HOW MICROBES CAUSE DISEASE

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HOW MICROBES CAUSE DISEASE.

Dr.Kedar Karki

TERMS RELATED TO VIRULENCE

MAJOR BACTERIAL VIRULENCE DETERMINANTS

Virulence determinants.

• A pathogen may have one or many virulent determinants. When a pathogen loses a crucial virulence determinant it becomes avirulent. Conversely, a normally non-virulent microbe may gain virulent genes and suddenly become virulent or a mildly pathogenic strain may gain an additional virulence determinant and become virulent. Virulent genes may occur through mutations in the microbes genome, or by picking up a plasmid .

Attachment system that bind pathogens to their hosts.

STEPS IN THE DISEASE PROCESS

• INFECTION =

• INCUBATION PERIOD =

• INITIAL SYMPTOMS =

SEPTICEMIA:

• This describes the case where the pathogen grows massively within the host. In effect the host becomes a virtual "culture tube" for the pathogen. Bacteria and viruses can be found in the blood and all the organs. Death often ensues when this happens.

TOXINS:

• Toxins are products of a pathogen that destroy/damage/inactivate one or more vital component of the host thus allowing the pathogen to survive and flourish.

EXOTOXINS

• EXOTOXINS are toxins that are SECRETED from the cell or leak out of the cell after it dies. Generally they are soluble proteins and thus are carried throughout the body in the blood or lymph, doing damage at a distance from the infection site. Toxins tend to target specific cells in the body. Some are enzymes and others are proteins that bind to and inhibit crucial cellular activities which eventually lead to the death of cells.

ENDOTOXINS

• Endotoxins have a general basic structure, but differ significantly in composition between species. Endotoxins are released in relatively small amounts as the cells grow, but in copious amounts when the cells die. Different endotoxins differ in their degree of toxicity, but all are heat stable and can tolerate autoclaving. Endotoxins harm many systems in the body and hence are very dangerous. They are often responsible for the cause of death of infections by G-v cells.

ENZYMES:

• Pathogens use a variety of enzymes to assist them in establishing infection and producing a disease. There are virulence determinant enzymes that dissolve the glue between cells, thus allowing the bacteria to spread rapidly through the tissue. There are enzymes (hemolysins) that #lyse red blood cells and others that lyse white blood cells. There are enzymes that degrade DNA, lipids and proteins.

ATTACHMENT SYSTEMS:

• Since many of the nonspecific defenses involve mechanically flushing away pathogens, a common virulence determinant of pathogens are cell components that stick the bacteria to the target cells.

SELF DESTRUCTION:

• Pathogens frequently cause disease by tricking the host cells into doing something they normally wouldn't do. One trick is to induce the host system to produce self-destructive chemicals that kill or inhibit its own cells.

CHANGING ANTIGENS:

• Pathogens often evade a host's immune system by frequently changing its surface antigens. The HIV, malaria and sleeping sickness pathogens use this strategy to avoid destruction. In these cases a pathogen produces a few antigenic variants as its population increases.

CAMOUFLAGE:

• In this case the pathogen camouflages itself so the host doesn't recognize the invader as being "nonself" and thus dangerous.

INITIAL SYMPTOMS =

• These refer to the first symptoms that clearly demonstrate an illness. Since symptoms vary widely between hosts this is a statistical matter.

ACUTE =

• This refers to the classical clinical or textbook symptoms, where the disease is in full flower and the patient is usually seriously or clearly ill.

The symptoms and outcome of every disease is dependent on a

mixture of many factors.

• The GENETICS of the host.

• The GENETICS infectious agent; virulent determinants, virulence plasmids etc.

• The PHYSICAL CONDITION of the host.

• The STRESS encountered by the host during the disease.

• The AGE & SEX of the host.

• The TREATMENT of the victim.

SUBCLINICAL or asymtomatic

• is very common for people to have a disease and not show any identifiable symptoms and yet to become as immune as another person who almost dies from the same disease.

RECOVERY =

• Period during which the symptoms decline and the patient recovers. Recovery may take many paths.

Six major ones are listed below.

• In many cases the etiological agent is totally eliminated and the patient returns to full health.

• • Some carriers appear to be fully recovered, but

the disease may be progressing slowly towards a fatal outcome, such as may occur with syphilis, #HIV and tuberculosis. Magic Johnson is probably such a case.

• In other cases, the patient shows a full recovery but the infectious agent is still present. Under these conditions the patient becomes a CARRIER and remains capable of shedding (spreading) the virulent form of the infectious agent for some period, perhaps for the remainder of their lives. This is the case for diseases like Typhoid, Herpes and HPV (Human Papilloma Virus).

• Some carriers, like those with herpes (shingles) and hepatitis have occasional outbreaks of the disease throughout their lives, but they are rarely fatal.

• In other cases, the patient recovers and eliminates the infectious agent, but their immune system has been damaged and they subsequently fall victim to an autoimmune disease like rheumatoid arthritis.

• In many cases a disease becomes CHRONIC. The victim makes a partial recovery, but they are still less well than normal and continuously demonstrate symptoms of ill-health or have frequent relapses. Many infestations (worms and other large parasites) take this path. Lyme disease can become chronic.

MAJOR BACTERIAL VIRULENCE DETERMINANTS

HOW MICROBES CAUSE DISEASE

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