

The difference between dry heat and moist heat lies in their mechanisms: dry heat sterilization requires higher temperatures and longer exposure times than moist heat, which uses steam under pressure (e.g., autoclaving).

Pasteurization is a process that uses mild heating to kill specific pathogens in food without altering its quality. It is used in dairy to kill pathogenic microbes while preserving the taste of milk.

High pressure controls microorganism growth by disrupting cellular structures and functions, often used in food processing (like high-pressure pasteurization) to extend shelf life without the need for heat.

An autoclave is a device that sterilizes equipment and supplies using steam under pressure, typically at 121°C for about 15-20 minutes. It works by raising the boiling point of water to kill pathogens effectively.

Low temperature inhibits microbial growth by slowing down metabolic reactions. Refrigeration (around 4°C) helps preserve food by slowing bacterial growth.

Desiccation is the process of removing moisture to inhibit microbial growth. Many bacteria require water, so drying food can prevent spoilage.

High salt and high sugar contribute to desiccation by creating hypertonic environments that draw water out of microbial cells, leading to their dehydration and inhibiting growth. An example is using salt to cure meats.

Filtration is a method to remove microorganisms from liquids or air by passing them through a filter with pore sizes that trap microbes. It is commonly used in water purification.

A high-efficiency particulate air (HEPA) filter captures at least 99.97% of particles that are 0.3 micrometers or larger, effectively trapping dust, pollen, mold, and bacteria in ventilation systems.

Ionizing radiation is a method of controlling microbial growth that uses high-energy radiation (like gamma rays) to damage DNA and cellular structures, leading to cell death. It is used for sterilizing medical supplies and food.

Nonionizing radiation includes ultraviolet (UV) light, which kills microbes by damaging their DNA, causing mutations that can impair reproduction. UV sterilizers are commonly used in laboratory and medical settings.

Important factors for chemical disinfectants include their concentration, contact time, temperature, pH, and presence of organic matter, which can inhibit their effectiveness. For instance, blood or saliva can reduce disinfectant action.

Methods to evaluate the effectiveness of disinfection are commonly use-dilution tests and the disk diffusion method that measure the ability to inhibit microbial growth.

Main types of chemical disinfectants discussed include alcohols, aldehydes, phenolics, halogens, and quaternary ammonium compounds.

Surfactants are agents that reduce surface tension and can disrupt membranes of microbes. For example, soaps and detergents use surfactants to help wash away bacteria and debris.

Detergents are surfactants that aid in cleaning; cationic detergents (positively charged) like benzalkonium chloride are effective against bacteria, while anionic detergents (negatively charged) like soap are better for removing dirt.

Acid anionic sanitizers, used in food processing, work by lowering pH and disrupting microbial cell walls; they are effective but can corrode metal surfaces if not used properly.

Phenolics are disinfectants derived from coal tar, which work by disrupting cell membranes and precipitating proteins. They are effective but have a strong odor and can be toxic.

Heavy metals control microbial growth by binding to proteins and disrupting metabolic processes. For example, silver ions are used in medical devices to reduce infection risk.

Examples of heavy metal uses include mercury for its antiseptic properties in the past, silver in wound dressings to prevent infection, and copper in water purification systems due to its antimicrobial properties.

The two halogens discussed in class are chlorine and iodine. Chlorine is commonly used in water treatment to kill pathogens, and iodine is used as an antiseptic for cleaning wounds.

Iodine works by oxidizing cellular components and causing protein denaturation. It is effective but can cause skin irritation and is not suited for sensitive areas.

Chlorine works by releasing hypochlorous acid in water, which kills bacteria and viruses on contact. It is widely used in disinfecting drinking water and swimming pools, though it may create harmful byproducts.

Alcohols, such as ethanol and isopropyl alcohol, kill microbes by denaturing proteins and disrupting membranes. They are effective on surfaces but can be flammable and are less effective in the presence of organic matter.

Essential oils, derived from plants, have antimicrobial properties and can inhibit microbial growth; for example, tea tree oil and eucalyptus oil are known for their antiseptic qualities.

Aldehydes, like formaldehyde and glutaraldehyde, are potent disinfectants used in medical settings and for preserving biological specimens. They work by cross-linking proteins and nucleic acids, effectively killing microbes.

Chemical food preservatives, such as nitrites and sulfites, are added to food products to prevent spoilage and bacterial growth. They extend shelf life but can cause sensitivities in certain individuals.

Peroxygens, like hydrogen peroxide, are powerful oxidizing agents that kill microbes by producing reactive oxygen species that damage cellular components. They are often used for disinfecting surfaces and wounds, but can be irritating to skin.

Antimicrobial drugs are substances that kill or inhibit the growth of microorganisms, including antibiotics and antivirals used to treat infections.

The five modes of action of antimicrobial drugs include:   - Inhibiting cell wall synthesis (e.g., penicillin)   - Disrupting cell membrane function (e.g., polymyxins)   - Inhibiting protein synthesis (e.g., tetracyclines)   - Inhibiting nucleic acid synthesis (e.g., fluoroquinolones)   - Inhibiting metabolic pathways (e.g., sulfonamides)

An antibiotic is a type of antimicrobial drug specifically against bacteria, such as amoxicillin.

Antibiotic resistance develops when bacteria evolve mechanisms to survive exposure to an antibiotic, often through mutation or acquiring resistance genes.

Bacterial cells can pass antibiotic resistance to other cells through horizontal gene transfer methods, such as transformation, transduction, or conjugation, enabling rapid spread of resistance traits.

Key terms related to microbial control include:   - Sterilization: elimination of all microorganisms.   - Disinfection: reduction of pathogens to safe levels.   - Antisepsis: prevention of infection on living tissues.   - De-germing: removal of microbes from a surface.   - Sanitization: lowering microbial counts to safe levels.   - Biocide: agent that kills living organisms.   - Germicide: agent specifically targeting pathogens.   - Bacteriostasis: inhibiting bacterial growth.   - Asepsis: absence of bacteria or other microorganisms.

Patterns of microbial death caused by treatments can often be described as logarithmic; a defined reduction in viable counts occurs over time when exposed to effective microbial control agents.

Effects of microbial control agents on cellular structures include damage to membranes, proteins, and nucleic acids, resulting in impaired function and cell death.

In comparing moist heat (boiling and pasteurization) with dry heat (like baking), moist heat is generally more effective at lower temperatures and shorter exposure times due to enhanced penetration and protein denaturation.

Filtration is used to remove microbes from liquids or gases by physical separation. Low temperatures suppress microbial growth by inhibiting metabolic rates, while high pressure can disrupt cell membranes. Desiccation and osmotic pressure can lead to cell death by dehydration.

Radiation kills cells by inducing DNA damage and other cellular changes through ionizing or non-ionizing radiation, effectively disrupting essential functions.

Use-dilution tests and disk diffusion methods are two standard techniques used to evaluate antimicrobial effectiveness based on the ability to inhibit microbial growth in varying conditions.

Methods of action and preferred uses of chemical disinfectants vary; some are effective against a broad range of microbes, while others are more specialized based on chemical composition and application.

Halogens used as antiseptics (like iodine) differ from those used as disinfectants (like chlorine) based on concentration and application context. Surface-active agents, such as detergents, are designed for cleaning and sanitizing various surfaces.

Glutaraldehyde is favored as a chemical disinfectant due to its broad-spectrum efficacy, ability to sterilize even in the presence of organic matter, and relatively low toxicity compared to other agents.

The type of microbe affects control methods as different microorganisms possess distinct structural and metabolic characteristics that influence susceptibility to various antimicrobial agents. For example, Gram-negative bacteria often require different approaches than Gram-positive bacteria due to differences in cell wall composition.

Microbial Traits Controlled by DNA: DNA governs various traits in microbes including metabolism, resistance to antibiotics, virulence factors, and structural components. For example, the presence of the gene for pectinase allows certain bacteria to degrade plant materials, facilitating their survival in specific environments.

Genotype vs Phenotype: The genotype represents the genetic makeup of an organism, encompassing all alleles present, while the phenotype refers to the observable characteristics and traits resulting from the genotype and its interaction with the environment. For instance, a plant may have a genotype for tallness (TT or Tt), but only those expressing that phenotype will be tall; others may be shorter (tt).

Central Dogma: The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein. It comprises three main processes: transcription (DNA to RNA), translation (RNA to protein), and replication (copying DNA).

DNA Replication and Gene Expression: DNA replication entails duplicating the entire DNA molecule so that each new cell receives a complete copy. Gene expression involves two stages: transcription, where DNA is transcribed into messenger RNA (mRNA), and translation, where mRNA is translated into a functional protein.

Location of Processes: In prokaryotic cells, replication, transcription, and translation occur in the cytoplasm as there's no nucleus. In eukaryotic cells, replication and transcription happen in the nucleus, while translation occurs in the cytoplasm.

Flow of Genetic Information: Gene expression relates to how genes are turned on and off, recombination refers to the exchange of genetic material, and DNA replication ensures genetic material is accurately copied for distribution in cell division.

Structure of Nucleotides: Nucleotides, the building blocks of DNA, consist of a phosphate group, a sugar (deoxyribose in DNA), and a nitrogenous base (adenine, thymine, cytosine, or guanine). The 1’ through 5’ carbons label location; the 3’ and 5’ refer to where the phosphate bonds occur, essential in DNA synthesis.

Bond Between Nucleotides: Nucleotides are linked by phosphodiester bonds formed between the 5' phosphate of one nucleotide and the 3' hydroxyl group of another, creating a sugar-phosphate backbone.

Antiparallel DNA Strands: DNA strands run in opposite directions (5' to 3' and 3' to 5'), which is crucial for replication and function. This orientation allows for complementary base pairing.

Base-Pairing Rules: Adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G) through hydrogen bonds, ensuring fidelity in DNA replication.

Determining 5' and 3' Ends: The 5' end of a DNA strand has a phosphate group attached to the 5' carbon, while the 3' end has a hydroxyl group attached to the 3' carbon of the sugar.

Semi-Discontinuous DNA Replication: DNA replication is termed semi-discontinuous because the leading strand is synthesized continuously, while the lagging strand is synthesized in short segments called Okazaki fragments.

Direction of DNA Replication: The template strand is read in the 3' to 5' direction, and the new strand is synthesized in the 5' to 3' direction.

Replication Fork: The replication fork is the area where the double helix unwinds, allowing replication to occur and creating a Y-shaped structure.

Leading vs Lagging Strand: The leading strand is synthesized continuously towards the replication fork, while the lagging strand is synthesized in short segments away from it, producing Okazaki fragments.

Okazaki Fragments: These are short sequences of DNA synthesized during the lagging-strand replication that are later joined by DNA ligase.

Primers: Primers are short RNA sequences needed to initiate DNA synthesis, providing a free 3’-hydroxyl group for DNA polymerase to extend.

Events at the DNA Replication Fork: 1. Helicase unwinds DNA. 2. Primase lays down RNA primers. 3. DNA polymerase synthesizes the new strand. 4. Ligase joins Okazaki fragments.

Enzymes in DNA Replication: Key enzymes include helicase (unwinds DNA), primase (adds RNA primers), DNA polymerase (synthesizes new DNA strands), and ligase (joins DNA fragments).

Origin of Replication: This is the specific DNA sequence where replication begins, allowing the unwinding and synthesis of DNA to start.

Bidirectional Replication: In bidirectional replication, DNA replication proceeds in two directions around the origin, creating two replication forks.

Vertical vs Horizontal Gene Transfer: Vertical gene transfer occurs when genetic information is passed from parent to offspring, while horizontal gene transfer involves genetic exchange between organisms of the same generation.

Transformation Steps: Transformation involves a bacterium taking up free DNA from its environment, then integrating it into its genome, which can occur naturally or be facilitated in labs.

Conjugation Steps: Conjugation involves direct transfer of DNA between bacteria via a pilus, allowing for genetic exchange; usually occurs in plasmids.

Template for Transcription: The DNA strand serving as the template for RNA synthesis is called the antisense strand, which is complementary to the mRNA produced.

Comparing RNA and DNA Polymers: RNA is usually single-stranded and contains ribose sugars and uracil instead of thymine, while DNA is double-stranded with deoxyribose and thymine.

Ribosomal RNA (rRNA): rRNA is a critical component of ribosomes, forming the structure and facilitating protein synthesis.

Messenger RNA (mRNA): mRNA carries the genetic information from DNA to the ribosome, where proteins are synthesized.

Transfer RNA (tRNA): tRNA transports amino acids to the ribosome during translation, matching them to the corresponding codons on the mRNA.

RNA Polymerase: This enzyme synthesizes RNA from a DNA template during transcription, catalyzing the formation of bonds between ribonucleotides.

Promoter: A location on DNA where RNA polymerase binds to initiate transcription, determining where and when a gene is expressed.

Terminator: This sequence signals the end of transcription, causing RNA polymerase to release the newly synthesized RNA.

Three Stages of Transcription: 1. Initiation (RNA polymerase binds to promoter). 2. Elongation (RNA strand elongates). 3. Termination (transcription ends at terminator).

Direction During Transcription: The DNA template is read in the 3' to 5' direction, while the newly synthesized RNA strand is made in the 5' to 3' direction.

RNA Processing: In eukaryotes, RNA undergoes processing to remove introns and splice exons together; this ensures proper protein translation and transportation out of the nucleus.

Exons and Introns: Exons are coding regions of RNA, while introns are non-coding sequences removed during RNA processing.

5’ Cap and 3’ Poly A Tail: The 5' cap protects mRNA from degradation and assists ribosome binding, while the poly A tail stabilizes mRNA and aids in export from the nucleus.

Amino Acids Structure: Amino acids are organic molecules with a central carbon, an amino group, a carboxyl group, and a variable R group determining its characteristics.

Bond Between Amino Acids: Peptide bonds form between the carboxyl group of one amino acid and the amino group of another, linking them in a polypeptide chain.

Codons: Codons are three-nucleotide sequences in mRNA that specify amino acids; for example, AUG codes for methionine, the start codon.

Sense vs Nonsense Codons: Sense codons code for amino acids, while nonsense codons (like UAA, UAG, UGA) signal the termination of the translation process.

Total Codons: There are 64 possible codons (61 coding and 3 stop codons), providing redundancy in the genetic code.

Redundancy in Codons: Multiple codons can code for the same amino acids due to the nature of the genetic code being degenerate, allowing flexibility in translation.

Nonsense Codons: The three nonsense codons are UAA, UAG, and UGA, which signal termination of translation.

Start Codon: AUG is the start codon that encodes for methionine, marking the beginning of protein synthesis.

Interpreting Codon Table: The codon table maps sequences of rRNA to corresponding amino acids, allowing for the decoding of mRNA during translation.

Ribosomes: Ribosomes are cellular structures that facilitate the assembly of amino acids into proteins by reading mRNA during translation.

Anticodons: Anticodons are triplet sequences on tRNA complementary to codons on mRNA, ensuring the correct amino acid is added during protein synthesis.

Direction of mRNA Template Reading During Translation: The mRNA template is read in the 5' to 3' direction by ribosomes during protein synthesis.

A, E, and P Sites on Ribosomes: The A site (aminoacyl) holds the incoming tRNA, the P site (peptidyl) holds the growing polypeptide, and the E site (exit) is where tRNAs exit the ribosome after donating their amino acids.

Translation Steps: 1. Initiation (mRNA, tRNA, and ribosomes assemble). 2. Elongation (tRNAs bring amino acids to the ribosome). 3. Termination (release factors help terminate synthesis).

Coupling of Transcription and Translation: In prokaryotes, transcription and translation can occur simultaneously in the cytoplasm, enhancing efficiency in gene expression.

Mutation, Mutagens, Spontaneous Mutation: Mutations are changes in DNA sequences, mutagens are agents that induce mutations (like radiation), and spontaneous mutations arise naturally during DNA replication due to errors.

Base Substitution Mutations: These involve the replacement of one nucleotide with another, possibly altering the amino acid sequence (e.g., A to T).

Missense vs Nonsense Mutations: Missense mutations change one amino acid in a protein, while nonsense mutations create a premature stop codon, truncating the protein.

Frameshift Mutations: Caused by insertions or deletions of nucleotides, these mutations shift the reading frame of the codons, often resulting in entirely different protein sequences.

Benefit of Mutations: Mutations can confer advantages, such as antibiotic resistance, promoting survival in challenging environments.

Definitions: Genetics studies heredity; a genome is the complete set of genetic material; chromosomes are structures containing DNA; genes are specific DNA sequences coding for proteins; the genetic code is the mapping of DNA sequences to amino acids; genotypes are genetic makeup; phenotypes are physical traits; and genomics is the study of genomes.

DNA as Genetic Information: DNA contains the instructions for building proteins and maintaining cell functions, serving as the hereditary material passed from parents to offspring.

Protein Synthesis: This involves transcription, where mRNA is synthesized from DNA, followed by translation, where ribosomes synthesize proteins based on mRNA codons.

Comparing Protein Synthesis: In prokaryotes, protein synthesis occurs in the cytoplasm with simultaneous transcription, while in eukaryotes, transcription occurs in the nucleus, followed by mRNA processing before translation in the cytoplasm.

Operons: Operons are groups of genes regulated together, often functioning in related biological pathways, like the lac operon in E. coli, controlling lactose metabolism.

Pre-Transcriptional Regulation: In bacteria, gene regulation can occur at the level of RNA polymerase binding to promoters to control transcription.

Post-Transcriptional Regulation: This includes RNA processing, where introns are spliced out and exons are joined, along with adding a 5’ cap and a poly A tail to the mRNA to enhance stability and translation efficiency.

Classifying Mutations: Mutations can be classified as point mutations (single nucleotide changes), frameshift mutations (insertions/deletions), or large-scale mutations (affecting larger DNA regions).

Repairing Mutations: Methods of repairing mutations include nucleotide excision repair (removing damaged regions) and mismatch repair (correcting base pair mismatches).

Effect of Mutagens: Mutagens can significantly increase the mutation rate by inducing changes in DNA, thereby increasing genetic diversity or the risk of diseases like cancer.

Functions of Plasmids and Transposons: Plasmids are extra-chromosomal DNA in bacteria, often carrying antibiotic resistance genes, while transposons are segments of DNA that can move within the genome, contributing to genetic diversity and evolution.