The three major portals of entry are the mucous membranes, the skin, and the parenteral route. In the category of mucous membranes, the main entry points are the respiratory, the digestive, and the genitourinary tract. This is because each of them is exposed to the outside world where organisms can be found. The skin is more difficult as a portal of entry because it is impenetrable to microorganisms and it must be broken in some way in order for them to enter. The parenteral route is where the skin is broken in some way, such as a cut, a surgical procedure, or an injection site, allowing entry.
There are several organisms that can enter in each of these different ways but an easy way to remember them is to think about the kinds of illnesses that are associated with each portal of entry. For example, diseases associated with entry through mucous membranes of the respiratory tract include pneumonia caused by Steptococcus species, tuberculosis caused by Mycobacterium tuberculosis, and influenza caused by the influenza virus. Digestive diseases associated with entry via the gastrointestinal tract include cholera caused by Vibrio cholerae, salmonellosis caused by Salmonella enterica, and enterohemorrhagic disease caused by E. coli. Urinary tract infections and sexually transmitted diseases involve entry through the mucous membranes of the genitourinary tract and include gonorrhea caused by Neisseria gonorrhoeae, herpes caused by the herpes simplex virus, and AIDS caused by the human immunodeficiency virus. For the skin and perenteral routes, any organisms routinely found on the skin could be included as if the skin is broken they can enter. Some diseases associated with the perenteral route include tetanus caused by Clostridium tetani, hepatitis caused by viruses Hepatitis B and C, and malaria caused by Plasmodium species which gain entry through mosquito bites.
Avoiding or defeating the host defenses is the third requirement for a successful infection and there are a variety of ways that bacteria can do this. These mechanisms can be either passive or active. Passive pathogen defenses include the capsule and cell wall components, such as the M protein, that can inhibit phagocytosis. Active protection can be provided by a variety of enzymes, such as leukocidins, hemolysins, and kinases, which help to defeat the host defenses.
The major differences are that exotoxins are actively produced by living Gram-positive cells while endotoxins are components of the Gram-negative cell outer layer and are released upon the death of these cells. In addition, exotoxins are proteins, are highly toxic, and can be fatal in small doses, while endotoxins are lipopolysaccharide complexes which are less toxic but can still be fatal at high doses. Exotoxins also have highly specific effects, do not produce fever, and can confer long-lasting immunity. In comparison, endotoxins cause nonspecific effects inducing local reactions such as fever, aches and possible shock, and do not confer immunity.