THE NORMAL FLORA CONCEPT
The phrase "normal flora" refers to the several types of bacteria and fungi that are long-term inhabitants of specific body locations, such as the skin, oropharynx, colon, and vagina. Although they can exist in asymptomatic humans, viruses and parasites (protozoa and helminths), the other two major families of microorganisms, are typically not regarded as part of the normal flora. One term used frequently to describe the common flora organisms is commensals. Commensals are organisms that live off of another host without causing harm to it.
The typical flora is commonly referred to as the human microbiome. The microbiome now includes a large number of newly discovered microorganisms that have been identified by advanced molecular diagnostic techniques. Researchers are looking at how these bacteria affect immune system performance and whether they are the source of disease.
The number and type of members of the typical flora differ from one site to the next. The internal organs are often sterile, even though the normal flora widely populates numerous regions of the body. Except for the rare passing organism, the central nervous system, blood, lower bronchi and alveoli, liver, spleen, kidneys, and bladder are all free of organisms.
There is a difference between the carrier state and these organisms' existence. Although technically speaking, everyone carries microbes, the phrase is not typically used in a medical context for that purpose. The word "carrier" suggests that a person may be a source of infection for others since they may harbor a potential pathogen. When someone has an asymptomatic infection or has recovered from a condition but still carries the organism and may shed it for an extended length of time, this term is most commonly used.
Additionally, a difference must be established between the permanent inhabitants of the typical flora and the organism that has colonized the individual.
Although the regular flora creatures have colonized us all in a sense, the term colonization usually refers to the acquisition of a new organism. The new organism may either produce an infectious disease or be destroyed by our host's defenses when it colonizes or attaches itself and grows, usually on a mucosal membrane. Moreover, the individual who has been infected by a new organism can spread it to other people, serving as a reservoir for infection.
Three important mechanisms exist in which components of the normal flora contribute to the preservation of health as well as the cause of disease:
(1) They have the potential to spread illness, particularly to those with weakened immune systems. These organisms may be infected in different areas of the body even when they are not in their normal anatomical place.
(2)They function as a safeguarding defense mechanism for the host. On the skin and mucosa, nonpathogenic resident bacteria occupy attachment sites that can obstruct the colonization of pathogenic bacteria. Colonization resistance is the capacity of normal flora members to inhibit the growth of infections.
Pathogens may proliferate and spread disease if the natural flora is inhibited. Antibiotics, for instance, can alter the usual flora of the colon, which promotes the growth of Clostridium difficile, an antibiotic-resistant bacteria, and the development of pseudomembranous colitis.
SKIN'S NORMAL FLORA
Staphylococcus epidermidis is the most common organism. Although it is not harmful to the skin, it can cause illness when it gets to particular areas, like prosthetic joints and mechanical heart valves. Compared to Staphylococcus aureus, its harmful sibling, it is significantly more commonly discovered on the skin. About 10^3–10^4 organisms/cm^2 of skin are present. While some of them are situated in hair follicles and serve as a reservoir to replenish the superficial flora after hand washing, the majority are found superficially in the stratum corneum. Propionibacterium and Peptococcus are examples of anaerobic organisms that are found in the lower dermal follicles, where there is less oxygen tension. One prevalent skin condition is Propionibacterium acnes anaerobe which has a role in the development of acne.
Yeast Candida albicans is also a part of the skin's natural flora. When needles puncture the skin, as they do in patients receiving intravenous medication or those with catheters, it can enter the bloodstream. For individuals with weakened cell-mediated immunity, it is a significant contributor to systemic infections.
RESPIRATORY TRACT'S NORMAL FLORA
The mouth, throat, and nose are home to a diverse range of species, but the lower bronchi and alveoli usually have few or none at all. Many kinds of staphylococci and streptococci populate the nose, with S. aureus being the most common pathogen. Healthcare workers might link nasal, cutaneous, or perianal carriage to sporadic disease outbreaks caused by this bacterium, especially in the newborn nursery.
A combination of Neisseria species, S. epidermidis, and viridans streptococci are found in the throat. These nonpathogens reside at attachment sites on the mucosa of the pharynx and prevent the growth of S. aureus, Neisseria meningitidis, and Streptococcus pyogenes, respectively.
Viridans streptococci account for roughly half of the bacteria found in the mouth. As a member of the viridans group, Streptococcus mutans is particularly interesting since it is present in dental plaque, which is the precursor to caries, in high concentrations (10 10 /g). The bacteria secrete high molecular weight, gelatinous glucans, which make up the plaque on the enamel surface. A significant amount tooth acid is produced by the confined bacteria, demineralizing the enamel and causing caries. Subacute bacterial endocarditis is primarily caused by viridans streptococci, including S. sanguinis. During dental surgery, these microorganisms may enter the circulation and adhere to injured heart valves.
In the gingival crevices, where oxygen concentration is very low, anaerobic bacteria, including species of Bacteroides, Prevotella, Fusobacterium, Clostridium, and Peptostreptococcus, are detected. These microorganisms have the potential to induce lung abscesses if inhaled, particularly in sick patients who don't take good care of their teeth. Furthermore, Actinomyces israelii, an anaerobic actinomycete that can result in abscesses of the jaw, lungs, or belly, finds its natural home in the gingival crevices.
FAUNAL TEXTURE OF THE INNER TRACT
Because of its low pH, the stomach of a normal fasting person contains relatively few microbes. Yeasts, including Candida albicans, lactobacilli, and streptococci are typically found in modest amounts in the small intestine.
The terminal ileum is home to a greater concentration of these organisms.
The majority of bacteria in the body are found in the colon. Bacteria make up around 20% of the feces, or about 10^11 organisms/g. Table 6-3 enumerates the principal microorganisms identified in the colon. Keep in mind that anaerobes make up over 90% of the fecal flora, with Bacteroides fragilis being the most significant. Coliforms are the most prevalent facultative bacteria, with Escherichia coli being the most significant type.
Extraintestinal illness is largely influenced by the normal flora of the intestinal system. For instance, B. fragilis is a significant cause of peritonitis linked to intestinal wall perforation following trauma, appendicitis, or diverticulitis, and E. coli is the primary cause of urinary tract infections. Other significant anaerobic pathogens are Fusobacterium and Peptostreptococcus. Other significant facultative bacteria are Pseudomonas aeruginosa, which can cause a variety of infections, especially in hospitalized patients with weakened host defenses, and Enterococcus faecalis, which causes endocarditis and urinary tract infections. P. aeruginosa can be found in soil, water, and 10% of typical feces.
The main natural flora may be suppressed by antibiotic medication (e.g., with clindamycin), which might lead to an overgrowth of a rare bacterium like the toxin-producing Clostridium difficile, which can cause severe colitis. When some antibiotics, such neomycin, are taken orally in advance of gastrointestinal surgery in an attempt to "sterilize" the gut, the normal flora gradually returns to normal levels after being significantly reduced for a few days.
NORMAL GENITOURINARY TRACT FLORA
Adult women's vaginal flora is mostly made up of Lactobacillus species. The acid that maintains the adult woman's vagina's low pH is produced by lactobacilli. Before puberty and after menopause, lactobacilli are infrequent, vaginal pH is high, and estrogen levels are low.
Since the inhibition of lactobacilli by antibiotics can result in the overgrowth of Candida albicans, lactobacilli appear to prevent the growth of potential pathogens. Candida vaginitis may arise from an overabundance of this yeast.
Members of the fecal flora can colonize the vagina, which is situated near the anus. For instance, germs like Enterobacter and Escherichia coli are present in the introitus of women who are prone to repeated urinary tract infections.
In the vagina, 15% to 20% of women who are fertile harbor group B streptococci. This bacteria, which is acquired during delivery, is a major contributor to sepsis and meningitis in infants. About 5% of women have S. aureus colonization in their vagina, which puts them at risk for toxic shock syndrome.
In a healthy individual, the urine in the bladder is sterile; but, as it passes through the outermost parts of the urethra, it frequently becomes polluted with nonhemolytic streptococci, S. epidermidis, coliforms, and diphtheroids. Mycobacterium smegmatis, an acid-fast bacterium, is present in the secretions surrounding the urethra of women and uncircumcised men. Staphylococcus saprophyticus, a urinary tract infection cause, is found in the skin surrounding the genitourinary system.
