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Questions and Answers
Which type of toxin is associated with being produced by gram-negative bacteria?
What is the primary composition of exotoxins?
What is the main action of endotoxins when released into the body?
Which specific feature distinguishes exotoxins from endotoxins in terms of their heat stability?
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Which type of toxin is known for causing endotoxic or septic shock?
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Which organism is known to produce exotoxins?
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What characteristic is associated with the immunogenicity of exotoxins?
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What do exotoxins and endotoxins have in common regarding their origin?
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Which characteristic defines an ideal antibacterial?
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Which method determines the minimum inhibitory concentration (MIC) of an antibiotic?
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What is NOT a complication of antimicrobial chemotherapy?
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Which of the following best describes chemoprophylaxis?
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What does the standardization of bacterial inoculum entail in susceptibility testing?
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Which of the following methods is NOT a quantitative method for antibiotic susceptibility testing?
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What type of bacteria exclusively metabolize anaerobically but are insensitive to the presence of O2?
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Which category of microorganisms prefer neutral pH for optimal growth?
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What is a common procedure in macro-broth dilution tests?
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What is a desirable characteristic of an antibiotic regarding resistance?
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What type of bacteria requires oxygen levels below 0.2 atmospheres for optimal growth?
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What is the optimal growth temperature range for thermophiles?
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Which of the following describes psychrotrophs?
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What term refers to microorganisms that thrive in environments with high salt concentrations?
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Which of the following describes the metabolism of obligate anaerobes?
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What is the primary method of growth for microorganisms classified as acidophiles?
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Which of the following statements about antiseptics is true?
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What is the primary purpose of decontamination?
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What is true about dry heat sterilization?
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What is the characteristic of moist heat sterilization at 100 oC?
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Which method is the best example of pasteurization?
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What pressure condition is required for steam sterilization using an autoclave?
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Which of the following is a limitation of boiling water for sterilization?
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What is the main method of sterilization used for glassware and metallic instruments?
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What role do commensal bacteria in the gut play concerning pathogenic organisms?
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Under which condition might commensal bacteria become harmful?
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What is the primary difference between infection and disease?
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Which of the following is NOT a mode of transmission for infections?
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What is a characteristic of carriers in terms of disease transmission?
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In what way can E. coli cause a urinary tract infection?
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Which statement accurately describes a permanent carrier?
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What is a potential portal of exit for pathogens from the host?
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Study Notes
Commensal Bacteria
- Commensal bacteria in the gut digest polysaccharides and produce certain vitamins.
- They inhibit the growth of pathogenic organisms by competing for nutrients.
- Under certain conditions, commensal bacteria can become pathogenic.
- Conditions that can lead to commensal bacteria causing disease include:
- Weakened immune systems (e.g., immunosuppressed, diabetic, or leukemic patients)
- Alteration of host tissues (e.g., Streptococcus viridans, a normal inhabitant of the mouth and throat, can cause endocarditis in individuals with pre-existing heart lesions)
- Changes in the natural habitat of the bacteria (e.g., E. coli, a normal resident of the intestine, can cause urinary tract infections if it enters the renal system via blood or lymphatics)
Infection and Disease
- Infection is the process of a parasite entering a relationship with a host.
- Disease is the destruction of host tissues caused by parasite invasion, toxin production, or other virulence factors.
- Factors required for disease development include:
- Source of infection: Human, animal, or environmental (soil)
- Mode of transmission: Insect bites, fecal contamination, inhalation of droplets, sexual contact, blood transfusion
- Portal of entry: Gastrointestinal, genitourinary, or respiratory tracts, skin, mucous membranes, abrasions, insect bites, or injections
- Parasite multiplication: Can occur locally at the portal of entry or spread through tissues, blood, or lymphatics to a target organ
- Portal of exit: Urine, stools, respiratory or genital discharges, blood (transmitted by insects or injections)
Carriers
- Carriers are individuals who harbor a pathogenic organism without exhibiting clinical symptoms.
- Carriers can transmit the organism to other susceptible individuals.
- Carriers can be transient (during the incubation period) or permanent (chronic), as in hepatitis B.
- Carriers are considered significant sources of infections for the following reasons:
- They don't show signs of the disease.
- They interact with the public without being recognized as carriers.
- They can be classified into two groups:
Toxins
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Exotoxins:
- Protein toxins secreted by living bacteria that diffuse into the surrounding medium (extracellular toxins).
- Production of most exotoxins is controlled by genes found on plasmids or bacteriophages.
- Exotoxins are highly specific in their actions and can be categorized as neurotoxins, enterotoxins, or cytotoxins based on their effects and target organs.
- Examples: Cl. tetani exotoxin (plasmid-coded), C. diphtheriae and Cl. botulinum toxins (phage-coded)
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Endotoxins:
- Integral components of the cell wall of gram-negative bacteria.
- Released when the bacteria die and disintegrate.
- Toxicity is associated with the lipid portion of the endotoxin.
- Organisms producing endotoxins include E. coli and Meningococci.
- Endotoxins are primarily responsible for endotoxic or septic shock.
- They cause generalized, non-specific toxic effects like fever, hypotension, disseminated intravascular coagulation (DIC), shock, and potentially death due to organ failure.
Key Differences Between Exotoxins and Endotoxins
| Property | Exotoxins | Endotoxins |
|---|---|---|
| Location of Genes | Plasmid or bacteriophage | Bacterial chromosome |
| Composition | Proteins | Lipopolysaccharides |
| Action | Specific (binds to specific receptors on specific host cells), No fever | Non-specific (fever and shock), No specific receptors |
| Heat Stability | Labile, destroyed at 60°C. Stable at 100°C for 1 hr | Stable, resistant to heat |
| Diffusibility | Diffusible, Excreted by living cells | Not diffusible, Integral part of the cell wall |
| Immunogenicity | Strong, induce high titer of antitoxin | Weak immunogenicity |
| Toxicity | Strong | Weak |
| Convertibility to Toxoid | Yes | No |
| Produced by | Gram-positive bacteria mainly | Gram-negative bacteria |
Other Noteworthy Points
- Toxoid: A toxin treated (usually with formalin) to remove its toxicity while retaining its immunogenicity.
- Peptidoglycans and teichoic acids, released when gram-positive cells die, can also cause effects similar to endotoxins.
Oxygen Requirements for Bacterial Growth
- Obligate aerobes: Require oxygen for growth and die in its absence.
- Obligate anaerobes: Cannot survive in the presence of oxygen.
- Facultative anaerobes: Can grow with or without oxygen.
- Aerotolerant anaerobes: Can tolerate oxygen but do not use it for metabolism.
- Microaerophiles: Require low oxygen levels and high carbon dioxide concentrations for optimal growth.
Effect of pH on Growth
- Acidophiles: Thrive in acidic environments (optimum pH below 7.0).
- Neutrophiles: Grow best at neutral pH (around 7.0).
- Alkaliphiles: Prefer alkaline environments (optimum pH above 7.0).
Effect of Temperature on Growth
- Mesophiles: Grow optimally at temperatures near 37°C (human body temperature).
- Thermophiles: Optimal growth between 45°C and 70°C.
- Extreme thermophiles (hyperthermophiles): Some Archaea thrive at temperatures above 80°C, with maximum temperatures reaching 115°C.
- Psychrophiles: Cold-loving organisms that can grow at 0°C.
- Psychrotrophs: Similar to psychrophiles but have an optimal growth temperature between 10°C and 15°C.
Water Availability and Bacterial Growth
- Water activity: Represents the availability of water for cellular processes.
- Halophiles: Microorganisms that require salt (NaCl) for growth.
Ideal Antibacterial Properties
- Selective target
- Narrow spectrum (does not kill normal flora)
- Minimal adverse reactions (toxicity, allergy)
- Various routes of administration
- Good absorption and distribution to the infection site
- Slow emergence of resistance
Complications of Antimicrobial Chemotherapy
- Development of drug resistance
- Drug toxicity
- Superinfection
- Hypersensitivity
Chemoprophylaxis
- Administration of medication to prevent disease or infection
Antibiotic Susceptibility Testing
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Quantitative Methods: Determine the minimum amount of antibiotic (minimum inhibitory concentration, or MIC) that inhibits the growth of a bacterial isolate. Methods include:
- Broth dilution methods (macrobroth and microbroth)
- Agar dilution methods
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Qualitative Methods: Determine the susceptibility or resistance of a bacterial isolate to a specific antibiotic.
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Automated Susceptibility Tests: Use automated systems to perform susceptibility testing. -Newer Non-Automated Susceptibility Tests: Include methods like E-tests and disc diffusion methods.
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Molecular Techniques: Rapidly identify antibiotic resistance genes.
Antiseptics
- Chemical agents non-toxic to living tissues. They can be applied to skin and mucous membranes but not suitable for systemic administration. -Examples: ethyl alcohol, chlorohexidine
Decontamination
- Reduces pathogenic microorganisms so that items are safe to handle, use, or dispose of.
- Methods of decontamination include cleaning, disinfection, and sterilization.
Methods of Sterilization
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Sterilization by Heat:
-
Dry Heat: Kills by destructive oxidation of essential cellular components. Less efficient than moist heat but less expensive and non-corrosive.
- Dry heat sterilizer: Isolated, double-walled chamber heated by electricity with a thermostat.
- Temperature: 160°C for 2 hours or 170°C for 1 hour.
- Used for glassware, ointments, powders, oils, and metallic instruments.
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Moist Heat: Kills by protein denaturation.
- Methods:
- Moist heat below 100°C: Pasteurization (e.g., milk at 63°C for 30 min or 72°C for 15-20 sec followed by rapid cooling)
- Moist heat at 100°C: Boiling for 20 min (kills vegetative bacteria and hepatitis B virus but not all bacterial spores)
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Steam sterilization above 100°C: Autoclave
- Closed chamber where pressure increases the boiling point of water above 100°C.
- Kills all microorganisms, including spores.
- Essential for sterilization of surgical instruments, lab equipment, and other materials requiring sterility.
- Methods:
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Description
This quiz explores the role of commensal bacteria in the gut, their benefits, and conditions under which they may become pathogenic. It also discusses the relationship between infection and disease, highlighting how parasites interact with hosts. Test your knowledge on these fundamental microbiological concepts.
