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 MICROBIOLOGY    everything you think about microbial's science......
Introduction
The gram-positive spore-forming bacilli are the Bacillus and Clostridium species. These bacilli are ubiquitous, and because they form spores they can survive in the environment for many years. Bacillus species are aerobes, whereas clostridia are anaerobes.
Of the many species of both Bacillus and Clostridium genera, most do not cause disease and are not well characterized in medical microbiology. Several species, however, cause important disease in humans. Anthrax, a prototype disease in the history of microbiology, is caused by Bacillus anthracis. Anthrax remains an important disease of animals and occasionally of humans, and B anthracis is a major agent of bioterrorism and biologic warfare. Bacillus cereus causes food poisoning and occasionally eye or other localized infections. Clostridia cause several important toxin-mediated diseases: Clostridium tetani, tetanus; Clostridium botulinum, botulism; Clostridium perfringens, gas gangrene; and Clostridium difficile, pseudomembranous colitis. Other clostridia are also found in mixed anaerobic infections in humans (see Chapter 22).

Bacillus Species
The genus bacillus includes large aerobic, gram-positive rods occurring in chains. Most members of this genus are saprophytic organisms prevalent in soil, water, and air and on vegetation, such as Bacillus cereus and Bacillus subtilis. Some are insect pathogens. B cereus can grow in foods and produce an enterotoxin or an emetic toxin and cause food poisoning. Such organisms may occasionally produce disease in immunocompromised humans (eg, meningitis, endocarditis, endophthalmitis, conjunctivitis, or acute gastroenteritis). B anthracis, which causes anthrax, is the principal pathogen of the genus.
Morphology & Identification
Typical Organisms

The typical cells, measuring 1 x 3–4  m, have square ends and are arranged in long chains; spores are located in the center of the nonmotile bacilli.
Culture
Colonies of B anthracis are round and have a "cut glass" appearance in transmitted light. Hemolysis is uncommon with B anthracis but common with the saprophytic bacilli. Gelatin is liquefied, and growth in gelatin stabs resembles an inverted fir tree.
Growth Characteristics
The saprophytic bacilli utilize simple sources of nitrogen and carbon for energy and growth. The spores are resistant to environmental changes, withstand dry heat and certain chemical disinfectants for moderate periods, and persist for years in dry earth. Animal products contaminated with anthrax spores (eg, hides, bristles, hair, wool, bone) can be sterilized by autoclaving.
Bacillus anthracis
Pathogenesis
Anthrax is primarily a disease of herbivores—goats, sheep, cattle, horses, etc; other animals (eg, rats) are relatively resistant to the infection. Humans become infected incidentally by contact with infected animals or their products. In animals, the portal of entry is the mouth and the gastrointestinal tract. Spores from contaminated soil find easy access when ingested with spiny or irritating vegetation. In humans, the infection is usually acquired by the entry of spores through injured skin (cutaneous anthrax) or rarely the mucous membranes (gastrointestinal anthrax), or by inhalation of spores into the lung (inhalation anthrax).
The spores germinate in the tissue at the site of entry, and growth of the vegetative organisms results in formation of a gelatinous edema and congestion. Bacilli spread via lymphatics to the bloodstream, and they multiply freely in the blood and tissues shortly before and after the animal's death.
B anthracis (see Figure 12–1) that does not produce a capsule is not virulent and does not induce anthrax in test animals. The poly-D-glutamic acid capsule is antiphagocytic. The capsule
gene is on a plasmid.

 

 Anthrax toxin is made up of three proteins: protective antigen (PA), edema factor (EF), and lethal factor (LF). PA binds to specific cell receptors, and following proteolytic activation it forms a membrane channel that mediates entry of EF and LF into the cell. EF is an adenylyl cyclase; with PA it forms a toxin known as edema toxin. LF plus PA form lethal toxin, which is a major virulence factor and cause of death in infected animals. When injected into laboratory animals (eg, rats) the lethal toxin can quickly kill the animals. The anthrax toxin genes are on another plasmid.
In inhalation anthrax ("woolsorter's disease"), the spores from the dust of wool, hair, or hides are inhaled, phagocytosed in the lungs, and transported by the lymphatic drainage to the mediastinal lymph nodes, where germination occurs. This is followed by toxin production and the development of hemorrhagic mediastinitis and sepsis, which are usually rapidly fatal. In anthrax sepsis, the number of organisms in the blood exceeds 107/mL just prior to death. In the Sverdlovsk inhalation anthrax outbreak of 1979 and the US bioterrorism inhalation cases of 2001 (see Chapter 48) the pathogenesis was the same as in inhalation anthrax from animal products.
Pathology
In susceptible animals, the organisms proliferate at the site of entry. The capsules remain intact, and the organisms are surrounded by a large amount of proteinaceous fluid containing few leukocytes from which they rapidly disseminate and reach the bloodstream.
In resistant animals, the organisms proliferate for a few hours, by which time there is massive accumulation of leukocytes. The capsules gradually disintegrate and disappear. The organisms remain localized.
Clinical Findings
In humans, approximately 95% of cases are cutaneous anthrax and 5% are inhalation. Gastrointestinal anthrax is very rare; it has been reported from Africa, Asia, and the United States following occasions where people have eaten meat from infected animals.
The bioterrorism events in the fall of 2001 (see Chapter 48) resulted in 22 cases of anthrax: 11 inhalation and 11 cutaneous. Five of the patients with inhalation anthrax died. All the other patients survived.
Cutaneous anthrax generally occurs on exposed surfaces of the arms or hands, followed in frequency by the face and neck. A pruritic papule develops 1–7 days after entry of the organisms or spores through a scratch. Initially it resembles an insect bite. The papule rapidly changes into a vesicle or small ring of vesicles that coalesce, and a necrotic ulcer develops. The lesions typically are 1–3 cm in diameter and have a characteristic central black eschar. Marked edema occurs. Lymphangitis and lymphadenopathy and systemic signs and symptoms of fever, malaise, and headache may occur. After 7–10 days the eschar is fully developed. Eventually it dries, loosens, and separates; healing is by granulation and leaves a scar. It may take many weeks for the lesion to heal and the edema to subside. Antibiotic therapy does not appear to change the natural progression of the disease. In as many as 20% of patients, cutaneous anthrax can lead to sepsis, the consequences of systemic infection—including meningitis—and death.
The incubation period in inhalation anthrax may be as long as 6 weeks. The early clinical manifestations are associated with marked hemorrhagic necrosis and edema of the mediastinum. Substernal pain may be prominent, and there is pronounced mediastinal widening visible on x-ray chest films. Hemorrhagic pleural effusions follow involvement of the pleura; cough is secondary to the effects on the trachea. Sepsis occurs, and there may be hematogenous spread to the gastrointestinal tract, causing bowel ulceration, or to the meninges, causing hemorrhagic meningitis. The fatality rate in inhalation anthrax is high in the setting of known exposure; it is higher when the diagnosis is not initially suspected.
Animals acquire anthrax through ingestion of spores and spread of the organisms from the intestinal tract. This is rare in humans, and gastrointestinal anthrax is extremely uncommon. Abdominal pain, vomiting, and bloody diarrhea are clinical signs.
Diagnostic Laboratory Tests
Specimens to be examined are fluid or pus from a local lesion, blood, and sputum. Stained smears from the local lesion or of blood from dead animals often show chains of large gram-positive rods. Anthrax can be identified in dried smears by immunofluorescence staining techniques.
When grown on blood agar plates, the organisms produce nonhemolytic gray to white colonies with a rough texture and a ground-glass appearance. Comma-shaped outgrowths (Medusa head) may project from the colony. Gram stain shows large gram-positive rods. Carbohydrate fermentation is not useful. In semisolid medium, anthrax bacilli are always nonmotile, whereas related nonpathogenic organisms (eg, B cereus) exhibit motility by "swarming." Virulent anthrax cultures kill mice or guinea pigs upon intraperitoneal injection. Demonstration of capsule requires growth on bicarbonate-containing medium in 5–7% carbon dioxide. Lysis by a specific anthrax   -bacteriophage may be helpful in identifying the organism.
An enzyme-linked immunoassay (ELISA) has been developed to measure antibodies against edema and lethal toxins, but the test has not been extensively studied. Acute and convalescent sera obtained 4 weeks apart should be tested. A positive result is a fourfold change or a single titer of greater than 1:32.
Resistance & Immunity
Immunization to prevent anthrax is based on the classic experiments of Louis Pasteur. In 1881 he proved that cultures grown in broth at 42–52 °C for several months lost much of their virulence and could be injected live into sheep and cattle without causing disease; subsequently, such animals proved to be immune. Active immunity to anthrax can be induced in susceptible animals by vaccination with live attenuated bacilli, with spore suspensions, or with protective antigens from culture filtrates. Animals that graze in known anthrax districts should be immunized for anthrax annually.
Four countries produce vaccines for anthrax. Russia and China use attenuated spore-based vaccine administered by scarification. The US and Great Britain use a bacteria-free filtrate of cultures adsorbed to aluminum hydroxide. The current US Food and Drug Administration approved vaccine contains cell-free filtrates of a toxigenic nonencapsulated nonvirulent strain of B anthracis. The amount of protective antigen present per dose is unknown and all three toxins' components (LF, EF, and PA) are present and adsorbed to aluminum hydroxide. The dose schedule is 0, 2, and 4 weeks, then 6, 12, and 18 months, followed by annual boosters. The vaccine is available only to the US Department of Defense and to persons at risk for repeated exposure to B anthracis. Because of significant controversy in the US military about the current anthrax vaccine and its use in areas where there is potential for biologic warfare, the military is developing a new recombinant protective antigen vaccine (rPA) adsorbed to aluminum hydroxide.
Treatment
Many antibiotics are effective against anthrax in humans, but treatment must be started early. Ciprofloxacin is recommended for treatment; penicillin G, along with gentamicin or streptomycin, has previously been used to treat anthrax.
In the setting of potential exposure to B anthracis as an agent of biologic warfare, prophylaxis with ciprofloxacin or doxycycline should be continued for 4 weeks while three doses of vaccine are being given, or for 8 weeks if no vaccine is administered.
Some other gram-positive bacilli, such as B cereus, are resistant to penicillin by virtue of  -lactamase production. Doxycycline, erythromycin, or ciprofloxacin may be effective alternatives to penicillin.
Epidemiology, Prevention, & Control
Soil is contaminated with anthrax spores from the carcasses of dead animals. These spores remain viable for decades. Perhaps spores can germinate in soil at pH 6.5 at proper temperature. Grazing animals infected through injured mucous membranes serve to perpetuate the chain of infection. Contact with infected animals or with their hides, hair, and bristles is the source of infection in humans. Control measures include (1) disposal of animal carcasses by burning or by deep burial in lime pits, (2) decontamination (usually by autoclaving) of animal products, (3) protective clothing and gloves for handling potentially infected materials, and (4) active immunization of domestic animals with live attenuated vaccines. Persons with high occupational risk should be immunized.
Bacillus cereus
Food poisoning caused by Bacillus cereus has two distinct forms: the emetic type, associated with fried rice, and the diarrheal type, associated with meat dishes and sauces. B cereus produces toxins that cause disease that is more an intoxication than a food-borne infection. The emetic form is manifested by nausea, vomiting, abdominal cramps, and occasionally diarrhea and is self-limiting, with recovery occurring within 24 hours. It begins 1–5 hours after ingestion of rice and occasionally pasta dishes. B cereus is a soil organism that commonly contaminates rice. When large amounts of rice are cooked and allowed to cool slowly, the B cereus spores germinate and the vegetative cells produce the toxin during log-phase growth or during sporulation. The diarrheal form has an incubation period of 1–24 hours and is manifested by profuse diarrhea with abdominal pain and cramps; fever and vomiting are uncommon. The enterotoxin may be preformed in the food or produced in the intestine. The presence of B cereus in a patient's stool is not sufficient to make a diagnosis of B cereus disease, since the bacteria may be present in normal stool specimens; a concentration of 105 bacteria or more per gram of food is considered diagnostic.
B cereus is an important cause of eye infections, severe keratitis, endophthalmitis, and panophthalmitis. Typically, the organisms are introduced into the eye by foreign bodies associated with trauma. B cereus has also been associated with localized infections and with systemic infections, including endocarditis, meningitis, osteomyelitis, and pneumonia; the presence of a medical device or intravenous drug use predisposes to these infections.
Other Bacillus species are rarely associated with human disease. It is difficult to differentiate superficial contamination with bacillus from genuine disease caused by the organism. Five Bacillus species (B thuringiensis, B popilliae, B sphaericus, B larvae, and B lentimorbus) are pathogens for insects, and some have been used as commercial insecticides. Genes from B thuringiensis coding for insecticidal compounds have been inserted into the genetic material of some commercial plants. This has been associated with concern on the part of environmental activists about genetically engineered plants and food products.
Clostridium Species
The clostridia are large anaerobic, gram-positive, motile rods. Many decompose proteins or form toxins, and some do both. Their natural habitat is the soil or the intestinal tract of animals and humans, where they live as saprophytes. Among the pathogens are the organisms causing botulism, tetanus, gas gangrene, and pseudomembranous colitis.
Morphology & Identification

 Culture
Clostridia are anaerobes and grow under anaerobic conditions; a few species are aerotolerant and will also grow in ambient air. Anaerobic culture conditions are discussed in Chapter 22. In general, the clostridia grow well on the blood-enriched media used to grow anaerobes and on other media used to culture anaerobes as well.
Colony Forms
Some clostridia produce large raised colonies (eg, C perfringens); others produce smaller colonies (eg, C tetani). Some clostridia form colonies that spread on the agar surface. Many clostridia produce a zone of hemolysis on blood agar. C perfringens typically produces multiple zones of hemolysis around colonies.
Growth Characteristics
Clostridia can ferment a variety of sugars; many can digest proteins. Milk is turned acid by some and digested by others and undergoes "stormy fermentation" (ie, clot torn by gas) with a third group (eg, C perfringens). Various enzymes are produced by different species (see below).
Antigenic Characteristics
Clostridia share some antigens but also possess specific soluble antigens that permit grouping by precipitin tests.
Clostridium botulinum
Clostridium botulinum, which causes botulism, is worldwide in distribution; it is found in soil and occasionally in animal feces.
Types of C botulinum are distinguished by the antigenic type of toxin they produce. Spores of the organism are highly resistant to heat, withstanding 100 °C for several hours. Heat resistance is diminished at acid pH or high salt concentration.
Toxin
During the growth of C botulinum and during autolysis of the bacteria, toxin is liberated into the environment. Seven antigenic varieties of toxin (A–G) are known. Types A, B, and E (and occasionally F) are the principal causes of human illness. Types A and B have been associated with a variety of foods and type E predominantly with fish products. Type C produces limberneck in birds; type D causes botulism in mammals. The toxin is a 150,000-MW protein that is cleaved into 100,000-MW and 50,000-MW proteins linked by a disulfide bond. Botulinum toxin is absorbed from the gut and binds to receptors of presynaptic membranes of motor neurons of the peripheral nervous system and cranial nerves. Proteolysis—by the light chain of botulinum toxin—of the target SNARE proteins in the neurons inhibits the release of acetylcholine at the synapse, resulting in lack of muscle contraction and paralysis. The SNARE proteins are synaptobrevin, SNAP 25, and syntaxin. The toxins of C botulinum types A and E cleave the 25,000-MW SNAP-25. Type B toxin cleaves synaptobrevin. C botulinum toxins are among the most toxic substances known: The lethal dose for a human is probably about 1–2  g. The toxins are destroyed by heating for 20 minutes at 100 °C.
Pathogenesis
Although C botulinum types A and B have been implicated in cases of wound infection and botulism, most often the illness is not an infection. Rather, it is an intoxication resulting from the ingestion of food in which C botulinum has grown and produced toxin. The most common offenders are spiced, smoked, vacuum-packed, or canned alkaline foods that are eaten without cooking. In such foods, spores of C botulinum germinate; under anaerobic conditions, vegetative forms grow and produce toxin.
The toxin acts by blocking release of acetylcholine at synapses and neuromuscular junctions (see above). Flaccid paralysis results. The electromyogram and edrophonium strength tests are typical.
Clinical Findings
Symptoms begin 18–24 hours after ingestion of the toxic food, with visual disturbances (incoordination of eye muscles, double vision), inability to swallow, and speech difficulty; signs of bulbar paralysis are progressive, and death occurs from respiratory paralysis or cardiac arrest. Gastrointestinal symptoms are not regularly prominent. There is no fever. The patient remains fully conscious until shortly before death. The mortality rate is high. Patients who recover do not develop antitoxin in the blood.
In the United States, infant botulism is as common as or more common than the classic form of paralytic botulism associated with the ingestion of toxin-contaminated food. The infants in the first months of life develop poor feeding, weakness, and signs of paralysis ("floppy baby"). Infant botulism may be one of the causes of sudden infant death syndrome. C botulinum and botulinum toxin are found in feces but not in serum. It is assumed that C botulinum spores are in the babies' food, yielding toxin production in the gut. Honey has been implicated as a possible vehicle for the spores.
Diagnostic Laboratory Tests
Toxin can often be demonstrated in serum from the patient, and toxin may be found in leftover food. Mice injected intraperitoneally die rapidly. The antigenic type of toxin is identified by neutralization with specific antitoxin in mice. C botulinum may be grown from food remains and tested for toxin production, but this is rarely done and is of questionable significance. In infant botulism, C botulinum and toxin can be demonstrated in bowel contents but not in serum. Toxin may be demonstrated by passive hemagglutination or radioimmunoassay.
Treatment
Potent antitoxins to three types of botulinum toxins have been prepared in horses. Since the type responsible for an individual case is usually not known, trivalent (A, B, E) antitoxin must be promptly administered intravenously with customary precautions. Adequate ventilation must be maintained by mechanical respirator, if necessary. These measures have reduced the mortality rate from 65% to below 25%.
Although most infants with botulism recover with supportive care alone, antitoxin therapy is recommended.
Epidemiology, Prevention, & Control
Since spores of C botulinum are widely distributed in soil, they often contaminate vegetables, fruits, and other materials. A large restaurant-based outbreak was associated with sautéed onions. When such foods are canned or otherwise preserved, they either must be sufficiently heated to ensure destruction of spores or must be boiled for 20 minutes before consumption. Strict regulation of commercial canning has largely overcome the danger of widespread outbreaks, but commercially prepared foods have caused deaths. A chief risk factor for botulism lies in home-canned foods, particularly string beans, corn, peppers, olives, peas, and smoked fish or vacuum-packed fresh fish in plastic bags. Toxic foods may be spoiled and rancid, and cans may "swell," or the appearance may be innocuous. The risk from home-canned foods can be reduced if the food is boiled for more than 20 minutes before consumption. Toxoids are used for active immunization of cattle in South Africa.
Botulinum toxin is considered to be a major agent for bioterrorism and biologic warfare (see Chapter 48).
Clostridium tetani
Clostridium tetani, which causes tetanus, is worldwide in distribution in the soil and in the feces of horses and other animals. Several types of C tetani can be distinguished by specific flagellar antigens. All share a common O (somatic) antigen, which may be masked, and all produce the same antigenic type of neurotoxin, tetanospasmin.
Toxin
The vegetative cells of C tetani produce the toxin tetanospasmin (MW 150,000) that is cleaved by a bacterial protease into two peptides (MW 50,000 and 100,000) linked by a disulfide bond. The toxin initially binds to receptors on the presynaptic membranes of motor neurons. It then migrates by the retrograde axonal transport system to the cell bodies of these neurons to the spinal cord and brain stem. The toxin diffuses to terminals of inhibitory cells, including both glycinergic interneurons and aminobutyric acid-secreting neurons from the brain stem. The toxin degrades synaptobrevin, a protein required for docking of neurotransmitter vesicles on the presynaptic membrane. Release of the inhibitory glycine and  -aminobutyric acid is blocked, and the motor neurons are not inhibited. Hyperreflexia, muscle spasms, and spastic paralysis result. Extremely small amounts of toxin can be lethal for humans.
Pathogenesis
C tetani is not an invasive organism. The infection remains strictly localized in the area of devitalized tissue (wound, burn, injury, umbilical stump, surgical suture) into which the spores have been introduced. The volume of infected tissue is small, and the disease is almost entirely a toxemia. Germination of the spore and development of vegetative organisms that produce toxin are aided by (1) necrotic tissue, (2) calcium salts, and (3) associated pyogenic infections, all of which aid establishment of low oxidation-reduction potential.
The toxin released from vegetative cells reaches the central nervous system and rapidly becomes fixed to receptors in the spinal cord and brain stem and exerts the actions described above.
Clinical Findings
The incubation period may range from 4–5 days to as many weeks. The disease is characterized by tonic contraction of voluntary muscles. Muscular spasms often involve first the area of injury and infection and then the muscles of the jaw (trismus, lockjaw), which contract so that the mouth cannot be opened. Gradually, other voluntary muscles become involved, resulting in tonic spasms. Any external stimulus may precipitate a tetanic generalized muscle spasm. The patient is fully conscious, and pain may be intense. Death usually results from interference with the mechanics of respiration. The mortality rate in generalized tetanus is very high.
Diagnosis
The diagnosis rests on the clinical picture and a history of injury, although only 50% of patients with tetanus have an injury for which they seek medical attention. The primary differential diagnosis of tetanus is strychnine poisoning. Anaerobic culture of tissues from contaminated wounds may yield C tetani, but neither preventive nor therapeutic use of antitoxin should ever be withheld pending such demonstration. Proof of isolation of C tetani must rest on production of toxin and its neutralization by specific antitoxin.
Prevention & Treatment
The results of treatment of tetanus are not satisfactory. Therefore, prevention is all-important. Prevention of tetanus depends upon (1) active immunization with toxoids; (2) proper care of wounds contaminated with soil, etc; (3) prophylactic use of antitoxin; and (4) administration of penicillin.
The intramuscular administration of 250–500 units of human antitoxin (tetanus immune globulin) gives adequate systemic protection (0.01 unit or more per milliliter of serum) for 2–4 weeks. It neutralizes the toxin that has not been fixed to nervous tissue. Active immunization with tetanus toxoid should accompany antitoxin prophylaxis.
Patients who develop symptoms of tetanus should receive muscle relaxants, sedation, and assisted ventilation. Sometimes they are given very large doses of antitoxin (3000–10,000 units of tetanus immune globulin) intravenously in an effort to neutralize toxin that has not yet been bound to nervous tissue. However, the efficacy of antitoxin for treatment is doubtful except in neonatal tetanus, where it may be lifesaving.
Surgical debridement is vitally important because it removes the necrotic tissue that is essential for proliferation of the organisms. Hyperbaric oxygen has no proved effect.
Penicillin strongly inhibits the growth of C tetani and stops further toxin production. Antibiotics may also control associated pyogenic infection.
When a previously immunized individual sustains a potentially dangerous wound, an additional dose of toxoid should be injected to restimulate antitoxin production. This "recall" injection of toxoid may be accompanied by a dose of antitoxin if the patient has not had current immunization or boosters or if the history of immunization is unknown.
Control
Tetanus is a totally preventable disease. Universal active immunization with tetanus toxoid should be mandatory. Tetanus toxoid is produced by detoxifying the toxin with formalin and then concentrating it. Aluminum-salt-adsorbed toxoids are employed. Three injections comprise the initial course of immunization, followed by another dose about 1 year later. Initial immunization should be carried out in all children during the first year of life. A "booster" injection of toxoid is given upon entry into school. Thereafter, "boosters" can be spaced 10 years apart to maintain serum levels of more than 0.01 unit antitoxin per milliliter. In young children, tetanus toxoid is often combined with diphtheria toxoid and pertussis vaccine.
Control measures are not possible because of the wide dissemination of the organism in the soil and the long survival of its spores.
Clostridia that Produce Invasive Infections
Many different toxin-producing clostridia (Clostridium perfringens and related clostridia) (Figure 12–3) can produce invasive infection (including myonecrosis and gas gangrene) if introduced into damaged tissue. About 30 species of clostridia may produce such an effect, but the most common in invasive disease is Clostridium perfringens (90%). An enterotoxin of C perfringens is a common cause of food poisoning.


 

Toxins
The invasive clostridia produce a large variety of

toxins and enzymes that result in a spreading infection.

Many of these toxins have lethal, necrotizing, and

hemolytic properties. In some cases, these are different

properties of a single substance; in other instances,

they are due to different chemical entities. The alpha

toxin of C perfringens type A is a lecithinase, and its

lethal action is proportionate to the rate at which it

splits lecithin (an important constituent of cell

membranes) to phosphorylcholine and diglyceride. The

theta toxin has similar hemolytic and necrotizing

effects but is not a lecithinase. DNase and

hyaluronidase, a collagenase that digests collagen of

subcutaneous tissue and muscle, are also produced.
Some strains of C perfringens produce a powerful

enterotoxin, especially when grown in meat dishes. When

more than 108 vegetative cells are ingested and

sporulate in the gut, enterotoxin is formed. The

enterotoxin is a protein (MW 35,000) that may be a

nonessential component of the spore coat; it is distinct

from other clostridial toxins. It induces intense

diarrhea in 6–18 hours. The action of C perfringens

enterotoxin involves marked hypersecretion in the

jejunum and ileum, with loss of fluids and electrolytes

in diarrhea. Much less frequent symptoms include nausea,

vomiting, and fever. This illness is similar to that

produced by B cereus and tends to be self-limited.
Pathogenesis
In invasive clostridial infections, spores reach tissue

either by contamination of traumatized areas (soil,

feces) or from the intestinal tract. The spores

germinate at low oxidation-reduction potential;

vegetative cells multiply, ferment carbohydrates present

in tissue, and produce gas. The distention of tissue and

interference with blood supply, together with the

secretion of necrotizing toxin and hyaluronidase, favor

the spread of infection. Tissue necrosis extends,

providing an opportunity for increased bacterial growth,

hemolytic anemia, and, ultimately, severe toxemia and

death.
In gas gangrene (clostridial myonecrosis), a mixed

infection is the rule. In addition to the toxigenic

clostridia, proteolytic clostridia and various cocci and

gram-negative organisms are also usually present. C

perfringens occurs in the genital tract of 5% of women.

Before legalization of abortion in the United States,

clostridial uterine infections followed instrumental

abortions. Clostridium sordellii has many of the

properties of C perfringens. C sordellii has been

reported to cause a toxic shock syndrome after medical

abortion with mifepristone and intravaginal misoprostol.

Endometrial infection with C sordellii is implicated.

Clostridial bacteremia is a frequent occurrence in

patients with neoplasms. In New Guinea, C perfringens

type C produces a necrotizing enteritis (pigbel) that

can be highly fatal in children. Immunization with type

C toxoid appears to have preventive value.
Clinical Findings
From a contaminated wound (eg, a compound fracture,

postpartum uterus), the infection spreads in 1–3 days to

produce crepitation in the subcutaneous tissue and

muscle, foul-smelling discharge, rapidly progressing

necrosis, fever, hemolysis, toxemia, shock, and death.

Treatment is with early surgery (amputation) and

antibiotic administration. Until the advent of specific

therapy, early amputation was the only treatment. At

times, the infection results only in anaerobic fasciitis

or cellulitis.
C perfringens food poisoning usually follows the

ingestion of large numbers of clostridia that have grown

in warmed meat dishes. The toxin forms when the

organisms sporulate in the gut, with the onset of

diarrhea—usually without vomiting or fever—in 6–18

hours. The illness lasts only 1–2 days.
Diagnostic Laboratory Tests
Specimens consist of material from wounds, pus, and

tissue. The presence of large gram-positive rods in

Gram-stained smears suggests gas gangrene clostridia;

spores are not regularly present.
Material is inoculated into chopped meat-glucose medium

and thioglycolate medium and onto blood agar plates

incubated anaerobically. The growth from one of the

media is transferred into milk. A clot torn by gas in 24

hours is suggestive of C perfringens. Once pure cultures

have been obtained by selecting colonies from

anaerobically incubated blood plates, they are

identified by biochemical reactions (various sugars in

thioglycolate, action on milk), hemolysis, and colony

form. Lecithinase activity is evaluated by the

precipitate formed around colonies on egg yolk media.

Final identification rests on toxin production and

neutralization by specific antitoxin. C perfringens

rarely produces spores when cultured on agar in the

laboratory.
Treatment
The most important aspect of treatment is prompt and

extensive surgical debridement of the involved area and

excision of all devitalized tissue, in which the

organisms are prone to grow. Administration of

antimicrobial drugs, particularly penicillin, is begun

at the same time. Hyperbaric oxygen may be of help in

the medical management of clostridial tissue infections.

It is said to "detoxify" patients rapidly.
Antitoxins are available against the toxins of C

perfringens, Clostridium novyi, Clostridium

histolyticum, and Clostridium septicum, usually in the

form of concentrated immune globulins. Polyvalent

antitoxin (containing antibodies to several toxins) has

been used. Although such antitoxin is sometimes

administered to individuals with contaminated wounds

containing much devitalized tissue, there is no evidence

for its efficacy. Food poisoning due to C perfringens

enterotoxin usually requires only symptomatic care.
Prevention & Control
Early and adequate cleansing of contaminated wounds and

surgical debridement, together with the administration

of antimicrobial drugs directed against clostridia (eg,

penicillin), are the best available preventive measures.

Antitoxins should not be relied on. Although toxoids for

active immunization have been prepared, they have not

come into practical use.
Clostridium difficile & Diarrheal Disease
Pseudomembranous Colitis

Pseudomembranous colitis is diagnosed by detection of

one or both C difficile toxins in stool and by

endoscopic observation of pseudomembranes or

microabscesses in patients who have diarrhea and have

been given antibiotics. Plaques and microabscesses may

be localized to one area of the bowel. The diarrhea may

be watery or bloody, and the patient frequently has

associated abdominal cramps, leukocytosis, and fever.

Although many antibiotics have been associated with

pseudomembranous colitis, the most common are ampicillin

and clindamycin. The disease is treated by discontinuing

administration of the offending antibiotic and orally

giving either metronidazole or vancomycin.
Administration of antibiotics results in proliferation

of drug-resistant C difficile that produces two toxins.

Toxin A, a potent enterotoxin that also has some

cytotoxic activity, binds to the brush border membranes

of the gut at receptor sites. Toxin B is a potent

cytotoxin. Both toxins are found in the stools of

patients with pseudomembranous colitis. Not all strains

of C difficile produce the toxins, and the tox genes

apparently are not carried on plasmids or phage.
Antibiotic-Associated Diarrhea
The administration of antibiotics frequently leads to a

mild to moderate form of diarrhea, termed antibiotic-

associated diarrhea. This disease is generally less

severe than the classic form of pseudomembranous

colitis. As many as 25% of cases of antibiotic-

associated diarrhea may be associated with C difficile.
References
Allen SDS, Emery CL, Lyerly DM: Clostridium. In: Manual

of Clinical Microbiology, 8th ed. Murray PR et al

(editors). ASM Press, 2003.
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source:Jawetz, Melnick, & Adelberg's Medical Microbiology, 24th Edition by Vishal