Pathology of Infection Flashcards
Introduction
Infectious pathology is the most common form of organic and cellular disease. Fortunately, it is often mild and the ‘pathology’ goes virtually unnoticed. Many forms of infection, however, are associated with significant tissue damage or loss of life and infectious disease is still a leading cause of death worldwide.
There are many infectious microorganisms with a variety of special abilities that enable them to infect, proliferate and move on to the next host. Whether or not you get an infection (i.e. show the signs and symptoms of infectious disease) depends upon a highly complex and regulated process that involves not only factors and features of the invading microorganism (e.g. its virulence or disease-producing potential), but also the integrity of a number of host defence mechanisms.
Both humoral and cell-mediated immune systems battle infectious organisms (e.g. humoral immunity more often with bacterial infections and T cells with viral infections) and occasionally cause tissue toxicity that leads to illness and disease. Since the immune system has a limited repertoire of tools, different origins of infectious pathology show a great deal of similarity and overlap in symptoms and pathology. Specific symptoms depend on the location of the invading organism and its predilection for specific organs or tissues.
Are all interactions with microorganisms bad?
Not all interactions between humans and microorganisms are harmful. The internal and external exposed surfaces of the body are normally inhabited by a multitude of bacteria, normal microflora, and the host is not adversely affected.
In fact, in some instances, the interaction is mutually beneficial, with the human host providing nutrition and the microorganism secreting essential metabolic byproducts (e.g. vitamin K in the intestinal tract) or keeping more virulent organisms at bay.
Criteria to determine whether bacteria caused disease
The treatment of infectious disease is most effective when there is identification of the pathogenic organism and its site(s) of infection. Clinical signs and symptoms are the first step in identification; for instance, a cough is usually indicative of respiratory infection; vomiting or diarrhea of a gastrointestinal infection.
In 1884 the German physician Robert Koch published a set of criteria to determine whether bacteria caused disease. There are some exceptions and limitations to these ‘postulates’ but in general, they are still useful in establishing the infectious origin of a disease.
Koch’s Postulates state that: The organism :
- must be found in lesions of the disease.
- must be isolated and cultured in vitro.
- must transmit the disease to another animal.
- must be recovered from lesions in that animal.
Some infectious agents can be directly observed in microscopic sections (e.g. the inclusion bodies formed by CMV (cytomegalovirus) or herpes virus; clumps of bacteria). Many agents require special stains to be visualized based on particular characteristics in their cell walls (e.g. Gram +ve or Gram -ve, acid-fast or silver stains, specific Ab-labelled immunohistochemical stains). Laboratory studies (e.g. isolation and growth of organisms from sputum, blood, stool or urine samples) and radiographic examination (e.g. chest X-ray) result in identification and determination of drug sensitivity of the offending organism in the majority of cases, but not always. The organism may prove difficult to identify or the site of primary infection remain unknown.
Increasingly DNA sequence analysis and PCR-based methods are used to identify microbes that prove elusive to grow in culture and to characterize specific virulent strains. Many of the genomes of bacteria, viruses, fungi and parasites have been completely sequenced. Surprisingly bacteria exchange hundreds of genes from one to another, particularly those genes involved in virulence.
THE ORGANISMS
- Prions
- Viruses
- Bacteria
a. Bacteriophages, Plasmids and Transposons
b. Chlamydia, Rickettsia, Mycoplasma
c. Fungi
d. Protozoa
e. Helminths
f. Ectoparasites
Prions
infectious proteins (or proteinacious infectious particles), lack DNA or RNA (e.g. Creutzfeld-Jakob disease (CJD), ‘Mad Cow’ disease (new variant CJD) or bovine spongioform encephalopathy (BSE))
represent an abnormal form of a normal host prion protein (PrP)
modify the host protein to undergo a conformational change conferring resistance to degradation
Viruses
obligate intracellular organisms, take over genetic apparatus of host cell for their own replication
the viral genome may be DNA or RNA surrounded by a protein coat (capsid)
illnesses may be acute (colds, influenza), chronic (hepatitis B or C) or latent (herpes zoster)
some viruses have the capacity to transform host cells into neoplastic cells (HPV)
Bacteria
have a cell membrane but lack membrane-bound nuclei or organelles; also bound by a cell wall
often classified by shape (round - cocci; rods - bacilli, spirochetes); reaction on Gram staining (gram +ve (have a thick cell wall that retains the stain) or Gram –ve (do not stain); or oxygen requirement for growth (aerobic or anaerobic)
pathogenic strains “in the wild”, but normal flora may infect if defenses are down
Bacteriophages, Plasmids and Transposons
are mobile genetic elements that infect bacteria and may impart bacterial virulence factors
Chlamydia, Rickettsia, Mycoplasma
atypical or intracellular bacteria, lacking certain features typical of bacteria (e.g. no cell wall)
Fungi
have thick cell walls and cell membranes
many are normal flora but are common opportunistic infections (e.g. Candida albicans)
Protozoa
single-celled organisms that may have complex life cycles (may cycle in multiple hosts and be transmitted from one host to another), e.g. plasmodium falciparum – malaria – replicate in insect vector before transmission to human host; giardia; toxoplasma
Helminths
parasitic worms, find 3 classes o roundworms (nematodes – ascaris, hookworms), flatworms (cestodes - pork, beef or fish tapeworms), flukes (trematodes - schistosomes)
Ectoparasites
ticks, fleas, lice - attach to and live on the skin
may cause disease directly or be vectors for other organisms (e.g. a deer tick transmits Lyme disease spirochete.
VIRULENCE
Virulence are factors that are characteristic of the organisms that allows it to colonize, proliferate, invade and destroy host tissues.
- Colonization
- Proliferation
- Tissue injury
- Dissemination and Transmission
Colonization
the organism must be able to colonize the site it was introduced to.
Dependent on:
a) size of inoculum
b) ability to adhere to the tissue - viruses: bind to cell surface receptors - bacteria: express adhesion gene products to enhance binding to cell surfaces.
