The term microbial flora refers to the population of microorganisms that are present in humans. Before getting into the various microorganisms, it is essential to understand some basic terminologies based on the host-parasite relationship. It is a complex relationship that can be either beneficial or harmful. The host is the large plant/animal where the parasite resides. This host acts as the reservoir for the parasite.

Commensalism-a type of relationship in which one organism is benefitted while the other is neither harmed nor benefitted.

Parasitism– a type of relationship in which one organism is benefitted while the other is harmed.

Mutualistic– a type of relationship in which both the organisms are benefitted. Ex-lichens

Pathogen– the organism that causes disease in the host.

Pathogenicity– the ability of the pathogen to cause disease in the host.

Virulence– the degree of pathogenicity, it depends on adherence (capsule, pilli, fimbriae, spikes) invasion, and toxicity.

Carrier state– an organism of high pathogenicity that remains normal

Colonization– multiplication of microorganisms without invoking the immune system

Opportunistic– pathogen that causes disease in impaired people but not in normal people

The flora is of two types, resident and transient. Residents are those that are relatively fixed whereas transient are those that occupy the skin for some days, are derived from the environment, and are not permanent. The resident microbes don’t cause harm if undisturbed, if disturbed, they can cause disease.

In order to establish a colony inside the human body, it is essential for the microorganism to enter the body. It does through the skin, nose, eyes, mouth, respiratory and genital cavities. Hence immunity is strong in these regions. Microbial flora won’t be present in any internal organs except the alimentary canal in neonates due to very high immunity.


100-10000 microorganisms are present in the skin and in sweat, sebaceous, and eccrine glands. High salt, low Ph, and drying make some unfavorable conditions for them. Staphylococcus epidermidis occupies the majority of the skin followed by S.aureus, micrococci, and diphtheroids that are responsible for causing acnes in children.


Very less in number due to constant blinking, high moisture, and lacrimal apparatus. Some species are Moraxella, diphtheroid, S.epdiermidis, and streptococci.

Nose and Nasopharynx

It is sterile at birth but starts to acquire flora at 3 months. It is the natural habitat for many bacteria. Some examples are Moraxella, diphtheroid, staphylococcus, and streptococci.

Respiratory tract

The upper tract is partially sterile by the action of phagocytes, mucous, and lysozyme. The species present are mostly gram-negative bacteria. The lower tract is completely sterile.

Oral cavity

Sterile at birth, consists of Moraxella and staphylococcus in neonates, streptococcus in children and spirochetes in puberty. Bacteria is present in the concentration of 10^8/ml of saliva.

Gastrointestinal tract

It is considered as the fermentation chamber for bacteria. Both beneficial and harmful bacteria are present in this region. Helicobacter pylori, Escherichia coli, Enterococcus, Lactobacilli, and Peptococcus are some prominent examples of flora in this region.

Genitourinary tract

Mycobacterium smegmatis is present in both males and females. Male sperm contains zinc which is antimicrobial, it contains alpha streptococcus. The vagina contains Lactobacillus acidophilus (Doderlein bacteria) only during the puberty period. This bacteria is absent before and after puberty due to the lack of glycogen and alkaline Ph.

The following image represents the characteristics that are associated with that of microbes.


Click to read about cellular adaptation

Click to read about environment