Normal Microbial Flora
The human body harbors several species of bacteria, viruses, fungi, and protozoa. The excellent majority of these are commensals, or “normal flora,” defined as organisms that live symbiotically on or inside the human great number but rarely cause disease.
Anatomic sites where bacteria are usually discovered include the skin (staphylococci and diphtheroids), oropharynx (streptococci, anaerobes), large intestine (enterococci, enteric bacilli), and vagina (lactobacilli). calculating when an isolate is a part from the typical flora instead of an invasive pathogen may be difficult.
For example, culture of staphylococci from a blood sample may represent skin contamination at the time of phlebotomy or may indicate a potentially life-threatening bloodstream infection. Helpful clues include symptoms and signs of virus (eg, cough, fever) and the presence of inflammatory cells (eg, polymorphonuclear cells in the sputum and an elevated proportion of immature neutrophils within the blood).
Isolation of an obligate pathogen for example Mycobacterium tuberculosis from any website is diagnostic of virus. Fortunately, associate of microorganisms are absolute pathogens. for example, Neisseria meningitidis, a major bacterial cause of meningitis, can be cultured from the oropharynx of as numerous as 10% of asymptomatic people, in which case it represents transient typical flora.
already if asymptomatic, the great number can serve as a carrier, transferring bacteria to susceptible people. Infections resulting from commensals that hardly ever cause illness (eg, Candida albicans) or organisms ubiquitous in the ecosystem which are generally not considered human pathogens (eg, Mycobacterium avium complicate; MAC) are termed opportunistic infections.
These infections occur nearly exclusively in immunocompromised hosts such as HIV-infected sufferers or transplant recipients. The agents are opportunists in that they take advantage of impaired great number immunity to cause virus but rarely cause illness in a healthy great number. The website from which an organism is cultured is basic in differentiating colonization from virus.
Growth of any microorganism from a usually sterile website such as blood, cerebrospinal fluid, synovial (joint) fluid, or thorough tissues of the body is diagnostic of virus. For example, Bacteroides, the predominant genus of bacteria within the colon, might cause intra-abdominal abscesses and sepsis when the integrity from the colonic mucosa is breached.
Staphylococcus epidermidis, a shared skin commensal, can cause bacteremia after intravascular catheter placement. Knowledge from the shared endogenous flora may be helpful in calculating the cause of an infection and may aid in the choice of empiric antibiotic therapy. When the delicate symbiosis between the commensal and the great number is disturbed, the typical flora may be overgrown by either endogenous or exogenous organisms.
This occurrence, which may be transient or persistent, is called colonization. for example, general-spectrum antibiotics will destroy normal vaginal flora, for example lactobacilli, and allow overgrowth of Candida (yeast) species. When substitute from the typical flora occurs within the hospital surroundings, the colonizers are said to be nosocomially acquired.
The distinction between hospital-acquired and community-acquired infections has clouded in recent years, simply because of an enhance in medical care within the house or skilled nursing facility among sufferers who before would have required long-term hospitalization.
For this reason, the broader term “healthcare-associated infections” is used to include both hospitalized patients and patients with frequent medical interactions (eg, residence in nursing home, outpatient hemodialysis, home intravenous antibiotics). Healthcare-associated infections are meaningful because the organisms are often resistant to multiple antibiotics.
Not uncommonly, colonization will progress to symptomatic infection. for example, people hospitalized for extended periods frequently become colonized with gram-negative bacteria such as Pseudomonas aeruginosa. These people are then at increased risk for life-threatening infections for example pseudomonas pneumonia.
great number defense mechanisms that serve to hinder colonization by pathogenic bacteria be make up of consistently (1) mechanical clearance, (2) phagocytic killing, and (3) depriving organisms of necessary nutrients. Successful colonizers have alternation to evade or conquer these defenses. for example, gonococci, the bacteria that cause gonorrhea, avoid excretion in the urine by adhering to the mucosal epithelium from the urogenital tract with pili.
Pneumococci resist phagocytosis by encapsulation inside a slime inner that impairs uptake by neutrophils. Some staphylococci elaborate enzymes known as hemolysins that destroy great number red blood cells, consequently giving them access to a needed source of iron. Colonization of sites that are usually sterile or have really associate of microbes is usually simpler because there’s no competition for nutrients from endogenous flora. However, great number defenses at these websites are frequently vigorous.
for example, the stomach is typically sterile because few microbes can survive at the typical gastric pH of 4.0. Nevertheless, if antacids are used to decline gastric acidity, colonization from the stomach and trachea with gram-negative bacteria rapidly occurs. The typical flora prevents colonization via several mechanisms. These organisms frequently have a selective assistance over colonizers in that they’re already established in an anatomic niche.
This method that they are bound to receptors on the great number cell and are able to metabolize local nutrients. Numerous species of the typical flora are capable to create bacteriocins, proteins which are toxic to other bacterial strains or species. Finally, the normal flora contributes production of antibodies that may cross-react with colonizing organisms.
for example, an antibody produced against E coli, a gram-negative bacterium typically found in the big intestine, cross-reacts with the polysaccharide capsule of a meningitis-producing strain of N meningitidis. When the normal flora is changed (eg, by the administration of general-spectrum antibiotics), one bacterial species might predominate or exogenous bacteria might gain a selective advantage, permitting colonization and predisposing the great number to infection.