The regions of the genitourinary tract that harbor resident microbes are the vagina and outer opening of the urethra in females and the anterior urethra in males (figure1). The internal reproductive organs are kept sterile through physical barriers such as the cervical plug and other host defenses. The kidney, ureter, bladder, and upper urethra are presumably kept sterile by urine flow and regular bladder emptying. The shortness of the urethra in females (about 3.5 cm long), frequently leads to urinary tract infections. The principal residents of the urethra are nonhemolytic streptococci, staphylococci, corynebacteria, and occasionally, coliforms.
Fig1. Microbiota of the reproductive tract. (a) Female and (b) male genitourinary residents (location indicated by color).
The vagina presents a notable example of how changes in physiology can greatly influence the composition of the normal microbiota. An important factor influencing these changes in women is the hormone estrogen. Estrogen normally stimulates the vaginal mucosa to secrete glycogen, which certain bacteria (primarily Lactobacillus species) ferment, thus lowering the pH to about 4.5. Before puberty a female produces little estrogen and little glycogen and has a vaginal pH of about 7. These conditions favor the establishment of diphtheroids,3 staphylococci, streptococci, and some coliforms. As hormone levels rise at puberty, the vagina be gins to deposit glycogen, and the microbiota shift to the acid- producing lactobacilli. It is thought that the acidic pH of the vagina during this time prevents the establishment and invasion of microbes with potential to harm a developing fetus. The estrogen glycogen effect continues, with minor disruption, throughout the childbearing years until menopause, when the microbiota return to a mixed population similar to that of prepuberty. These transitions are not abrupt but occur over several months to years.
Maintenance of the Normal Microbiota
There is no question that the normal residents are essential to the health of humans and other animals. When living in balance with their host, the microbiota create an environment that may prevent infections and can enhance certain host defenses. In general, the microbes replace themselves naturally on a regular basis to maintain the types and numbers in their zones. However, because the exact content of the microbiota is not fixed, a number of changes can disrupt this balance. Use of broad-spectrum antibiotics, changes in diet, and underlying disease all have the potential to alter the makeup of the microbiota and tilt the system toward disease. A growing trend in therapy is the use of live cultures of known microbes in the form of probiotics. This essentially involves introducing pure cultures of known microbes into the body through ingestion or inoculation. The microbes chosen for this process are considered nonpathogenic.
For a look into laboratory studies that address the effects of microbiota, see 13.2 Making Connections.
