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Bacterial pathogens that cause infections or disease in humans have innate characteristic mechanisms with which they use to suppress the immune response of their host in their bid to establish a disease process. Initiation of an infectious disease process leads to the development of clinical signs and symptoms in the human host, and these syndromes helps to announce the presence of a bacterium or group of bacteria that are responsible for the disease process. Some of the basic characteristics of pathogenic bacteria are highlighted in this section.



Colonization is the first stage of bacterial infection. It is the establishment of a bacterial pathogen at the appropriate site of entry into the host. Portal of entry of a pathogen into the body of a host may include the nose, mouth, eyes, ear, vagina, or broken skin. Bacterial pathogens usually colonize host tissues that are in contact with the external environment, and this makes it easier for the pathogenic microbe to penetrate host cells and tissues in order to start the process of disease development. Colonization of host cells and tissues is paramount for a pathogen to establish an infectious disease process in the body of a human or animal host.


Pathogenic bacteria have the ability to be transferred from one person to another through direct body contacts, indirect body contacts, and feacal-oral route, through the bite of an insect vector or through airborne means. Fomites and droplet infections can also transfer pathogenic bacteria from one individual to another. Fomites are inanimate or non-living objects that can help to transmit infectious disease agents or infections from one person to another and from one location to another. They include door handles, sinks, spoons, plates, water, tables, chairs, as well as other inanimate surfaces or objects and food that help to transmit infectious agents and/or diseases to susceptible human or animal hosts. Some bacteria species produce spores which allow them to withstand harsh conditions until they find a suitable environment (e.g., the human body) where they can thrive effectively and dissipate their deleterious activities. Other infections can also be acquired by visiting or staying in certain places (e.g., hospital environments where nosocomial bacteria can be picked up). Pathogenic bacteria can also cross the placenta of a pregnant woman and go on to infect the foetus in utero, and they are a source of concern in hospital wards and operating rooms where they cause a range of infectious diseases. 


Adherence is the process by which pathogenic bacteria attaches itself to the body surfaces of its human host. The word adherence can also be used synonymously with attachment and adhesion. Bacteria produce chemical substances (e.g., adhesins) that allow them to bind specifically or non-specifically to surfaces (both inanimate and animate surfaces). Adhesins are macromolecules that bind a bacterium to a specific surface either in vivo or in vitro.The binding of pathogenic bacteria to the cell of a human host is a key requirement for disease development. Adhesion of bacterial cells to implants (e.g., catheters) in the body of a human host promotes the formation of biofilms which is of medical importance. Biofilm consists of multilayered microbial cell clusters embedded in an extracellular polysaccharide matrix, which facilitates the adherence of pathogenic microorganisms to biomedical, industrial and other surfaces in the environment. Generally, biofilms helps protect the pathogenic microorganisms from host immune system and antimicrobial attack. In other words, for a bacterium to be able to initiate a disease process in a human host, it must first of all attach successfully to specific host cells in order to release its virulence factors that promote its injurious effects in the host. And one of the ways of achieving this is through the use of adhesins and biofilms – which are important characteristics of pathogenic microorganisms.


Toxigenicity is the ability of bacteria to produce toxins that contribute to the development of a disease or infection. Toxins are products of pathogenic microbes which at low concentrations act on cells or tissues of a host to cause systemic damage. Pathogenic bacteria are known for their ability to produce a wide variety of toxins upon invading a host cell, and these substances help to increase their pathogenicity and virulence in vivo. Bacteria that produce toxins are said to be toxigenic in nature. Toxins, when produced, may be transported by blood and lymph fluids; and they cause cytotoxic effects at tissue sites distant from the original point of entry, invasion or growth of the bacteria producing the toxins. Toxigenicity or toxigenesis is one of the qualities of pathogenic bacteria which help to increase their pathogenicity and virulence, and thus contribute to their ability to cause disease or infection in the host.

The toxins produced by pathogenic bacteria can be of two types: (1) endotoxins – which are intracellular and are not elaborated by living intact microbial cells and (2) exotoxins – which are extracellular and are elaborated by living microbial cells. Endotoxins are cell-associated toxins, and they are released by a dead bacterial cell. They are lipopolysaccharides (LPS), and are located on the outer membrane of Gram-negative bacteria. Exotoxins are produced by a living bacterial cell, and they are usually proteins that act enzymatically. Pathogenic bacteria that produce toxins include Clostridium, Corynebacterium, Vibrio, Shigella, Bordetella, Escherichia, Pseudomonas Bacillus and Staphylococcus.


Pathogenicity is simply defined as the mechanism of infectious disease development caused by a pathogenic microbe (in this case a pathogenic bacterium) in a human host. It is the structural and biochemical mechanisms whereby pathogenic bacteria cause disease in a human host. Pathogenicity is the ability of a microbe to cause disease in a host. Certain features of pathogenic bacteria that help to enhance pathogenicity include fimbriae, pili, cell wall components, LPS and capsules.   


Virulence factors are microbial products (e.g., toxins) that enhance the degree of pathogenicity of a pathogenic microbe in a host. It is the capacity of a bacterial pathogen to cause disease. A virulent bacterial pathogen causes severe symptoms of a disease than a less virulent pathogenic microbe in the same individual. Virulence is usually the combination of invasiveness, toxigenicity and the state of immunity of the host in relation to the infecting bacterial pathogen.


Pathogenic bacteria have intrinsic factors that allow it to evade the immune system of its host. A pathogenic bacterium has the potential to produce disease or infection in a human host, but it will only do so if it has enough virulence factors or power to enter the host cells/tissues and overcome its defense mechanisms, i.e., the immune system. Otherwise, the immune system of the host will defend against the invading bacterial pathogen, thus restoring the body to its normal function. This is very possible if the invading pathogen is not too virulent in nature, and if the host’s immune system is still intact and strong.


Pathogenic bacteria must be able to enter its host tissues or cells, multiply and spread to other nearby cells, tissues and organs in order to properly establish a disease process. The process of microbial invasion into a host cell is important for the establishment of an infection or disease process because if the pathogenic organism fails to have access into the host or the host cell, then, the disease process may likely not occur. Invasion of the host or host cell is therefore important for the pathogen since it allows the organism the chance to evade other host molecules which might be directed towards it.

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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