Chapter Capsule

A. Spirochetes Spirochetes are helical, flexible bacteria that move by periplasmic flagella. Several Treponema species are obligate parasitic spirochetes with 8 to 12 regular spirals; best observed under dark-field microscope; cause treponematoses.

1. Direct observation of treponeme in tissues and blood tests important in diagnosis.

2. Cannot be cultivated in artificial media; treatment by large doses of penicillin or tetracycline.

B. Major Treponematoses

1.Syphilis: Treponema pallidum causes complex progressive disease in adults and children.

a. Sexually transmitted syphilis is acquired through close contact with a lesion; untreated sexual disease occurs in stages over long periods.

(1) At site of entrance, multiplying treponemes produce a primary lesion, a hard ulcer or chancre, which disappears as the microbe becomes systemic.

(2) Secondary syphilis occurs when spirochete infects many organs; marked by skin rash, fever, damage to mucous membranes. Both primary and secondary syphilis are communicable. Latent period establishes pathogen in tissues.

(3) Final, non-communicable tertiary stage is marked by tumors called gummas and life-threatening cardiovascular and neurological effects.

b. Congenital syphilis is acquired transplacentally; disrupts embryonic and fetal development; survivors may have respiratory, skin, bone, teeth, eye, and joint abnormalities if not treated.

2.Nonsyphilitic treponematosis: Slow progressive cutaneous and bone diseases endemic to specific regions of tropics and subtropics; usually transmitted under unhygienic conditions.

a. Bejel is a deforming childhood infection of the mouth, nasal cavity, body, and hands.

b. Yaws occurs from invasion of skin cut, causing a primary ulcer that seeds a second crop of lesions.

c. Pinta is superficial skin lesion that depigments and scars the skin.

C.Leptospira interrogans

L. interrogans has very regular coils and a prominent hook; causes leptospirosis, a worldwide zoonosis acquired through contact with urine of wild and domestic animal reservoirs. Spirochete enters cut, multiplies in blood and spinal fluid.

1. Causes muscle aches, headache.

2. Second phase is marked by Weil’s syndrome, which involves kidneys and liver.

3. Requires early treatment with penicillin or tetracycline.

D.BorreliaBorrelia species are loose, irregular spirochetes that cause borreliosis; infections are vector-borne (mostly by ticks). Borrelias cause recurrent fever and other symptoms because the spirochete repeatedly changes antigenically and forces the immune system to keep adapting.

1.B. hermsii is zoonotic (in wild rodents) and carried by soft ticks; B. recurrentis has a strictly human reservoir and is carried by lice; both species cause relapsing fever.

2.Borrelia burgdorferi: A zoonosis carried by mice and spread by a hard tick (Ixodes) that lives on deer and mice; causes Lyme disease, a syndrome that occurs endemically in several regions of the United States. Tick bite leads to fever and a prominent ring-shaped rash; if allowed to progress, may cause cardiac, neurological, and arthritic symptoms; can be controlled by antibiotics and by avoiding tick contact.

E. Curved Bacteria (Vibrios) Vibrios are short spirals or sausage-shaped cells with polar flagella.

1.Vibrio cholerae: Causes epidemic cholera, a human disease that originated in Asia but is now distributed worldwide in natural waters. Organism is ingested in contaminated food and water; microbe infects the surface of epithelial cells in small intestine, is not invasive; severity of cholera due to potent cholera toxin that causes electrolyte and water loss through secretory diarrhea; resulting dehydration leads to muscle, circulatory, and neurological symptoms; treatment with oral rehydration (electrolyte and fluid replacement) completely restores stability; vaccine available.

2.Vibrio parahaemolyticus: Causes food infection associated with seawater and seafood; symptoms similar to mild cholera; for prevention, food must be well cooked and refrigerated during storage.

3.Campylobacter species: C. jejuni is a common cause of severe gastroenteritis worldwide; acquired through food or water contaminated by animal feces; enteritis is due to enterotoxin; C. fetus causes diseases in pregnant women and fatal septicemia in neonates; Helicobacter pylori may be etiologically involved in diseases of the stomach lining such as gastritis and ulcers.

F. Obligate Intracellular Parasitic Bacteria

1. Rickettsias and chlamydias are tiny, gram-negative rods or cocci that are metabolic and intracellular parasites; diseases treatable with tetracycline and chloramphenicol. Rickettsia causes rickettsioses; most are zoonoses spread by arthropod vectors.

a.Epidemic typhus: Caused by Rickettsia prowazekii; carried by lice; associated with human overcrowding; disease starts with high fever, chills, headache; rash occurs; Brill-Zinsser disease is a chronic, recurrent form.

b.Endemic (murine) typhus: Zoonosis caused by R. typhi, harbored by mice and rats; occurs sporadically in areas of high flea infestation; symptoms like epidemic form, but milder.

c.Rocky Mountain spotted fever: Etiologic agent is R. rickettsii; zoonosis carried by dog and wood ticks (Dermacentor); most cases on eastern seaboard; infection causes distinct spotted, migratory rash; acute reactions include heart damage, CNS damage; disease prevented by drug therapy and avoidance of ticks.

d. The genus Ehrlichia contains two species of rickettsias that have recently been discovered in humans. These tick-borne bacteria cause human monocytic and granulocytic ehrlichiosis.

2. Characteristics of ectoparasitic, blood-sucking arthropod vectors: Transmitters of parasites between various vertebrate reservoirs; vectors may pass pathogen to offspring transovarially or to hosts through bite, feces, or mechanical injury and scratching.

a. Ticks are arachnids; hard ticks have a hard shield and soft ticks lack it. They cling to host, feed, and inoculate the host with saliva containing the pathogen; carry Rocky Mountain spotted fever, Q fever, borrelioses, viral fevers.

b. Fleas are flattened insects with jumping legs and a blood-probing mouth; extremely resistant and nonspecific to host; carry murine typhus, bubonic plague.

c. Lice are insects that cling to body hair and gently pierce skin; infection occurs when louse is crushed by scratching and rubbed into skin; carry epidemic typhus, trench fever, and relapsing fever.

G. Diseases Related to the Rickettsioses

1. Q fever: Caused by Coxiella burnetii; agent has a resistant spore form that can survive out of host; a zoonosis of domestic animals; transmitted by air, dust, unpasteurized milk, ticks; usually inhaled, causing pneumonitis, fever, hepatitis.

2. Bartonella: A closely related genus; is the cause of trench fever, spread by lice, and cat-scratch disease, a lymphatic infection associated with a clawing injury by cats.

H. Chlamydia Organisms in the genus Chlamydia pass through a transmission phase involving a hardy elementary body and an intracellular reticulate body that has pathologic effects.

1.Chlamydia trachomatis: A strict human pathogen that causes eye diseases and STDs. Ocular trachoma is a severe infection that deforms the eyelid and cornea and may cause blindness. Conjunctivitis occurs in babies following contact with birth canal; prevented by ocular prophylaxis after birth.

2.C. trachomatis causes very common bacterial STDs: nongonococcal urethritis in males, and cervicitis, salpingitis (PID), infertility, and scarring in females; also lymphogranuloma venereum, a disfiguring disease of the external genitalia and pelvic lymphatics.

3.C. pneumoniae causes an atypical pneumonia that is a serious complication in asthma patients.

4.Chlamydia psittaci: Causes ornithosis, a zoonosis transmitted to humans from bird vectors; highly communicable among all birds; pneumonia or flulike infection with fever, lung congestion.

I. Mollicutes/Mycoplasmas Mycoplasmas naturally lack cell walls and are thus highly pleomorphic; not obligate parasites but require special lipids; fuse tightly to host membranes during infection; diseases treated with tetracycline, erythromycin.

1.Mycoplasma pneumoniae: Causes primary atypical pneumonia; pathogen slowly spreads over interior respiratory surfaces, causing fever, chest pain, and sore throat; M. hominis and Ureaplasma urealyticum are normal colonists of most persons; may cause urethritis, PID, and other reproductive tract diseases.

2.L forms: Bacteria that normally have cell walls but have transiently lost them through drug therapy. They may be involved in certain chronic diseases.

A. Hard-Tissue Disease Dental caries is a slow, progressive infection of irregular areas of enamel surface; begins with colonization of tooth by slime-forming species of Streptococcus and cross adherence with Actinomyces; process forms layer of thick, adherent material called plaque; this complex layer harbors dense masses of bacteria and extracellular substances.

1. Acid formed by agents in plaque dissolves the inorganic salts in enamel.

2. If unabated, this produces a caries lesion, which may invade dentin and root canal and destroy tooth.

B. Soft-Tissue Disease Periodontal disease involves periodontium (gingiva and surrounding tissues); starts when plaque forms on root of tooth and is mineralized to a hard concretion called calculus; this irritates tender gingiva; inflammatory reaction and swelling create gingivitis and pockets between tooth and gingiva invaded by bacteria (spirochetes and gram-negative bacilli); tooth socket may be involved (periodontitis), and the tooth may be lost.