Medmicro Chapter 15 Bacillus

Peter C. B. Turnbull

General Concepts

Clinical Manifestations

Anthrax is caused by Bacillus anthracis. Humans acquire the disease directly

from contact with infected herbivores or indirectly via their products. The

clinical forms include ( 1 ) cutaneous anthrax (eschar with edema), from

handling infected material (this accounts for more than 95 percent of cases);

(2) intestinal anthrax, from eating infected meat; and (3) pulmonary anthrax,

from inhaling spore-laden dust. Several other Bacillus spp, in particular B

cereus and to a lesser extent B subtilis and B licheniformis, are periodically

associated with bacteremia/septicemia, endocarditis, meningitis, and infections

of wounds, the ears, eyes, respiratory tract, urinary tract, and

gastrointestinal tract. Bacillus cereus causes two distinct food poisoning

syndromes: a rapid-onset emetic syndrome characterized by nausea and vomiting,

and a slower-onset diarrheal syndrome.

Structure and Classification

Bacillus species are rod-shaped, endospore-forming aerobic or facultatively

anaerobic, Gram-positive bacteria; in some species cultures may turn

Gram-negative with age. The many species of the genus exhibit a wide range of

physiologic abilities that allow them to live in every natural environment. Only

one endospore is formed per cell. The spores are resistant to heat, cold,

radiation, desiccation, and disinfectants. Bacillus anthracis needs oxygen to

sporulate; this constraint has important consequences for epidemiology and

control. In vivo, B anthracis produces a polypeptide (polyglutamic acid) capsule

that protects it from phagocytosis. The genera Bacillus and Clostridium

constitute the family Bacillaceae. Species are identified by using morphologic

and biochemical criteria.

Pathogenesis

The virulence factors of B anthracis are its capsule and three-component toxin,

both encoded on plasmids. Bacillus cereus produces numerous enzymes and

aggressins. The principal virulence factors are a necrotizing enterotoxin and a

potent hemolysin (cereolysin). Emetic food poisoning probably results from the

release of emetic factors from specific foods by bacterial enzymes.

Host Defenses

The reasons for marked differences in susceptibility to anthrax among different

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animal species are not known. The protective actions of the live-spore animal

vaccine or the human chemical vaccines are based on induction of humoral and

cell-mediated immunity to the protective antigen component of anthrax toxin.

Epidemiology

Individuals at risk for anthrax include those in contact with infected animals

or animal products. Episodes of B cereus food poisoning occur sporadically

worldwide and result from ingestion of contaminated food in which the bacteria

have multiplied to high levels under conditions of improper storage after

cooking.

Diagnosis

Cutaneous anthrax is diagnosed on the basis of the characteristic papule (early)

or eschar (later) with extensive surrounding edema, backed by a history of

exposure to animals or their products. Diagnosis is confirmed by observation of

characteristic encapsulated bacilli in polychrome methylene blue-stained smears

of blood, exudate, lymph, cerebrospinal fluid, etc., and/or by culture. Other

Bacillus infections are diagnosed by culture of the bacteria.

Control

Anthrax: Control in animals is essential for control in humans. In endemic

areas, animals that die suddenly should be handled cautiously and livestock

should be vaccinated annually. A human vaccine is available for individuals in

high-risk occupations. Anthrax is readily treated with antibiotics (e.g.,

penicillin, tetracycline, chloramphenicol, gentamicin, or erythromycin).

Other Bacillus Infections: Control is by good hygiene. Treatment is with

non-ß-lactam antibiotics for Gram-positive bacteria. Food poisoning is

controlled by adequate cooking, avoidance of recontamination of cooked food, and

proper storage (efficient refrigeration).

Pharmaceutical, Agricultural, and Industrial Importance

Many of the physiologic properties and specialized metabolites of Bacillus

species are used in the pharmaceutical, agricultural, and food industries. On

the other hand, the resistance of the spores to sterilization and disinfection

makes them problem contaminants in foods, medical supplies, surgical procedures,

etc.

INTRODUCTION

Bacillus species are aerobic, sporulating, rod-shaped bacteria that are

ubiquitous in nature. Bacillus anthracis, the agent of anthrax, is the only

obligate Bacillus pathogen in vertebrates. Bacillus larvae, B lentimorbus, B

popilliae, B sphaericus, and B thuringiensis are pathogens of specific groups of

insects. A number of other species, in particular B cereus, are occasional

pathogens of humans and livestock, but the large majority of Bacillus species

are harmless saprophytes.

Anthrax has afflicted humans throughout recorded history. The fifth and sixth

plagues of Egypt described in Exodus are widely believed to have been anthrax.

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The disease was featured in the writings of Virgil in 25 BC and was familiar in

medieval times as the Black Bane. It was from studies on anthrax that Koch

established his famous postulates in 1876, and vaccines against anthraxthe best

known being that of Pasteur (1881)were among the first bacterial vaccines

developed.

Bacillus species are used in many medical, pharmaceutical, agricultural, and

industrial processes that take advantage of their wide range of physiologic

characteristics and their ability to produce a host of enzymes, antibiotics, and

other metabolites. Bacitracin and polymyxin are two well-known antibiotics

obtained from Bacillus species. Several species are used as standards in medical

and pharmaceutical assays.

The spores of the obligate thermophile B stearothermophilus are used to test

heat sterilization procedures, and B subtilis subsp globigii, which is resistant

to heat, chemicals, and radiation, is widely used to validate alternative

sterilization and fumigation procedures. Certain Bacillus species are important

in the natural or artificial degradation of waste products. Some Bacillus insect

pathogens are used as the active ingredients of insecticides.

Because the spores of many Bacillus species are resistant to heat, radiation,

disinfectants, and desiccation, they are difficult to eliminate from medical and

pharmaceutical materials and are a frequent cause of contamination. Bacillus

species are well known in the food industries as troublesome spoilage organisms.

Clinical Manifestions

Although anthrax remains the best-known Bacillus disease, in recent years other

Bacillus species have been increasingly implicated in a wide range of infections

including abscesses, bacteremia/septicemia, wound and burn infections, ear

infections, endocarditis, meningitis, ophthalmitis, osteomyelitis, peritonitis,

and respiratory and urinary tract infections. Most of these occur as secondary

or mixed infections or inimmunodeficient or otherwise immunocompromised hosts

(such as alcoholics and diabetics), but a significant proportion are primary

infections in otherwise healthy individuals. Some of these infections are severe

or lethal. Of the species listed in Table 15-1, most frequently implicated in

these types of infection is B cereus, followed by B licheniformis and B

subtilis. Bacillus alvei, B brevis, B circulans, B coagulans, B macerans, B

pumilus, B sphaericus, and B thuringiensis cause occasional infections. As

secondary invaders, Bacillus species may exacerbate preexisting infections by

producing either tissue-damaging toxins or metabolites such as penicillinase

that interfere with treatment.

Bacillus cereus is well known as an agent of food poisoning, and a number of

other Bacillus species, particularly B subtilis and B licheniformis, are also

incriminated periodically in this capacity.

Anthrax

Anthrax is primarily a disease of herbivores. Humans acquire it as a result of

contact with infected animals or animal products. In humans the disease takes

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one of three forms, depending on the route of infection. Cutaneous anthrax,

which accounts for more than 95 percent of cases worldwide, results from

infection through skin lesions; intestinal anthrax results from ingestion of

spores, usually in infected meat; and pulmonary anthrax results from inhalation

of spores.

Cutaneous anthrax usually occurs through contamination of a cut or abrasion,

although in some countries biting flies may also transmit the disease. After a

2- to 3-day incubation period, a small pimple or papule appears at the

inoculation site. A surrounding ring of vesicles develops. Over the next few

days, the central papule ulcerates, dries, and blackens to form the

characteristic eschar (Fig. 15-1). The lesion is painless and is surrounded by

marked edema that may extend for some distance. Pus and pain appear only if the

lesion becomes infected by a pyogenic organism. Similarly, marked lymphangitis

and fever usually point to a secondary infection. In most cases the disease

remains limited to the initial lesion and resolves spontaneously. The main

dangers are that a lesion on the face or neck may swell to occlude the airway or

may give rise to secondary meningitis. If host defenses fail to contain the

infection, however, fulminating septicemia develops. Approximately 20 percent of

untreated cases of cutaneous anthrax progress to fatal septicemia. However, B

anthracis is susceptible to penicillin and other common antibiotics, so

effective treatment is almost always available.

FIGURE 15-1 Evolution of an anthrax eschar in a 4-year-old boy. (A&B) the lesion

when first seen (day 0). Note the arm swollen from the characteristic edema. (C)

Day 6. (D) Day 10. (E) Day 15. Although penicillin treatment was begun

immediately and the lesion was sterile by about 24 hours, it continued to evolve

and resolve as seen. (Photographs kindly supplied by W.E. Kobuch, M.D., St.

Luke's Hospital, Lupane, Bulawayo, Zimbabwe.)

Intestinal anthrax is analogous to cutaneous anthrax but occurs on the

intestinal mucosa. As in cutaneous anthrax, the organisms probably invade the

mucosa through a preexisting lesion. organisms spread from the mucosal lesion to

the lymphatic system. In pulmonary anthrax, inhaled spores are transported by

alveolar macrophages to the mediastinal lymph nodes, where they germinate and

multiply to initiate systemic disease. Gastrointestinal and pulmonary anthrax

are both more dangerous than the cutaneous form because they are usually

identified too late for treatment to be effective.

Herbivorous animals, the primary hosts of B anthracis, contract the infection by

ingesting spores on forage plants; the spores are derived from soil or dust or

are deposited on leaves by flies after feeding on an anthrax-infected carcass.

If the spores enter a lesion in the gastrointestinal mucosa, they germinate and

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are taken into the bloodstream and lymphatics, finally producing systemic

anthrax, which is usually fatal.

Symptoms prior to fulminant systemic anthrax may be absent or mild, consisting,

for example, of malaise, low fever, and mild gastrointestinal symptoms in the

case of gastrointestinal disease. During this phase the organism is multiplying

and producing toxin in the regional lymph nodes and spleen. Released toxin

causes breakdown of these organs probably of the spleen in particular. This

causes the sudden onset of hyperacute illness with dyspnea, cyanosis, high

fever, and disorientation, which progress in a few hours to shock, coma, and

death. Although symptoms vary somewhat with the host species, this final acute

phase is marked by a high-grade bacteremia. In humans, blood cultures are not

always positive.

Bacillus Food Poisoning

Bacillus cereus can cause two distinct types of food poisoning. The diarrheal

type is characterized by diarrhea and abdominal pain occurring 8 to 16 hours

after consumption of the contaminated food. It is associated with a variety of

foods, including meat and vegetable dishes, sauces, pastas, desserts, and dairy

products. In emetic disease, on the other hand, nausea and vomiting begin 1 to 5

hours after the contaminated food is eaten. Boiled rice that is held for

prolonged periods at ambient temperature and then quick-fried before serving is

the usual offender, although dairy products or other foods are occasionally

responsible. The symptoms of food poisoning caused by other Bacillus species (B

subtilis, B licheniformis, and others) are less well defined. Diarrhea and/or

nausea occurs 1 to 14 hours after consumption of the contaminated food. A wide

variety of food types have proved responsible in recorded instances.

A Bacillus food poisoning episode usually occurs because spores survive cooking

or pasteurization and then germinate and multiply when the food is inadequately

refrigerated. The symptoms of B cereus food poisoning are caused by a toxin or

toxins produced in the food during this multiplication. Toxins have not yet been

identified for other Bacillus species that cause food poisoning.

Structure and Classification

The family Bacillaceae, consisting of rod-shaped bacteria that form endospores,

has two principal subdivisions: the anaerobic spore-forming bacteria of the

genus Clostridium, and the aerobic or facultatively anaerobic spore-forming

bacteria of the genus Bacillus frequently known as ASB (aerobic spore-bearers).

Bacterial cells of Bacillus cultures are Gram positive when young, but in some

species become Gram negative as they age.

Most Bacillus species are saprophytes. Table 15-1 lists the identifying

characteristics of some of the species most likely to be encountered by the

physician. Not only are Bacillus endospores resistant to hostile physical and

chemical conditions, but also various species have unusual physiologic

properties that enable them to survive or thrive in harsh environments, ranging

from desert sands and hot springs to Arctic soils and from fresh waters to

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marine sediments. The genus includes thermophilic, psychrophilic, acidophilic,

alkaliphilic, halotolerant, and halophilic representatives, which are capable of

growing at temperatures, pH values, and salt concentrations at which few other

organisms could survive.

Figure 15-2 shows the structure of a generalized Bacillus endospore (details of

the structure differ from species to species). One spore is produced per

vegetative cell. The central protoplast, or germ cell, carries the constituents

of the future vegetative cell, accompanied by dipicolinic acid, which is

essential to the heat resistance of the spore. Surrounding the protoplast is a

cortex consisting largely of peptidoglycan (murein), which is also important in

the heat and radiation resistance of the spore. The inner layer, the cortical

membrane or protoplast wall, becomes the cell wall of the new vegetative cell

when the spore germinates. The spore coats, which constitute up to 50 percent of

the volume of the spore, protect it from chemicals, enzymes, etc.

FIGURE 15-2 Cross section of a Bacillus spore.

The events involved in sporulation of vegetative cells and in germination of

spores are complex and are influenced by factors such as temperature, pH, and

the availability of certain divalent cations and carbon- and nitrogen-containing

compounds. Spores formed under different conditions have different stabilities

and degrees of resistance to heat, radiation, chemicals, desiccation, and other

hostile conditions.

Pathogenisis

The pathogenicity of B anthracis depends on two virulence factors: a

poly-y-D-glutamic acid polypeptide capsule, which protects it from phagocytosis

by the defensive phagocytes of the host, and a toxin produced in the log phase

of growth. This toxin consists of three proteins: protective antigen (PA) (82. 7

kDa), lethal factor (LF) (90.2 kDa), and edema factor (EF) (88.9 kDa). Host

proteases in the blood and on the eukaryotic cell surface activate protective

antigen by cutting off a 20-kDa segment, exposing a binding site for LF and EF.

The activated 63 kDa PA polypeptide binds to specific receptors on the host cell

surface, thereby creating a secondary binding site for which LF and EF compete.

The complex (PA+LF or PA+EF) is internalized by endocytosis and, following

acidification of the endosome, the LF or EF cross the membrane into the cytosol

via PA-mediated ion-conductive channels. This is analogous to the A-B

structure-function model of cholera toxin with PA behaving as the B (binding)

moiety (Fig.15-3). EF, responsible for the characteristic edema of anthrax, is a

calmodulin-dependent adenylate cyclase. (Calmodulin is the major intracellular

calcium receptor in eukaryotic cells.) The only other known bacterial adenylate

cyclase is produced by Bordetella pertussis (see Ch 31), but the two toxins

share only minor homologies. LF appears to be a zinc-dependent metalloprotease

though its substrate and mode of action have yet to be elucidated.

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FIGURE 15-3 Mechanism of action of the anthrax toxin. The toxin is composed of

three proteins. Protective antigen (PA) binds to an appropriate site on the host

cell membrane. A cell surface protease cleaves off a 20-kDa piece from the

protective antigen and thereby exposes a secondary binding site for which lethal

factor (LF) and edema factor (EF) compete. The complex (PA+LF or PA+EF) is

internalized by receptor-mediated endocytosis, and acidification of the endosome

results in the transfer of the LF or EF across the endosome membrane into the

cytosol where they carry out their catalytic actions. (Model by S.H. Leppla,

Ph.D., Laboratory of Microbial Ecology, National Institutes of Health, Bethesda,

MD.)

The toxin and capsule of B anthracis are encoded on two large plasmids called

pXO 1 (110 MDa) and pX02 (60 MDa), respectively. Strains lacking either of these

plasmids have greatly reduced virulence (Fig.15-4). The attenuated live vaccine

strain developed by Sterne in 1937, which is still the basis of most anthrax

vaccines for livestock, lacks pX02 and is therefore Cap- Tox+. The protection

afforded by such vaccines apparently is related primarily to antibodies specific

for the protective antigen component of the toxin. In contrast, the attenuated

vaccine strains developed by Pasteur 110 years ago were inadvertently cured of

pXO1 (by subculturing at 42° to 43°C); these Pasteur strains are therefore Cap+

Tox-. Strains of this type do not induce protective immunity; the partial

effectiveness of Pasteur's vaccines is now believed to have been due to the

residual uncured (Cap+ Tox+) cells they contained, and this would also explain

the partial virulence of these strains.

FIGURE 15-4 Genetics of virulence factor production by B. anthracis. Plasmids

pX01 and pX02 encode, respectively, the anthrax toxin and capsule. Curing the

bacteria of pX01 produces an encapsulated, nontoxigenic strain that is

nonprotective. Curing of pX02 produces a toxigenic nonencapsulating strain that

can be used as a protective vaccine. Production of protective antigen is

essential for a strain to be protective.

The only other Bacillus species for which virulence factors have been identified

is B cereus. A 38 to 46-kDa protein complex has been shown in animal models to

cause necrosis of the skin or intestinal mucosa (Fig. 15-5), to induce fluid

accumulation in the intestine, and to be a lethal toxin. This protein is

believed to be responsible for the necrotic and toxemic nature of severe B

cereus infections and for the diarrheal form of food poisoning. Bacillus cereus

also produces two hemolysins; one of these, cereolysin (58 kDa), is a potent

necrotic and lethal toxin. Although this toxin is neutralized by serum

cholesterol, it probably contributes to the pathogenesis of B cereus infections.

Little is known about the other hemolysin at present. Phospholipases produced by

B cereus may act as exacerbating factors by degrading host cell membranes

following exposure of their phospholipid substrates in wounds or other

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infections. The agent responsible for the emetic type of B cereus food poisoning

has not been clearly identified. The emesis may be induced by breakdown products

resulting from the action of one or more B cereus enzymes on the food.

FIGURE 15-5 Necrosis of rabbit ileal mucosa 4 hours after introduction of a

toxigenic cell-free culture filtrate of B cereus. (A) Gross appearance of the

luminal surface of the ileum compared with a section of control ileum. (B)

Histologic appearance of a cross-section of the toxin-exposed ileum. (From

Turnbull PCB: Studies on the production of enterotoxins by Bacillus cereus. J

Clin Pathol 29:941, 1976, with permission.)

Host Defenses

Anthrax has been documented in a wide variety of warm-blooded animals. Some

species, such as rats, chickens, and dogs, are quite resistant to the disease,

whereas others (notably herbivores such as cattle, sheep, and horses) are very

susceptible. Humans have intermediate susceptibility. The specific mechanisms of

resistance in the more resistant species are not known.

Protective immunity against anthrax requires antibodies against components of

anthrax toxin, primarily protective antigen. Both the noncellular human vaccines

and live-spore animal vaccines confer protection by eliciting antibodies to

protective antigen. The poly-g-D-glutamic acid capsule of B anthracis is poorly

immunogenic, and antibodies to the polysaccharide and other components of the

cell wall are not protective.

Nothing is known about immune responses to food poisoning or other types of

infections with Bacillus species other than B anthracis. These types of

infection are rare, and effective vaccines against them have not been developed.

Epidemiology

The ultimate reservoir of B anthracis is contaminated soil, in which spores

remain viable for long periods. Herbivores, the primary hosts, become infected

when foraging in a contaminated region. Because the organism does not depend on

an animal reservoir, it cannot readily be eradicated from a region, and anthrax

remains endemic in many countries. Humans become infected almost exclusively

through contact with infected animals or animal products. Human anthrax is

traditionally classified as either nonindustrial or industrial anthrax,

depending on whether the disease is acquired directly from animals or indirectly

during handling of contaminated animal products. Nonindustrial anthrax usually

affects people who work with animals or animal carcasses, such as farmers,

veterinarians, knackers, and butchers, and is almost always cutaneous.

Industrial anthrax, acquired from handling contaminated hair, hides, wool, bone

meal, or other animal products, has a higher chance of being pulmonary as a

result of the inhalation of spore-laden dust.

The development of an effective animal vaccine in the 1930s, together with

improved factory hygiene, introduction of procedures for sterilizing imported

animal products, replacement of animal products with man-made alternatives, and

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the availability since the mid-1960s of a human vaccine, has resulted in a

greatly reduced incidence of the disease in North America. Human anthrax is now

very rare in the United States. However, major epidemics still break out in

endemic countries, normally following an outbreak in livestock. Nonendemic

countries must remain alert for episodes of anthrax arising from imported animal

products.

Diagnosis

The clinical diagnosis of anthrax is confirmed by directly visualizing or

culturing the anthrax bacilli. Fresh smears of vesicular fluid, fluid from under

the eschar, blood, lymph node or spleen aspirates, or (in meningitic cases)

cerebrospinal fluid are stained with polychrome methylene blue (M'Fadyean's

stain) and examined for the characteristic square-ended, blue-black bacilli

surrounded by a pink capsule (Fig. 15-6). (It should be remembered that B

anthracis organisms are not invariably detected in stained blood smears of

humans dying of anthrax.) Alternatively, the bacilli may be cultured from these

specimens and checked for sensitivity to the anthrax gamma phage, for penicillin

sensitivity, and for capsule formation. Colonies grown overnight at 37°C on

blood agar are gray or white, nonhemolytic, with a dry, ground-glass appearance;

they are at least 3 mm in diameter and sometimes have tails (Fig. 15-7).

Capsules can be seen in polychrome methylene blue-stained smears of cultures

grown on nutrient agar containing 0.7 percent sodium bicarbonate and incubated

overnight under CO2 (e.g., in a candle jar); encapsulated colonies are mucoid.

Alternatively, 2 ml of blood (such as commercial defibrinated horse blood)

inoculated with a pinhead quantity of material from a suspected colony and

incubated at 37°C yields readily demonstrable encapsulated bacilli in 6 hours.

Culturing may be unsuccessful if the patient has been treated with antibiotics.

FIGURE 15-6 Blood smears from a guinea pig that died of anthrax, stained with

M'Fadyean stain (polychrome methylene blue). The capsule (C) is pink around the

dark-blue bacilli. Although not obvious from this photograph, anthrax bacilli

frequently have square ends.

FIGURE 15-7 Colonies of B anthracis on a blood agar plate. Note the

characteristic tackiness of colonies that allow them to be teased upright with a

loop (foreground) and the characteristic tailing seen in the background

(arrows). (Photograph kindly supplied by R.W. Charlton. From Turnbull PCB,

Kramer JM, Melling J: Bacillus. p. 187. In Parker MT, Duerden BI (eds):

Systematic Bacteriology. Topley and Wilson's Principles of Bacteriology,

Virology and Immunity. Vol. 2. Edward Arnold, Sevenoaks, England, 1990, with

permission.)

Isolation of B anthracis from old specimens or from animal or environmental

material being examined for public health purposes is more difficult,

particularly if, as is often the case, B cereus or other Bacillus species are

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present in substantial numbers. The specimen should be examined both unheated

and heated to 60°C to 65°C for 15 min with subculture to both blood or nutrient

agar and specialized selective agars. Very rarely it may be necessary to use

mouse or guinea pig inoculation to isolate B anthracis. Up to about 0.2 ml of

the specimen (or an aqueous extract of the specimen) is injected subcutaneously

into a mouse, or intramuscularly or subcutaneously in a guinea pig (more

sensitive than a mouse); the encapsulated bacilli can be seen in a smear of

blood aspirated from the heart of the animal at death, and the bacteria are

readily observed in and isolated from this blood. If soil samples are being

used, the animals should be injected 24 hours earlier with tetanus and gas

gangrene antitoxin.

When a specimen from an individual not suspected clinically of having anthrax

yields substantial numbers of Gram-positive bacilli, the specimen should be

cultured and tested as shown in Figure 15-8 to determine the Bacillus species

present. The most common Bacillus species may be identified by the

characteristics in Table 15-1. Incrimination of a Bacillus species as the cause

of an infection is usually based on its presence in large numbers at the

infection site, especially in the absence of other known pathogens. Since

Bacillus species are common environmental organisms, their presence in small

numbers is not generally considered significant. For this reason, the use of

selective or enrichment systems for isolating clinically relevant, nonanthrax

Bacillus species is confined to just a few situations, such as the retrospective

examination of feces several days after a food poisoning incident (by which time

the offending Bacillus organism may be present in only small numbers).

FIGURE 15-8 Flow chart for identification of principal Bacillus species.

Control

To comprehend the strategies used to control anthrax, it is important to

understand the cycle of infection in susceptible animals. As a susceptible

animal with anthrax approaches death, its blood contains as many as 109

bacilli/ml (depending on the species). Necrosis of the walls of small blood

vessels during the acute phase of the illness leads to hemorrhages and to

characteristic bloody exudations from the mouth, nose, and anusa highly

diagnostic sign. These exudates carry vast numbers of the bacilli, which

sporulate on exposure to air and produce a heavily contaminated environmental

site that is potentially capable of infecting other animals for many years.

Because sporulation of B anthracis requires oxygen and therefore does not occur

inside a closed carcass, regulations in most countries forbid postmortem

examination of animals when anthrax is suspected. The vegetative cells in the

carcass are killed in a few days by the process of putrefaction. Nevertheless,

in the case of livestock, legislation invariably requires that the carcass be

burned or buried in quicklime (calcium oxide). However, it is becoming

increasingly apparent in the new era of sensitivity about environmental

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contamination that implementation in the past (sometimes many decades) of the

order to bury in quicklime has left us a legacy of burial sites which are

contaminated with viable anthrax spores and it is to be hoped that this

instruction will be removed from veterinary public health orders.

Livestock in endemic areas are effectively protected by yearly inoculations with

a vaccine made from spores of a live attenuated strain (see above). Noncellular

vaccines for human use are available for individuals in high-risk occupations.

They appear to have contributed to the decline in incidence of industrial

anthrax since they became available in the 1960s, but animal studies suggest

that there are limitations to their ability to protect against anthrax. The

human vaccine available in the United States is an aluminum hydroxide-adsorbed

cell-free filtrate of a B anthracis culture grown to maximize the yield of

protective antigen and minimize the quantities of lethal factor, edema factor,

and other unwanted metabolites.

Bacillus anthracis is susceptible to penicillin and to almost all other

broad-spectrum antibiotics. Because it is easily recognized, cutaneous anthrax

is almost always treated early and cured. Gastrointestinal and pulmonary anthrax

infections are difficult to identify before the fulminant phase and therefore

carry a high mortality. In uncomplicated anthrax cases, adequate treatment

consists of 500 mg of penicillin V taken orally every 6 hours for 5 days, or 600

mg (1 million units) of procaine penicillin administered intramuscularly every

12 to 24 hours for 5 days. In severe cases, 1,200 mg (2 million units) of

penicillin G should be administered intravenously every 6 hours, reverting to

the intramuscular regime of 600 mg every 12 to 24 hours once recovery starts. If

pulmonary anthrax is suspected, continuous-drip administration is advisable.

Tetracyclines (tests in animals indicate doxycycline is good), chloramphenicol,

gentamicin, or erythromycin may be used if the patient has penicillin

hypersensitivity. The fluoroquinolone, ciprofloxacin, has also been shown to be

effective in monkeys and guinea pigs and would be expected to be effective in

treatment of cases of human anthrax.

Avoidance of other types of Bacillus infections is largely a matter of observing

proper hygiene. Bacillus cereus and its close relatives B thuringiensis and B

mycoides produce potent ß-lactamases and thus are not responsive to penicillin,

ampicillin, or the cephalosporins. They are mostly resistant to trimethoprim as

well. These species are generally sensitive to standard empirical treatment with

an aminoglycoside combined with vancomycin and to chloramphenicol, erythromycin,

tetracycline, clindamycin, and sulfonamides.

Bacillus food poisoning, like all types of food poisoning, can largely be

prevented by proper food handling. Food should be cooked adequately; cooked food

should not be recontaminated from uncooked food (separate utensils and cutting

surfaces should be used for cooked and uncooked food); and, of particular

importance, cooked food should be stored under proper refrigeration.

References

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Claus D, Berkeley RCW: Genus Bacillus Cohn 1872, 174AL. p. 1105. In Sneath PHA,

Mair NS, Sharpe ME, Holt JG (eds): Bergey's Manual of Systematic Bacteriology.

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Agriculture Agricultural Handbook no. 427. U.S. Department of Agriculture,

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Klimpel KR, Arora N, Leppla SH: Anthrax toxin lethal factor contains a zinc

metalloprotease consensus sequence which is required for lethal toxin activity.

Molecular Microbiol 13:1093, 1994

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anthracis lethal factor. FEMS Microbiol Lett 124:343, 1994

Kramer JM, Gilbert RJ: Bacillus cereus and other Bacillus species. p.21. In

Doyle MP (ed): Foodborne Bacterial Pathogens. Marcel Dekker, New York, 1989

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Sourcebook of Bacterial Proteins. Academic Press, New York

Norris JR, Berkeley RCW, Logan NA et al: The genera Bacillus and

Sporolactobacillus. p. 1711. In Starr MP, Stolp H, Truper HG (eds): The

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Clin Pathol 29:941, 1976

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Clinical Microbiology, 6th ed. American Society for Microbiology, Washington, DC

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(eds): Systematic Bacteriology. Topley and Wilson's Principles of Bacteriology,

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