TRANSMISSION AND DISSEMINATION OF MICROBES
Host Barriers to Infection
The outcome of infection is determined by the ability of the microbe to infect, colonize, and damage host tissues and the ability of host defense mechanisms to eradicate the infection. Host
barriers to infection prevent microbes from entering the body and consist of innate and adaptive immune defenses  (see Fig. 6-1 , Chapter 6). Innate immune defense mechanisms exist
before infection and respond rapidly to microbes. These mechanisms include physical barriers to infection, phagocytic cells and natural killer cells, and plasma proteins, including the
complement system proteins and other mediators of inflammatory responses (cytokines, collectins, acute phase reactants). Adaptive immune responses are stimulated by exposure to
microbes and increase in magnitude, speed, and effectiveness with successive exposures to microbes. Adaptive immunity is mediated by T and B lymphocytes and their products ( Chapter
Microbes can enter the host by inhalation, ingestion, sexual transmission, insect or animal bites, or injection. The first barriers to infection are intact host skin and mucosal surfaces and
their secretory products. In general, respiratory, gastrointestinal, or genitourinary tract infections occur in healthy persons and are caused by relatively virulent microorganisms that are
capable of damaging or penetrating intact epithelial barriers. In contrast, most skin infections in healthy persons are caused by less virulent organisms entering the skin through damaged
sites (cuts and burns).
The dense, keratinized outer layer of skin is a natural barrier to infection, and the low pH of the skin (about 5.5) and the presence of fatty acids inhibit growth of microorganisms other than
residents of the normal flora. Human skin is normally inhabited by a variety of bacterial and fungal species, including some potential opportunists, such as Staphyloccus epidermidis and
Canadida albicans. Although skin is usually an effective barrier, certain types of fungi (dermatophytes) can infect the stratum corneum, hair, and nails, and a few microorganisms are able
to traverse the unbroken skin. For example, Schistosoma larvae released from freshwater snails penetrate swimmers' skin by releasing collagenase, elastase, and other enzymes that dissolve
the extracellular matrix. Most microorganisms, however, penetrate through breaks in the skin, including superficial pricks (fungal infections), wounds (staphylococci), burns (Pseudomonas
aeruginosa), and diabetic and pressure-related foot sores (multibacterial infections). Intravenous catheters in hospitalized patients can produce local or systemic infection (bacteremia).
Needle sticks can expose the recipient to potentially infected blood and may transmit HBV, HCV, or HIV. Some pathogens penetrate the skin via an insect or animal bite. For instance,
bites by fleas, ticks, mosquitoes, mites, and lice break the skin and transmit arboviruses (causes of yellow fever and encephalitis), rickettsiae (Rocky Mountain spotted fever), bacteria
(plague, Lyme disease), protozoa (malaria, leishmaniasis), and helminths (filariasis). Animal bites can lead to infections with bacteria or with rabies virus.
Date: 2016-04-22; view: 1046