Chapter 2: Host - Parasite Interactions

Symbiosis

A long-term association between two organisms of different species

Mutualism

Both partners benefit

- Ex. E. Coli and humans

Parasitism

One partner benefits, other is harmed

Commensalism

One partner benefits, other unaffected

- Ex. S. epidermidis on skin

What does S. epidermidis eat?

Dead skin cells

Pathogenicity

The ability of a microbe to produce disease in an individual

True pathogens

Organisms that cause disease in healthy individuals a high percentage of the time

- Ex. Yersinia pestis, Bacillus anthracis

Opportunistic pathogens

Organisms that are generally part of the normal flora and typically cause infection only when a person is immunocompromised or the environment changes.

- Ex. H. influenzae, S. epidermidis

Iatrogenic infections

An infection that occurs as a result of medical treatment or procedure

- Ex. E. coli UTI after placement of indwelling catheter

The Human Microbiome

The sum total of every microorganism living in and on an individual

- There are trillions

- 10 bacteria to every 1 human cell

When does the human microbiome begin to develop?

Begins developing as soon as a baby is born because the uterus is completely sterile

What benefits does the human microbiome provide?

Immune support, digestion, vitamins; however, this varies based on the site on the body and many factors

Carrier State

A pathogenic bacterium part of a human's microbiome

- Doesn't cause the host harm, but can be passed on to other humans

- S. typhi → causes typhoid fever, a significant and deadly illness before there were antibiotics to treat it

- N. meningitidis → causes meningitis

- Can be a transient or chronic condition

Transient

Happens quickly and ends quickly

Opportunistic Infections

Opportunistic infections that occur due to a change in the microbiome or immune status

- Candida albicans

- Clostridioides difficile (C. diff)

Candida albicans

a normal type of yeast that lives in the human body, grows where mucous membranes are

Clostridioides difficile (C. diff)

a bacterium that can infect your colon and cause severe inflammation and watery diarrhea

Normal Microbiota of the skin

Major genera: Staphylococcus and Micrococcus

- Some grow best in areas of high sebum and/or sweat production

- Known to cause odors or acne

- Mostly aerobic organisms

- Primary opportunistic pathogen: Staphylococcus aureus

Normal Microbiota of the Oral Cavity

Major groups: Streptococcus, and anaerobes

- Plaque creates low oxidation-reduction potential on tooth surface

- Strict anaerobes can grow

Normal Microbiota of the Respiratory Tract

Upper respiratory tract - mouth nasopharynx, oropharynx, and larynx

- Mostly colonized with Streptococci, Corynebacterium, and anaerobes

Lower respiratory tract - trachea, bronchi, and pulmonary parenchyma

- Mostly sterile

Normal Microbiota of the GI Tract

Major organisms: Gram-negative rods and anaerobic bacteria

GI Tract - esophagus, stomach, small intestines, and colon

- Colon has over 70% of all microbes found in the body

- 35,000 bacterial species

Normal Microbiota of the Genitourinary Tract

Sterile sites: Kidneys, ureters, bladder, fallopian tubes, uterus, testicles, urethra (most of it)

Non sterile sites: Distal urethra (especially in women), vagina

Major organisms: Lactobacilli, anaerobes, and skin flora

Sterile sites

No organisms present

Distal

Close to the surface

Lactobaciliil

Produces lactic acid, which helps maintain the pH balance of the vagina

Pathogenicity

Whether or not a pathogen is present or not

Pathogenesis

The steps by which an organism causes infection

1. Bacterial Exposure

2. Colonization → enter host

3. Immune Evasion

4. Infection → disease

If an organism can complete these inside a human host, it is considered a human pathogen

Infectivity

How easily a pathogen is able to encounter and invade a host (exposure and colonization)

AKA how contagious something is

ID 50: Median Infectious Dose

The number of particles required to cause infection in 50% of the population

- The one that takes less particles to cause disease is the better pathogen because it's more infective

Lower ID 50

Higher infectivity

Higher ID 50

Less infectivity

Virulence

How much damage an organism does to its host

LD 50: Median Lethal Dose

The number of particles required to cause death in 50% of an exposed population

Virulence Factors

Molecules produced by pathogens like bacteria and viruses that enable them to colonize a host, evade the immune system, and cause disease

Host Resistance Factors:

First line of defence

Physical, chemical, biological

Physical

Skin, nasal hair, eyelashes, eyelids, mucous membranes, mucociliary clearance, urination

Chemical

Low pH

Skin - pH 5.5

Gastric acid - pH 1-3

Vagina - pH 4.4

Antimicrobial molecules

- IgA, Sebum, mucous, lysozyme, lactoperoxidase, beta defensins, pepsin

Biological

Microbiome

Second line of defence

Cells, proteins, other responses