Lesson 1, Topic 1
In Progress


April 11, 2024

Learning Objective: Differentiate among various pathogens.
      Pathogens are disease-causing organisms. This section discusses several types of pathogens, including unusual pathogenic bacteria, fungi, protozoa, parasites, helminths, and viruses.

Unusual Pathogenic Bacteria
Learning Objective: Describe the unusual characteristics of chlamydia, mycoplasma, and rickettsia organisms.
Typical pathogenic bacteria measure 1000 to 5000 nanometer (nm). Chlamydia, mycoplasma, and rickettsia are tiny, unusual bacteria that fall between the size ranges of typical pathogenic bacteria and viruses (TABLE 35.2).
            • Chlamydia: Require host cells for growth.
            • Rickettsia: Transmitted by blood-sucking insects and cannot multiply outside a living host cell.
            • Mycoplasmas: Do not contain peptidoglycan in their cell wall. For example, Mycoplasma pneumonia, which causes “walking pneumonia,” is a gram-negative bacterium that does not have a cell wall.

Pathogenic Fungi
Learning Objective: Describe pathogenic fungi.
      Mycology is the study of fungi and the diseases they cause (see TABLE 35.2). Fungi are eukaryotes that are larger than bacteria and have a nucleus. Fungi include yeasts and molds. Fungi are present in the soil, air, and water, but only a few species cause disease. They are transmitted by the following:
            • Direct contact with infected persons
            • Prolonged exposure to a moist environment
            • Inhalation of contaminated dust or soil
Some fungal infections affect the skin, hair, or nails. Tinea infections, such as ringworm and athlete’s foot, are examples of these types of fungal infections. Some fungi can penetrate the tissues of the internal body structures and cause serious diseases of the mucous membranes, heart, and lungs.
      A diagnosis of a fungal skin infection is usually based on culturing skin scrapings or microscopic observation of skin scrapings. Before microscopic observation, the samples are treated with potassium hydroxide (KOH) to dissolve nonfungal material, making the fungal elements easier to observe. Fungal infections must be treated with antifungal medications.

35.2 Critical Thinking Application
Fungi include two other types of organisms. Name these organisms. Share your answers with the class.

Pathogenic Protozoa
Learning Objective: Describe pathogenic protozoa.
      Protozoa are single-celled parasitic organisms that contain a nucleus. They range in size from microscopic to macroscopic (visible to the naked eye) (see TABLE 35.2). They are present in moist environments and in bodies of water, such as lakes and ponds. Protozoa are transmitted through contaminated feces, food, and drink. Some pathogenic protozoa inhabit the bloodstream; others inhabit the intestines and genital tract. Diagnosis usually is based on the patient’s signs and symptoms and on microscopic examination of stool or blood.

Pathogenic Parasites
Learning Objective: Describe pathogenic parasites.
      Parasitology includes the study of all parasitic organisms that live on or in the human body (see TABLE 35.2). In a parasitic relationship, the host is harmed, and the parasite thrives. Parasites are transmitted by ingestion, direct penetration of the skin, and injection by an arthropod, also referred to as a vector. A parasite cannot be identified accurately based on a single test or specimen. Parasites are frequently identified in feces, blood, urine, sputum, tissue fluid, or tissue biopsy samples.

Pathogenic Helminths
Learning Objective: Describe pathogenic helminths.
Helminths are parasites called worms. Helminths live on or in another living organism. They sustain themselves at the expense of the host organism. They can live in animals or humans. Worms are usually transmitted through the soil, by infected clothing or fingernails, after contact with infected persons, or through contaminated food/water. Helminths go through the same life cycle as other worms. The adult worm lays eggs (ova). The ova develop into larvae, which grow into adult worms, and the cycle begins again. Diagnosis usually is based on a microscopic examination of feces for ova (eggs) and parasites, and on patient signs and symptoms (FIGURE 35.4).

TABLE 35.2

Common Diseases Caused by Pathogens

FIGURE 35.4  (A) Roundworms. (B) Whipworms. From Stepp CA, Woods MA: Laboratory procedures for medical office personnel, Philadelphia, 1998, Saunders.

Pathogenic Viruses
Learning Objective: Describe pathogenic viruses.
      Many scientists do not consider viruses to be microorganisms because they are not alive. Viruses are not able to metabolize or reproduce unless they are inside of a host cell. Viruses have their own enzymes but must use the host cell organelles and macromolecules to reproduce and metabolize. Because of the absolute need for a host cell, a virus can be considered an obligate intracellular pathogen.
      Viruses consist of a genetic core covered by a protein coat called a capsid. Some viruses have an additional spiked layer of protection over the capsid called an envelope. A viral genetic core is made up of either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA). The RNA or DNA contains information about how and what the host cell needs to produce to form a new virus. The genetic material is like a recipe for a new viral particle.
      A virus cannot be cultured on solid nutrient media, such as those used to culture bacteria and fungi. Viruses must be cultured in fertilized eggs or in a tissue culture, which is done by referral, research, or large hospital laboratories.
Usually, instead of culturing a specimen for a virus, the patient’s blood sample is tested for a specific antibody related to the possible viral infection. For example, in the diagnosis of Lyme disease, patient serum is tested for the specific antibody produced in response to the Lyme antigen . This form of testing is referred to as serology or immunology testing (discussed later in the chapter). TABLE 35.2 lists common diseases caused by viruses.

35.3 Critical Thinking Application
Looking at TABLE 35.2, pick four viral diseases that you are familiar with. Write down the disease, the virus that causes it, and two ways to prevent the infection. Be ready to share your answers with the class.