Miscellaneous Bacterial Agents of Disease: Study Notes (Chapter 21, Talaro's Foundations in Microbiology)

Miscellaneous Bacterial Pathogens

Overview

Miscellaneous bacterial pathogens are those that do not fit the typical categories of gram-positive or gram-negative rods or cocci. This group includes spirochetes, curviform bacteria, obligate intracellular parasites, and mycoplasmas. These organisms are responsible for a variety of human diseases, often with unique morphologies and pathogenic mechanisms.

System Profile: Major Miscellaneous Bacterial Pathogens

The following table summarizes the main pathogens, their associated diseases, and affected organ systems:

Organism	Skin/Skeletal	Nervous/Muscle	Cardiovascular/Lymphatic/Systemic	Gastrointestinal	Respiratory	Urogenital
Treponema pallidum	Skin rash (secondary syphilis)	Tertiary syphilis	Secondary/tertiary syphilis			Chancre (primary syphilis)
Leptospira			Leptospirosis			Leptospirosis
Borrelia burgdorferi	Lyme disease	Lyme disease	Lyme disease
Vibrio cholerae			Septicemia	Secretory diarrhea
Vibrio parahaemolyticus	Necrotizing wound infections		Septicemia	Gastroenteritis
Campylobacter jejuni				Gastroenteritis
Helicobacter pylori				Gastric, peptic, and duodenal ulcers

The Spirochetes

General Characteristics

Spirochetes are helical, gram-negative bacteria with endoflagella located in the periplasmic space. Their unique motility is due to flexing and rotation, allowing crawling movements. Most are free-living saprobes or commensals, but some are significant pathogens.

• Major pathogenic genera: Treponema, Leptospira, Borrelia

• Structure: Endoflagella, outer membrane, cell body

Genus Treponema

Thin, regular, coiled cells found in the oral cavity, intestinal tract, and perigenital regions. Pathogenic species are strict parasites with complex growth requirements and require live cells for cultivation.

Treponema pallidum and Syphilis

• Transmission: Sexual and transplacental

• Host: Humans are the natural host

• Survival: Extremely fastidious and sensitive; cannot survive long outside the host

Stages of Syphilis

Stage	Average Duration	Clinical Setting	Diagnosis	Treatment
Incubation	3 weeks	No lesion; treponemes adhere and penetrate epithelium, multiply, disseminate	Asymptomatic phase	Not applicable
Primary	2 to 6 weeks	Chancre at inoculation site; intense treponemal activity in body; chancre later disappears	Dark-field microscopy; VDRL, FTA-ABS	Benzathine penicillin G, $2 imes 10^6$ units; doxycycline
Primary latency		Healed chancre; little scarring; treponemes in blood; few, if any, symptoms	Serological tests (+)	As above
Secondary	2 to 6 weeks after chancre leaves	Skin, mucous membrane lesions; hair loss; fever, lymphadenopathy; symptoms can persist for months	Dark-field testing of lesions; serological tests (+)	As above
Latency	6 months to 8 or more years	Treponemes quiescent unless relapse occurs; lesions can reappear	Seropositive blood test	As above
Tertiary	Variable, up to 20 years	Neural, cardiovascular symptoms; gummas develop in organs; seropositivity	Treponeme may be demonstrated by DNA analysis of tissue	As above

Primary Syphilis

• Chancre formation at site of inoculation (mouth/genitals)

• Highly contagious fluid from chancre

• Chancre heals as spirochete enters bloodstream

Secondary Syphilis

• Spirochete multiplies in bloodstream

• Rash on skin, palms, soles; fever, headache, sore throat

• Rash is painless, non-itchy, can persist for months

• Rash disappears spontaneously

Latency and Tertiary Syphilis

• ~30% enter latent period (up to 20 years)

• Tertiary syphilis: damage to multiple tissues/organs

• Gummas: Painful, swollen tumors may develop

Congenital Syphilis

• T. pallidum crosses placenta (any trimester)

• Symptoms: nasal discharge, skin eruptions, bone deformation, nervous system abnormalities

• Late form: stigmata in bones, eyes, inner ear, joints; Hutchinson's teeth (notched, barrel-shaped)

Diagnosis and Testing

• Symptoms may mimic other diseases; overlap with other STDs

• Diagnosis: symptoms, history, microscopy, serology

• Staining: silver stains for microscopy

• Serological tests: RPR, VDRL, FTA-ABS, TP-PA

• Treatment: Penicillin G

Nonsyphilitic Treponematoses

• Bejel: T. pallidum endemicum; childhood infection of mouth, nasal cavity, body, hands (Middle East, North Africa)

• Yaws: T. p. pertenue; skin ulcer, secondary lesions (Africa, Asia, South America)

• Pinta: T. p. carateum; superficial skin lesion, depigmentation, scarring (Latin America)

Genus Leptospira and Leptospirosis

• Tight, regular coils with hook at one/both ends

• Medically important species: L. interrogans

• Transmission: Contact with urine from infected animals

• Symptoms: Sudden high fever, chills, headache, muscle aches, conjunctivitis, vomiting

• Targets kidneys, liver, brain, eyes; long-term infection may affect kidneys/liver

• ~100 cases/year in U.S.

Genus Borrelia

• Large spirochetes, 3-10 loose coils

• Transmitted by arthropod vectors

• B. hermsii: Relapsing fever (tick-borne, wild mammals)

• B. burgdorferi: Lyme disease (tick-borne, complex lifecycle with mice, deer)

Borrelia hermsii – Relapsing Fever

• Reservoirs: squirrels, chipmunks

• Symptoms: high fever, chills, headache, fatigue, nausea, vomiting, muscle aches, abdominal pain

• Extensive organ damage possible

• Immune response leads to recurrent relapses as parasite changes antigens

• Treatment: Doxycycline, erythromycin

Relapsing Fever Pattern

1. Primary infection induces high fever

2. Initial antibody response reduces fever

3. Reinfection by mutant Borrelia causes relapse

4. Immune reaction to new antigen slows symptoms

5. New antigenic form causes another relapse

6. Antibody response again reduces symptoms, followed by relapse

Borrelia burgdorferi – Lyme Disease

• Complex lifecycle: mice, deer, ticks

• Transmission: tick bites

• Symptoms: bull's eye rash (50-70%), fever, headache, stiff neck, dizziness

• If untreated: cardiac, neurological symptoms, polyarthritis

• Treatment: Early—doxycycline/amoxicillin; Late—ceftriaxone/azithromycin

Curviform Bacteria

General Characteristics

• Gram-negative, adapted for intestinal survival

• Vibrio: Comma-shaped rods, single polar flagellum

• Campylobacter: Short spirals/curved rods, one flagellum

• Helicobacter: Tight spirals, several polar flagella

• Generally cause enteric diseases

Vibrio cholerae

• Comma-shaped bacteria; cause of cholera

Epidemiology of Cholera

• Favored by warm, monsoon, alkaline, saline conditions

• Pandemic since 1961 (El Tor biotype)

• Spread by contaminated food/water

• Infects mucous barrier of small intestine (noninvasive)

• Cholera toxin causes secretory diarrhea ('rice water stool'), leading to dehydration and systemic symptoms

• Treatment: Rehydration, doxycycline, azithromycin

Pathogenesis of Cholera: Toxigenic Diarrhea

• Normal absorption: Cl-, HCO3-, Na+, K+, H2O enter intestinal cell and blood vessel

• Cholera toxin disrupts normal physiology, causing massive electrolyte and water loss into intestine

• Mechanism: Toxin binds to cell, activates adenyl cyclase, increases cAMP, stimulates ion pumps, leading to secretory diarrhea

• Positive ions (Na+, K+) and water follow anions into lumen

Other Vibrio Pathogens

• Vibrio vulnificus: Cutaneous infection, gastroenteritis; severe in diabetics, liver disease, HIV

• Virulence factors: collagenase, metalloproteinase, lipase, endotoxin

• Vibrio parahaemolyticus: Less serious gastroenteritis, cutaneous infection

• Treatment: Fluid/electrolyte replacement; antimicrobials as needed