Symbiotic Relationships Involving Microorganisms

Symbiosis

- Defined as two dissimilar organisms living together in a close association.

- The organisms in the relationship are referred to as symbionts.

- Many microorganisms participate in symbiotic relationships.

Neutralism

- Refers to a symbiotic relationship in which neither symbiont is affected by the relationship.

Commensalism

- A symbiotic relationship that is beneficial to one symbiont and of no consequence to the

other.

- Many organisms in the indigenous microbiota of humans are considered to be commensals.

Mutualism

- A symbiotic relationship that is beneficial to both symbionts; examples include lichens

(composed of an alga, a filamentous fungus and sometimes a yeast) and the relationship

humans have the intestinal bacterium Escherichia coli).

Parasitism

- A symbiotic relationship that is beneficial to one symbiont (the parasite) and detrimental to the other symbiont (the host).

➔ A host is a living organism that harbors another living organism.

➔ The parasite may or may not cause disease in the host.

● A change in conditions can cause one type of symbiotic relationship to shift to another type.

Various Symbiotic Relationships

Indigenous Microbiota of Humans

- Includes all the microbes (bacteria, fungi, protozoa, and viruses) that reside on and within a person; sometimes referred to as the human microbiome.

- Our indigenous microbiota is composed of as many as 10,000 different species.

- It is estimated that bacterial protein genes are 360 times more abundant than human genes.

- Blood, lymph, spinal fluid, and most internal tissues and organs are normally free of microbes (i.e., they are sterile).

- Transient microbiota take up temporary residence on and within humans.

- Dysbiosis - destruction of the resident microbiota disturbs the delicate balance between host and microorganisms.

Areas of the Body Where Most of the Indigenous Microbiota Reside

Microbiota of the Skin

- Primarily bacteria and fungi—about 30 different types; most common = Staphylococcus spp.

- Number and variety of microorganisms depend on:

➔ Amount of moisture present

➔ pH

➔ Temperature

➔ Salinity

➔ Presence of chemical wastes and other microbes

Microbiota of the Ears and Eyes

- Middle ear and inner ear are usually sterile; the outer ear and auditory canal contain the same microbes as on the skin.

- Eye is lubricated and cleansed by tears, mucus, and sebum - few microbes present.

Microbiota of the Respiratory Tract

- Respiratory tract is divided into:

Upper Respiratory Tract

- (nasal passages and throat) Has an abundance of microbes; many are harmless, some are opportunistic pathogens.

- Carriers harbor virulent pathogens in their nasal passages or throats, but do not have the diseases usually caused by these pathogens.

- Example: people harboring the bacteria that cause diphtheria, pneumonia,

meningitis, and pertussis (whooping cough).

Lower Respiratory Tract

- (larynx, trachea, bronchi, and lungs)

- usually free of microbes

Microbiota of the Oral Cavity (Mouth)

- Oral cavity affords a shelter for numerous anaerobic and aerobic bacteria; remaining food particles provide a rich nutrient medium for bacteria.

- Careless dental hygiene may cause:

➔ Dental caries (tooth decay)

➔ Gingivitis (gum disease)

➔ Periodontitis

- The most common organisms within the indigenous microbiota of the mouth are various species of α- hemolytic streptococci and different anaerobes.

Microbiota of the Gastrointestinal (GI) Tract

- The GI tract is designed for digestion of food, absorption of nutrients, and elimination of

undigested materials.

- The colon (large intestine) contains the largest number and variety of microbes of any colonized area of the body: an estimated 500 to 600 different species - primarily bacteria.

- Colon is anaerobic; bacteria in colon are mostly obligate, aerotolerant, and facultative

anaerobes.

- Many members of the colon microbiota are Opportunists.

Microbiota of the Genitourinary (GU) Tract

- The GU tract consists of the kidneys, ureters, urinary bladder, urethra, and parts of the

female/male reproductive systems.

- Kidney, ureters, and urinary bladder are usually sterile; the distal urethra and its external opening harbor many microbes, including bacteria, yeasts, and viruses.

- Most frequent causes of urethral infections include Chlamydia trachomatis, Neisseria

gonorrhoeae, and mycoplasmas.

- The male and female reproductive systems are usually sterile, with the exception of the Vagina.

Beneficial and Harmful Roles of the Indigenous Microbiota

● Humans derive many benefits from their indigenous microbiota (e.g., vitamins K and

B12).

Microbial Antagonism

- This means “microbes versus microbes”

- Many members of our indigenous microbiota are beneficial, in that they prevent other microbes from becoming established.

- Involve the production of antibiotics and bacteriocins (antibacterial proteins); an example is colicin, produced by E. coli.

Opportunistic Pathogens & Biotherapeutic Agents

- Opportunistic pathogens are those microbes that “hang around,” waiting for the opportunity

to cause infection.

- The delicate balance of the indigenous microbiota can be disturbed by antibiotics, other types of chemotherapy, and changes in pH.

- Bacteria and yeasts used to stabilize the microbial balance are called biotherapeutic

agents or probiotics.

Microbial Communities

- Microbes are often organized into biofilms - complex communities of assorted organisms. 

Biofilms

- Biofilms consist of a variety of different species of bacteria plus a gooey polysaccharide that the bacteria secrete; the bacteria grow in tiny clusters called microcolonies, separated by water channels.

- Biofilms have medical significance; they form on urinary catheters, prosthetic joints, venous

and arterial catheters, and medical equipment and can cause diseases like endocarditis and prosthetic joint infections.

- Microbes commonly associated with biofilms on medical devices include the yeast, Candida albicans, and bacteria such as Staphylococcus aureus, Enterococcus spp., Klebsiella pneumoniae, and Pseudomonas aeruginosa.

- Biofilms are very resistant to antibiotics and disinfectants. Antibiotics that are effective

against pure cultures of organisms have been found to be ineffective against those same organisms within an actual biofilm.

- Bacteria in biofilms produce different types of proteins that may not be produced by the

bacteria in pure culture.

- Bacteria in biofilms produce different types of proteins that may not be produced by the

bacteria in pure culture.

Biofilm Development

1. Adhesion to surface

2. Formation of monolayer and production of “slime”

3. Microcolony formation, with multi-layering cells

4. Mature biofilm, with characteristic “mushroom”

formed of polysaccharide. Note cells starting to detach, reverting to planktonic cells and

completing the cycle.

Synergistic Infections

Synergism

- When two or more organisms “team up” to produce a disease that neither could cause by

itself

- Diseases are called synergistic infections, polymicrobial infections, or mixed infections:

➔ Acute necrotizing ulcerative gingivitis (ANUG, trench mouth, or Vincent disease).

➔ Bacterial vaginosis (BV)

Agricultural Microbiology

Role of microbes in Elemental Cycles

- Endosymbionts - bacteria found within other microorganisms

- Microorganisms play an important role in the cycling of nutrients and elements such as

nitrogen, carbon, oxygen, sulfur, and phosphorus.

➔ N2 is converted by nitrogen-fixing bacteria and cyanobacteria into ammonia (NH3) and ammonium ion (NH4).

- Some nitrogen-fixing bacteria (e.g., Rhizobium and Bradyrhizobium spp.) live in

and near the root nodules of legumes such as alfalfa, soybeans, and peanuts.

- Nitrifying bacteria:

➔ Nitrosomonas

➔ Nitrosospira

➔ Nitrosococcus

➔ Nitrosolobus

➔ Nitrobacter spp.

- Denitrifying bacteria:

➔ Pseudomonas

➔ Bacillus

Other soil microbes

- Bacteria, fungi, algae, protozoa, viruses, and viroids; many are decomposers.

- A variety of human pathogens live in soil, including Clostridium spp. (such as C. tetani and C. botulinum) and the spores of Bacillus Anthracis.

Infectious diseases of farm animals

- caused by a wide variety of pathogens

- can be transmitted to humans

- economic concern to farmers and ranchers

Microbial diseases of plants

- Most plant diseases are caused by fungi, viruses, viroids, and bacteria.

Biotechnology

- Any technological application that uses biological systems, living organisms, or derivatives thereof, to make or modify products or processes for specific use

- Many antibiotics and drugs are produced in pharmaceutical companies by fungi and bacteria (e.g., penicillin).

Bioremediation

- Use of microbes to clean up various types of wastes, including industrial and toxic wastes, and environmental pollutants (e.g., herbicides and pesticides)

- Methanotrophs (bacteria that normally consume methane in the environment) have been used to remove highly toxic solvents such as trichloroethylene and tetrachloroethylene from the soil.

Epidemiology

- Study of disease

- Epidemiologists - study the factors that determine the frequency, distribution, and determinants of diseases in human populations.

Epidemiologic Terminology

Communicable Disease

- Infectious disease that can be transmitted from one person to another

Contagious Disease

- Communicable disease that is easily transmitted from person to person.

Zoonotic Diseases

- Diseases that humans acquire from animal sources

Incidence of a particular disease

- Number of new cases of that disease in a defined population during a specific time period.

Morbidity Rate

- Number of new cases of a particular disease that occurred during a specified time period

per a specifically defined population (usually per 1,000, 10,000, or 100,000 population)

Period Prevalence

- Number of cases of a disease existing in a given population during a specific time period

(e.g. during the year 2010)

Point Prevalence

- Number of cases of a disease existing in a given population at a particular moment in

time (e.g right now)

Mortality Rate

- Ratio of the number of people who died of a particular disease during a specified time

period per a specified population

Sporadic Disease

- Occurs only occasionally within the population of a particular geographic area (e.g., tetanus)

Endemic Disease

- Always present within the population of a particular geographic area (e.g. staphylococcal and streptococcal infections)

Epidemic Disease

- Greater than usual number of cases of a disease in a particular region, usually within a short period of time (e.g., the influenza epidemic of 2017/2018)

Pandemic

- Disease that is occurring in epidemic proportions in many countries simultaneously.

Examples:

➔ Influenza

➔ HIV/AIDS

➔ Tuberculosis

➔ Malaria

Interactions between Pathogens, Hosts, and the Environment

Infectious disease occurs depends on:

➔ Factors pertaining to the pathogen

- virulence of pathogen, mode of entry, and number of organisms

➔ Factors pertaining to the host

- health status, nutritional status, hygiene, age, travel, lifestyle

➔ Factors pertaining to the environment

- physical factors such as climate, season, and geographic location; availability of appropriate reservoirs; sanitary and housing conditions; and availability of potable water.

The Chain of Infection

Six components in the infectious disease process:

➔ Pathogen

➔ Source of the pathogen (reservoir)

➔ Portal of exit

➔ Mode of transmission

➔ Portal of entry

➔ Susceptible host

Reservoirs of Infection

- Sources of microbes that cause infectious diseases

Living Reservoir

- humans, pets, farm animals, insects, and arachnids (e.g., ticks and mites)

Human Carriers:

➔ Passive carriers

➔ Incubatory carriers

➔ Convalescent carriers

➔ Active carriers

Animals

- Zoonotic Diseases/Zoonoses - Infectious diseases that humans acquire from animal sources

May be acquired by:

➔ Direct contact with an animal

➔ Inhalation or ingestion of the pathogen

➔ Injection of the pathogen by an arthropod

Arthropods

- Many different types of arthropods serve as reservoirs of infection, including insects (e.g., fleas, mosquitoes, lice) and arachnids

- Vectors - arthropods involved in the transmission of infectious diseases

Nonliving Reservoir

- Air, soil, dust, contaminated water and foods, and fomites

- Fomites - inanimate objects capable of transmitting pathogens

Most Common Modes of Transmission of Infectious Diseases

➔ Direct skin-to-skin contact

➔ Direct mucous membrane-to-mucous membrane contact by kissing or sexual intercourse

➔ Indirect contact via airborne droplets of respiratory secretions, usually produced by sneezing or coughing

➔ Indirect contact via food and water contaminated by fecal matter

➔ Indirect contact via arthropod vectors

➔ Indirect contact via fomites

➔ Indirect contact via transfusion of contaminated blood or blood products or by parenteral injection using non sterile syringes or needles

Public Health Agencies

World Health Organization (WHO)

- A specialized agency of the United Nations founded in 1948

- Missions: to promote technical cooperation for health among nations; to carry out programs to control and eradicate diseases; and to improve the quality of human life

➔ Investigates outbreaks of Ebola virus, etc.

➔ Eradicated smallpox

➔ Attempting to eradicate polio and Dracunculiasis

Centers for Disease Control and Prevention

(CDC)

- A federal agency administered by the U.S. Department of Health and Human Services; located in Atlanta, Georgia; established in 1946

- Mission: “to collaborate to create the expertise, information, and tools that people and communities need to protect their health”

- Certain infectious diseases, known as nationally notifiable diseases, must be reported to the CDC.

➔ Publishes Morbidity and Mortality Weekly Report (MMWR).

Measures for prevention and control of epidemics:

➔ Increase host resistance through the development and administration of vaccines that induce active immunity and maintain it in susceptible persons.

➔ Ensure that persons exposed to a pathogen are protected against the disease.

➔ Segregate, isolate, and treat those who have contracted a contagious infection to prevent

the spread of the pathogen to others.

➔ Identify and control potential reservoirs and vectors of infectious diseases.

Bioterrorism and Biologic Warfare Agents

Biologic Warfare Agents

- Microbes purposely used to harm others in times of war

Bioterrorism Agents

- Pathogens used to create fear, chaos, illness, and death in situations other than war

Examples:

➔ Bacillus anthracis (the cause of anthrax)

➔ Clostridium botulinum (the cause of botulism)

➔ Smallpox virus (Variola major)

➔ Yersinia pestis (the cause of plague)

Water Supplies and Sewage Disposal

Water

- most essential resource necessary for the survival of humanity

2 General types of water pollution:

➔ Chemical pollution

➔ Biologic pollution (e.g., fecal material and

garbage)

● The 1993 cryptosporidiosis epidemic in Milwaukee, Wisconsin, was the largest waterborne epidemic in the United States.

Water Treatment

- Water is tested for fecal contamination by checking for the presence of coliform bacteria

(coliforms), such as E. coli and other members of the family Enterobacteriaceae.

Major Steps:

➔ Sedimentation

➔ Coagulation

➔ Filtration

➔ Chlorination

Sewage Treatment

- Raw sewage consists mainly of water, fecal material, garbage, and bacteria.

- Includes primary, secondary, and tertiary sewage treatments