2. Bacterial Pathogenesis

Severity of disease

Pathogens can cause a variety of different diseases, with some being more severe than others. Human bodies are nutrient-rich and can provide a pathogen with an ideal environment in which to grow and multiply.

The severity of infections that pathogens cause will vary. Some infections may be mild, while others can be life threatening. For example, the common cold is a mild viral infection compared with the lethal Ebola virus disease.

Diseases resulting from bacterial pathogens include:

  • tuberculosis
  • meningitis
  • food poisoning
  • gonorrhea
  • typhoid
  • chlamydia

Some scientists believe that viruses are not living organisms. Some reasons for this include:

  • they do not have cells
  • they cannot reproduce without invading a living cell
  • they do not actively respond to changes in their environment

Viral infections include:

  • influenza
  • rotaviruses
  • measles
  • mumps
  • HIV
  • coronaviruses that cause the common cold
  • SARS-CoV-2Trusted Source, which causes COVID-19

About 300 species of fungiTrusted Source are pathogenic to humans. As with bacteria and viruses, they can have a significant effect on human health.

Fungi cause many different types of illness, including:

  • asthma
  • skin and nail infections
  • lung infections, such as pneumonia
  • bloodstream infections
  • meningitis

Protozoa are responsible for most protist diseases. Protozoa are single cell microorganisms that feed on other microorganisms, organic tissues, and debris. Protist diseases include:

  • dysentery
  • malaria
  • African trypanosomiasis, or sleeping sickness

Parasitic worms cause many diseases, which includeTrusted Source:

  • lymphatic filariasis
  • onchocerciasis
  • schistosomiasis
2. Bacterial Pathogenesis

How do pathogens spread?

Pathogens can spread in a variety of different ways. For example, direct skin-to-skin contact during sex can lead to sexually transmitted infections (STIs).

Coughing or sneezing can cause pathogens to spread through tiny droplets in the air. These droplets can contain microorganisms, which other people breathe in.

Microorganisms can also travel straight into the gut when a person consumes contaminated food or water.

Bites from infected insects can also spread disease. For example, ticks with a bacterial infection can cause Lyme disease if they bite someone, and mosquitoes with a viral infection can cause Zika virus disease.

2. Bacterial Pathogenesis

What are pathogens?

Pathogens are organisms that can cause disease. The different types of pathogens and the severity of the diseases that they cause are very diverse.

In this article, we look at different pathogens, how they affect people, and the diseases they cause. We also explain how pathogens spread and how to reduce the risk of infection.

Different types of pathogens

EM image of feline calicivirus which is one type of pathogen
Bacteria, viruses, and fungi are all types of pathogens.

A pathogen brings disease to its host. Another name for a pathogen is an infectious agent, as they cause infections. As with any organism, pathogens prioritize survival and reproduction.

The human body’s immune system acts as a defense against pathogens. The body can easily fight off some pathogens, but others are potentially fatal.

There are five main types of pathogens:


Bacteria are microscopic pathogens that reproduce rapidly after entering the body. They can release toxins that damage tissues and cause illness.

Doctors typically prescribe antibiotics to treat bacterial infections, but some bacteria are becoming resistant to these drugs.

Not all bacteria are pathogenic, though. In the body, there are many types of harmless bacteria, and some may even support essential bodily functions.


Smaller than bacteria, a virus invades a host cell. It then replicates, producing hundreds and thousands of new viruses that go on to infect more host cells. Viruses can pass from person to person in various ways, including:

  • via respiratory droplets that travel through the air
  • through contact with the blood of a person with the infection
  • through contact with the bodily fluids of someone with the infection


There are thousands of species of fungi, some of which cause disease in humans. Common fungal skin conditions include athlete’s foot and ringworm. These conditions are contagious and can spread through person-to-person contact.

A study in Trends in MicrobiologyTrusted Source found that fungal pathogens are evolving a capacity for memory. They can use signals in the body to anticipate imminent threats to their survival, against which they can then prepare themselves.


These single cell organisms cause disease in their host. They infect other organisms to survive and reproduce.

Protist pathogens affect plants and food crops. Foods containing protists can cause dysentery, which is an infection of the intestines that causes diarrhea.

Protist pathogens can also be parasitic and live in other organisms, such as mosquitoes. Protists cause malaria through mosquito bites.

Parasitic worms

Parasitic worms, also known as helminths, are large enough for people to see with the naked eye, and they can live in many areas of the body. Some worms include:

  • Flatworms: These include tapeworms, which reside in the intestines.
  • Thorny-head worms: This type of worm lives in the intestines.
  • Roundworms: These worms can survive in the gastrointestinal tract and lymphatic system.
2. Bacterial Pathogenesis

Bacterial Pathogenesis and Antibiotic Resistance: A Growing Problem

Across the span of human history, pathogenic bacteria have been one of the main killers of human and animal populations, causing untold misery to human civilization. Throughout the 20th Century, medical science managed to make significant progress against pathogenic bacteria and the diseases they cause through the use of antibiotics.

However, due to antibiotics being over-prescribed by medical professionals and the over-use of antibiotics within the dairy and meat industries, there are several worrying setbacks in the fight against bacterial infection. New drug-resistant strains of bacteria including MRSA have emerged over the past few decades, and ‘old’ diseases such as tuberculosis have now adapted and are on the rise in developing countries where previously they were close to eradication.

Currently, medical professionals are researching several alternatives to antibiotics to increase the effectiveness of medical science in combating infection.

In Conclusion

The relationship between host and pathogen is a complex one that is constantly evolving. Pathogenicity depends on a variety of factors both belonging to the microorganism and the host and any perturbations in this relationship can have profound effects on the progression and severity of the disease caused.

Whilst modern medicine has largely managed to effectively treat a bacterial infection, growing issues such as antibiotic resistance show that there is clearly some way to go before we can completely eradicate disease.

2. Bacterial Pathogenesis

Bacterial Virulence Factors

Virulence factors aid in the bacteria’s ability to invade, cause disease, and evade host defense mechanisms. Some are listed below.

Adherence Factors: The ability to physically attach to mucosal sites can increase bacteria’s pathogenicity. Many pathogenic bacteria use pili to adhere to host cells.

Capsules: Protective capsules exist on many pathogenic bacteria, which helps to protect them against host defense mechanisms including phagocytosis.

Endotoxins and exotoxins: Lipopolysaccharide endotoxins can cause fever, blood pressure changes, and lethal shock. Exotoxins are secreted by pathogenic bacteria and include neurotoxins and cytotoxins.

Siderophores: Some pathogenic bacteria can use siderophores to compete with the host for iron, which is an essential growth factor.

Intracellular Growth

Most pathogenic bacteria multiply in tissue fluids rather than host cells, but some species (for example Rickettsia) can only multiply and proliferate within eukaryotic cells. Salmonella bacteria invade cells but do not require them for growth.

Host-mediated Pathogenesis

Sometimes, tissue damage can result from the host’s own immune system when responding to invasion and bacterial toxins. This is the case with diseases such as tuberculosis.

2. Bacterial Pathogenesis

Factors in Bacterial Pathogenicity

As mentioned, disease progression within a host organism is a complex and dynamic process. There is a multitude of factors both about the pathogen and the host which affect bacterial pathogenicity and thus cause any number of different outcomes for a previously healthy organism.

Bacterial Infectivity

It is in the best interests of the pathogen not to kill the host as its objective is to multiply. However, if there is an imbalance between host resistance and bacterial virulence, this may lead to increased pathogenicity and potentially fatal outcomes.

Host Susceptibility

A healthy, intact immune system, including phagocytic cells, is vital for the best possible defense against pathogenic infections. Initially, resistance is conferred by non-specific mechanisms, with specific immunity developing over time. Any number of factors can lead to a compromised immune system and therefore increased host susceptibility.

Factors that can affect host susceptibility include conditions such as HIV which cause immunosuppression, aging (older people are more susceptible to disease than younger patients) medication and treatment for conditions such as cancer, and in some cases diet and nutrition and the health of the host’s microbiome.

Host Resistance

Host resistance plays a large part in the severity of infection progression. Numerous attributes of the host come into play when the host is exposed to pathogens, both chemical and physical. These include mucosal surface secretions with several antibacterial factors and phagocytic cells which mediate inflammatory responses through complex signaling pathways.

Many lymphoid cells participate in this inflammatory response. To be able to effectively infect an organism, pathogenic microorganisms must be able to overcome these defense mechanisms and host attributes.

2. Bacterial Pathogenesis

What is Bacterial Pathogenesis?

The processes which lead to disease are complex and have many contributing factors both internal and external. This article will provide an overview of bacterial pathogenesis and its contributions to the development of disease within a host organism.

Pathogenic Bacteria

Pathogenicity – An overview.

A pathogen is a term that refers to a microorganism that causes disease in an organism. Pathogenicity is the ability of the pathogen to produce disease. Pathogenicity is expressed by microbes using their virulence, or the degree of the microbe’s pathogenicity.

Genetic, biochemical, and structural features that lead to the ability of the pathogen to cause disease are known as its determinants of virulence.  Genetic and molecular factors play a large part in a microorganism’s pathogenicity.

The relationship between host and pathogen is dynamic – each modifies the function and activities of the other. The relationship and, therefore, the degree to which disease progresses in the host organism is determined by the pathogen’s resistance and the resistance and susceptibility of the host organism, largely in part due to the effectiveness of the host’s defense mechanisms.

Bacteria can largely be organized into three groups. Frank or primary pathogens are considered to be probable agents of disease. Opportunistic pathogens become pathogenic following the perturbation of a host’s defense mechanisms (for example by wounds, aging, and disease.) Finally, there are nonpathogens, such as Lactobacillus acidophilus, which are present naturally within the host and rarely or never cause disease.