Rickettsias, Chlamydias, Spirochetes, and Vibrios: Pathogenic Gram-Negative Bacteria

Rickettsias

General Characteristics

Rickettsias are a group of extremely small, non-motile, aerobic, Gram-negative bacteria. They are obligate intracellular parasites, meaning they must live and reproduce inside host cells. Their cell walls contain only a small amount of peptidoglycan, making them appear almost wall-less. Rickettsias are typically transmitted via arthropod vectors such as ticks, fleas, and lice.

• Obligate intracellular parasites: Require host cells for survival and reproduction.

• Transmission: Arthropod vectors (e.g., ticks, lice, fleas).

• Unusual genetics: Possess genes for protein synthesis, ATP production, and reproduction.

Table: Characteristics of Some Rickettsias

Organism	Primary Vector	Reservoirs	Disease
R. rickettsii	Hard ticks (Dermacentor)	Ticks, rodents	Rocky Mountain Spotted Fever
R. prowazekii	Human body louse	Humans	Epidemic typhus
R. typhi	Fleas	Rodents	Murine (endemic) typhus
Ehrlichia chaffeensis	Hard ticks	Ticks	Human monocytic ehrlichiosis
Anaplasma phagocytophilum	Hard ticks	Ticks	Human granulocytic anaplasmosis

Rickettsia rickettsii and Rocky Mountain Spotted Fever (RMSF)

R. rickettsii is the causative agent of Rocky Mountain Spotted Fever, the most severe and common rickettsial illness in the United States. Transmission occurs via hard ticks among humans and rodents. Most infected individuals develop a rash on the trunk and appendages, and approximately 5% of patients die if untreated.

• Symptoms: Rash, fever, headache, muscle pain.

• Pathogenesis: Bacteria trigger endocytosis in endothelial cells, escape the endosome, divide in the cytosol, and infect adjacent cells. Damage to endothelial cells leads to blood leakage, low blood pressure, and insufficient oxygen/nutrient delivery.

• Treatment: Antimicrobials (e.g., doxycycline).

• Prevention: Avoiding tick exposure.

Vectors

• Mechanical vectors: Transmit pathogens on their body without being infected.

• Biological vectors: Transmit pathogens via bites or secretions; the vector is infected.

Other Rickettsias

• Rickettsia prowazekii: Causes epidemic typhus, transmitted by human body louse (Pediculus), occurs in crowded, unsanitary conditions.

• Rickettsia typhi: Causes murine (endemic) typhus, transmitted by fleas from rodents, not usually fatal.

• Ehrlichia chaffeensis and Anaplasma phagocytophilum: Cause human monocytic ehrlichiosis and anaplasmosis, respectively. Transmitted by ticks, resemble RMSF but without rash, and have three stages in leukocytes: elementary body, initial body, and morula.

Chlamydias

General Characteristics

Chlamydias are small, Gram-negative bacteria that lack peptidoglycan in their cell walls. They have two membranes and are obligate intracellular parasites, growing only within vesicles of host cells. Chlamydias are considered energy parasites because they lack metabolic enzymes to produce their own ATP.

• No cell wall: Penicillins and cephalosporins are ineffective.

• Developmental cycle: Involves elementary bodies (EBs, infectious form) and reticulate bodies (RBs, replicative form).

Table: Characteristics of the Smallest Microbes

Feature	Rickettsias	Chlamydias	Mycoplasmas	Viruses
Cellular Structure	Small cells with little peptidoglycan	Small, wall-less cells with two membranes	Small, wall-less pleomorphic cells	Acellular
Diameter	0.3 μm	EB: 0.2–0.4 μm; RB: 0.6–1.5 μm	0.1–0.8 μm	0.01–0.3 μm
Lifestyle	Obligate intracellular	Obligate intracellular	Free-living	Obligate intracellular
Replication	Binary fission	Binary fission of RBs	Binary fission	Chemical assembly
Nucleic Acids	DNA and RNA	DNA and RNA	DNA and RNA	DNA or RNA
Functional Ribosomes	Present	Present	Present	Absent
Metabolism	Present	Present	Limited	None
ATP-Generating System	Present	Absent	Present	None
Phylum	Proteobacteria	Chlamydiae	Firmicutes	None

Chlamydia trachomatis

Chlamydia trachomatis is the most common sexually transmitted infection (STI) in the United States. It infects humans exclusively and enters through abrasions or lacerations in conjunctiva and mucous membranes. It can cause lymphogranuloma venereum and trachoma.

• Lymphogranuloma venereum: STI with genital lesions, lymph node swelling, and possible complications such as PID and proctitis.

• Trachoma: Leading cause of nontraumatic blindness worldwide due to scarring of conjunctiva and corneal abrasions.

Developmental Cycle of Chlamydia

• Elementary body (EB): Infectious, extracellular form.

• Reticulate body (RB): Non-infectious, replicative, intracellular form.

• Both forms occur within the phagosome of a host cell.

Diagnosis, Treatment, and Prevention

• Diagnosis: Demonstration of bacteria inside cells, Giemsa stain for inclusion bodies, cell culture, fluorescent antibodies, or nucleic acid probes.

• Treatment: Antibiotics (doxycycline, erythromycin for 21 days for STI; azithromycin cream for newborns with trachoma).

• Prevention: Vaccine in development, abstinence or monogamy, prompt use of antibacterial agents to prevent blindness.

Spirochetes

General Characteristics

Spirochetes are thin, tightly coiled, helically shaped bacteria that move in a corkscrew fashion due to axial filaments. This motility enables them to burrow through host tissues. Three genera cause human disease: Treponema, Borrelia, and Leptospira.

• Axial filaments: Specialized flagella for movement.

• Pathogenic genera: Treponema, Borrelia, Leptospira.

Treponema pallidum and Syphilis

Treponema pallidum is the causative agent of syphilis, a disease transmitted almost exclusively by sexual contact. Untreated syphilis progresses through four stages: primary, secondary, latent, and tertiary. Congenital syphilis can occur if an infected mother transmits the pathogen across the placenta.

• Primary phase: Chancre formation (10–21 days).

• Secondary phase: Sore throat, fever, rash, lymphadenopathy.

• Latent phase: Asymptomatic, can last decades.

• Tertiary phase: Inflammation, hyperimmune response, gummas, neurological and cardiac complications.

• Diagnosis: Antibody tests; tertiary syphilis is difficult to diagnose.

• Treatment: Penicillin is effective for early stages and congenital syphilis, but not for tertiary syphilis.

• Prevention: No vaccine; abstinence and safe sex.

Borrelia burgdorferi and Lyme Disease

Borrelia burgdorferi causes Lyme disease, transmitted by Ixodes ticks. If untreated, Lyme disease progresses through three stages: expanding red rash, neurological and cardiac dysfunction, and arthritis.

• Symptoms: Bull's-eye rash, neurological symptoms, arthritis.

• Treatment: Doxycycline for adults, penicillin for children in early stages.

• Prevention: Avoid tick exposure, use DEET, cover skin.

Leptospira interrogans and Leptospirosis

Leptospira interrogans is a motile, obligately aerobic spirochete found in wild and domestic animals. Leptospirosis occurs when the bacteria enter through skin or mucous membranes and travel via the bloodstream, causing hemorrhaging, fever, and kidney dysfunction.

• Transmission: Contact with contaminated water or animal urine.

• Treatment: Intravenous penicillin.

• Prevention: Vaccine available for livestock and pets; eradication is impractical due to animal reservoirs.

Pathogenic Gram-Negative Vibrios

Vibrio cholerae and Cholera

Vibrio cholerae is the causative agent of cholera, a severe diarrheal disease. It is found in areas with poor sewage and water treatment. The most important virulence factor is cholera toxin, which causes rapid loss of fluids and electrolytes.

• Symptoms: Rice-water stool, dehydration, electrolyte imbalance.

• Treatment: Fluid and electrolyte replacement, antimicrobial drugs (e.g., tetracycline).

• Prevention: Proper sewage and water treatment, oral cholera vaccine.

Campylobacter jejuni

Campylobacter jejuni is a common cause of gastroenteritis in the US, often transmitted by consuming contaminated poultry, milk, or water. It produces bloody diarrhea, fever, malaise, and abdominal pain.

• Treatment: Azithromycin or ciprofloxacin.

• Prevention: Proper food handling and preparation.

Helicobacter pylori

Helicobacter pylori colonizes the stomach and is associated with peptic ulcers and gastric adenocarcinoma. It produces urease to neutralize stomach acid and uses flagella to burrow through the stomach lining.

• Diagnosis: Positive urease test, biochemical tests.

• Treatment: Antimicrobial drugs (metronidazole, amoxicillin, tetracycline, clarithromycin) plus proton pump inhibitor or bismuth for at least 2 weeks.

• Prevention: Good hygiene, adequate sewage treatment, proper food handling.

Additional info:

• Penicillins and cephalosporins are ineffective against Chlamydia due to lack of cell wall.

• Syphilis and chlamydia remain major public health concerns, with increasing rates in recent years.