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