Bacteria

Bacteria are classified into three major categories, according to their shape, gram-staining properties, and requirements for oxygen. Round or spherical bacteria are referred to as cocci, rod-shaped bacteria are referred to as bacilli, and spiral or corkscrew-shaped bacteria are called spirochetes. Some bacteria grow in chains (streptococci), some grow in pairs (diplococci), and some grow in clusters (staphylococci).

Bacteria that require oxygen to live and reproduce are aerobic; those that cannot tolerate the presence of oxygen are anaerobic. When bacteria enter the body, they trigger the immune system to produce antibodies (proteins that fight and destroy antigens). Some bacteria produce poisonous substances called endotoxins (toxins that are found within the bacteria and are released when the cell breaks apart); others produce exotoxins (excreted by both gram-negative and gram-positive bacteria).

Different bacteria thrive under different environmental conditions. Some form spores (a protective covering over the original cell) to protect themselves against destruction from heat, cold, lack of water, toxic chemicals, and radiation. Examples of spore-forming infectious diseases are anthrax and botulism. Other bacteria thrive best in water, such as Pseudomonas species or Legionella. The bacterium that causes tuberculosis can survive for years in many environments. Some bacteria, such as Staphylococcus aureus, can survive in very high temperatures.

Different methods can be used to prevent infection and its spread. A physiologic response can stimulate the immune system to produce a fever and the body’s immune response is triggered to destroy the invading pathogenic microorganism through phagocytosis (ingestion and digestion of bacteria). Another method is to clean, sterilize, or boil inanimate objects, such as a glass or surgical instruments, to prevent the spread of infection if the items are used by or on a patient. The correct antimicrobial agent must be given and completed as prescribed, otherwise the patient is at increased risk for developing infection with a multidrug-resistant organism.

Viruses

Viruses are not cells. They do not have cell walls and do not reproduce like other microbes. They cannot be treated with antibiotics or antifungals. They are composed of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA); have an outside coating made of protein; and are dependent on the cell they have invaded in order to survive and reproduce. Some viruses use the cytoplasm from the cell they have attacked to develop an “envelope” that makes it harder for the body’s immune system to destroy them. Viruses have the ability to keep changing their protein markers, called antigens, making it difficult for the virus to be neutralized or killed by white blood cells (WBCs). Once viruses have established themselves in the body, they can trigger an immune response that is harmful to the cells. They can also damage cells by preventing protein synthesis from occurring. New viral elements can be released into the circulation either by the virus breaking down the wall of the cell it has invaded and releasing itself or from small offshoots that have burst, thereby infecting other cells.

Protozoa

Protozoa are one-celled parasitic organisms that have the ability to move. There are four main types, named by their method of travel within their environment. They are called either amebas, ciliates, flagellates, or sporozoa. These microorganisms are typically found in water and soil. Many protozoa species have the ability to lie dormant. Although thousands of species exist, only a few are pathogenic to humans. To cause disease, some protozoans have to be ingested whereas others are introduced into the body through the bite of a vector, such as a mosquito.

Fungi

Fungi are very small, primitive organisms that grow on living plants, animals, and other decaying organic material. They thrive in warm, moist environments. Fungal infections in humans are called mycoses and are classified into three main types: (1) cutaneous mycoses, which grow in the outer layer of the skin; (2) subcutaneous mycoses, which involve the deeper layers of the skin, subcutaneous tissues, and sometimes bone; and (3) systemic or deep mycoses involving internal organs.

Fungal infections are difficult to eradicate once they have invaded a host, because fungi tend to form spores that are resistant to ordinary antimicrobial agents. Antifungal agents can be given topically or systemically, but can be toxic to the liver and the nervous system; therefore the course of treatment must be carried out cautiously and over a long period.

Fungal infections commonly found in immune-competent hosts include coccidioidomycosis (caused by Coccidioides immitis), histoplasmosis (caused by Histoplasma capsulatum), and blastomycosis (caused by Blastomyces dermatitidis). They are all systemic mycoses caused by inhalation of airborne spores. Once in the lung, the spores take root and then can spread to any part of the body. However, it is important to note that the majority of fungal infections are self-limited and do not cause clinical disease.

Opportunistic fungal infections (infections that occur in a person with a depressed immune system) are more typically found in patients who have some form of immune compromise and typically include species from Candida, Cryptococcus (can infect any organ in the body, including the brain and the meninges), and Aspergillus (found in soil, dust, and decomposing organic material).

Mycoplasmas

Mycoplasmas, once thought to be a virus, are very small organisms that do not have a cell wall. They are more like an extracellular parasite because they attach themselves to epithelial cells that line the body cavities and outer surfaces, such as the skin. They tend to be slow growing. For example, Mycoplasma pneumoniae can take up to 3 weeks to incubate before signs or symptoms begin to appear.

Helminths

Helminths are worms (either round, flat, or hook-like) and flukes. All are parasitic and are typically spread via the fecal-oral route. Pinworms are most commonly found in children and cause significant itching in the perianal area due to the eggs being laid outside the rectum. Flatworms, such as a tapeworm, can grow up to 50 feet long and live in the intestines. Hookworm and fluke infestations can easily penetrate the skin, and found in the blood, and invade organs such as the liver and lungs. Flatworms and flukes can cause significant weight loss and debilitation.

Prions

Although quite rare (one case per 1 million persons), prions are usually spread through eating meat, especially brain tissue that has been infected, or in even rarer cases, through corneal transplantation from a donor who had a prion infection (Centers for Disease Control and Prevention, 2010). Prions are extremely resistant to the typical methods used for killing most viruses, bacteria, and fungi. These organisms require a type of special cleaning and sterilization that can be especially hard on surgical instruments.

Mechanical Barriers

Mechanical barriers are intact skin and mucous membranes. They are the primary defense the body has against invading microorganisms and infection. Skin, being the largest organ of the body, serves as a first line of defense against harmful agents in the environment. It functions as a protective covering for the more delicate and vulnerable underlying tissues and organs.

The portals of exit and entry provide the means by which pathogens move in and out of the body. For example, pathogens most frequently exit or enter the body where the skin and mucous membranes meet, such as through the mouth, nose, and gastrointestinal or genitourinary tracts, as well as through a cut in the skin.

Chemical Barriers

Chemical barriers assist the skin and mucous membranes in fighting off invasive organisms by the secretion of tears, saliva, and mucus. Lactic and fatty acids, which inhibit the growth of bacteria, are excreted via sweat and the sebaceous glands. Secretions from the mucous membranes lining the respiratory, gastrointestinal, and reproductive tracts contain an abundance of a bactericidal enzyme called lysozyme. This same enzyme is found in tears and saliva. Stomach acid and digestive enzymes kill off most swallowed microorganisms. Mucus produced by the respiratory tract helps capture a variety of inhaled particles. Cilia (tiny hairs), which line the respiratory tract, trap organisms and debris and then propel them up and out of the body with a wavelike action.

Fever

Fever is one of the primary mechanisms the body has to prevent infection from an invading microorganism. Once the immune system has determined that an invasion is trying to occur, it signals the hypothalamus in the brain to raise the body temperature. In an effort to fight off the infection, the body increases the heart rate and respiratory rate due to the increased metabolic and oxygen demand at the cellular level. Shivering occurs to increase the core body temperature, but the surrounding environment feels cooler than the new core body temperature, and the patient may complain of “freezing to death.” It is at this point in the inflammatory response that fever is noticeably increased. In an attempt to decrease the body’s temperature via evaporation, diaphoresis (sweating) occurs, and it is not unusual for the patient to not want to be covered by a blanket or sheet. This increased heat in the body creates a hostile environment to the microorganisms, and a person whose immune system is intact is able to destroy them more efficiently. Once the threat of infection is no longer present, the immune system again signals the hypothalamus and the body is able to start cooling down on its own.

Nutrition

Poor nutrition predisposes a person to developing an infection because the body may not have sufficient protein stores to generate enough antibodies to help fight off an infection. The very young and the elderly have a less efficient immune system, and that is why it is important to ensure that these age groups have received the appropriate vaccinations and immunizations. Excessive stress is another factor that influences a person’s immune status. Stress can increase blood cortisol levels, which will decrease the anti-inflammatory response of the body.

Antigens

An antigen is a form of protein found on the outside of cells that has the ability to identify it as “self” (native) or “non-self” (foreign). Antigens can stimulate the immune response to wipe out microorganisms.

Bone Marrow

The bone marrow is a major component in the body’s defense system. Bone marrow plays an important role in the manufacturing of blood products that help the body defend itself against infection. These products are called leukocytes, neutrophils, macrophages, and lymphocytes.

Leukocytosis

Leukocytosis is an increased number of leukocytes (white blood cells), usually seen at the beginning of an infection when the person’s immune system has not been overly stressed. Leukocytosis is seen more often with bacterial than viral infections. When infection does occur, the bone marrow is stimulated to produce and release more leukocytes to help the body fight infection.

Phagocytosis

The process of phagocytosis is a form of innate immunity. This is the body’s first line of defense at the cellular level. Within the first few hours of the onset of the inflammatory process, the monocytes swell up (becoming macrophages) and migrate to the site of inflammation. Neutrophils, which are a type of leukocyte, are also released and have the ability to kill both aerobic and anaerobic organisms. After the macrophages and neutrophils engulf and destroy bacteria and other foreign matter, they die, producing an exudate (discharge) that is composed of tissue, fluid, dead cells, and their by-products. This exudate, usually yellow or green in color, is commonly known as pus and is a sign of infection.

Macrophages

Macrophages are monocytes (large leukocytes) that have left the bloodstream and have migrated into the tissues. They ingest and destroy pathogens and clear away the cellular debris and dead neutrophils in the latter stages of an infection. Macrophages cleanse the lymphatic fluid as it passes through the lymph nodes and perform a similar action on the blood as it passes through the liver and spleen.

Liver Cells