Lecture15: Microbe-Human Interactions

What are microbe-host interactions?

-Contact between microbes and hosts

-Can be beneficial, neutral, or pathogenic

Resident biota/normal flora

Microbes colonized in/on a host that do not normally cause disease

Colonization

The act of microbes taking up long-term residence in a human host

Infection

The entry and multiplication of microbes in host tissues

Disease

Any deviation from health, including when a microbial infection damages or disrupts host tissues and organs

Microbial Antagonism

Normal flora prevent intruder infection

How do we acquire normal flora?

Microbes present in utero

Acquired during birth, breastfeeding, caregivers

Environment

Pathogenicity

An organism’s potential to cause disease

True Pathogens

Capable of causing disease in health people with normal immune defenses

Opportunistic Pathogens

Cause disease when the host’s defenses are compromised

Polymicrobial

Meaning multiple microbes contribute to the disease

Virulence

Describes the severity of disease caused by a microbe

Ability to establish itself in host and cause damage

Virulence Factor

A characteristic or structure of microbe that contributes to virulence

Steps of establishing infection

1. Portals of entry

2. Attachment and interaction with microbiome

3. Surviving host defenses

4. Causing disease

5. Vacating host

Step 1: Portals of entry

A portal of entry is a route by which a microbe enters a host

Typically, a cutaneous (skin) or membranous route

Microbes adapt to use a certain portal of entry

Infectious Dose (ID)

The minimum number of microbes required to establish infection

Step 2: Attachment and Interaction

Microbe may have specific requirements/structures for attachment

Adhesion

Microbes need to gain a stable foothold on host tissues

Step 3: Surviving host defenses

Host immune system recognized “foreign” microbes

Attacks with phagocytes, other immune cells, antibodies

Step 4: Causing disease

1. Directly through enzymes or toxins

2. Indirectly by inducing host defenses and producing an excessive response

3. Epigenetic changes made to host cells

Extracellular enzymes

Exoenzymes

Toxins

Specific chemical products of microbes that are poisonous to other organisms

Exotoxins

Proteins with specificity for a target cell and powerful effects

Endotoxin

One molecule, LPS, causes a variety of effects on tissues and organs

Epigenetics

Alterations to DNA that impact how or if a gene is expressed, but the DNA sequence does not change

Microbes may secrete molecules that interact with DNA to shut off host genes

Will disease occur? — Microbe

-Virulence Factors

-Infectious Dose

-Correct Portal of Entry

Will disease occur? — Host

-Genetic variability of host defense

-Previous exposure

-General health

Sign

Any objective evidence of disease as noted by an observer

Symptom

Subjective evidence of disease as sensed by the patient

Syndrome

When a disease can be identified or defined by certain signs and symptoms

The inflammatory response

Includes cells and chemicals that respond nonspecifically to disruptions in the tissue

Signs of infection in the blood

-Changes in the number of circulating white blood cells

-Presence of a microbe or its products in the blood

Asymptomatic or Subclinical

Infections commonly cause no symptoms

Step 5: Portals of Exit

Microbes are released from the host through secretion, excretion, discharge, or sloughed tissue.

The more microbes released, the more likely infection is to spread

Latency

Infectious agent retreats into a dormant state and may recur

Sequelae

Consequences in the form of long-term or permanent damage to tissues or organs

4 distinct phases

1. Incubation

2. Prodrome

3. Acute

4. Convalescence

Incubation

From initial contact to first symptoms

Established timeline for most infections

Prodrome

Earliest notable symptoms

Vague discomfort, but may have specific symptoms

Acute

Microbe is multiplying quickly

Greatest virulence—well established infection

Convalescence

Recovery period

Symptoms decline as body responds to infection

Continuation

Only present in some infections

Organism is present or symptoms continue