Defintion of pathogen

Pathogen
A pathogen is defined as any organism capable of invading the body and causing disease.

Such an organism is said to be pathogenic.

Koch isolated and identified organisms such as Mycobacterium tuberculosis and Bacillus anthracis, where the isolation of the organism only occurred in the presence of disease.

It is to define these bacteria as pathogens.

They also fulfill Koch’s further definition of a pathogen that introduction of a pure culture of the organism into the healthy host can cause the disease.

This definition works well for many bacteria, but does nor fully describes the complex interactions between microbes and humans which more recent understanding has revealed.

For instance, Escherichia coli is found in huge numbers in the healthy human bowel, and could therefore be defined as non-pathogenic.

E. coli is also important cause of diarrhoeal disease and potent enterotoxins and other pathogenic determinants have been described in some strains.

E. coli can therefore behave as a pathogen or as a colonizer depending on various circumstances.

A broader definition of a ‘biological agent’ used in European Union legislation is: ‘any microorganism, cell culture or toxin capable of entering the human body and causing harm’.

Changes in medical practice mean that increasing numbers of patients are immunocompromised as the result of either disease or treatments.

In such patients organisms which are usually non-pathogenic, such as saprophytic fungi, may act as pathogens.

Intensive therapy medicine, with insertion of intravascular cannulae, allows Staphylococcus epidermitis, a normal part of the skin flora, to enter the cannula and cause blood-borne infection: behaving as a pathogen.
Definition of pathogen

Infective Parasitic Pathogen: Toxoplasma gondii

Toxoplasma gondii is a protozoan parasite which can be transmitted by fecal-oral contamination. Cats are the original hosts for this protozoa. They excrete microscopic in active forms of this protozoa in their feces. Farm animals (notably sheep and pigs) become infected by consuming feed and water contaminated by barn cats’ fecal material. 

Active forms of this parasite then multiply with the farm animals and encyst themselves in the brain, heart muscle, other skeletal muscle, and liver. The cysts are microscopic and can exists as long as the farm animal lives. 

When these animals are slaughtered to provide meat, the raw meat contains the cysts which can then infected humans if it is eaten raw or not heated sufficiently to inactive various forms of this parasite. About 30% of all fresh pork is infected and is the main meat source of Toxoplasma gondii in the United Sates. 

If cutting and grinding equipment is not thoroughly washed and sanitized, other raw meats such as ground beef can also become contaminated. About 5% of the ground beef sold in supermarket contains some pork because the grinding equipment is not properly cleaned between grinding operations. Cysts of this protozoa are also found in wild game meats such as elk, moose, and venison. 

Symptoms of the disease in humans are fever, muscle aches, headaches, loss of appetite and sore throat. Other symptoms will appear, depending upon the internal organs involved. In pregnant women, these parasites can be carried by way of the placenta to fetal tissue. If fetuses are infected, spontaneous abortions may occur. Most infected infants show no obvious symptoms at birth, but will show signs of eye damage and mental retardation later in life. 

It is estimated that there are over 3,300 cases of congenital Toxoplasmosis each year resulting in 450 deaths of infants and young children. Other surviving infected children are mentally retarded as a result of this parasitic infection. 

Infective Parasitic Pathogen: Toxoplasma gondii

The definition of Pathogen

The definition of Pathogen
A pathogen is defined as any organism capable of invading the body and causing diseases. Such an organism is said to be pathogenic.

Robert Koch isolated and identified organisms such as Mycobacterium tuberculosis and Bacillus anthracis, where the isolation of the organism only occurred in the presence of disease.

It is easy to define these bacteria as pathogens. They also fulfill Koch’s further definition of a pathogen, that introduction of a pure culture of the organism into a health host can cause the disease.

This definition works well for many bacteria, but does not fully describe the complex interactions between microbes and humans which more recent understanding has revealed.

For instance, Escherichia coli is found in huge numbers in the healthy human bowel, and could therefore be defined as non-pathogenic.

E. coli is also an important cause of diarrheal disease and potent enterotoxin and other pathogenicity determinants have been described on some strains.

E. coli can therefore behave as a pathogen or as a colonizer, depending on various circumstances.

Changes in medical practice mean that increasing numbers of patients are immunocompromised as the result of either disease or treatments.

In such patients organisms which are usually non-pathogenic, such as saprophytic fungi, may act as pathogens.

Intensive therapy medicine, with insertion of intravascular cannulae allows Staphylococcus epidermis, a normal part of the skin flora, to enter the cannula and cause blood borne infection: behaving as a pathogen.
The definition of Pathogen

Lower respiratory tract infection

Lower respiratory tract infection
The principal function of the respiratory tract is gas exchange. It is therefore expose to the gaseous environment continuing particulate organic material such as bacteria, viruses and spores.

The entire respiratory tract is constantly exposed to air but the majority of particles are filtered out in the nasal hairs and by inertial impaction with mucus-covered surfaces in the posterior nasopharynx.

The epiglottis, its closure reflex and the cough reflex all reduce the risk of microorganisms reaching the lower respiratory tract.

Particles small enough to reach the trachea and bronchi stick to the respiratory mucus lining their walls and are propelled towards the oropharynx by the action of cilia (the mucociliary escalator).

Antimicrobial factors present to respiratory secretions further disable inhaled microorganisms.

They include lysozyme, lactoferrine and secretory IgA.

Particles in the size range 5-10 um may penetrate further into the lungs and even reach the alveolar air spaces.

Alveolar macrophages are available there to phagocytes potential pathogens. If these are overwhelmed, neutrophils can be recruited via the inflammatory response.

The defenses of the respiratory tract are a reflection of its vulnerability to microbial attack.

Acquisition of microbial pathogens is primarily by inhalation, but aspiration and mucosal and haematogenous spread also occur.

Individuals with healthy lungs rarely have any bacteria beyond the carina.

Respiratory pathogens have developed a range of strategies to overcome host defences. Influenza virus for example has specific surface antigens that adhere to mucosal epithelial cells.

The virus also undergoes periodic genetic reassortment , resulting in expression of novel adhesions to which the general population has no effective immunity.

Streptococcus pneumonia and Haemophilus influence both produce an enzymes (IgA protease) capable of disabling mucosal IgA.

Both of these species , other capsulated bacteria and mycobacteria are all resistant to phagocytosis. Penetration of local tissue is usually required before damage occurs. Although viruses using the common cold appear to be an exception.
Lower respiratory tract infection

Campylobacter jejuni

Campylobacter jejuni Campylobacter jejuni is considered to be a pathogen principally of veterinary significance only slightly more than decade ago, Campylobacter jejuni (formerly known as Vibrio fetus) was known to cause abortion in sheep. 

Following the development of procedures for detecting the organism in stool specimens, Campylobacter jejuni became recognized as a leading cause of acute bacterial gastroenteritis in humans. 

Campylobacter jejuni is a gram-negative, microaerophilic, thermophilic rod that grows best at 42°C (107°F) and low oxygen concentrations. 

These characteristics are adaptations for growth in its normal habitat – the intestines of warm-blooded birds and mammals. 

Several closely related species with similar characteristics, C. coli, C. fetus, and C. upsalienis, may also cause disease in man but are responsible for less than one percent of human infections annually. Evidence suggests that Campylobacter is responsible for at least as many cases of enteric illness as Salmonella. 

Indeed, it is now believed that campylobacteriosis is more common in the United States than salmonellosis and shigellosis combined. Common symptoms of campylobacter enteritis include profuse diarrhea (sometimes containing blood), abdominal cramps and nausea. 

Human volunteer and retrospective studies of food associated outbreaks revealed that ingesting relatively small numbers (only a few hundreds cells) of Campylobacter jejuni can produce illness. 

Campylobacter jejuni