Pathogenic bacteria enter the human body in various ways and through various medium or routes. After exposure to an infectious agent, the body becomes contaminated by the agent, and the pathogenic microorganism eventually develops disease or infection in the susceptible human or animal host following a defined pattern (Figure 1). It is the duty of clinical microbiologists to isolate and characterize the infecting pathogenic microorganism(s), and differentiate same from normal microflora so that the appropriate treatment can be administered in order to restore the host to its normal working condition prior to its invasion by the pathogen.


Some of the portal of entry of infectious agents into our body including bacteria, viruses and fungi are as follows:

Figure 1. Flowchart of disease development in a host. This pathway shows the development of disease in the body after exposure to an infectious agent.Photo courtesy: https://www.microbiologyclass.com
  • THE SKIN: The human skin is usually the first physical barrier that prevents the entry of pathogenic microorganisms into the body but any crack, abrasion, wound or cut on it allows infectious agents and their particles to easily gain entry into the body to cause disease. The bite of animals or arthropods (e.g. insects) can also serve as route via which some infectious agents gain entry into the human body. Pathogenic microorganisms must prevail over the normal intact skin (which is the primary defense against diseases) before they can cause infection or disease, and this can only happen when the skin is exposed either through injuries or burns. However, the human skin is occasionally dry throughout the day, and its dry nature does not allow pathogenic bacteria to easily thrive on it because these organisms require moisture for their growth. Most skin infections (e.g. acne) have been attributed to the over-secretion of sebum, an oily substance which is an ideal environment for most Gram-positive bacteria such as Propionibacterium species to thrive. Inflammatory reactions stimulated by the growth of the bacterium within the skin glands promote the development of acne or pimple on the surface of the skin especially on the face.     
  • THE GASTROINTESTINAL TRACT/ALIMENTARY CANAL: The gastrointestinal tract (GIT) or alimentary is largely responsible for the digestion of food but this very important part of the body can also serve as route via which pathogenic microorganisms enter the body. Pathogens can enter the body via the GIT through the ingestion of microbe-contaminated food or food products; and this entry is usually through the mouth. Poor hygiene during the preparation and processing of foods gives pathogens the chance to enter food products and either contaminate them directly or produce harmful toxins that cause clinical disease in humans after the consumption of such foods.
  • THE RESPIRATORY TRACT: The respiratory tract is usually the preferred route via which most human pathogens including bacteria, viruses and especially fungi are transmitted to susceptible human hosts. For example, most pathogenic fungi gain entry into the body via the inhalation of fungal spores or conidia which are ubiquitously found in the atmosphere. The respiratory tract is also the preferred route via which some bacterial and viral pathogens gain entry into the human body. Diseases or infections transmitted via the respiratory tract are generally known as droplet infections because the causative agents of such diseases (e.g. Corynebacterium diphtheriae, Mycobacterium tuberculosis, Haemophilus influenzae and Bordetella pertussis) are passed for infected persons to susceptible non-infected human hosts through the droplets of saliva, sputum or aerosols emanating from the diseased individuals.
  • THE GENITAL TRACT: Most sexually transmitted diseases (STDs) such as gonorrhea, genital warts, urethritis, genital herpes infection and syphilis are transmitted via the genital tract especially when people indulge in illicit unprotected sex with an infected partner. Some STDs are congenital in acquisition in the sense that the disease can be acquired or transmitted to the newborn during delivery as the child passes through an infected vagina. Examples of such STDs that are congenital in nature include neonatal pneumonia, neonatal syphilis, neonatal sepsis, neonatal meningitis, and neonatal ophthalmia. Pathogens can also enter the body via the urinary tract.
  • THROUGH BLOOD TRANSFUSION: Blood transfusion is the medical procedure in which blood drawn from a healthy individual (the donor) is transferred to a sick patient (the receiver) via the vein in order to take care of a possible blood loss in the receiver. This procedure is an important medical practice that helps to revitalize anaemic patients (i.e. those with a condition of blood loss) but it can also serve a route via which pathogenic microorganisms enter the body. When blood is not properly screened for the presence of infectious agents before it is being transfused, the receiver becomes infected by even more deadly pathogens such as the causative agents of AIDS and hepatitis amongst other diseases contracted via this medium.   
  • THROUGH CONGENITAL ROUTE: Some infectious agents or diseases are transmitted vertically from mother to the unborn child or foetus via the placenta. A variety of sexually transmitted infections (STIs) such as HIV/AIDS are usually transmitted via this route. Such infections are known as congenital infections because they are transmitted from an infected mother to her unborn child in utero (i.e. in the womb or within the uterus).  
  • THROUGH THE EYES OR CONJUNCTIVA: Infectious agents can also gain entry into the body through the eye either by direct contact or indirectly with contact with contaminated fomites. The eyes of neonates can become contaminated with pathogens during delivery especially in cases where the mother’s vagina is harbouring pathogens.  
  • THROUGH PARENTERAL ROUTES: The use of shared syringes during the administration of parenteral drugs medically or by drug users can provide a convenient medium for the transmission of pathogenic microorganisms from one person to another. Examples of infectious agents transmitted via this route include human immunodeficiency virus (HIV) and hepatitis virus amongst others. Syringes for the medical administration of parenteral drugs should not be shared between patients but instead, different syringes should be used for different patients and for different procedures to prevent the transmission of microbes in the process.   

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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