Normal microbial Flora

The human body is home to different species of bacteria, viruses, fungi and protozoa. The great majority of these are commensals, or "normal flora," defined as organisms on or within the human host symbiotic life but rarely cause disease.

Anatomical sites where bacteria are usually discovered include the skin (staphylococci and diphtheroids), Oropharynx (streptococci, anaerobic spore-forming bacteria), colon (intestinal enterococci, bacilli) and vagina (lactobacilli).

To determine when an isolate is part of the typical flora instead of an invasive pathogen can be difficult. For example, cultivation of staphylococci skin contamination from a blood sample at the time of phlebotomy may represent or may indicate a potentially life-threatening bloodstream infections.

Useful clues include symptoms and signs of virus (e.g., cough, fever) and the presence of inflammatory cells (e.g., polymorphonuclear cells in the sputum and an increased share of immature neutrophils in the blood).

Isolation of an obligate pathogen Mycobacterium tuberculosis, for example, a website is diagnostic of virus. Fortunately, few of microorganisms are absolute pathogens. For example, Neisseria meningitidis, a leading bacterial cause of meningitis, can be grown from the oropharynx of so numerous as 10% of asymptomatic people, in which case the transient typical flora represents.

Even if asymptomatic, the host can serve as a carrier, transferring bacteria to sensitive people. Infections due to commensals which hardly lead to disease (e.g. Candida albicans) or organisms are ubiquitous in the environment that are generally not considered to be human pathogens (e.g., Mycobacterium avium complex; MAC) are referred to as opportunistic infections.

These infections occur almost exclusively in immune hosts such as HIV-infected patients or transplant recipients. The agents are opportunists in that they may be impaired host immunity trigger virus but rarely trigger disease in a healthy host advantage. The website from which an organism is grown is essential in differentiating colonization of virus.
Growth of a micro-organism of a mostly sterile customers such as blood, cerebrospinal fluid, synovial fluid (joint) or deep tissues of the body is diagnostic of virus. Bacteroides, the predominant genus of bacteria in the colon, for example, can activate intra-abdominal abscess and sepsis when the integrity of the Colon mucosa is violated.

Staphylococcus epidermidis, a common skin Commensalism, may lead to bacteremia after intravascular catheter placement. Knowledge of the common endogenous flora can be helpful in determining the cause of an infection and in the choice of empirical antibiotic therapy may aid. When the delicate symbiosis between the Commensalism and interfere with the host, the typical flora become overgrown by endogenous or exogenous organisms.

This phenomenon, which can be transient or persistent, is called colonization. For example, broad spectrum antibiotics destroy normal vaginal flora, such as lactobacilli, and let overgrowth of Candida (yeast) species. As a replacement for the typical flora within the environment of the hospital occurs, the settlers said it acquired nosocomially.

The distinction between hospitals and community-acquired infections has blurred in recent years, simply because a improve in medical care inside the home or skilled nursing facility among patients who previously would have long term hospitalization necessary.

For this reason, the broader term "healthcare-associated infections" is used to include both hospital patients and patients with frequent medical interactions (for example, residence in nursing home, outpatient hemodialysis, home intravenous antibiotics). Healthcare-associated infections are important because the organisms often resistant to multiple antibiotics.

Symptomatic infection, not uncommon colonization will progress. For example, people's Hospital for prolonged periods of time often colonized with gram-negative bacteria such as Pseudomonas aeruginosa. These people are then increased risk for life-threatening infections for example pseudomonas pneumonia.

Host defense mechanisms that serve to inhibit colonization by pathogenic bacteria consist of (1) mechanical cleanup, (2) and (3) Phagocytic killing, depriving it of organisms of necessary nutrients. Successful settlers have adapted to circumvent or overcome this defense. For example, Avoid gonococcal, the bacteria that cause gonorrhea, excretion in the urine by adhering to the mucosal epithelium of the urogenital tract with pili.

Pneumococcal resist phagocytosis by Encapsulation within a layer of mucus that impairs absorption by Neutrophil granulocytes. Some staphylococci extensive enzymes known as hemolysins which host destroy red blood cells, allowing them access to a necessary source of iron. Colonization of sites that are usually sterile or have couple of microbes is usually easier because there is no competition for nutrients from endogenous flora. Host Defense on these websites are, however, often powerful.

For example, the stomach normally sterile because some microbes can survive on a typical stomach pH of 4.0. Nevertheless, if antacids are used to reduce stomach acid acidity, colonization of the stomach and the trachea with gram-negative bacteria occurs quickly. The typical flora prevents colonization through several mechanisms. These organisms often have a selective advantage over settlers in that they are already in an anatomical niche are established.

This means that they are bound to receptors on the host cell and are able local metabolize nutrients. Numerous species of flora are able to make the typical Bacteriocins, proteins that are toxic to other bacterial strains or species. Finally, the normal flora promotes the production of antibodies that can react with the organisms colonize cross-.

For example, cross-reacts an antibody produced against E coli, a gram-negative bacterium normally found in the large intestine, with the polysaccharide capsule of a tribe meningitis-production of N meningitidis. When the normal flora is changed (for example, by the administration of broad-spectrum antibiotics), one bacterial species can dominate or exogenous bacteria can get a selective advantage, admits colonization and predisposing the host to infection.