The urinary tract is almost always invaded from the exterior by way of the urethra. The regular flushing of the urinary tract with urine serves as a defense against invading microorganisms. Urine in the bladder is normally sterile or contains only small numbers of fastidious bacteria; however, successful pathogens (e.g., N. gonorrhoeae, E. coli) adhere to the urinary epithelium, overcoming the host defense of regular flushing. Anatomy plays an important role in infection. Women have many more urinary tract infections than men because the distance between the urinary bladder and skin (i.e., the length of the urethra) is 5 cm in women, in contrast with 20 cm in men. Obstruction of urinary flow, as in benign prostatic hyperplasia, or reflux can compromise normal defenses and increase susceptibility to urinary tract infections. Urinary tract infections can spread further up from the bladder to the kidney and cause acute and chronic pyelonephritis.
From puberty until menopause, the vagina is protected from pathogens by a low pH resulting from catabolism of glycogen in the normal epithelium by lactobacilli. Antibiotics can kill the lactobacilli, allowing overgrowth of yeast, with resultant vaginal candidiasis.
Some microorganisms proliferate locally, at the site of initial infection, whereas others penetrate the epithelial barrier and spread to distant sites by way of the lymphatics, the blood, or nerves (
Fig. 9.6 ). In contrast to those that disseminate, pathogens that cause superficial infections stay confined to the lumen of hollow viscera (e.g., V. cholerae) or interact exclusively with epithelial cells (e.g., papillomaviruses, dermatophytes).
Microbes can spread within the body in several ways:
Lysis and invasion. Some extracellular bacteria, fungi, and helminths secrete lytic enzymes that destroy tissue and allow direct invasion. For example, S. aureus secretes hyaluronidase, which degrades the extracellular matrix between host cells. Invasive microbes initially follow tissue planes of least resistance and drain to regional lymphatics. S. aureus may travel from a localized abscess to the draining lymph nodes. This can sometimes lead to bacteremia and spread to deep organs (heart, bone).
Through blood and lymph. Microorganisms may be spread either in extracellular fluid or within host cells. Some viruses (e.g., poliovirus, HBV), most bacteria and fungi, some protozoa (e.g., African trypanosomes), and all helminths are transported in the plasma. Leukocytes can carry herpesviruses, HIV, mycobacteria, Leishmania, and Toxoplasma. The parasites Plasmodium and Babesia are found within red cells.
Cell-to-cell transmission. Most viruses spread locally from cell to cell by replication and release of infectious virions, but others may propagate from cell to cell by causing fusion of host cells, or by transport within nerves (as with rabies virus and varicella-zoster virus).
The consequences of bloodborne spread of pathogens vary widely depending on the virulence of the organism,
Portal of entry Infection (skin, respiratory, gastrointestinal,
M. tuberculosis virus, rabiesin macrophages Lymphatics