Chapter 13 - Selected Genera of Medical Importance

13 - 1 Microorganisms are friend and foe

Humans are born into an environment laden with microorganisms, and colonization of the human body begins at the time of birth. Colonization simply implies the establishment of microorganisms on the body surface which, by extension, continues internally (oral cavity, gastrointestinal tract, ear canals, etc.). Throughout life, the skin and mucous membranes exposed to the outside world harbor a variety of indigenous bacteria, the normal flora.

A few hours after birth, the establishment of the normal flora on the surfaces of the body is well under way. Organisms acquired by an infant during passage through the birth canal are replaced by organisms derived from persons who attend the infant and from ingested foods. During the first day of life, many organisms find their niche for the life of the individual. Others take months or years to reach populations found in normal adults.

The number of different species of microorganisms living on the surfaces of the body is very large and even includes species that have not been fully characterized and classified. Some examples of the principal resident bacteria include Staphylococcus, Micrococcus and Propionibacterium found on the skin; Lactobacillus, Streptococcus, Neisseria, Corynebacterium and Bacteroides in the mouth; and Bacteroides, Clostridium, Lactobacillus, Streptococcus and many species of the Family Enterobacteriaceae in the intestinal tract. Usually the normal flora cause no disturbances in the health of their host. In fact, they often benefit the host by outcompeting pathogenic bacteria, yeasts and protozoa which are encountered occasionally

In the healthy individual it is not normal for microorganisms to grow in areas that are inside the body; the lungs, heart, brain, spleen and muscles are typically free of microbes. Penetration of these areas by the normal flora or pathogens will incur the wrath of the immune system and result in a diseased state. Your body has a vast array of defenses to keep microbes out of the internal regions and these are described in Animal Defenses Against Microbes in the microbiology textbook.

In the diagnosis of bacterial diseases of the human body, a general knowledge of the normal flora is essential. Exogenous pathogens must be distinguished from the indigenous species for the correct interpretation of bacteriological findings. Also, an increasing number of clinically-diagnosed bacterial diseases involves bacteria that are indigenous yet potentially pathogenic. Given an opportunity to infect an individual whose resistance is depressed, or, more specifically, to colonize an uncommon site (such as E. coli in the eye or urinary tract), certain of the normal flora may produce an endogenous bacterial disease. Examples of factors which lower the resistance of a host include radiation damage, prolonged use of antibiotics or steroids, and the debilitating effects of AIDS or other diseases. As the lives of more patients with major illnesses are prolonged by improved methods of treatment, and as more infections caused by virulent exogenous bacteria are controlled by effective antimicrobial drugs, endogenous bacterial diseases have become more common.

For more information on medically important bacteria, see Bacterial Pathogens in the microbiology textbook.

13 - 2 <i>Staphylococcus, Streptococcus</i> and <i>Neisseria</i>

The coccus form of the bacteria included here does not necessarily indicate any taxonomic relationship. However, all of these organisms are parasites of humans, and some are pathogenic. The isolation from human sources of these and related organisms and their differentiation also involve some common procedures and problems.

The Family Micrococcaceae consists of gram-positive, spherical bacteria which typically divide in two or more planes to form clusters of cells. Two of the genera, Staphylococcus and Micrococcus, are normal inhabitants of the human skin and are differentiated by their oxygen relationship Staphylococcal infections are usually caused by S. aureus which can be isolated from the mucous membranes of the nasopharynx in about one-third of the human population. Diseases produced by S. aureus include boils, impetigo, pneumonia, osteomyelitis, mastitis, meningitis, endocarditis, bacteremia, enterocolitis and urogenital infections. Many strains produce an extracellular enterotoxin capable of producing food poisoning and toxic-shock syndrome.

Clinical specimens suspected of containing staphylococci should be streaked onto appropriate selective-differential media such as Baird-Parker Agar or Vogel-Johnson Agar. Typical colonies are gram-stained and observed for gram-positive cells in staphylococcal arrangements. The catalase test is useful in distinguishing staphylococci from streptococci which are morphologically similar and often isolated from lesions. Many strains of S. aureus form yellowish to golden-colored colonies, hence the species name. The major criterion employed to distinguish S. aureus from other staphylococci found on humans is the coagulase test. Production of coagulase, an extracellular enzyme which clots plasma, is a characteristic of nearly all strains.

Streptococcus is a member of the lactic acid group of bacteria and forms the dominant bacterial flora of the mouth and pharyngeal areas of humans and some animals. Streptococcus cells are gram-positive, catalase-negative and spherical or ovoid in shape. Because the cells divide in one plane, they are usually seen in pairs or short to long chains. Pathogenic streptococci are responsible for numerous diseases; some examples follow:

Diagnosis of suspected streptococcal infection is usually made from material of the anterior nasal or nasopharyngeal areas of the throat, or from pus, sputum, spinal fluid, discharges, exudates, urine, blood or milk. As streptococci tend to be fastidious organisms, a rich medium such as Blood Agar is inoculated with the specimen in order to obtain isolated colonies. If the specimen is taken from a site containing a wide variety of indigenous or contaminating organisms (e.g., the rectal area or wounds), media with one or more selective agents are streaked. After incubation, the plates are examined for typical streptococcal colonies which tend to be white, glistening and relatively small.

Most strains of Streptococcus which are responsible for diseases are hemolytic on Blood Agar. Hemolytic reactions are classified as α, β or γ according to the appearance of zones around isolated colonies growing on or in the medium:

β-hemolytic strains of S. pyogenes are the principal causes of strep throat and scarlet fever. Rheumatic fever and glomerulonephritis develop as secondary complications. α- and γ-hemolytic streptococci are occasionally associated with chronic diseases or are non-pathogenic. S. pneumoniae (the pneumococci) is α-hemolytic and causes the majority of bacterial pneumonia cases in humans. Pneumococci can be isolated from the pharynx of at least 30% of non-diseased humans. An important characteristic which distinguishes pneumococci from other α-hemolytic streptococci is its sensitivity to the antibiotic optochin.

Recently, certain genetically and biochemically-distinct groups of species have been removed to new genera by Streptococcus taxonomists: Enterococcus includes the enteric or fecal streptococci (e.g., E. faecalis and E. durans), and Lactococcus includes the dairy-related lactic streptococci (e.g., L. lactis). The test for hemolysis and the associated terms continue to apply to these new genera.

Neisseria

The Family Neisseriaceae consists of gram-negative organisms, usually appearing as cocci. Cells of the genus Neisseria tend to be arranged in pairs with each cell flattened where it is in contact with its mate, hence a coffee-bean shape is generally seen and is highly diagnostic in clinical samples. The principal habitats of neisseriae associated with humans are the mucous membranes. Two species are pathogenic: N. meningitidis and N. gonorrhoeae, each named after its disease.

Specimens for bacteriological examination include pus, urethral exudate, urine and cerebrospinal fluid. Pathogenic neisseriae are difficult to culture in the laboratory as they have complex nutritional requirements and are very fragile away from their host. A clinical specimen must be cultured within a few hours of its taking. Incubation must be at 35-37°C in a very humid atmosphere with 5-10% carbon dioxide.

All species of Neisseria produce the enzyme oxidase; the oxidase test is very important in the identification of this organism and easily sets Neisseria apart from the preceding two genera discussed (as does the negative gram reaction). Neisseria is also catalase-positive, distinguishing it from Streptococcus.