Gram-Negative Cocci and Medically Important Spirochetes: Neisseria, Treponema, Borrelia, and Leptospira

Gram-Negative Cocci

Genus Neisseria

The genus Neisseria includes Gram-negative cocci of major medical importance, particularly Neisseria gonorrhoeae and Neisseria meningitidis. These bacteria are often found in pairs (diplococci) or short chains and are characterized by the absence of flagella, the presence of capsules in pathogenic species, and pili/fimbriae for attachment and conjugation. They are aerobic or microaerophilic, oxidase positive, and most are catalase positive. Pathogenic species require enriched complex media and increased CO2 for growth.

• Key Features: Gram-negative, diplococci, oxidase positive, capsule (in pathogens), pili/fimbriae, aerobic/microaerophilic.

• Habitat: Some species are normal flora in the oral/respiratory tract, complicating diagnosis.

Neisseria gonorrhoeae: The Gonococcus

Neisseria gonorrhoeae is the causative agent of gonorrhea, a common sexually transmitted infection (STI). It possesses several virulence factors that contribute to its pathogenicity.

• Virulence Factors:

  • Endotoxin (lipooligosaccharide)

  • Pili: Mediate attachment, conjugation, and resistance to neutrophil phagocytosis

  • Porin B: Interferes with neutrophil degranulation

  • IgA protease: Cleaves IgA antibodies

  • Antigenic variation: Multiple serotypes

Epidemiology and Pathology of Gonorrhea

Gonorrhea is strictly a human infection and is among the top five STIs globally. It is highly prevalent in young adults and adolescents, with over 500,000 cases reported annually in the US and more than 82 million new cases worldwide each year. The infectious dose is low (100–1,000 organisms in males), and the bacterium does not survive long outside the host.

• Transmission: Sexual contact, perinatal (to newborns during birth)

• Symptoms in Males: Urethritis, yellowish discharge, scarring, infertility; extragenital infections (anal, oral, conjunctivitis, septicemia, arthritis, skin lesions)

• Symptoms in Females: 50% symptomatic (discharge, vaginitis, urethritis, pelvic inflammatory disease), 50% asymptomatic; increased risk of sterility, ectopic pregnancy, and HIV transmission

• Extragenital Infections: Similar to males

Gonorrhea in Newborns

Newborns can acquire gonococcal infection during passage through the birth canal, leading to ophthalmia neonatorum (eye inflammation and potential blindness). Prophylactic eye drops are used for prevention, though shortages have been reported.

Diagnosis and Control of Gonorrhea

• Diagnosis:

  • Gram stain: Gram-negative diplococci, often intracellular, from exudate (urethral, vaginal, cervical, or eye)

  • NAAT (Nucleic Acid Amplification Test): Sensitive but may not distinguish resistance or normal flora

• Treatment: Ceftriaxone (3rd generation cephalosporin) is the main reliable therapy; resistance is rising

• Prevention: Reportable disease, safe sex practices, prophylaxis for newborns

Neisseria meningitidis: The Meningococcus

Neisseria meningitidis is a leading cause of bacterial meningitis, especially in children and young adults. It is characterized by rapid onset and high mortality if untreated.

• Virulence Factors: Capsule, adhesive fimbriae, IgA protease, endotoxin

• Serotypes: 12 capsular antigen types; A, B, C, Y, W cause most disease

• Reservoir: Human nasopharynx; higher carriage in institutional settings (e.g., dormitories)

• Pathogenesis: Bacteria cross the blood-brain barrier, causing meningitis and meningococcemia (sepsis with hemorrhage and petechiae)

• Complications: Permanent neurological damage (10–20%), death (10–15%)

Diagnosis, Treatment, and Prevention of Meningococcal Disease

• Diagnosis:

  • Gram stain of CSF or blood

  • Cultures for differentiation and susceptibility

  • Oxidase test (positive)

  • Rapid antigen tests for capsular polysaccharides

  • Serotyping with specific antibodies

  • PCR (NAA tests): Detects DNA but not resistance

• Treatment: IV or IM 3rd generation cephalosporins; prophylaxis for close contacts

• Prevention: Vaccines (conjugate for A, C, W, Y; recombinant protein for B; combination vaccines for high-risk groups)

Medically Important Spirochetes

General Characteristics

Spirochetes are Gram-negative, spiral-shaped bacteria. Most are saprobes or commensals, but some are important human pathogens. They lack typical LPS in their outer membrane but have lipoproteins. Major genera include Treponema, Borrelia, and Leptospira.

Genus Treponema

Treponema species are thin, regularly coiled cells with periplasmic flagella. They are microaerophilic or anaerobic and are found in the oral cavity, intestinal tract, and perigenital regions. Pathogenic species are strict parasites and require live cells for cultivation.

• Metabolism: Some species rely solely on glycolysis for ATP production

• Genome: Small, about 1,000 genes

Treponema pallidum subsp. pallidum: Syphilis

Treponema pallidum is the causative agent of syphilis, a chronic, systemic STI. It is microaerophilic, extremely fastidious, and sensitive to environmental conditions. Transmission occurs via sexual contact or transplacentally.

• Pathogenesis: Binds to epithelium, multiplies, penetrates capillaries, and disseminates via circulation

• Clinical Stages: Primary, secondary, tertiary (late)

Primary Syphilis

Characterized by a hard, painless chancre at the site of inoculation and regional lymphadenopathy. Incubation is 10–90 days, and the lesion heals spontaneously in 3–6 weeks in about one-third of cases.

Secondary Syphilis

Known as "the great pretender" due to its varied manifestations. Presents with flu-like symptoms, generalized rash (often on palms and soles), and mucocutaneous lesions (condylomata lata). Rash disappears spontaneously and may enter a latent phase. The Jarisch-Herxheimer reaction (fever, aches) may occur after penicillin treatment.

Tertiary (Late) Syphilis

Occurs in about 30% of untreated cases, years to decades after initial infection. Characterized by gummas (necrotic lesions), cardiovascular syphilis (arteritis, aneurysms, heart valve damage), and neurosyphilis (dementia, stroke, seizures, blindness).

Congenital Syphilis

Transmitted transplacentally, leading to stillbirth, neonatal death, or severe deformities (nasal discharge, skin lesions, bone and teeth abnormalities, "late" syphilis features). Incidence is rising in some regions.

Diagnosis and Treatment of Syphilis

• Diagnosis:

  • Symptoms and history

  • Microscopy: Darkfield or fluorescent antibody staining

  • Serology: RPR, VDRL (nonspecific); TP-PA, FTA-ABS, EIA, Western blot (specific)

• Treatment: Penicillin (Benzathine penicillin for early syphilis; penicillin G for late/tertiary)

• Prevention: Safe sex practices, screening, and treatment of pregnant women

Nonsyphilitic Treponematoses

• Bejel: Treponema pallidum subsp. endemicum; deforming childhood infection in Middle East/North Africa

• Yaws: Treponema pallidum subsp. pertenue; skin and bone lesions, Central Africa/South America/Indonesia

• Pinta: Treponema carateum; superficial skin lesions, Central/South America

Treponema denticola

Major contributor to periodontal disease, usually found in biofilms with other organisms. It is anaerobic and produces adhesins, peptidases, and pore-forming proteins that help evade host defenses and degrade host tissues.

Borrelia: Arthropod-Borne Spirochetes

Borrelia species are large, irregularly coiled spirochetes transmitted by arthropod vectors. They stain poorly with Gram stain but can be visualized with special stains.

• Relapsing Fever: B. recurrentis (louse-borne), B. hermsii (tick-borne)

• Lyme Disease: B. burgdorferi, B. mayonii (Ixodes tick vector)

Lyme Disease

Nonfatal, slowly progressive syndrome. Characterized by erythema migrans (bull's-eye rash), fever, headache, stiff neck, and, if untreated, cardiac, neurological, and arthritic complications. Treated with tetracyclines or beta-lactams. Prevention includes tick avoidance and prompt removal.

Relapsing Fevers

Reservoirs are wild rodents; vectors are ticks or lice. Characterized by recurring fever due to antigenic variation of outer surface proteins. Treated with tetracyclines or penicillins; Jarisch-Herxheimer reaction may occur.

Leptospira

Leptospira species are thin, coiled spirochetes with hooked ends. They are obligate aerobes, require specialized media, and are maintained in small animal reservoirs. Infection occurs via skin contact with contaminated water, leading to endothelial damage and, in severe cases, Weil disease (renal/hepatic dysfunction, vasculitis, myocarditis). Diagnosed by microscopic agglutination test; treated with doxycycline.

Genus	Key Disease(s)	Transmission	Main Features
Neisseria gonorrhoeae	Gonorrhea	Sexual, perinatal	Urethritis, PID, conjunctivitis, infertility
Neisseria meningitidis	Meningitis, meningococcemia	Respiratory droplets	Rapid onset, petechiae, shock
Treponema pallidum	Syphilis	Sexual, transplacental	Chancre, rash, gummas, neurological
Borrelia burgdorferi	Lyme disease	Tick bite	Bull's-eye rash, arthritis, carditis
Leptospira interrogans	Leptospirosis	Contact with animal urine/water	Fever, renal/hepatic dysfunction