Porth's Essentials of Pathophysiology, 4e

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Infection and Immunity

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are designated as gram-positive organisms if they are stained purple by a primary basic dye (usually crystal violet); those that are not stained by the crystal violet but are counterstained red by a second dye (safranin) are called gram-negative organisms. Staining character- istics and microscopic morphology are used in combi- nation to describe bacteria. For example, Streptococcus pyogenes , the agent of pharyngitis and rheumatic fever, is a gram-positive streptococcal organism that is spheri- cal, grows in chains, and stains purple by Gram stain. Legionella pneumophila , the bacterium responsible for Legionnaires’ disease, is a gram-negative rod. Another means of classifying bacteria according to microscopic staining properties is the acid-fast stain . Because of the unique fatty acid content and composition of their cell membrane, certain bacteria are resistant to the decolorization of a primary stain (either carbol fuch- sin or a combination of auramine and rhodamine) when treated with a solution of acid alcohol. These organisms are termed acid-fast and include a number of significant human pathogens, most notably Mycobacterium tubercu- losis (the cause of tuberculosis) and other mycobacteria. For purposes of taxonomy (i.e., identification and classification), each member of the bacterial kingdom is categorized into a small group of genetically related organisms called the genus , and further subdivided into distinct individuals within the genus called spe- cies. The genus and species assignment of the organ- ism is reflected in its name (e.g., Staphylococcus [genus] aureus [species]). Spirochetes. The spirochetes are an eccentric category of bacteria that are mentioned separately because of their unusual cellular morphology and distinctive mech- anism of motility. Technically, the spirochetes are gram- negative rods but are unique in that the cell’s shape is helical and the length of the organism is many times its width (Fig. 14-7). A series of filaments are wound about the cell wall and extend the entire length of the cell. These filaments propel the organism through an aque- ous environment in a corkscrew motion. Spirochetes are composed of four genera: Leptospira, Borrelia, Treponema , and Brachyspira . Each genus has saprophytic and pathogenic strains. The pathogenic leptospires infect a wide variety of wild and domestic animals. Infected animals shed the organisms into the environment through the urinary tract. Transmission to humans occurs by contact with infected animals or urine-contaminated surroundings. Leptospires gain access to the host directly through mucous membranes or breaks in the skin and can produce a severe and potentially fatal illness called Weil syndrome. In con- trast, the borreliae are transmitted from infected ani- mals to humans through the bite of an arthropod vector such as lice or ticks. (A vector is any organism that car- ries a pathogen from a source to a host.) Included in the genus Borrelia are the agents of relapsing fever ( Borrelia recurrentis ) and Lyme disease ( Borrelia burgdorferi ). Included in the genus Borrelia are the agents of relapsing fever ( Borrelia recurrentis ) and Lyme disease ( Borrelia burgdorferi is the most common in North America).

disassemble a clogged sink drain to see a perfect exam- ple of a bacterial biofilm. The physical appearance of a colony of bacteria grown on an agar plate can be quite distinctive for dif- ferent species. Some produce pigments that give colonies a unique color. Some bacteria produce highly resistant spores when faced with an unfavorable environment. The spores can exist in a quiescent state almost indefi- nitely until suitable growth conditions are encountered, at which time the spores germinate and the organism resumes normal metabolism and replication. Classification. Bacteria are extremely adaptable life forms. They are found not just in humans and other hosts, but in almost every environmental extreme on earth. However, each individual bacterial species has a well-defined set of growth parameters, including nutri- tion, temperature, light, humidity, and atmosphere. Bacteria with extremely strict growth requirements are called fastidious. For example, Neisseria gonorrhoeae , the bacterium that causes gonorrhea, cannot live for extended periods outside the human body. Aerobes are bacteria that require oxygen for growth and metabolism; anaerobes cannot survive in an oxygen-containing envi- ronment. An organism capable of adapting its metabo- lism to aerobic or anaerobic conditions is facultatively anaerobic. It is of interest to note that the vast majority of bacterial species found in humans and in the surrounding environment have not been cultivated on artificial media. In the laboratory, bacteria are generally classified according to the microscopic appearance and staining properties of the cell. The Gram stain, originally devel- oped in 1884 by Danish bacteriologist Christian Gram, is still the most widely used staining procedure. Bacteria FIGURE 14-6. Electron micrograph depicting large numbers of Staphylococcus aureus bacteria, which were found on the luminal surface of an indwelling catheter. Of importance are the sticky-looking substances woven between the round cocci bacteria, which were composed of polysaccharides and are known as biofilm.This biofilm has been found to protect the bacteria that secrete the substance from attacks by antimicrobial agents such as antibiotics (magnified × 2363). (From the Centers for Disease Control and Prevention Public Health Images Library. No. 7488. Courtesy of Rodney M. Donlan, Janice Carr.)