Bacteria Health Article

Pathogenic bacteria

Although normal flora bacteria are not normally pathogenic, disease may result from invasion of normal flora into normally sterile areas or if the host immune system is deficient. When bacteria that normally reside in the GI tract (such as E. coli) are introduced to the urinary tract, for example, a urinary-tract infection may result. This is considered an endogenous infection.

Exogenous infections result from invasion of noncommensal organisms (i.e., those not normally found on the human body). Transmission of exogenous bacteria may occur by various routes, including inhalation of aerosolized organisms, ingestion (e.g., contaminated food or utensils), or direct contact of a wound or mucous membrane with organisms.

When bacteria first enter the body, local inflammation may be the first sign of infection. Physical symptoms such as pain, erythema (redness), edema (swelling), or pus formation result from the response of the immune system against the invading bacteria. If the bacteria spread to the bloodstream (bacteremia), they may disseminate to and colonize at various sites in the body.

VIRULENCE FACTORS. Bacteria have developed numerous mechanisms that allow them to invade a host and colonize an otherwise inhospitable site to cause disease. Many of these mechanisms enhance their ability to cause disease in humans; such traits are called virulence factors. Some common virulence factors include:•

• Bacterial growth. The byproducts of normal bacterial growth may cause tissue destruction if colonization has occurred in a normally sterile site. For example, Clostridium perfringens is a normal flora bacteria of the GI tract but may cause gas gangrene if it infects a wound or trauma site.
• Release of toxins. Some pathogenic bacteria produce proteins (toxins) that are inevitably toxic to the host. An endotoxin is composed of lipopolysaccharides found in the outer membrane of gram-negative bacteria. Exotoxins are proteins produced intracellularly and secreted by either gram-negative or gram-positive bacteria.
• Capsule formation. The polysaccharide layers of a capsule form a protective shield around a bacteria and help the cell to evade immune response.
• Internalization. Some bacteria are able to escape intra-cellular killing when internalized by phagosomes and go on to survive in the cytoplasm (e.g., Mycobacterium tuberculosis). In this way they are protected from anti-body-mediated immune responses.
• Granuloma formation. A granuloma is a lesion formed in response to infection by some intracellular pathogens. Viable bacteria are walled off in the granuloma and thus prevented from further colonization.
• Antigenic mimicry. A bacterial cell may be able to trick the immune system by presenting antigens (molecules recognized by antibodies) that are similar to host antigens. Immunological cells therefore have difficulty distinguishing between the bacterium and a host cell.

Antibiotic resistance

The emergence of bacterial strains that are resistant to treatment by current antibiotics is an important public-health concern. Antibiotics are chemical substances produced by microorganisms that inhibit bacterial growth or kill bacterial cells. Narrow-spectrum antibiotics target only a limited variety of bacteria, while broad-spectrum antibiotics have the ability to inhibit or kill a wide variety of bacteria.

Bacteria can resist the action of antibiotics using one or more of four basic mechanisms:

• inactivation or modification of the drug
• modifying the drug's target binding site
• decreasing uptake of the drug into the cell
• altering the biochemical pathway that the drug is targeting

In many developing countries, antibiotics are freely distributed as over-the-counter drugs, leading to their widespread use for viral or noninfectious illnesses. This practice, coupled with the overuse of antibiotics in veterinary medicine, farming, and plant culture, has contributed to the spread of antibiotic-resistant bacteria.

Common diseases and disorders

The following list describes some of the most common bacteria that are pathogenic to humans.

• Staphylococcus. Staphylococci are gram-positive bacteria found as part of the normal flora of most individuals. S. aureus is the causative agent of many infections, including toxic shock syndrome (TSS), staphylococcal food poisoning, impetigo, and furuncles (boils). S. saprophyticus causes urinary-tract infections in sexually active women. S. epidermidis may infect damaged or artificial heart valves and cause a condition called endocarditis.
• Streptococcus. Streptococci are gram-positive bacteria that commonly colonize the oropharynx (the area of the throat at the back of the mouth). Example syndromes include pharyngitis (sore throat), scarlet fever, necrotizing fasciitis (streptococci are popularly known as the "flesh-eating bacteria"), and rheumatic fever. S. pneumoniae is a common cause of bacterial pneumonia and meningitis.
• Neisseria. N. gonorrhoeae is the causative agent of gonorrhea, a leading sexually transmitted disease (STD). N. meningitidis is a leading cause of adult meningitis.
• Escherichia. E. coli is the most commonly encountered species of this genus. The bacteria is a common cause of gastroenteritis (inflammation of the lining of the stomach and intestines) but also causes urinary-tract infections and neonatal meningitis.
• Salmonella. Most Salmonella infections result from ingestion of contaminated food and lead to enteritis. The febrile (fever-inducing) illness typhoid fever is caused by S. typhi.
• Vibrio. The most commonly known Vibrio infection is cholera, caused by V. cholerae. Spread by ingestion of contaminated food or water, cholera infection is an important cause of diarrheal disease in developing countries.
• Clostridium. C. perfringens causes a variety of human diseases, including myonecrosis (gas gangrene), clostridial food poisoning, and soft-tissue infections (cellulitis and fasciitis). Tetanus (also known as lockjaw) is caused by C. tetani; C. botulinum causes food-borne botulism.
• Mycobacterium. Tuberculosis, caused by infection with M. tuberculosis, is a highly prevalent pulmonary disease. Hansen's disease (also known as leprosy) is caused by M. leprae.
• Chlamydia. Chlamydiae, once thought to be viruses because of their small size, cause numerous human diseases. C. trachomatis is the causative agent of conjunctivitis (inflammation of the outer surface of the eye), infant pneumonia, and urogenital chlamydia. Bronchitis, pneumonia, and sinusitis are often caused by C. pneumoniae.