Pathogenic Microorganisms: Introduction, Types, Source of infection, symptoms and Diseases


Pathogens are defined as organism that are taxonomically widely diverse that cause diseases to their respective hosts with the severity of symptoms of diseases. It comprises not only viruses’ bacteria and fungi but also unicellular and multicellular eukaryotes.

Generally, depending on their relationship to the host, microorganism can be classified as saprophytes and parasites. Saprophytes are free-living microbes, which live on dead organism matter. Mostly, they are not capable of multiplying on living tissues and are thus not associated with infectious diseases. While parasites are established and multiply in living hosts. But a majority of parasitic microorganisms are harmless to host. For example, the large number of microorganisms are living on the skin, on the mucous membrane and in the digestive tract of man and animals are not causing any visible harm to the host. These parasitic microorganisms are called normal flora of the body or commensals, which live in complete harmony with the host without producing diseases in the host are called pathogens. Sometimes, when the resistance of the host is lowered, the normal flora of the body behaves as facultative pathogens.

 Infection: Infection is the entering and multiplication of a parasite in the host without necessarily producing the disease.

 Infectious diseases: It result from the interaction of parasites.

 Pathogenicity: It is the ability of microorganism to cause disease.

Virulence: It is the degree of pathogenicity exhibited by virulent organisms when introduced in to the host in very small quantity or, in other words, it is the capacity of a given strain of microbial species to produce diseases.

 Primary pathogens: They are the parasites which are capable of penetrating or evading the normal host defense mechanism of a healthy susceptible individual.

 Secondary pathogens or opportunistic parasites: These parasitic microorganisms are not capable of penetrating the normal defense mechanism of a a healthy susceptible host. But it can infect or cause diseases when host defense mechanism has been weakened as by wounds, prolonged diseases, old age, poisons, etc.


 Basically, there are two types of pathogens.

Extracellular parasites

They usually resist phagocytic engulfment and multiply in body fluids and tissues spaces.

Streptococcus pneumoniae, an encapsulated extracellular bacterium, produces a rapidly progressing acute pneumonia which quickly kills a person. The patient who survives this rapidly recovers because of the production of antibodies that neutralize the capsules antiphagocytic property.

Intracellular parasites

They usually multiply inside the host cells. The intracellular parasites, usually produce chronic diseases with less rapid onset, much longer recovery time is required even when the chemotherapy and a greater likelihood for clinical relapse. E.g., Mycobacterium tuberculosis. These features of intracellular infections are largely due to the protection provided by the host cell to parasite. Human antibodies have little access to intracellular pathogens and fail to eradicate infection and thus aid patient recovery. Intracellular pathogens are attacked by cell medicated immunity which eliminates the microbes by killing the infected host cells. Some of these pathogens may escape destruction by cell mediated immunity and cause recurrences months or years after recovery. Intracellular infection is also poorly controlled by chemotherapeutics agents that either fail to penetrate the infected host cells or are ineffective in cytoplasm.

 Source of Infection:


 This is the most common route for transfer of infection from one host to another. Salmonella and other members of the Enterobacteriaceae family remain in the intestine. These intestinal organisms leave the body in the feces and pollute the environment, such as water, soil, and food. since, these organisms remain viable for many weeks or many months, they are likely to be ingested by other susceptible hosts and to colonies in the intestinal tract after ingestion.

 Bacillus anthracis, Clostridium species, Leptospiral, Pseudomonas, Vibrio cholerae, Shigella are list of many species of bacteria that are transmitted by ingestion and excreted in faeces.


This is another prevalent route of infection, particularly in crowded environments, which aids in the inhalation of airborne bacteria for respiratory infections caused by Klebsella and Pseudomonas. These pathogens remain viable after inhalation in various locations in the respiratory tract or in a lymph node and causes infection. For example, certain viral diseases such as influenzas, small pox and chicken pox are transmitted by the respiratory route.

Mycobacterium tuberculosis, Streptococcus pneumonia, Corynebacterium diphtheriae, and Bordetella pertussis are few examples of pathogens transmitted through inhalation.

Sexually transmitted diseases

 The important diseases spread by this method are AIDS, Gonococcal urethritis, etc. These infections may also be transmitted indirectly when freshly contaminated bedding comes into touch with susceptible animals. Different pathogens infecting sexually are Herpes virus, HIV virus, Neisseria gonorrhea, Trichomonas vaginalis, etc.

Diseases transmitted by skin wounds

Staphylococcus, Streptococcus, Corynebacterium, Spherophorus, and other pyogenic bacteria commonly enter by this pathway. Many toxigenic species of clostridium like C. tetani (tetanus), C. oedematiens infect the host through the skin wound.  Again, for new pathogens like Actinobacillus lignieresii, Actinomycetes bovis, Nocordia farcinia, Bacillus anthracis, etc. skin wounds produce good source of entry into the host tissues.

 Insect bites

Many flies and insects transmit a number of diseases from one host to another. The most important diseases transmitted by insect bites are malaria, yellow, fever, Dengue, etc. Important diseases spread by non-biting insects and flies include typhoid, cholera, sleeping sickness, kala azar, spotted fever, dysentery, and others.  Important diseases spread by non-biting insects and flies include typhoid, cholera, sleeping sickness, kala azar, spotted fever, dysentery, and others. Insects that spread infection may be infected themselves, or they may simply operate as passive or mechanical carriers of infections. These are also called vectors. These insects spread infection by biting or depositing infective organisms on the skin, food, or other things.

Fig: Pathogenic Microorganisms

Development of Disease:

The development of diseases in an animal depends upon the interaction between three factors, namely the etiological agents, the host and the environment. The virulence capacity of these etiological agents (parasites) is generally attributed to three factors, namely infectivity, invasiveness, and toxigenicity.

Infectivity: The infectiousness of a microorganism depends upon its ability to get establishes upon or within the host by overcoming the defensive barriers of the host such as antibodies and phagocytes.

Invasiveness: Once microorganism infect the host, its existence depends on the production of chemical components, metabolic products and enzymes that can interact normal boy defenses. When parasite invades, complex changes occur and produces an infectious disease in the host.

 Toxigenicity: The normal cellular components or metabolic products of pathogenic organisms, which interfere with the activity of host cells are toxins, some highly pathogenic species are not very invasive but their main property is toxigenicity. Microbial toxins are either discharged into the surrounding fluids (exotoxins) or remain attached to the producing cell (endotoxins).

Development of Symptoms:

  • Fever
  • Toxemia
  • Diarrhea
  • Fatigue
  • Loss of appetite
  • Inflammation
  • Necrosis
  • Hypertrophy
  • Coughing
  • Difficulty in breathing

Defense mechanism against microorganism:

Innate Immunity

Physical Barriers: Skin and mucous membranes serve as physical barriers, preventing pathogens from entering the body.

Cellular Defenses: Pathogens are engulfed and destroyed by cells like macrophages, neutrophils, and natural killer cells.

Chemical Barriers: Stomach acid, enzymes in tears and saliva, and antimicrobial peptides all have antimicrobial capabilities that kill or limit the growth of germs.

Inflammatory Response: Inflammation is triggered in reaction to infection or damage, which helps isolate and remove microorganisms while also promoting tissue regeneration.

Adaptive Immunity

Cell-mediated Immunity: T lymphocytes identify and remove contaminated cells.

Humoral Immunity: B cells generate antibodies that bind to and destroy infections, preparing them for elimination by other immune cells.

Memory Response: Following an initial exposure to a pathogen, the immune system produces memory cells, allowing for a faster and more efficient response on subsequent exposures.

Leave a Comment

Your email address will not be published. Required fields are marked *