Microorganisms that survive on another living thing (human or animal) or inanimate item without producing disease are referred to as normal flora. The human body, like most other ecosystems on the planet, provides a vital habitat for millions of microorganisms such as bacteria, fungus, protozoa, and viruses. 

From shortly after birth until death, every human being’s skin and mucous membranes are home to a varied microbial flora. The human body, which has around 1013 cells, is home to about 1014 microorganisms on a regular basis. The typical microbial flora is made up of this bacterial population. The typical microbial flora is very stable, with certain genera occupying different bodily areas at different times in a person’s life. Microorganisms from the typical flora may help, damage, or exist as commensals with the host. Despite the fact that most members of the natural microbial flora found on human skin, nails, eyes, oropharynx, genitalia, and gastrointestinal system are innocuous in healthy people, these organisms commonly cause disease in sick people. Most researchers do not consider viruses and parasites to be part of the normal microbial flora since they are not commensals and do not help the host.

The Human Microbiome Project takes on the task of sequencing the human microbiota’s genome, with an emphasis on the microbes that live in the skin, mouth, nose, digestive system, and vaginal area. When it released its early results in 2012, it marked a watershed moment in the project’s history.


The host’s anatomy, physiology, susceptibility to infections, and morbidity are all influenced by the normal flora.

Until germ-free animals became accessible, it was not generally known that the normal flora has a significant impact on the host’s well-being. The investigator got germ-free animals by caesarean delivery and kept them in special isolators, allowing him to raise them in an environment devoid of detectable viruses, bacteria, and other organisms. Two intriguing discoveries regarding animals grown in germ-free environments were made. First, the animals which were free of germs survived nearly twice as long as their traditionally kept counterparts, and second, the principal reasons of mortality in the two groups were different. Infection commonly killed traditional animals, whereas intestinal atonia killed germ-free animals regularly.

Despite the fact that the bacterial flora may be unfavourable, research on antibiotic-treated animals show that the flora protects people against infections. Streptomycin was used to decrease the normal flora, and subsequently animals were infected with streptomycin-resistant Salmonella. Normally, around 106 organisms are required to develop a gastrointestinal infection, however infectious illness was caused by less than 10 organisms in streptomycin-treated mice with altered flora. After birth, humans’ typical flora normally develops in an orderly sequence, or succession, leading to stable populations of bacteria that make up the normal adult flora. The nature of the local environment, which is influenced by pH, temperature, redox potential, and oxygen, water, and nutrient levels, is the most important element influencing the makeup of the typical flora in a body area. The local setting is similar to a concerto in which one main instrument generally takes centre stage.

So, what does the regular flora have to do with anything? The flora appears to impact human anatomy, physiology, longevity, and, ultimately, cause of death, according to animal and human research. Although the causal link between flora and mortality and illness in humans is well established, the human microflora’s participation in these processes requires additional investigation.


The nature of the microenvironment influences the makeup of the dermal microflora, which differs from site to site. Each of the three areas of skin has its own bacterial flora: (1) axilla, perineum, and toe webs; (2) hand, face, and trunk; and (3) upper arms and legs. Partially occluded skin sites (axilla, perineum, and toe webs) contain more germs than non-occluded skin sites (legs, arms, and trunk). Gram-negative bacteria populate the axilla, perineum, and toe webs more commonly than dry skin regions.

The quantity of germs on a person’s skin remains essentially constant; bacterial survival and colonisation depend on a combination of factors, including skin exposure to a specific environment and innate and species-specific bactericidal action. Bacterial adhesion to epithelial surfaces also requires a high degree of specificity. Staphylococci, which make up the majority of the nasal flora, have a clear advantage over Viridans streptococci when it comes to colonising the nasal mucosa. Viridans streptococci, on the other hand, are rarely found in significant quantities on the skin or in the nose, but they dominate the oral flora.

The density of bacteria on the skin is inconsistent in the microbiology literature; one explanation for this is the diversity of techniques used to collect skin germs. For a particular skin area, the scrub technique produces the highest and most accurate counts. The majority of microorganisms dwell in the Stratum corneum surface layers and the higher portions of hair follicles. Some microbes, on the other hand, live in the deeper parts of the hair follicles and are immune to standard disinfection methods.


A typical nail’s microbiology is quite similar to that of the skin. Depending on what the nail comes into touch with, dust particles and other foreign things may become trapped behind it. These dust particles may contain fungus and bacilli in addition to existing skin flora. The most common fungus found beneath the nails are Aspergillus, Penicillium, Cladosporium, and Mucor.


Dental caries and periodontal disease, which afflict roughly 80% of the people in the Western world, are caused by the oral flora. Many brain, face, and respiratory problems are caused by anaerobes in the oral flora, which are commonly characterised by abscess development.

Anaerobes, Staphylococci, Neisseriae, Diphtheroids, and other bacteria are found in the pharynx and trachea, as well as those found in the normal oral cavity. The pharynx may also include pathogenic organisms such as Haemophilus, mycoplasmas, and pneumococci. Anaerobic microbes are also commonly reported. Pathogens (Neisseria meningitides, C. diphtheriae, Bordetella pertussis, and others) frequently colonise the upper respiratory tract, and it may be regarded the primary point of assault for such organisms.


Bacteria find the stomach to be a hostile habitat. It comprises of germs that have been ingested with food as well as those that have been expelled from the mouth. Helicobacter species that may colonise the stomach are linked to gastritis type B and peptic ulcer disease. In most people, aspirates of duodenal or jejunal fluid contain around 103 organisms per millilitre. The majority of the bacteria cultivated (streptococci, lactobacilli, Bacteroides) are considered transients. The lack of organisms in the upper GI system may be explained in part by rapid peristalsis and the existence of bile. Bacterial populations begin to rise farther up the jejunum and into the ileum, reaching 106 to 108 organisms/ml at the ileocecal junction, with Streptococci, Lactobacilli, Bacteroides, and bifidobacteria predominating.

Although infections are inhibited by the natural flora, several of its members can cause disease in humans. Intra-abdominal abscesses and peritonitis are caused by anaerobes in the digestive system. Appendicitis, cancer, infarction, surgery, or gunshot wounds all cause bowel perforation, which nearly invariably seed the peritoneal space and surrounding organs with the normal flora. Anaerobes can also wreak havoc on the gastrointestinal tract.

The animal microbiota has more information than the human microbiome. According to animal studies, unique filamentous bacteria adhere to ileal epithelial cells and alter host membranes with few or no negative consequences. Microbes have been found on gastrointestinal surfaces and in the Lieberkuhn crypts in dense layers. Other research suggests that the gut flora might influence immune response.


Flora which is prevalent in the vaginal area is determined by the host’s age, pH, and hormone levels. During the first month of life, Lactobacillus spp. is predominant in female newborns (vaginal pH, about 5). Glycogen release appears to stop around the age of one month and continues till puberty. Diphtheroids, S. epidermidis, Streptococci, and E. coli prevail at a higher pH during this time (approximately pH 7). Glycogen secretion restarts at puberty, the pH decreases, and women develop an adult flora dominated by Lactobacillus acidophilus, Corynebacteria, Peptostreptococci, Staphylococci, Streptococci, and Bacteroides. The pH increases again after menopause and the flora recovers to that of prepubescent females due to less secretion of glycogen. Yeasts (Torulopsis and Candida) are present in the vaginal area on a regular basis (10 to 30% of women); they can grow and can induce vaginitis.


Flora of the Conjunctiva is limited. Around 17 to 49% of culture specimens are found to be negligible. Lysozyme, which is produced in tears, may have a role in bacterial control by disrupting with the development of their cell walls. Corynebacteria, Neisseriae, and Moraxellae are cultivated when positive samples indicate bacteria. Staphylococci and streptococci are also prevalent, and Haemophilus parainfluenzae has been found in 25 percent of the total of Conjunctival samples, according to latest findings.


The normal human flora has been briefly described; however, the infectious processes of a species and the clinical syndromes in which they have been implicated have not yet been explored. A breach in mucosal surface frequently leads to the host becoming infected by various members of the natural flora. Infections with species of the regular human flora include caries, periodontal disease, abscesses, foul-smelling discharges, and endocarditis. Furthermore, host impairment or host defences might cause the normal flora to fail to inhibit transitory infections or for members of the normal flora to infiltrate the host. Both the cases may result into the death of the host.