BIOS 250 - Unit 3: Controlling Microbes

Epidemiology

The study of where and when infectious diseases occur in a population and how they are transmitted and maintained in nature

Morbidity

• A state of illness

• Disease rate in a population

Prevalence

• The total number or proportion of individuals in a population ill with a specific disease

• Disease rate in a population

• Total number of existing cases / Total population

Incidence

• The number of individuals with new infections of a particular disease in a given period of time

• New cases over a period

• Total number of new cases / Total population

Mortality

• Death

• Death rate in a population

Sporadic disease

• An illness that occurs at relatively low levels with no discernible pattern or trend, frequently with no geographic focus

• Occasional

• Ex: Tetanus, rabies

Endemic disease

• An illness that is constantly present (often at low levels) in a population

• Ex: Malaria in some regions

Epidemic disease

• An illness with a higher-than-expected incidence in a given period within a given population

• Sudden increase

• Ex: Seasonal flu

Pandemic disease

• An epidemic that is worldwide as opposed to regional

• Global spread

• Ex: COVID-19

Components of the Epidemiological Triad

• Host

• Agent

• Environment

Common source spread

A mode of disease transmission in which every infection originates from the same source

Propagated source spread

• The progression of an infectious disease from person to person, either indirectly or directly, through a population of susceptible individuals as one infected individual transmits the agent to others, who transmit it to others yet again

• Multiple sources of infection

• Spread is longer lasting

Point source spread

A form of common source spread in which the transmission of a disease from the source occurs for a brief period that is less than the pathogen’s incubation period

Continuous source spread

A mode of disease transmission in which every infection originates from the same source and that source produces infections for longer than one incubation period

Intermittent source spread

A mode of disease transmission in which every infection originates from the same source and that source produces infections for a period before stopping and then starting again

Observational study

• A type of scientific study that involves measurement of study subjects on variables hypothesized to be associated with the outcome of interest, but without any manipulation of the subjects

• Measures associations

Experimental study

• A type of scientific study that involves manipulation of the study subjects by the researcher through application of specific treatments hypothesized to affect the outcome while maintaining rigorously controlled conditions

• Proves causal relationship

Modern day challenges in epidemiology

• Emerging infectious diseases

• Antibiotic resistance

• Globalization and travel

Fomite

Inanimate item that may harbor microbes and aid in disease transmission

BSL-1

• Microbes are not known to cause disease in healthy hosts and pose minimal risk to workers and the environment

• Fewest precautions

• Ex: Nonpathogenic strains of Escherichia coli

BSL-2

• Microbes are typically indigenous and are associated with diseases of varying severity

• They pose a moderate risk to workers and the environment

• Ex: Staphylococcus aureus

BSL-3

• Microbes are indigenous or exotic and cause serious or potentially lethal diseases through respiratory transmission

• High risk

• Ex: Mycobacterium tuberculosis

BSL-4

• Microbes are dangerous and exotic, posing a high risk of aerosol-transmitted infections, which are frequently fatal without treatment or vaccines

• Few labs are at this level

• Ex: Ebola and Marburg viruses

Sterilization

• Killing/removal of all living organisms (pathogens) from inanimate objects (including endospores)

• Common application: Preparation of surgical equipment and of needles used for injection

• Common agents: Pressurized steam (autoclave), chemicals, radiation

Disinfection

• Killing/removal of most/all organisms (pathogens) from inanimate objects (not including endospores)

• Common application: Cleaning surfaces like laboratory benches, clinical surfaces, and bathrooms

• Common agents: Chlorine bleach, phenols (e.g., Lysol), glutaraldehyde

Antisepsis

• Killing/removal of most/all pathogens from the surface of living tissues (not including endospores)

• Common application: Cleaning skin broken due to injury; cleaning skin before surgery

• Common agents: Boric acid, isopropyl alcohol, hydrogen peroxide, iodine (betadine)

Sanitization

• Reducing the microbial population to levels deemed safe for public health

• Common application: Commercial dishwashing of eating utensils, cleaning public restrooms

• Common agents: Detergents containing phosphates (e.g., Finish), industrial-strength cleaners containing quaternary ammonium compounds

Physical control methods

• Heat

• Cold

• Filtration

• Radiation

Dry-heat sterilization

Protocol that involves the direct application of high heat

Moist-heat sterilization

Protocol that involves steam under pressure in an autoclave, allowing the steam to reach temperatures higher than the boiling point of water

Boiling

• Moist-heat

• Does not sterilize

• Disinfection

Autoclave

• Specialized device for the moist-heat sterilization of materials through the application of pressure to steam, allowing the steam to reach temperatures above the boiling point of water

• Destroys endospores

• Sterilization typically 121°C and 15 psi in 20 min

• Flash sterilization at higher temperature can be used

Pasteurization

• Form of microbial control using heat that is applied to foods

• Kills pathogens and reduces the number of spoilage-causing microbes while maintaining food quality

• Not all organisms are killed

Refrigeration

• Slows growth

• Reducing fluidity of microbial membranes

Freezing

• Stops microbial growth

• Microbial cells can grow once thawed

Membrane filtration

Method to remove bacteria from liquid, typically heat-sensitive solutions, using filters with an effective pore size of 0.2 µm or smaller, depending on need

HEPA filter

• High-efficiency particulate air filter with an effective pore size that captures bacterial cells, endospores, and viruses as air passes through, removing them from the air

• Removes nearly all microbes from air (0.3 µm)

Ionizing radiation

• High-energy form of radiation that is able to penetrate surfaces and sterilize materials by damaging microbial cell components and DNA

• Produces reactive oxygen species

• Ex: Gamma rays, x-rays

Ultraviolet radiation

• Damages DNA (disinfect)

• Poor penetrating power (glass and plastic will block)

Sterilant

• Destroys all microorganisms and endospores

• Oxidizes primary metabolites

• Destroys cell walls and spores

• Penetrates biofilms

• Ex: Peracetic acid

High-level disinfectant

• Can’t destroy endospores, struggles with biofilms

• Denatures proteins through oxidation

• Oxidizes lipids

• Need high concentrations to penetrate biofilms

• Ex: Bleach

Intermediate-level disinfectant

• Can’t destroy some non-enveloped viruses and all endospores

• Denatures proteins via dehydration

• Dissolves lipids, causing membrane rupture

• Not effective against biofilms

• Ex: 70% isopropyl alcohol

Low-level disinfectant

• Can’t destroy all viruses, bacteria, fungi, or endospores

• Disrupts and dissolves cell membranes

• Alters some membrane proteins

• Not biofilm-effective

• Not effective against some Gram negatives

• Ex: Lysol (quaternary ammonium compounds)

Alcohols

• Mode of action: Denatures proteins and dissolves lipids

• Use: Skin antiseptic, surface disinfectant

• Level: Intermediate

• Ex: Ethanol, isopropanol

Phenolics

• Mode of action: Denatures proteins, disrupting lipid membranes

• Use: Household disinfectants

• Level: Intermediate

• Ex: Triclosan, Lysol

Halogens

• Mode of action: Oxidizes cellular components, denaturing enzymes, and disrupting membranes

• Use: Water treatment, wound antiseptics

• Level: High (concentration matters)

• Ex: Iodine, chlorine

Aldehydes

• Mode of action: Cross-links proteins and DNA, making them inactive

• Use: Sterilization of medical instruments

• Level: High (sterilants)

• Ex: Formaldehyde, glutaraldehyde

Antimicrobial drugs

Chemical compounds, including naturally produced drugs, semisynthetic derivatives, and synthetic compounds, that target specific microbial structures and enzymes, killing specific microbes or inhibiting their growth

Antibiotics

• Taken after getting infected

• Targets bacteria directly

Vaccines

• Taken before getting infected

• Immune system kills pathogen

Selective toxicity

Desirable quality of an antimicrobial drug indicating that it preferentially kills or inhibits the growth of the target microbe while causing minimal or no harm to the host

Broad-spectrum antimicrobial

Drug that targets many different types of microbes

Narrow-spectrum antimicrobial

Drug that targets only a specific subset of microbes

Superinfection

Secondary infection that may develop as a result of long-term, broad-spectrum antimicrobial use

MIC

Lowest concentration of antibiotic that inhibits growth

MLC

Lowest concentration of antibiotic that kills

Bactericidal antibiotics

Antibiotics that kill target organisms

Bacteriostatic antibiotics

• Antibiotics that prevent growth of organisms

• Immune system would eventually remove intruding microbe

Dosage

Amount of medication given during a certain time interval

Drug half-life

Rate at which 50% of drug is eliminated from blood (kidneys)

Short half-life

Give drug multiple times

Long half-life

May have side effects so few doses

Drug interactions

Result of two drugs together

Synergistic interaction

Increases drugs’ effects

Antagonistic interaction

Reduces drugs’ effects

Oral

Pills

Topical

Skin/mucous membranes

Parenteral

Intravenous/intramuscular injection

Modes of action

The way a drug affects microbes at the cellular level

Beta-lactams

• Group of antimicrobials that inhibit cell wall synthesis

• Ex: Penicillins, cephalosporins, carbapenems, and monobactams

• Inhibit transpeptidase that help form peptide bridges between adjacent glycan chains

Vancomycin

• Glycopeptide

• Too large, so it blocks transglycosylase and transpeptidase activity

• Narrow spectrum (only targets Gram-positive)

Bacitracin

• Inhibits bactoprenol transport

• Narrow spectrum

Chloramphenicol

Protein synthesis inhibitor with broad-spectrum activity that binds to the 50S subunit, inhibiting peptide bond formation

Tetracyclines

• Class of protein synthesis inhibitors that bind to the 30S subunit, blocking the association of tRNAs with the ribosome during translation

• Broad spectrum

Aminoglycosides

• Protein synthesis inhibitors that bind to the 30S subunit and interfere with the ribosome’s proofreading ability, leading to the generation of faulty proteins that insert into and disrupt the bacterial cytoplasmic membrane

• Broad spectrum

Antimetabolites

• Compounds that are competitive inhibitors for bacterial metabolic enzymes

• Sulfonamide and trimethoprim block folic acid anabolism

Polymyxin

• Lipophilic polypeptide antibiotic that targets the lipopolysaccharide component of gram-negative bacteria and ultimately disrupts the integrity of their outer and inner membranes

• Not good selective toxicity

• Used in topical ointments

• Last resort intravenous use

Rifampin

• Semisynthetic member of the rifamycin class that blocks bacterial RNA polymerase activity, inhibiting transcription

• Can cause liver toxicity

Fluoroquinolones

• Class of synthetic antimicrobials that inhibit the activity of DNA gyrase, preventing DNA replication

• Semi-synthetic

• Floxacins

• Targets topoisomerases (DNA gyrase, topoisomerase IV)

Imidazoles

• Class of antifungal drugs that inhibit ergosterol biosynthesis

• Synthetic antifungal

• Fungistatic at low concentrations, fungicidal at high concentrations

Ergosterol

Fungal version of cholesterol and hoponoids

Acyclovir

• Synthetic antiviral guanosine analog

• Inhibits DNA replication

• Specific toxicity because only activated by viral enzymes, which then targets viral DNA polymerase

• Neurotoxic metabolites (high doses can lead to confusion and tremors)

Antiretroviral therapy (ART)

• HIV treatment strategy

• Fusion inhibitors, protease inhibitors, reverse-transcriptase inhibitors, and integrase inhibitors

Fusion inhibitor

• Antiviral drug that blocks the fusion of HIV receptors to the coreceptors required for virus entry into the cell, specifically, chemokine receptor type 5

• Inhibits fusion of viral particle

Protease inhibitor

• Class of antiviral drugs, used in HIV therapy and hepatitis C therapy, that inhibits viral-specific proteases, preventing viral maturation

• Stops assembly of viral particles

Integrase inhibitors

• Antiviral drugs that block the activity of the HIV integrase responsible for recombination of a DNA copy of the viral genome into the host cell chromosome

• Stop DNA from being incorporated into host DNA

Reverse transcriptase inhibitor

• Classes of antiviral drugs that involve nucleoside analog competitive inhibition and non-nucleoside noncompetitive inhibition of the HIV reverse transcriptase

• Goes after viral polymerases

Active immunity

Stimulation of one’s own adaptive immune responses

Passive immunity

Adaptive immune defenses received from another individual or animal

Adaptive immunity

• Third-line defense characterized by specificity and memory

• B cells and T cells

• Specialized

• Learns specific pathogen then “remembers”

• Can change over time

Artificial active immunity

Immunity acquired through exposure to pathogens and pathogen antigens through a method other than natural infection

Artificial passive immunity

Transfer of antibodies produced by a donor to another individual for the purpose of preventing or treating disease

Natural active immunity

Immunity that develops as a result of natural infection with a pathogen

Natural passive immunity

Transfer of maternal antibodies to fetus (transplacentally) or infant (via breastmilk)

Herd immunity

A reduction in disease prevalence brought about when few individuals in a population are susceptible to an infectious agent

Variolation

The historical practice of inoculating a healthy patient with infectious material from a person infected with smallpox in order to promote immunity to the disease

Live attenuated vaccine

• Vaccine with live pathogen that has been attenuated to become less virulent in order to produce an active but subclinical infection

• Pathogen weakened genetically

• Positives: Can be passed to others, strong memory

• Negatives: Can lead to a full-blow disease if reversion occurs, need gene knowledge

• Ex: Chickenpox, measles, tuberculosis

Inactivated vaccine

• Vaccine composed of whole pathogen cells or viruses that have been killed or inactivated through treatment with heat, radiation, or chemicals

• Protein antigens preserved

• Positives: No active infection, weaker symptoms

• Negatives: Weaker memory, need high doses/multiple boosters, cannot be passed on

• Ex: Cholera, hep A, rabies, influenza

Subunit vaccine

• Vaccine that contains only key antigens as opposed to whole pathogens

• Minimal side effects but need boosters

• Ex: Hep B

Conjugate vaccine

• A vaccine consisting of a polysaccharide antigen conjugated to a protein to enhance immune response to the polysaccharide

• Add a strong antigen to a weak antigen to help boost establishment of memory

• Ex: Add capsule protein to antigen