Chlamydia trachomatis

Chlamydia trachomatis is a small Gram-negative bacterium that is an obligate intracellular parasite.

The various diseases caused by C. trachomatis infection are associated with different serotypes of the bacteria: urogenital infections and inclusion conjunctivitis (serotypes D through K), lymphogranuloma venereum (serotypes L1, L2, and L3), and an ocular infection of children, trachoma (serotypes A, B, and C). The venereal infections caused by C. trachomatis will be discussed here.

C. trachomatis exists in two forms during its unique life cycle. The infectious form, the elementary body (EB), is a metabolically inactive, sporelike structure. The EB is taken up by host cells, primarily by receptor-mediated endocytosis.

The bacteria prevent fusion of the endosome and lysosome, but the mechanism of this is not known. Inside the endosome, the EB differentiates into a metabolically active form, called the reticulate body (RB). Using energy sources and amino acids from the host cell, the RB replicates and ultimately forms new EBs that are capable of infecting additional cells.

Genital infection by C. trachomatis (serotypes D through K) is the most common bacterial sexually transmitted disease in the world.

In 2001, approximately 780,000 cases of genital chlamydia were reported to the Centers for Disease Control; this is about twice the number of cases of gonorrhea.

Before the identification of C. trachomatis, patients infected with this organism were diagnosed with nongonococcal urethritis (NGU). Indeed, C. trachomatis is the cause of about half the cases of NGU.

Other organisms that cause NGU include Ureaplasma urealyticum, Mycoplasma hominis, Mycoplasma genitalium, and Trichomonas vaginalis.121 In the past, it was recommended that patients with urethritis who did not respond to treatment for N. gonorrhoeae be tested or treated for C. trachomatis. Since the current CDC recommendations call for treatment of both bacteria in patients who are diagnosed with either infection, such "relapses" should no longer occur.

Genital C. trachomatis infections (other than lymphogranuloma venereum, discussed below) are associated with clinical features that are similar to those caused by N. gonorrhoeae. Patients may develop epididymitis, prostatitis, pelvic inflammatory disease, pharyngitis, conjunctivitis, perihepatic inflammation, and, among people engaging in anal intercourse, proctitis.

Unlike N. gonorrhoeae urethritis, C. trachomatis urethritis in men may be asymptomatic, so infected men might not seek treatment. Both N. gonorrhoeae and C. trachomatis frequently cause asymptomatic infections in women. C. trachomatis urethritis can be diagnosed by culture of the bacteria in human cell lines, but amplified nucleic acid tests performed on genital swabs or urine specimens are more sensitive and have supplanted cultures.

Genital infection with the L serotypes of C. trachomatis causes lymphogranuloma venereum, a chronic, ulcerative disease.

Lymphogranuloma venereum

Lymphogranuloma venereum is a sporadic disease in the United States and Western Europe, but it is endemic in parts of Asia, Africa, the Caribbean region, and South America. The infection is initially manifested by a small, often unnoticed, papule on the genital mucosa or nearby skin.

Two to 6 weeks later, growth of the organism and the host response in draining lymph nodes produce swollen, tender lymph nodes, which may coalesce and rupture. If not treated, the infection can subsequently cause fibrosis and strictures in the anogenital tract. Rectal strictures are particularly common in women.

Morphology

The morphologic features of C. trachomatis urethritis are virtually identical to those of gonorrhea. The primary infection is characterized by a mucopurulent discharge containing a predominance of neutrophils. Organisms are not visible in Gram-stained smears or sections.

The lesions of lymphogranuloma venereum contain a mixed granulomatous and neutrophilic inflammatory response, with a variable number of chlamydial inclusions in the cytoplasm of epithelial cells or inflammatory cells. Regional lymphadenopathy is common, usually occurring within 30 days of the time of infection.

Lymph node involvement is characterized by a granulomatous inflammatory reaction associated with irregularly shaped foci of necrosis and neutrophilic infiltration (stellate abscesses). With time, the inflammatory reaction is dominated by nonspecific chronic inflammatory infiltrates and extensive fibrosis.

The latter, in turn, may cause local lymphatic obstruction with lymphedema and strictures. In active lesions, the diagnosis of lymphogranuloma venereum may be made by demonstration of the organism in biopsy sections or smears of exudate.

In more chronic cases, the diagnosis rests with the demonstration of antibodies to the appropriate chlamydial serotypes in the patient’s serum.