Lab Exam 2
Effects of UV Light
Methods for Controlling Microbial Growth
Chemical: Utilizes various chemical compounds to kill or inhibit microbes.
Disinfectants: Used on inanimate surfaces to destroy most microbes, but not necessarily endospores.
Antiseptics: Applied to living tissues (skin, mucous membranes) to reduce the number of microbes.
Antibiotics: Naturally produced or synthetic substances that target specific microbial processes within a host.
Physical: Employs physical agents to control microbial populations.
Heat: Denatures proteins and melts lipids (e.g., sterilization via autoclave, pasteurization).
Filtration: Physically removes microbes from liquids or air, often used for heat-sensitive materials.
Radiation: Uses energy waves or particles to damage microbial cellular components.
Radiation Types
Ionizing radiation (gamma rays, X-rays, electron beams):
Possesses high energy to eject electrons from atoms, creating ions.
Has the greatest impact on microbial growth due to deep penetration.
Produces highly reactive free radicals (, ) when interacting with water.
These free radicals cause extensive damage to vital cellular components such as DNA (strand breaks), proteins (denaturation), and membranes (lipid peroxidation), leading to cell death.
Non-ionizing radiation (UV light):
Has lower energy and limited penetration, primarily affecting surfaces.
Kills microbes primarily by damaging DNA through the formation of abnormal covalent bonds.
UV Radiation – How It Works
Primary Mechanism: The absorption of UV energy by DNA directly causes molecular damage.
Formation of thymine (or cytosine) dimers: This occurs when two adjacent pyrimidine bases (thymine or cytosine) on the same DNA strand bond covalently, forming a cyclobutane ring structure.
Dimers distort the DNA helix: The abnormal bonding causes a bulge in the DNA backbone.
Block replication and transcription: DNA polymerase and RNA polymerase cannot accurately read past these dimers, halting DNA synthesis and gene expression.
Lead to mutations or cell death: If unrepaired, these errors can lead to non-functional proteins, genetic mutations, and ultimately cell death.
Types of UV Radiation: Classified by wavelength, each with different properties.
UV-C (200–280 nm):
This range, particularly around , is the most lethal (germicidal) form of UV radiation for microbes because its wavelength is efficiently absorbed by DNA.
It is largely blocked by Earth’s ozone layer, so it is primarily generated artificially for sterilization.
UV-B (280–315 nm):
Causes sunburn and DNA damage in human cells, contributing to skin cancer.
Less effective at killing microbes compared to UV-C, as its absorption by DNA is lower.
UV-A (315–400 nm):
Has the longest wavelength and penetrates deepest into skin.
Not significantly germicidal; primarily associated with skin aging.
Importance: Wavelengths causing sunburn (primarily UV-B) do not necessarily kill bacteria as effectively as UV-C, indicating specific germicidal wavelengths are crucial.
Important Concepts
Microbes have varying susceptibility to UV radiation: Bacterial endospores (e.g., Bacillus spp., Clostridium spp.) are significantly more resistant to UV damage than vegetative cells due to their protective layers and DNA repair mechanisms.
For complete sterilization, spores must be destroyed: UV radiation alone is generally not sufficient for true sterilization unless prolonged exposure and optimal conditions are met, especially against endospores.
Limitations of UV: Its effectiveness is highly restricted by its poor penetration ability.
Cannot pass through plastic, glass, paper, or many solid materials: This means UV light is only effective for surface sterilization or for transparent liquids/gases, and any shadowing significantly reduces its efficacy.
Line-of-sight exposure: Requires direct exposure of the microbial surface to UV light.
Uses of UV Radiation
Used widely for disinfection rather than sterilization due to limitations.
Surface sterilization: For laboratory benchtops, biosafety cabinets, and medical instruments that can be directly exposed.
Air purification systems: Incorporated into HVAC systems to reduce airborne pathogens in hospitals, laboratories, and other public spaces.
Treatment of drinking water: UV systems are an effective, chemical-free method for inactivating waterborne pathogens (bacteria, viruses, protozoa) in municipal and point-of-use water purification.
Healthcare settings such as:
Operating rooms: Used to disinfect surfaces and air between surgical procedures.
Intensive Care Units (ICUs): Helps reduce hospital-acquired infections (HAIs) by deactivating pathogens on surfaces.
Notably used to inactivate SARS-CoV-2 and other viruses by damaging their genetic material, preventing replication.
Safety Considerations
UV radiation is harmful to human cells: It can damage skin and eyes by inducing DNA mutations (leading to skin cancer, premature aging) and protein denaturation (causing cataracts).
Safety precautions include:
Wearing appropriate personal protective equipment (PPE) such as UV-blocking goggles (to protect eyes from photokeratitis and cataracts) and long-sleeved clothing.
Avoiding direct exposure: Ensuring no part of the body is exposed to active UV lamps.
Not looking directly at UV lamps: Even brief exposure can cause retinal damage.
Avoiding exposure of hands to UV sources: Leading to burns and increased skin cancer risk.
Implementing safety interlocks on UV equipment that automatically shut off the lamp when a door or cover is opened.
Antimicrobial Sensitivity Testing (Kirby-Bauer)
Definitions: Clarify terms related to antimicrobial agents and their effects.
Antimicrobial: A general term for substances that kill or inhibit microbial growth. This includes antibiotics (natural products), synthetic drugs, and semi-synthetic derivatives.
Sensitive (Susceptible): Indicates that an antibiotic effectively kills or inhibits an organism at concentrations achievable in the body, suggesting it would be a suitable treatment option.
Resistant: Indicates an antibiotic is ineffective against an organism, meaning the microbe can grow in the presence of the drug, rendering it unsuitable for treatment.
Spectrum of Activity: Refers to the range of microbial types an antimicrobial is effective against.
Narrow spectrum antimicrobials: Effective against a limited range of bacteria. They are often preferred when the pathogen is identified to minimize disruption to the normal microbiota.
Example: Penicillin G (effective primarily against Gram-positive bacteria like Staphylococcus and Streptococcus, and some spirochetes).
Gram-negative antimicrobials: Specifically target Gram-negative bacteria, which have an outer membrane that often makes them naturally resistant to some drugs.
Example: Polymyxin B (disrupts the outer and inner membranes of Gram-negative bacteria).
Broad spectrum antimicrobials: Effective against a wide range of both Gram-positive and Gram-negative bacteria. Useful when the causative agent is unknown or in polymicrobial infections, but can contribute to antibiotic resistance and dysbiosis.
Examples:
Ciprofloxacin (a fluoroquinolone that inhibits bacterial DNA gyrase).
Tetracycline (inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit).
Trimethoprim (inhibits bacterial folic acid synthesis).
Combination therapy: The use of two or more antimicrobials simultaneously to achieve better therapeutic outcomes, broaden the spectrum, or combat resistance.
Amoxicillin + clavulanic acid (Augmentin): Clavulanic acid is a beta-lactamase inhibitor that protects amoxicillin from enzymatic degradation by beta-lactamase-producing bacteria, thus restoring amoxicillin's effectiveness.
Clindamycin can also treat Methicillin-resistant Staphylococcus aureus (MRSA) by inhibiting protein synthesis; its use requires careful consideration due to potential for Clostridioides difficile infection.
Kirby-Bauer Procedure
Bacteria are spread evenly on a 150 mm Mueller-Hinton agar plate using a sterile cotton swab to create a confluent lawn. This ensures uniform growth and accurate measurement of inhibition zones.
Antibiotic-impregnated disks of known concentration are aseptically placed on the agar surface using a dispenser or sterile forceps.
The plate is incubated, typically at for 18-24 hours. The antibiotic diffuses radially outwards from the disk into the agar, creating a concentration gradient (highest near the disk, lowest furthest away).
A clear circular area around the disk, called the zone of inhibition, indicates the concentration of antibiotic was sufficient to prevent visible bacterial growth. No zone indicates resistance or no effect.
Interpreting Results
Measure the diameter of the zone of inhibition in millimeters (mm) using a ruler or caliper, including the disk itself.
Compare this measurement to a standardized chart (Clinical and Laboratory Standards Institute - CLSI guidelines) specific for each antibiotic and bakteri spesies. These charts provide interpretive criteria (breakpoints) related to clinical outcomes.
Results classifications: Based on the zone diameter compared to breakpoints.
Susceptible (S): The organism is inhibited by the concentration of the antimicrobial agent achievable at the site of infection with normal dosing, implying effective treatment.
Intermediate (I): The organism may be inhibited if a higher dose or prolonged exposure to the drug is used, or if the drug concentrates at the infection site. Often requires clinical judgment.
Resistant (R): The organism is not inhibited by achievable concentrations of the antimicrobial agent, indicating the drug is unlikely to be clinically effective.
A larger zone indicates a more effective antimicrobial agent (at that concentration) against the tested organism, while a smaller or absent zone indicates less effectiveness or resistance.
Quantitative Dilution & Colony Counting – Plate Count (HPC)
Purpose: To estimate the number of viable (living) bacteria in a sample, often used for environmental or food microbiology.
Only living cells can form colonies, providing a better indicator of disease risk or microbial load compared to methods that count both live and dead cells.
Countable Plates Criteria: Essential for accurate quantification.
Plates yielding 30–300 colonies are considered statistically reliable and countable. Below 30, random errors are too significant. Above 300, colonies become too crowded, making accurate counting difficult and potentially masking some colonies.
TFTC (Too Few To Count): Yields fewer than 30 colonies; implies the original sample had a very low bacterial count or was over-diluted.
TNTC (Too Numerous To Count): Yields more than 300 colonies; implies the original sample was too concentrated or under-diluted.
Key Assumption: Each colony corresponds to one originally viable bacterial cell, or colony-forming unit (CFU). This assumption holds true if cells are well-separated and not aggregated.
Results are reported as CFU/mL (colony-forming units per milliliter) or CFU/g (per gram of solid sample).
Dilution Procedures: Serial dilutions are used to reduce the bacterial concentration in a sample to a countable range.
Example: Diluting 1 mL of sample into 9 mL of sterile diluent results in a 1:10 dilution ().
Example: Diluting 0.1 mL of sample into 9.9 mL of sterile diluent results in a 1:100 dilution ().
Serial dilutions (e.g., …) increase the overall dilution factor by multiplying the individual dilution factors.
CFU/mL Formula: Used to calculate the original concentration of bacteria in the undiluted sample.
A simplified version when 1mL is plated is: where final dilution reciprocal is so if the dilution factor is it will be
Example from notes: If 86 colonies were counted on a plate that came from a dilution, and was plated, the calculation would be:
(This assumes the "Final dilution" in the problem context means the reciprocal of the dilution factor).
Optical Density Measurement – Spectrophotometer
What It Measures: Cell density through turbidity evaluation, which is the cloudiness of a liquid suspension caused by scattering light.
As a beam of light passes through a bacterial suspension, cells scatter and/or absorb the light, reducing the amount of light that reaches a detector.
Key Points: Provides a rapid, indirect estimation of bacterial population size.
Fast and indirect measurement method: Results are obtained quickly, but it doesn't directly count individual cells.
Cannot distinguish between live and dead cells: Turbidity is caused by the physical presence of cells, regardless of their viability.
Utilizes absorbance or percentage transmittance for evaluations: A spectrophotometer measures either how much light is absorbed by the sample (absorbance, OD) or how much light passes through the sample (% transmittance). Higher turbidity corresponds to higher absorbance and lower transmittance.
A blank sample (sterile broth without bacteria) is used for calibration purposes to zero the instrument and subtract background absorbance from the medium itself.
Oxygen Requirements Test (BHI / FTM Tubes)
These tests use media like Fluid Thioglycollate Medium (FTM) or Brain Heart Infusion (BHI) agar deeps, which contain a reducing agent (like thioglycollate) that creates an oxygen gradient from high at the top to anaerobic at the bottom.
Growth Patterns: Indicate the microbe's aerobic or anaerobic preferences and capabilities.
Obligate aerobe: Requires oxygen for growth because they possess enzymes (catalase, superoxide dismutase) to detoxify reactive oxygen species (ROS). Grows only at the very top of the medium where oxygen concentration is highest.
Facultative anaerobe: Can grow with or without oxygen; they prefer oxygen because aerobic respiration yields more ATP, but can switch to fermentation or anaerobic respiration in its absence. Grows throughout the medium, but more concentrated at the top where oxygen is present.
Obligate anaerobe: Cannot tolerate oxygen, as they lack the enzymes to neutralize toxic ROS and are poisoned by oxygen. Grows only at the bottom of the medium, far from oxygen.
Aerotolerant anaerobe: Does not use oxygen for growth but can tolerate its presence because they possess some ROS-detoxifying enzymes (often only superoxide dismutase). Grows evenly throughout the medium, irrespective of oxygen concentration.
Microaerophile: Requires a low, specific concentration of oxygen () for growth and is harmed by atmospheric concentrations (). Grows in a thin band just below the surface of the medium, where oxygen levels are optimal.
Effects of Temperature, Osmotic Pressure & pH
Temperature: Microbes have optimal growth temperatures; deviations can denature enzymes or solidify membranes.
Psychrophiles: Grow best at cold temperatures (e.g., to ).
Mesophiles: Grow best at moderate temperatures (e.g., to ), including most human pathogens.
Thermophiles: Grow best at high temperatures (e.g., to ).
Hyperthermophiles: Grow best at extremely high temperatures (e.g., to ).
Osmosis: The net movement of water across a semipermeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration), driven by differences in solute concentration.
Types of solutions affecting microbial cells:
Hypertonic: A solution with a higher solute concentration outside the cell than inside. Water leaves the cell, leading to plasmolysis (shriveling of the cytoplasm), which inhibits growth.
Hypotonic: A solution with a lower solute concentration outside the cell than inside. Water enters the cell, leading to increased internal pressure (turgor pressure). Bacterial cells with cell walls are protected from lysis, but cells without walls (like animal cells) may burst.
Isotonic: A solution with solute concentrations equal to that inside the cell. No net movement of water occurs, and cells maintain normal shape and function.
Salt Tolerance: Microbes vary greatly in their ability to tolerate and require salt.
Halophiles: Organisms that require high salt concentrations (e.g., NaCl up to or ) for growth, as salty environments provide their optimal osmotic conditions.
Facultative halophiles (halotolerant): Organisms that do not require high salt concentrations but can grow in its presence (typically above up to NaCl). They possess mechanisms to balance osmotic pressure internally.
Most pathogens cannot withstand high salt concentrations, making salt a historical preservative. However, some notable exceptions exist:
Staphylococcus spp.: Such as S. aureus, are facultative halophiles and can grow on media with high salt, like Mannitol Salt Agar.
Vibrio spp.: Many Vibrio species (e.g., V. parahaemolyticus) are halophilic and require salt for growth, often found in marine environments.
pH: The concentration of hydrogen ions () in a solution, affecting enzyme activity and protein structure.
Acidophiles: Grow optimally at low pH (e.g., pH to ).
Neutrophiles: Grow optimally at neutral pH (e.g., pH to ), including most human pathogens.
Alkaliphiles: Grow optimally at high pH (e.g., pH to ).
Polymerase Chain Reaction (PCR)
Purpose: An in vitro molecular technique used to amplify (make many copies of) specific segments of DNA from even a very small initial sample for further study, detection, and analysis (e.g., diagnostics, forensics).
Steps of PCR: Repeated in cycles (typically 25-35) in a thermocycler, which precisely controls temperature changes.
Denaturation: The reaction mixture is heated to to to break the hydrogen bonds between complementary DNA strands, unwinding the DNA double helix into single strands.
Annealing: The temperature is rapidly lowered to to , allowing short, synthetic DNA primers to bind (anneal) to their complementary sequences on the single-stranded template DNA. Primers define the region to be amplified.
Extension: The temperature is raised to to , the optimal temperature for Taq polymerase. The enzyme synthesizes new DNA strands by adding nucleotides complementary to the template strand, extending from the primers. This results in a doubling of the target DNA fragment in each cycle.
Key Reagents Required: Each component is crucial for successful amplification.
Template DNA: The DNA sample containing the target sequence to be amplified.
Primers: Two short, single-stranded DNA oligonucleotides (forward and reverse) that are complementary to the ends of the target DNA sequences and define the start and end points of amplification.
Taq polymerase: A heat-stable DNA polymerase enzyme isolated from Thermus aquaticus, which can withstand the high temperatures of the denaturation step without denaturing itself.
Nucleotides (dNTPs): Deoxynucleotide triphosphates (dATP, dCTP, dGTP, dTTP) are the building blocks used by Taq polymerase to synthesize new DNA strands.
Buffer: Provides the optimal chemical environment (pH, salt concentration) for the Taq polymerase activity.
Identification of Unknown Bacteria
Blood Agar Hemolysis Patterns: Blood agar is an enriched differential medium used to observe the hemolytic activity of bacteria due to the production of hemolysins (exotoxins that lyse red blood cells).
Alpha-hemolysis: Partial hemolysis of red blood cells, resulting in a greenish or brownish discoloration around colonies. This is due to the oxidation of hemoglobin to methemoglobin (e.g., Streptococcus pneumoniae).
Beta-hemolysis: Complete lysis of red blood cells, leading to a clear, transparent zone around colonies. This indicates vigorous hemolysin production (e.g., Streptococcus pyogenes, Staphylococcus aureus).
Gamma-hemolysis (Non-hemolytic): No lysis or discoloration of red blood cells around colonies, indicating the absence of hemolysin production (e.g., Enterococcus faecalis).
Biochemical Tests for Identification: These tests leverage specific metabolic capabilities of bacteria to aid in identification.
Catalase Test: Detects the presence of the enzyme catalase, which breaks down hydrogen peroxide () into water and oxygen gas. (30) Bubbles (from production) upon adding to a bacterial sample indicate a positive result (e.g., Staphylococcus spp. are positive, Streptococcus spp. are negative).
Oxidase Test: Detects the presence of cytochrome c oxidase, an enzyme involved in the electron transport chain of some aerobic organisms. A purple coloration within seconds after applying a reagent (tetramethyl-p-phenylenediamine) to bacterial colonies or a filter paper indicates a positive result (e.g., Pseudomonas spp., Neisseria spp. are positive; most Enterobacteriaceae are negative).
MacConkey Agar Test: A selective and differential medium. It is selective for Gram-negative bacteria (bile salts and crystal violet inhibit Gram-positives). It is differential for lactose fermentation.
Lactose fermenters (e.g., E. coli, Klebsiella) produce acid from lactose, which lowers the pH, causing the neutral red indicator to turn pink or red, often with bile salt precipitation.
Non-lactose fermenters (e.g., Salmonella, Shigella) remain colorless or yellowish and typically grow less robustly.
Triple Sugar Iron (TSI) Test: A differential medium that tests for three primary characteristics: glucose, lactose, and/or sucrose fermentation, gas production (), and hydrogen sulfide () production.
Reactions (color changes, gas bubbles, black precipitate) in the slant (aerobic) and butt (anaerobic) provide a profile for identification.
production is indicated by a black precipitate, formed when hydrogen sulfide gas reacts with ferrous sulfate in the medium.
Urease Test: Detects the enzyme urease, which hydrolyzes urea into ammonia, , and water. Ammonia increases the pH of the medium.
A pink coloration (due to phenol red indicator turning pink at alkaline pH) indicates a positive result (e.g., Proteus spp. are strongly positive).
ELISA & Rapid Diagnostic Testing
ELISA (Enzyme-Linked Immuno-Sorbent Assay): A highly sensitive and specific laboratory test that measures the concentration of antigens (pathogen components) or antibodies (host immune response) present in a sample (e.g., blood, urine).
The assay relies on specific antibody-antigen binding and uses an enzyme-linked secondary antibody to produce a detectable signal (typically a color change) in the presence of the target molecule. The intensity of the color change during the assay indicates the amount of target present.
Types of ELISA: Vary in their setup to detect different targets.
Direct ELISA: Detects antigen directly using an enzyme-linked primary antibody.
Indirect ELISA: Detects antibody in a sample using an antigen-coated plate and an enzyme-linked secondary antibody that binds to human antibodies.
Sandwich ELISA: Detects antigen by "sandwiching" it between two antibodies: a capture antibody coated on the plate and an enzyme-linked detector antibody that binds to a different epitope on the antigen. This method is highly sensitive and specific for antigen detection.
Competitive ELISA: Measures the concentration of an antigen by competing with a labeled version of the same antigen for binding sites on specific antibodies, allowing for quantitative analysis. This format is particularly useful for small molecules.