Listeria Monocytogenes

This microorganism is a potential pathogen for both humans and animals. Most human cases occur in patients with debilitating disease or in prenatal or neonatal infants. Sepsis, meningitis, and disseminated abscesses occur in infected patients. Meat, vegetables, and various milk products are the most common sources of infection.

Erysipelothrix Rhusiopathiae

This species is transmitted occasionally from infected pigs to farmers or veterinarians, in whom it causes primarily inflammatory infections of the skin. Septicemia and endocarditis may develop secondarily.

Propionibacterium Acnes

A common inhabitant of the crypts of the skin, this species may contribute to acne.

Streptobacillus Moniliformis

This species may infect individuals through the bite of infected rodents. After local ulcerative inflammation, life-threatening septicemia may develop.

Calymmatobacterium Granulomatis

This microorganism causes granuloma inguinale, a venereal disease with local tissue destruction in the genital, inguinal, and perianal region.

Clinical Manifestations

Listeriosis is a serious disease for humans, with a mortality greater than 25 percent. There are two main clinical manifestations, sepsis and meningitis (Fig. 16-1). Meningitis is often complicated by encephalitis, which is exceptional among bacterial infections. Occasionally, pyogenic infections of various organs have been found. Relapses may occur after apparent recovery.

Figure 16-1

Pathogenesis of listeriosis.

Structure, Classification, and Antigenic Types

All Listeria species are small, Gram-positive rods, which are sometimes arranged in short chains. In direct smears they may be coccoid, so they can be mistaken for streptococci. Longer cells can be suggestive of corynebacteria. Flagella are produced at room temperature rather than at 37° C. Hemolysin production is an important marker for L monocytogenes, although it is not definitive, as L ivanovii and L seeligeri are likewise hemolytic on blood agar. Further biochemical characterization is necessary to distinguish between the different Listeria species.

It may be desirable for epidemiologic purposes to identify a particular strain by serotyping to characterize surface antigens, such as O antigens (teichoic acids) and H antigens (proteins). The serovars 1/2a and 4b are responsible for up to 90 percent of all cases of listeriosis.

A particular property of L monocytogenes is the ability to multiply at low temperatures (Fig. 16-2). Bacteria therefore can accumulate in contaminated food stored in the refrigerator.

Figure 16-2

Multiplication of L monocytogenes in broth at low temperature.

Pathogenesis

Listeria monocytogenes is presumably ingested with raw, contaminated food (Fig. 16-2). An invasion factor secreted by the pathogenic bacteria enables them to penetrate host cells of the epithelial lining. Since this microorganism is widely distributed, this event may occur rather often. Normally, the immune system eliminates the infection before it spreads. Indeed, most adults who have no history of listeriosis have T lymphocytes primed specifically by Listeria antigens. If the immune system is compromised, however, systemic disease may develop. Listeria monocytogenes multiplies not only extracellularly but also intracellularly within macrophages after phagocytosis and even within parenchymal cells which are entered by induced phagocytosis. It therefore belongs to the large group of facultatively intracellular pathogens (Table 16-1).

Table 16-1

List of Some Facultative or Obligate Intracellular Microorganisms.

Survival within the phagosomes and eventual escape into the cytoplasm are mediated by a toxin, which also acts as a hemolysin. This toxin is one of the so-called SH-activated hemolysins, which are produced by a number of different bacteria such as serogroup A streptococci, pneumococci, and Clostridium perfringens. Obviously, nature has preserved the genetic code for this bacterial product in several species, and consequently the hemolysins from these different bacteria have common biochemical, biologic, and antigenic properties. Nonhemolytic variants of L monocytogenes are completely avirulent, as are the nonhemolytic species L innocua and L welshimeri. Hemolysin is not the only Listeria virulence factor, however, since the hemolytic Listeria species besides L monocytogenes (i.e., L seeligeri and L ivanovii) possess rather limited pathogenicity. The hemolysin gene is located on the chromosome within a cluster of other virulence genes which are all regulated by a common promotor. These additional genetic determinants are necessary for further steps in the intracellular life cycle of L monocytogenes. One particular gene product promotes the polymerization of actin, a component of the host cell cytoskeleton, on the bacterial surface. In this peculiar environment within host cells, surrounded by a sheet of actin filaments, the bacteria reside and even multiply. The growing actin sheet functions as a propulsive force which drives the bacterium across the intracellular pathways until it finally reaches the surface. Then, the host cell is urged to form slim, long protrusions containing living L. monocytogenes. Those cellular projections are engulfed by adjacent cells, even by non-professional phagocytes such as parenchymal cells. By such a mechanism a direct cell-to-cell spread in an infected organ may occur without an extracellular stage.

Host Defenses

Because it multiplies intracellularly, L monocytogenes is largely protected against humoral immune factors such as antibodies, and the effective host response is cell- mediated, involving both lymphokines (especially interferon) produced by CD4⁺ (T-helper) cells and direct lysis of infected cells by CD8⁺ (cytotoxic) T lymphocytes. Both of these fundamental defense mechanisms are expressed in the microenvironment of the infective foci. Histologically, these foci are organized as granulomas, characterized by a central accumulation of epitheliod cells (macrophages) with irregularly shaped nuclei and large, delicately structured cytoplasm and by peripheral lymphocytes recognizable by a round nucleus and a narrow border of intensely staining cytoplasm (Fig. 16-3).

Figure 16-3

Infective focus in the liver of mice 7 days after infection with L monocytogenes. Note the granulomatous reaction characterized by central accumulation of epitheloid cells (macrophages) and the presence of some dark, round cells (lymphocytes) in the periphery. (more...)

Epidemiology

Listeria species are found in living and nonliving matter. Various foodstuffs of vegetable and animal origin are sources of infection. Animal and human carriers also have been described. Most human cases of listeriosis develop in immunocompromised hosts: newborns, old people, cancer patients, and transplant recipients. Reports of sporadic cases of listeriosis are becoming more frequent as the number of persons at risk, especially because of immunosuppression by medical therapy, increases. Outbreaks of listeriosis are due mainly to a common source of contaminated food.

Listeriosis also may be transmitted congenitally across the placenta. The immunocompetent mother suffers at worst a brief, flu-like febrile illness, but the fetus, whose defense system is still immature, becomes seriously ill. Depending on the stage of gestation, the fetus is either stillborn or born with signs of congenital infection. Typically, multiple pyogenic foci are found in several organs (granulomatosis infantiseptica). The onset of listeriosis is delayed (i.e., a few days after birth) when infection is acquired during labor by bacteria colonizing the genital tract of the mother.

Diagnosis

Listeria monocytogenes is implicated when monocytosis is observed in the peripheral blood as well as the cerebrospinal fluid. Early diagnosis may be obtained by finding pleocytosis with Gram-positive rods in a Gram stain of smears of the cerebrospinal fluid. Final proof is obtained by culture. Serologic tests are highly unreliable.

Control

Hygienic food processing and storage may reduce the risk of listeriosis. Individuals in high-risk groups (i.e., immuno-compromised individuals and pregnant women) should avoid uncooked food or should at least marinate salads for a long time in a vinegar-based dressing to kill adherent bacteria.

Since a cell-mediated immune response (the most potent weapon against L monocytogenes) is induced only by injection of living antigen, vaccination is difficult. Even an attenuated living vaccine is dangerous for persons with impaired defenses, the proper target group. Completely avirulent live bacteria do not trigger an effective, cell-mediated immune response.

Antimicrobial agents are the mainstay of treatment. Most of the common antibiotics, except cephalosporins, are active against L monocytogenes in vitro. In practice, ampicillin combined with an aminoglycoside has given the best results. However, because infection occurs mainly in infirm patients and because intracellular bacteria are hardly accessible to most drugs, the cure rate is low. Furthermore, Listeria cells, although inhibited, are not killed by ampicillin. High doses for prolonged periods are indicated.

Clinical Manifestations

The most common human infection by E rhusiopathiae is erysipeloid, a well-defined, violet or wine-colored inflammatory lesion of the skin of the fingers or hand (Fig. 16-4). Itching is typical. Infrequently, septicemia develops, followed by various organ manifestations such as endocarditis or arthritis without fever.

Figure 16-4

Disease manifestations of E rhusiopathiae, P acnes, C granulomatis, and S moniliformis.

Structure and Classification

Erysipelothrix rhusiopathiae is a slender, Gram-positive rod similar to L monocytogenes. In general, E rhusiopathiae rods are longer, especially in rough variants. They grow on routine culture media under aerobic conditions, but preferentially in a CO₂ atmosphere. In contrast to L monocytogenes, they are nonmotile, nonhemolytic, and catalase negative. The production of H₂S is highly indicative, since very few other Gram-positive bacteria have this property.

Pathogenesis

A minor skin injury may facilitate the penetration of E rhusiopathiae after contact with infected material. After an incubation of 1 to 4 days the local lesion develops; spontaneous recovery occurs in 2 to 3 weeks. Septicemia has been observed without previous local lesions so that an oral infection is assumed. Endocarditis may develop in a few cases.

Epidemiology

Erysipelothrix rhusiopathiae is found in mammals, poultry, and fish. Individuals who have occupational exposure to such animals (i.e., farmers, veterinarians, slaughterhouse workers, and fish handlers) are at risk.

Diagnosis

The typical, nonpyogenic lesions on occupationally exposed persons suggest erysipeloid. Since there is no wound, a swab is not useful. Bacteria can be cultured from a biopsy of the progressing, inflamed edge of the lesion. Blood culture is indicated in the setting of sepsis and endocarditis.

Control

Penicillin is the drug of choice to treat serious infections. Since local skin infection is self-limited, therapy is not essential.

Clinical Manifestations

The pathogenic role of Propionibacterium acnes is still disputed. Although it is often detected in anaerobic blood cultures, it normally colonizes the skin crypts and is transported to cultures by pure chance (Fig. 16-4). Nevertheless, in compromised patients even this nonpathogenic species may induce pathologic reactions, such as endocarditis. In skin lesions P acnes is often found with other pathogenic bacteria, such as Staphylococcus aureus fs or actinomycetes, and is thought to support the damaging effect of those pathogens. It is doubtful whether P acnes alone is able to induce acne.

Structure and Classification

The club-shaped, Gram-positive rods of P acnes resemble the diphtheroids but, unlike the latter, are slow-growing and anaerobic, so that their presence in blood cultures is detected after about a week.

Pathogenesis

Propionibacterium acnes produces several metabolic products, hemolysin, and various enzymes such as lipase and neuraminidase, which are excreted into the surroundings. These metabolites may clear the way for other bacteria. Furthermore, P acnes degrades sebaceous matter to produce fatty acids that stimulate an inflammatory reaction.

Epidemiology

Propionibacterium acnes normally colonizes the deep crypts of the skin, where the availability of oxygen is reduced. The same applies to mucous membranes of the oroanal areas. They may be transported to other sites by chance.

Control

Practically all common antibiotics, including penicillins, erythromycin, and tetracyclines, can be used to treat P acnes infections.

Clinical Manifestations

Streptobacillus moniliformis causes the clinical disease called rat bite fever. At the site of the rodent bite, an ulcer appears; this may heal spontaneously (Fig. 16-4). Occasionally, the infection spreads to the regional lymph nodes, and bacteremia has been observed. General malaise and fever may be present after a few days. This generalized disease may be fatal. Colonization of various parts of the body, such as joints or endocardium, may lead to chronic disease accompanied by local symptoms. Rat bite fever also is caused by Spirillum minus, a very different bacterium (see Ch. 35).

Structure and Classification

Streptobacillus moniliformis is a Gram-negative, nonmotile rod of variable length. The individual cells are not regularly shaped or stained, and thus pleomorphism is seen in smears. There is a tendency for spontaneous development of cell-wall-deficient L-forms (see Ch. 2). Consequently, growth on artificial media depends on certain additives, such as serum or ascitic fluid, which are not always present in common culture media. Growth is best under a CO₂ atmosphere.

Pathogenesis

Humans usually become infected with S moniliformis through the bite of an infected rat. Ingestion of contaminated food has rarely been incriminated as the source of infection.

Host Defenses

The nonspecific resistance mechanisms in the skin and draining lymph nodes prevent dissemination. The low pH in the stomach normally guarantees that S moniliformis cannot survive gastric passage.

Epidemiology

S moniliformis belongs to the common bacterial flora of the nasopharynx of rats, from which it reaches humans directly by a rat bite or indirectly via food.

Diagnosis

The coincidence of fever after a rat bite draws attention to this infection. Positive cultures can be obtained from blood or synovial fluid. Mice are highly susceptible to S moniliformis, exhibiting a rapid lethal infection after inoculation. Because of the existence of L-forms, special media must be used for culture, since on conventional agar plates L-form colonies hardly are visible and are likely to be overlooked.

Clinical Presentation

C granulomatis causes granuloma inguinale. This infection typically is localized in the genital region (Fig. 16-4). It spreads to adjacent areas, and the regional lymph nodes also may be inflamed. Persistent granulomatous lesions tend to ulcerate, destroying skin and subcutaneous tissue.

Structure and Classification

Calymmatobacterium granulomatis is a Gram-negative, nonmotile rod. The capsule that surrounds the bacterial cell appears similar to that of Klebsiella. Addition of egg yolk and incubation in a CO₂ atmosphere are required for growth on artificial media.

Pathogenesis

Calymmatobacterium granulomatis is normally present in the gut flora and may be transmitted to the genital area by autoinoculation or sexual contact. After penetrating the skin the bacteria induce an inflammatory reaction, which may lead to destruction of the infected tissue. Within the inflammatory foci C granulomatis is found mainly intracellularly (Table 16-1 ) inside tissue macrophages (Donovan bodies). This is highly typical for granuloma inguinale. Superinfection of ulcers with other pathogenic organisms is possible.

Host Defenses

Antibodies against C granulomatis are produced during acute infection; their role in defense remains unclear. Cell-mediated defense mechanisms, expressed by a granulomatous reaction, are important in recovery.

Epidemiology

Granuloma inguinale occurs most frequently in people living under poor socioeconomic conditions (e.g., in the tropics). In the United States, infection of blacks is seven times more frequent than infection of whites. Transmission by sexual contacts is most common. Other sexually transmitted diseases, such as syphilis, may be associated.

Diagnosis

Microscopic evidence of intracellular Gram-negative encapsulated rods in ulcerative skin wounds of the genitoinguinal region is highly indicative for granuloma inguinale. Since experience with C granulomatis is lacking in most laboratories, cultural diagnosis probably will fail. Eventually, the yolk sac of 5-day-old chicken embryos can be inoculated directly with the infected material.

Control

Infection can be prevented by cleanliness or by avoiding sexual contacts with infected persons. Antibiotics active against intracellular bacteria, such as tetracycline or erythromycin, are effective in treatment.