Microbiology: Principles and Explorations

Sixth Edition

Chapter 14:Host-Microbe Relationships and

Disease Processes

Copyright © 2005 by John Wiley & Sons, Inc.

Jacquelyn G. Black

Host-Microbe Relationships

• Pathogen: A parasite capable of causing disease in a host

• Host: Any organism that harbors another organism

• Symbiosis: An association between two (or more) species

• Symbiosis includes: mutualism, commensalism, and parasitism

• Mutualism: Both members of the association living together benefit from the relationship

• Parasitism: One organism, the parasite, benefits from the relationship, whereas the other organism, the host, is harmed by it (e.g. bacteria, viruses, protozoa, fungi, helminths)

• Commensalism: Two species live together in a relationship such that one benefits and the other one neither benefits nor is harmed

Many of the bacteria on human skin are mutualistic

Parasite Infection: Female Pinworms Leaving the Anus of a 5-year-old child to lay eggs on the adjacent skin

Contamination, Infection, and Disease

• Can be viewed as a sequence of conditions in which the severity of the effects microbes have on their hosts increases

• Contamination: means that the microorganisms are present

• Infection: refers to the multiplication of any parasitic organism within or on the host’s body

• Disease: A disturbance in the state of health wherein the body cannot carry out all its normal functions

Pathogens, Pathogenicity, and Virulence

• Pathogenicity: the capacity to produce disease

• Virulence: refers to the intensity of the disease produced by pathogens, and it varies among different microbial species

• The virulence of a pathogen can increase by animal passage, the rapid transfer of the pathogen through animals of a species susceptible to infection by that pathogen

• Attenuation: the weakening of the disease-producing ability of the pathogen

Normal (Indigenous) Microflora

• Organisms that live on or in the body but do not cause disease

• Have well-established associations with humans

• Two categories of organisms can be distinguished:

1. Resident microflora: comprise microbes that are always present on or in the human body

2. Transient microflora: microbes that can be present under certain conditions in any of the locations where resident microflora are found

Locations of resident microflora of the human body

Opportunists

• Organisms that take advantage of particular opportunities to cause disease. Conditions that create opportunities for such organisms include:

1. Failure of the host’s normal defenses (immunocompromised)

2. Introduction of the organisms into unusual body sites

3. Disturbances in the normal microflora (microbial antagonism)

Koch’s Postulates

• Must be satisfied in order to prove that a specific organism is the causative agent of a particular disease

1. Specific causative agent must be observed in every case of a disease

2. Agent must be isolated from a diseased host and grown in pure culture

3. Agent from pure culture is inoculated into a healthy host, the agent must cause same disease

4. Agent must be reisolated from the inoculated, diseased experimental host and ID as being identical to the original causative agent

Demonstration that a bacterial disease satisfies Koch’s Postulates

Kinds of Diseases

• Human diseases are caused by infectious agents, structural or functional genetic defects, environmental factors, or any combination of these causes

• Infectious Diseases: caused by infectious agents such as bacteria, viruses, fungi, protozoa, and helminths

• Noninfectious Diseases: caused by any factor other than infectious organisms

Classification of Diseases

1. Inherited Diseases are caused by errors in genetic information

2. Congenital Diseases are structural and functional defects present at birth

3. Degenerative Diseases are disorders that develop in one or more body systems as aging occurs

4. Nutritional Deficiency Diseases lower resistance to infectious diseases and contribute to the severity of infections

5. Endocrine Diseases are due to excesses or deficiencies of hormones

Classification of Diseases (Continued)

6. Mental Diseases can be caused by a variety of factors (emotional, psychogenic or infection)

7. Immunological Diseases such as allergies, autoimmune diseases, and immunodeficiencies

8. Neoplastic diseases involve abnormal cell growth that leads to harmless or cancerous tumors

9. Iatrogenic diseases are caused by medical procedures and/or treatments

10. Idiopathic diseases are diseases whose cause is unknown

The Disease Process

• Virulence factors are structural or physiological characteristics that help organisms cause infection and disease

• Factors include:

1. Structures such as pili for adhesion to cells and tissues

2. Enzymes that help in evading host defenses

3. Protect the organism from host defenses

4. Toxins that can directly cause disease

Direct Actions of Bacteria

• Adherence or attachment: A critical point in the production of bacterial disease

• Adhesins are proteins or glycoproteins found on attachment pili (fimbriae) and capsules

• Colonization refers to the growth of microorganisms on epithelial surfaces, such as skin or mucous membranes or other host tissues

• Invasiveness is the ability to invade and grow in host tissues (hyaluronidase enzyme is the spreading factor)

Enzymatic Virulence Factors Help Bacteria Invade Tissues and Evade Host Defenses

Hyaluronidase: enzyme digests hyaluronic acid, a gluelike substance that helps hold the cells of certain tissues together

Coagulase triggers blood plasma clotting, allowing bacteria protection from immune defenses

Streptokinase dissolves blood clots

Bacterial Toxins

• Any substance that is poisonous to other organisms

• Exotoxins are soluble substances secreted into host tissues

• Some exotoxins are enzymes (e.g. hemolysin)

• Leukocidins are exotoxins that damage white blood cells

• Endotoxins are part of the cell wall and are released into host tissues from gram negative bacteria

Types of hemolysis:

A: Alpha or partial hemolysis of red blood cells results in a greenish zone around colonies of Streptococcus pneumoniae

B: Streptococcus pyogenes colonies release B-hemolysins, which produce complete breakdown of hemoglobin, causing clear zones to form around colonies on blood agar

A B

Clinical Use of Botulinum Toxin

• Help victims of dystonia which refers to a group of neurological disorders characterized by abnormal, sustained, involuntary movements

• Blepharospasm: patient’s eyes remain tightly closed at all times. Toxin blocks nerve impulses to muscles thereby relieving spasms of eyelids

• Oromandibular dystonia in which the patient’s jaws are clenched so tightly that the jaw bones may break are being helped by botulinum toxin injections

Clinical Use of Botulinum Toxin to Treat Blepharospasm

A Cosmetic Use of Botulinum Toxin, rapidly gaining in favor, is removal of wrinkles, especially “frown” wrinkles in the center of the forehead

Intoxications

• Diseases that result from the ingestion of a toxin rather than infections

• Many exotoxins have a special attraction for particular tissues:

1. neurotoxins: act on tissues of the nervous system to prevent muscle contraction (botulism) or muscle relaxation (tetanus)

2. Enterotoxins: act on tissues of the gut

• Toxoid: an altered toxin that has lost its ability to cause harm but that retains antigenicity

How Viruses Cause Disease

• Viruses can replicate only after they have attached to cells and then penetrated specific host cells

• Cytopathic effect (CPE): In tissue culture systems, once inside a cell, viruses cause these observable changes

• Productive viral infection: occurs when viruses enter a cell and produce infectious offspring

• Abortive viral infection: occurs when viruses enter a cell but are unable to express all their genes to make infectious offspring

An Example of the Cytopathic Effect (CPE)

Uninfected mouse cells Mouse cells infected with stomatitis virus

• Latent Viral Infections are characteristic of herpesviruses. A weakened immune system allows the virus to multiply

• Persistent Viral Infections involve a continued production of viruses over many months or years. The hepatitis B virus (HBV) infects the liver in such a chronic fashion that there may be no outward signs of an infection

How Fungi, Protozoa, and Helminths Cause Disease

• Most fungal diseases result from fungal spores that are inhaled or enter cells through a cut or wound

• Certain fungi produce mycotoxins

• Some protozoans invade and reproduce in red blood cells, and Giardia intestinalis attaches to tissues and ingests cells and tissue fluids. Virulence factor: adhesive disk

• Helminths are extracellular parasites that inhabit intestines or other body tissues and many release toxic waste products and antigens in their excretions

Giardia intestinalis: The suction forces of the adhesive disk are so strong that they leave markings behind on the intestinal surface

Signs, Symptoms, and Syndromes

• Most diseases are recognized by signs and symptoms

• Sign: a characteristic of a disease that can be observed by examining the patient (e.g. swelling, redness, rashes, coughing, pus, runny nose, vomiting)

• Symptom: a characteristic of a disease that can be observed or felt only by the patient (e.g. pain, shortness of breath, nausea, sore throat, headache)

• Syndrome: a combination of signs and symptoms that occur together and are indicative of a particular disease or abnormal condition

• Sequelae: even after recovery, some diseases leave after-effects (e.g. valve damage)

Types of Infectious Disease

• Acute disease develops rapidly and runs its course quickly (e.g. measles and colds)

• Chronic disease develops more slowly than an acute disease, is usually less severe, and persists for a long, indeterminate period (e.g. Tuberculosis)

• Subacute disease is intermediate between an acute and a chronic disease (e.g. gingivitis)

• Latent disease is characterized by periods of inactivity either before signs and symptoms appear (e.g. herpes virus)

Stages of an Infectious Disease

• Incubation period is the time between infection and appearance of signs and symptoms

• Prodromal phase is a short period during which nonspecific, often mild, symptoms such as malaise and headache

• Prodrome is a symptom indicating the onset of a disease

• Invasive phase is period during which the individual experiences the typical signs and symptoms of the disease

• Decline phase is the period of illness during which host defenses and effects of treatment overcome the pathogen

• Convalescent period tissues are repaired, healing takes place, and body regains strength and recovers

Stages in the Course of an Infectious Disease

Incubation periods of selected infectious diseases

Trends in deaths from infectious diseases: Infectious disease mortality decreased markedly in the U.S. during most of the 20th century

Changes in the Causes of Death in the U.S. from 1900 to 2000