How Do Bacteria Infect?

Bacteria are one of six types of microorganisms that can be seen only with a microscope. Most bacteria are beneficial to human health and bodily processes, and less than 1 percent of microbes are estimated to be pathogenic or disease-causing. Bacteria are classified by their shape, and the three most common types are bacilli, for "rod-shaped"; cocci, for "dot-shaped"; and spirilla, for "spiral-shaped." Bacteria infect humans through the process known as the chain of infection.

Chain of Infection Process

• The epidemiological chain of infection describes the infection process and contains four basic components (see diagram in the References section): the pathogenic agent, [pathogenic] reservoir or source, the mode of transmission, and the host (susceptible). The pathogen resides in a place that enables it to live, called the reservoir or source. Many different types of environments can be reservoirs, such as a swamp or even an animal like a rodent or cat. For example, during the horrific Bubonic Plague in the fourteenth century, the reservoir was the rat population that served as a home for the fleas carrying the bacteria. The mode of transmission is how the bacteria get into the host, which in this case is a human body. Touching someone who is infected is an example of direct contact transmission. Touching a nonliving object (fomite) that still contains the pathogen, such as toys that have been sneezed on and not cleaned or shared drinking cups, is an indirect means of transmission. The vector mode of transmission involves being bitten by an infected insect or animal, such as a mosquito. The goal is for the pathogenic bacteria to find a way to enter the host's body, perhaps through a wound, scratch or small puncture in the skin. Natural entry routes also exist in the form of openings or orifices of the human body.

First Response

• According to Tamparo and Lewis, an infection is the invasion and multiplication of pathogenic microorganisms such as bacteria in the body. When this happens, the body's first protective response occurs--the inflammatory response. Inflammation, with its characteristic redness, swelling and heat, begins at the site where the bacteria invaded through an opening in the skin. White blood cells known as phagocytes approach the site and try to engulf or digest the antigen (or foreign agent, the bacteria) by the process of phagocytosis. Next, the immune system kicks in with a B-cell immune response that involves antibodies as well as a T-cell or cell-mediated immunity.