Types of Pathogens

Pathogens are microorganisms that cause various diseases in plants and animals. Most pathogens are parasites, i.e. they harm their hosts. The main five categories of pathogens are viruses, bacteria, fungi, protozoa and helminths.

Pathogens are infectious agents. They can enter the body by various means. They can be soil-borne, foodborne, airborne or can spread through direct contacts, such as through body fluids or sexual contact. Their mode of replication inside the body and pathogenesis is different for various types of pathogens. Pathogens have various mechanisms to adapt to the internal environment in their hosts, for their survival. They hinder the normal functioning of the body and may cause various morphological or physiological damages.

Let’s learn more about various types of pathogens and some of the common diseases caused by them.

Bacteria

Bacteria are the most abundant microorganisms present almost everywhere. They are prokaryotes. The size ranges from 0.15 to 700 μm. Although most bacteria are beneficial to us, some bacteria are pathogenic and cause diseases in plants and animals. They can produce toxins or elicit a strong immune response to damage host cells. Some of the important characteristics of bacteria are as follows:

• Bacteria are the members of the kingdom Monera.
• They are free-living as well as parasites.
• On the basis of their shapes, they are classified as bacillus (rod-shaped), coccus (spherical), spirillum (spiral) and vibrium (comma-shaped).
• They are autotrophs as well as heterotrophs.
• The bacterial cell membrane is made up of phospholipids. The cell wall is made up of peptidoglycan (polysaccharides and amino acids).
• Bacteria have prokaryotic cells. They lack a nucleus and other membrane-enclosed organelles.
• The bacterial genome is located in an irregular shaped region called nucleoid in the cytoplasm.
• Electron transport occurs through the plasma membrane during processes, such as respiration and photosynthesis.
• Bacteria are classified as Gram-positive and Gram-negative, based on their reaction to the Gram stain and the structure of the cell envelope.
• Gram-positive bacteria have a thick cell wall. It consists of thick peptidoglycan layers and teichoic acid.
• Gram-negative bacteria have a thin cell wall. It is made up of a thin peptidoglycan layer and lipopolysaccharides.
• A layer of glycocalyx surrounds the cell wall and makes the outermost layer of the cell envelope. The structure and constituents of the glycocalyx layer differ from one bacterium to another. It can be in the form of a loosely held slime layer or a thick and tough capsule.
• Many bacteria possess flagella. It is used for motility.
• Fimbriae are present in many Gram-negative bacteria and some Gram-positive bacteria. They have a role to play in attachment and virulence.
• Bacteria mainly reproduce by binary fission. It is a kind of asexual reproduction.
• Bacteria also show primitive types of sexual reproduction through direct DNA transfer from one bacterium to another by the process of conjugation, transformation or transduction (virus-mediated).

Some of the common diseases caused by pathogenic bacteria are as follows:

Diseases Caused by Bacteria in Humans:

Diseases Caused by Bacteria in Plants:

Viruses

Viruses are not considered as living organisms. They can only replicate within the host. They contain DNA or RNA as a genome, which is enclosed within a protein coat. They are very small in size and the size of the virus ranges from 20 to 300 nm. Some of the important characteristics of viruses are as follows:

• Viruses are obligate parasites. They are inactive outside the host cell.
• The genetic material is either DNA or RNA. The genome may be circular or linear.
• The genetic material could be single or double-stranded RNA or DNA. Generally, plants are infected with single-stranded RNA viruses and animals are infected with single or double-stranded RNA or double-stranded DNA viruses.
• The genetic material is present inside the protein coat called the capsid.
• Many viruses have an outside envelope made up of lipids.
• Viruses that infect bacteria are called bacteriophages.

Some of the common diseases caused by viruses are as follows:

Diseases Caused by Viruses in Humans:

Diseases Caused by Viruses in Plants:

Protozoans

Protozoans are unicellular eukaryotes. They lack a cell wall like animals. They are sometimes referred to as “one-celled animals”. They are heterotrophs. They are either free-living or parasitic. They cause various diseases in humans. Some of the important characteristics of protozoans are as follows:

• They are single-celled eukaryotes.
• The size ranges from 1 µm to several mm.
• They are heterotrophic. They show phagocytosis and pinocytosis.
• They have various organs for locomotion, such as flagella, pseudopodia and cilia.
• On the basis of their locomotory organs, they are classified as Flagellates or Mastigophora, Amoebae or Sarcodina (have pseudopodia), Sporozoa (spore-producing and lack locomotory organs), Ciliates, or Ciliophora.
• They reproduce by both asexual and sexual means.

Some of the common diseases caused by parasitic protozoans are as follows:

Diseases Caused by Protozoans in Humans:

Diseases Caused by Protozoans in Plants:

Helminths

Helminths include multicellular parasitic worms, such as roundworms, flatworms, tapeworms, etc. They can be seen by the naked eye. Worms live in the host’s body, e.g. in the gastrointestinal tract, lymphatic system, etc. They derive nourishment from the host and find shelter inside the body. They can live in the body of the host for years by manipulating the immune system of the host. Parasitic worms cause various diseases to humans and other animals.

Some of the common diseases caused by helminths in humans are as follows:

Diseases Caused by Helminths in Humans:

Fungi

Fungi are a group of eukaryotic organisms. They are heterotrophs. They are mostly saprophytes that absorb organic matter from dead and decaying substrates. Parasitic fungi derive nutrients from living plants and animals. They have a cell wall made up of chitin. They reproduce by vegetative, asexual and sexual reproduction. Some of the common features of fungi are as follows:

• They are found in warm and humid places. Fungi are free-living or parasites. They are found in soil, air and water.
• Fungi are mostly filamentous. The filamentous body is called hyphae. Mycelium is the network of hyphae.
• Some hyphae are aseptate and contain many nuclei. These are known as coenocytic hyphae.
• The fungal cell wall is made up of chitin and other polysaccharides.
• Vegetative reproduction takes place by fragmentation, budding and fission. They reproduce asexually and sexually by spore formation. Conidia, sporangiospores and zoospores are asexual spores and oospores, ascospores and basidiospores are sexual spores.

Some of the common diseases caused by pathogenic fungi are as follows:

Diseases Caused by Fungi in Humans:

Diseases Caused by Fungi in Plants:

This was in detail about types of pathogens and diseases caused by them. Explore notes on other important concepts related to NEET, only at BYJU’S.

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