The Spirochetes

 

• Gram-negative, but Gram stain not useful; the width of many spirochetes is at or just below the resolving power of the light microscope. They are observed by dark-field microscopy or by staining with special reagents.

• long, slender, flexible spiral-shaped organisms; most appear as helical coils.

• The outer envelope is a membrane similar in structure to the outer membrane of gram-negative bacteria and the larger organisms stain as gram-negative. However, no endotoxin has been demonstrated.

• motile; move in corkscrew fashion; flagella, which lie between the outer membrane and the peptidoglycan layer, are referred to as periplasmic flagella or axial fibrils. They are attached subterminally at each pole of the cell and extend along the body of the organism. Allow bacteria to move in corkscrew fashion; especially suited to movement in viscous environments.

• some are free-living; others are obligate parasites

• many have not yet been cultivated in vitro.

Three genera are pathogenic for man:

Treponema (syphilis)

Borrelia (relapsing fever (omitted) and Lyme disease)

Leptospira (leptospirosis) - Omitted from lecture notes

Each has a distinct morphology and method of movement.

 

Syphilis - Treponema pallidum

Nonpathogenic treponemes can be found in the oral cavity, GI tract, and urogenital tract of animals - cultivable.

T. pallidum, the etiologic agent of syphilis, is morphologically indistinguishable from the nonpathogenic treponemes - not cultivated in vitro

 

outer membrane -> phospholipid bilayer with few OMPs

doubling time 30-33 h in vivo

rapid death outside the host

Epidemiology: Syphilis is endemic in all parts of the world. Exclusive to humans; no other known natural hosts.

STD’s in US: chlamydia > gonorrhea > papillomavirus > herpes > hepatitis B > syphilis > HIV

Transmission - sexual contact; blood transfusions, congenitally.

<10 organisms produce infection (35 - 50% of those exposed become infected).

The organism penetrates mucous membranes but only penetrates the skin through small breaks. Limited survival outside of the host.

Clinical: Syphilis is a slowly evolving chronic disease that fluctuates between short symptomatic stages and rather prolonged asymptomatic stages.

Following exposure - 10-90 day incubation period

Multiplication at the site of entrance results in the formation of a characteristic primary lesion known as a chancre (penis of male and cervix of female). The chancre begins as a papule and breaks down to form a superficial ulcer with a firm base - a hard chancre (contrast with soft chancre caused by Haemophilus ducreyi).

Chancre - no exudate, inflammation or tenderness; painless and heals spontaneously within several weeks even w/o treatment ("recovery")

However, organisms that escape invade the regional lymph nodes resulting in their enlargement. The organisms always disseminates - reaches the bloodstream and establishes a systemic infection.

Secondary syphilis - 2 to 10 weeks later.

Result of systemic dispersal of the organism

flu-like symptoms, low-grade fever, and enlargement of the lymph nodes

Followed by a generalized skin rash that may also involve the mucous membranes. Treponemes can be found in the skin rash. Lesions may develop in any of the organs. Sores contain treponemes and are infectious.

Secondary syphilis resolves spontaneously within weeks to months. The mechanisms of host defense are unknown but probably include cellular immune mechanisms, as well as immobilizing and opsonizing antibodies.

 

Tertiary syphilis occurs in 25-40% of untreated patients. This occurs 3-30 years after the primary infection, and it is a noninfectious stage of the disease. Pronounced immunological response of the host.

Tertiary lesions (gummas) contain few organisms but may result in necrosis, scar formation, and extensive tissue damage. Gummas are granulomatous lesions of the skin, subcutaneous tissue, mucous membranes, bone, or viscera. Serious manifestations usually result from lesions in the CNS, in the cardiovascular system, or in the eyes.

 

 

Congenital syphilis - T. pallidum, unlike most microbes, readily passes the placenta, so a syphilitic mother can transmit the disease during pregnancy to her unborn child.

Acute infection, characterized by extensive invasion of nearly all body organs. 50% of those infected are aborted or stillborn. Survivors may have birth defects, latent infection (40%), or death (untreated 25%). 1000 cases/year in U.S.

 

Serologic tests for syphilis – Cannot cultivate T. pallidum, so these are important in diagnosis because clinical manifestations may be confusing or exudative material is not present.

Treatment: Pen used for treatment of all stages of disease. Alternatives: Te, Em, Cm.

Virulence: (no toxins)

a. Treponemes readily attach to endothelial cells and pass through blood vessel walls; facilitates dissemination; can also pass through placenta. Nonpathogenic treponemes do not attach.

Treponemal adhesins (OMPs) - located on the pointed ends of the cell; recognize and bind to fibronectin, a glycoprotein present on eukaryotic cell surfaces. T. pallidum also attaches to laminin and collagen, integral components of the basement membrane.

b. Some of the histopathology associated with syphilitic lesions results from activation of host defenses. Antibodies, in association with complement or macrophages and T lymphocytes that interact with organisms, may indirectly damage the host tissues.

 

c. T. pallidum elaborates a hyaluronidase that degrades the hyaluronic acid within the ground substance to facilitate the spread of the organisms - hematogenous dissemination.

d. The organisms coat themselves with host serum proteins, especially fibronectin. May initially protect them from immune system. This coat may interfere with treponemal killing by the classic (antibody-dependent) complement pathway. In addition. T. pallidum carries surface-associated sialic acid. This component retards the activation and killing by the alternative (non-antibody-dependent) complement pathway.

 

Immunity - poorly understood. How does the bacterium establish persistent (decades) infection despite a vigorous host response?

Immunity does develop in untreated syphilis. In human experiments around the 1900's, 75% of 1000 syphilis patients did not progress beyond primary syphilis. Both humoral and cellular components are necessary for acquired resistance to treponemal infection. Passive transfer of immune serum does not protect experimental syphilitic rabbits against challenge inoculation.

 

Prophylaxis - best achieved the use of condoms and by avoiding sexual contact with infectious persons. No vaccine yet despite intense research in this area.

Identify sexual contacts of patient (that occurred during the incubation period). Treat case contacts.

 

 

 

Borrelia spp.

 

 

Borrelia burgdorferi

Lyme disease is named after a unique cluster of cases that occurred in 1975 in children in Lyme, Connecticut. In 1982 - etiologic agent discovered by Willy Burgdorfer. He isolated the spirochetes from the midgut of ticks; organisms reacted with sera from patients with Lyme disease.

Lyme disease is now the leading arthropod-borne disease in the U.S. and Europe.

1982 - 497 cases

1995 - >13,083 cases in 47 states

Transmission: In New England the deer tick is usually responsible for disease transmission. Need prolonged tick feeding: 12-24h. Only ~30% of patients recall the tick bite. No person to person transmission.

Humans are usually infected during the spring and summer seasons; the disease may also affect dogs (arthritis) and other domestic animals. The spirochete has been isolated from ticks and from the white-footed mouse and white-tailed deer.

Clinical: A history of tick bites, outdoor activities, or pets carrying ticks is associated with most cases of Lyme disease. The illness may be brief and inconsequential or chronic and disabling.

 

3 disease stages:

1. Localized lesion at the site of the tick bite: a red macular or papular lesion (erythema migrans) develops within 3 to 30 days in ~70% of patients. It expands into a distinctive large annular lesion with partial central clearing. The lesion resolves in about 3 weeks. Other symptoms include fever, headache, stiff neck, myalgia, fatigue, joint pain, and malaise.

2. Dissemination by hematogenous spread. 5-15% of untreated patients have flu-like symptoms, arthralgia, neurologic or cardiac involvement within a few months.

3. Chronic infection. May occur months to years later; affects skin, joints and CNS. Likely reflects ability of bacteria to establish themselves in a protected niche, despite host immune response.

 

Diagnosis: clinical presentation (many variations) and history of travel in endemic areas. High or rising ab titers against B. burgdorferi are also helpful (major surface antigens OspA and OspB are not entirely stable). Skin biopsies or blood cultures sometimes reveal the organism, but there appear to be few bacteria in host. Difficult, but not impossible, to cultivate.

Antigenic variation may also occur in Lyme borreliosis. OspA is a surface-associated proteins that has been shown to be protective in experimental animal vaccine trials. ospA and ospB genes are on a 49 kb linear plasmid molecule.

Serology helpful, but negative for 2 mo. Must do ELISA and confirm results with a Western blot.

 

Treatment: Doxycycline (semisynthetic Te) - effective in the early stage of the disease. The chronic stage requires more intensive treatment.

Prevention: Control of insect vectors; prevent against tick bites; insect repellents with DEET.

Vaccine: OspA (surface associated molecule with unknown function)

December 1998: FDA approves Lyme disease vaccine

The FDA approved the first vaccine against Lyme disease on Monday. The vaccine, SmithKline Beecham's LYMErix, contains a genetically engineered protein from the surface of Borrelia burgdorferi bacteria. This protein, called OspA, stimulates the immune system to produce antibodies "that appear to disable the bacteriums' ability to infect the individual."

The vaccine is approved for use in people between the ages of 15 and 70, and is given in three doses over a one-year period.

FDA Commissioner Dr. Jane E. Henney said that while the vaccine may be a good option for people who spend a lot of time outdoors where the ticks that spread Lyme disease are common, she also warned that the vaccine "is not 100% effective, so other precautions should be taken. These precautions include the traditional methods of protecting against the infection, such

as wearing protective clothing, using tick repellent, and removing attached ticks.

Side effects of the vaccine include redness, soreness and swelling at the injection site, and sometimes mild to moderate flu-like symptoms.

Unlike typical vaccines that work once an infection is inside the body, the Lyme vaccine essentially blocks the bacteria's transmission at skin level.