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Bacteriology at UW-Madison |
Neisseria gonorrhoeae, the
Gonococcus, and
Gonorrhea
Gonorrhea is a prevalent sexually transmitted disease
(std)
caused by the bacterium Neisseria gonorrhoeae. Infections
are acquired by sexual contact and usually affect the mucous membranes
of the urethra in males and the endocervix and urethra in females,
although
the infection may disseminate to a variety of tissues.
Although many species of Neisseria are normally found in the upper respiratory tract of humans, Neisseria gonorrhoeae is never part of the normal flora. The bacterium is only found after sexual contact with an infected person (or direct contact, in the case of infections in the newborn). In the vocabulary of the public health and medical microbiology N. gonorrhoeae is often referred to as the "gonococcus".
Figure 1. Left: Neisseria
gonorrhoeae Gram stain of pure culture;
Right: Neisseria
gonorrhoeae
Gram stain of a pustular exudate.
Neisseria gonorrhoeae is a relatively small Gram-negative coccus, usually seen in pairs with adjacent flattened sides (Figure 1 Left and Fig 2 below). The organism is frequently observed inside of phagocytic cells (neutrophils) that have become part of the gonorrhea pustular exudate (Figure 1 Right).
Figure 2. Neisseria
gonorrhoeae
N. gonorrhoeae is a relatively fragile organism, susceptible to temperature changes, drying, uv light, and other environmental conditions. It is also a nutritionally "fastidious" bacterium so it requires blood or hemoglobin and several amino acids and vitamins in order to grow. In the laboratory, cultures must be grown at 35-36 degrees in an atmosphere of 3-10% added CO2.
For centuries thereafter, gonorrhea and syphilis were confused, resulting from the fact that the two diseases were often present together in infected individuals. Paracelsus (1530) thought that gonorrhea was an early symptom of syphilis. The confusion was further heightened by the classic blunder of English physician John Hunter, in 1767. Hunter intentionally inoculated himself with pus from a patient with symptoms of gonorrhea and wound up giving himself syphilis!
The causative agent of gonorrhea, Neisseria gonorrhoeae, was first described by A. Neisser in 1879, in the pustular exudate of a case of gonorrhea. The organism was grown in pure culture in 1885, and its etiological relationship to human disease was later established using human volunteers in order to fulfill the experimental requirements of Koch's postulates.
Ocular infections by N. gonorrhoeae can have serious consequences of corneal scarring or perforation. Ocular infections (ophthalmia neonatorum) occur most commonly in newborns who are exposed to infected secretions in the birth canal. Part of the intent in adding silver nitrate or an antibiotic to the eyes of the newborn is to prevent ocular infection by N. gonorrhoeae.
Endocervical infection is the most common form of uncomplicated gonorrhea in women. Such infections are usually characterized by vaginal discharge and sometimes by dysuria. About 50% of women with cervical infections are asymptomatic. Asymptomatic infections occur in males, as well. Males with asymptomatic urethritis are an important reservoir for transmission of the disease Asymptomatic males and females are a major problem as unrecognized carriers of the disease, which occurs in the U.S. at an estimated rate of over one million cases per year.
In the male, the organism may invade the prostate resulting in prostatitis, or extend to the testicles resulting in orchitis. In the female, cervical involvement may extend through the uterus to the fallopian tubes resulting in salpingitis, or to the ovaries resulting in ovaritis. As many as 15% of women with uncomplicated cervical infections may develop pelvic inflammatory disease (PID). The involvement of testicles, fallopian tubes or ovaries may result in sterility. Occasionally, disseminated infections occur. leading to arthritis, endocarditis and meningitis.
Rectal infections (proctitis) with N. gonorrhoeae occur in about one-third of women with cervical infection. They most often result from autoinoculation with cervical discharge and are rarely symptomatic. Rectal infections in homosexual men usually result from anal intercourse and are more often symptomatic. Partners must be treated as well to avoid reinfection.
Although it has been shown that antiibodies are produced in response to infection by N. gonorrhoeae, (meaning that the immune system has also been activated) immunity against reinfection has not been clearly shown. In any case, immunity would only be specific for the infecting strain of bacterium so reinfections may occur.
Not everyone exposed to N. gonorrhoeae acquires the disease. This may be due to variations in the number of infectious organisms, to natural resistance, or to specific immunity. A 50% infective dose (ID50) of about 1,000 bacteria has been determined based on experimental urethral inoculation of male volunteers. No data is available for females.
Nonspecific factors have been implicated in natural resistance to gonococcal infection. In women, changes in the genital pH and hormones may increase resistance to infection at certain times of the menstrual cycle. Urine contains bactericidal and bacteriostatic components against N. gonorrhoeae. Factors in urine that may be important are pH, osmolarity, and the concentration of urea.
Penicillin resistance in N. gonorrhoeae was first described in 1976. The prevalence of penicillin-resistant strains has increased dramatically in the United States since 1984.
Tetracycline resistance of N. gonorrhoeae was first noted in 1986 and has now been reported in most parts of the world.
Gonorrhea is usually contracted from a sex partner who is either asymptomatic or has only minimal symptoms. It is estimated that the efficiency of transmission after one exposure is about 35 percent from an infected woman to an uninfected man and 50 to 60 percent from an infected man to an uninfected woman. More than 90 percent of men with urethral gonorrhea will develop symptoms within 5 days; fewer than 50 percent of women with genital gonorrhea will do so. Women with asymptomatic infections are at higher risk of developing pelvic inflammatory disease and disseminated gonococcal infection.