Medical Microbiology

LEARNING OBJECTIVES	PROCEDURES
Microorganisms and Disease
1. List and explain the stages of an infectious disease. 2. List and describe the three classifications of bacteria based on shape. 3. Give examples of infectious diseases caused by the following types of cocci: Staphylococci Streptococci Diplococci 4. State examples of infectious diseases caused by bacilli, spirilla, and viruses.
Microscope
5. Explain the function of each of the following parts of a compound microscope: base, arm, stage, light source, substage condenser, iris diaphragm, body tube, coarse adjustment, and fine adjustment. 6. Identify the function of each of the following microscope lenses: low-power, high-power, and oil-immersion. 7. List the guidelines for proper care of the microscope.	Use a microscope. Properly handle and care for a microscope.
Microbiologic Specimen Collection
8. Explain the purpose of obtaining a specimen, and identify body areas from which a specimen can be taken for microbiologic examination. 9. List ways to prevent contamination of a specimen by extraneous microorganisms. 10. Explain the precautions a medical assistant should take to prevent infection from a pathogenic specimen.	Collect a throat specimen.
Microbiologic Tests
11. Explain the importance of the early diagnosis of streptococcal pharyngitis. 12. Explain the purpose of and describe the procedure for a sensitivity test. 13. Explain the purpose of a microbiologic smear. 14. Explain the purpose of Gram staining. 15. Identify infectious diseases caused by gram-positive bacteria and gram-negative bacteria. 16. Give examples of methods to prevent and control infectious diseases in the community.	Perform a streptococcus test using a rapid strep test.

Introduction to Microbiology

Microbiology is the scientific study of microorganisms and their activities. As described in Chapter 17, microorganisms are tiny living plants and animals that cannot be seen by the naked eye, but must be viewed under a microscope. Anton van Leeuwenhoek (1632-1723) designed a magnifying glass strong enough for viewing microorganisms. He was the first individual to observe and describe protozoa and bacteria (Figure 34-1). Leeuwenhoek’s magnifying glass was the precursor of modern microscopes used today to study microorganisms. A microscope allows the observer to see individual microbial cells and to differentiate and identify microorganisms.

Figure 34-1 Bacteria drawn by van Leeuwenhoek in 1684.

For the most part, microbiology deals with unicellular, or one-celled, microscopic organisms. All of the life processes necessary to sustain the microbe are performed by one cell. Among them are the ingestion of food substances and their use for energy, growth, reproduction, and excretion.

Microorganisms are ubiquitous; they are found almost everywhere—in the air, in food and water, in the soil, and in association with plants, animals, and human life. Although vast numbers of microorganisms exist, only a relatively small number are pathogenic and able to cause disease.

When a pathogen infects a host, it often produces a set of symptoms peculiar to that disease. Scarlet fever is characterized by a sore throat, swelling of the lymph nodes in the neck, a red and swollen tongue, and a bright red rash covering the body. These symptoms aid the physician in diagnosing the disease. The medical assistant must be alert to all symptoms that the patient describes and must relay this information to the physician through careful and concise charting of these symptoms in the patient’s medical record.

If the physician is not able to diagnose the disease from the patient’s clinical signs and symptoms, laboratory tests may be used to help the physician identify the pathogen. Identification of the pathogen leads to proper treatment of the disease. Categories of laboratory tests used to identify a pathogen include the following:

• Microbial culture

• Biochemical tests

• Microscopy

• DNA testing (also known as PCR testing)

Although most laboratory tests used to identify a pathogen are performed at an outside laboratory, the medical assistant is frequently responsible for collection of the specimen that will be transported to the outside laboratory.

This chapter provides an introduction to microbiology, including a description of proper microbiologic collection and handling techniques that must be followed to ensure a high-quality specimen. Identification of a pathogen using microbial culturing, biochemical testing, and microscopy is also discussed in this chapter. DNA testing to detect pathogens (e.g., chlamydia and gonorrhea) was previously described in Chapter 23. Before undertaking this study, the medical assistant should review Chapter 17, which discusses introductory concepts that are basic to this chapter.

Bacteria can be classified according to their shape into three basic groups (Figure 34-2). Round bacteria are known as cocci. Cocci can be categorized further as diplococci, streptococci, or staphylococci, depending on their pattern of growth. Rod-shaped bacteria are bacilli. Spiral and curve-shaped bacteria are spirilla, and they include spirochetes and vibrios.

Figure 34-2 Classification of bacteria based on shape.

Cocci

Staphylococci are round bacteria that grow in grapelike clusters (Figure 34-3, A). The species Staphylococcus epidermidis is widely distributed and is normally present on the surface of the skin and the mucous membranes of the mouth, nose, throat, and intestines. S. epidermidis is usually nonpathogenic; however, a cut, abrasion, or other break in the skin can allow invasion of the tissues by the organism, resulting in a mild infection.

Staphylococcus aureus is commonly associated with pathologic conditions such as boils, carbuncles, pimples, impetigo, abscesses, Staphylococcus food poisoning, and wound infections. Infections caused by staphylococci usually cause much pus formation (suppuration) and are termed pyogenic infections.

Streptococci are round bacteria that grow in chains (Figure 34-3, B). Before the advent of antibiotics, streptococcal infections were a major cause of human death. Diseases caused by streptococci include streptococcal sore throat (“strep throat”), scarlet fever, rheumatic fever, pneumonia, puerperal sepsis, erysipelas, and skin conditions such as carbuncles and impetigo.

Diplococci are round bacteria that grow in pairs. Pneumonia, gonorrhea, and meningitis are infectious diseases caused by diplococci.

What Would You Do? What Would You Not Do?

Case Study 1

Paula Hutchinson brings her 8-year-old daughter Caitlin to the medical office. Caitlin has had a fever, sore throat, and difficulty eating for the past 2 days. The physician orders a rapid strep test. Caitlin refuses to open her mouth so that the specimen can be collected. She says that she doesn’t want that “swab thing” in her mouth because she’s afraid it will make her throw up. Paula wants to know why the strep test must be run. She says that Caitlin’s throat is very red with white patches and wants to know why the physician doesn’t just prescribe an antibiotic for her without running the test.

Bacilli

Bacilli are rod-shaped bacteria that are frequently found in the soil and air (Figure 34-3, C). Some bacilli are able to form spores, a characteristic that enables them to resist adverse conditions such as heat and disinfectants. Diseases caused by bacilli include botulism, tetanus, gas gangrene, gastroenteritis produced by Salmonella food poisoning, typhoid fever, pertussis (whooping cough), bacillary dysentery, diphtheria, tuberculosis, leprosy, and plague.

Escherichia coli is a species of bacillus that is found among the normal flora of the large intestine in enormous numbers (Figure 34-3, D). It is normally a harmless bacterium; however, if it enters the urinary tract as a result of lowered resistance, poor hygiene practices, or both, it may cause a urinary tract infection.

Spirilla

Spirilla are spiral or curve-shaped bacteria. Treponema pallidum, a spirochete, is the causative agent of syphilis (Figure 34-3, E). This microorganism cannot be grown in commonly available culture media; the diagnosis of syphilis is generally made using serologic tests. A serologic test is performed on the serum of the blood. Cholera is caused by another type of spirillum, Vibrio cholerae. Immunization and proper methods of sanitation and water purification have all but eliminated cholera in the United States.