Inoculum size is defined as the number of invading pathogenic microorganisms that is sufficient enough to initiate an infectious disease process (i.e., an infection) in a susceptible human host. Pathogens must reach or exceed a particular threshold before they will be able to produce clinical signs and symptoms which are unique to a given disease process. Normally, the penetration of the delicate skin of a human being by a small number of pathogenic microorganism is less often than not capable of causing an infection or a disease in the host (though some exceptions can exist such as in HIV-1 infection where a minute microbe is required to establish a disease). It is noteworthy that the human body is prepared with immune system (innate or acquired) that normally wards-off pathogenic microbes upon gaining entry into the body (especially if the number of penetrating organisms is too small to overpower the host’s immunity). The host immune system is usually the first responder to the invading pathogen prior to administering any formal therapeutic support to the individual.


Pathogenic microorganisms enter the body of a human host via many sources or routes including the nose (inhalation), skin penetration (wound, abrasion and cuts), ingestion (through food and water), surgical means, insect bites (e.g., Plasmodium parasites), through blood transfusions and possibly through the transplant of organs. The inoculum size of an organism coupled to other host and pathogen factors is a determining factor whether an infection will occur or not after prior entry of a pathogen into a human host. Antiseptics are normally applied on wounds or on the skin prior to surgery or injection in order to effectively reduce the number of microorganisms on the site of cut or surgical penetration so that unwarranted or infectious amount of pathogens do not gain entry and initiate an infection in the host, during or after the surgery.

Virtually every anatomical site of the human body are preferred location for the entry of pathogens into the body of a human host, but some pathogenic microbes enter their host only through a particular site and the inoculum size of the invading organism is very critical to the initiation of an infection or a disease process. The continuous exposure of a human host to pathogenic microorganisms can increase its inoculum size over time, thus giving the invading pathogenic microbe the chance to establish an infection which can progress into a disease if not therapeutically and diagnostically managed.   

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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