How Microorganisms Enter a Host

How Microorganisms Enter a Host

• Germs must penetrate host defenses to damage tissues and cause disease

• Mucous membranes are a common entry point

• The respiratory tract is the most common portal of entry for microbes that are inhaled through the nose or mouth, like influenza and colds

• The gastrointestinal tract is another entry for germs in food or water

• Most are destroyed by stomach acid or bile, but some, like Vibrio cholerae, can survive

• Sexually Transmitted Infections enter through the genitourinary tract

• Conjunctiva, the membrane covering the eye, can be penetrated

• Some pathogens require a broken mucous membrane, such as a cut or micro-abrasion, and others can penetrate unbroken membranes

• Intact skin is impenetrable by most microbes

• Some microbes can enter through hair follicles or wounds

• Some fungi can grow directly on the skin without needing to enter through breaches

• The parenteral route involves microbes being deposited directly into tissues when the skin or membranes are broken

• For example, a tick bite can introduce bacteria, like Lyme Disease, or an intravenous injection with a contaminated syringe can introduce HIV

Number of Microbes Introduced

• If too few microbes enter the body, the immune system can easily fight them off and prevent disease
• The likelihood of disease increases with the number of microbes introduced

Infectious Dose

• The virulence of a microbe can be expressed numerically using the infectious dose
• ID50 is the amount of bacteria needed to cause disease in 50% of the population
• For Bacillus anthracis, the ID50 depends upon the route of entry
• 10-20 endospores via a cut in the skin
• 10,000 to 20,000 endospores via inhalation
• 250,000 to 1,000,000 endospores via ingestion
• The potency of a toxin is expressed as the lethal dose
• LD50 is the dose that kills 50% of the infected population

Mechanisms of Pathogenicity

• Adherence involves surface molecules that allow a pathogen to stick to the surface

• Pathogens often adhere to specific receptors on the host cell surface

• Examples include fimbriae or glycocalyx of bacteria and viral proteins

• Invasiveness is the ability of a pathogen to invade and multiply in healthy tissues

• Two types of molecules promote invasiveness: extracellular enzymes and invasins

Extracellular Enzymes (Exoenzymes):

• These enzymes erode the surface of host cells and damage tissues, providing nutrients, and protection for microbes
• Specific Examples:
• Hemolysin destroys red blood cells
• Fibrinolysin degrades fibrin coats
• Collagenase degrades connective tissue
• Coagulase promotes blood clots around the bacterial cell

Invasins

• Surface proteins cause rearrangements of the host cell cytoskeleton, forcing the host cell to take in the bacterium, thus protecting the bacterial cell from host defenses

Bacterial Toxins

• A toxin is a poisonous substance produced by a microorganism
• A toxoid is an inactive toxin
• Toxoids can be used as vaccines
• For example, the DTaP vaccine contains diphtheria, pertussis, and tetanus toxoids
• Exotoxins are toxins secreted from the bacteria that are heat sensitive and can be extremely toxic
• Clostridium botulinum produces botulism toxin, where 1mg can kill 1,000,000 guinea pigs

Categories of Exotoxins:

• Neurotoxins interfere with nerve impulses
• Botulinum toxin, produced by Clostridium botulinum, causes flaccid paralysis by permanently relaxing muscles, eventually stopping the heart and breathing
• Tetanus toxin, produced by Clostridium tetani, causes rigid paralysis, resulting in uncontrollable muscle contractions (spasms) and lockjaw, with death usually due to spasms of the respiratory muscles
• Enterotoxins interfere with salt absorption in the small intestine
• These cause cells to pump out water to counteract the high salt concentration, resulting in watery diarrhea
• For example, Montezumas revenge (E. coli) and Cholera (Vibrio cholera)
• Cytotoxins kill cells by interfering with protein synthesis
• For example, diphtheria toxin causes respiratory illness

Endotoxins

• Are part of the outer membrane of gram-negative bacteria
• Lipopolysaccharide (LPS) does not cause problems when embedded in the membrane
• Endotoxin (LPS) is released when the bacterium dies, worsening symptoms
• Endotoxins can cause fever, hemorrhaging, and shock
• They are less potent than exotoxins
• For example, Salmonellosis (Salmonella enterica) is food poisoning due to millions of dead bacteria, where symptoms are caused by endotoxin

Comparing Endotoxins and Exotoxins

• Exotoxins:
• Proteins that are released outside of the cell
• Extremely toxic and often lethal
• Endotoxins:
• Lipopolysaccharides (LPS) only released when the bacteria dies
• Not as toxic and usually not lethal

Viral Infections:

• Viruses evade defenses by invading and growing inside of cells, making them inaccessible to the immune system
• Viruses may disguise themselves to look like their hosts
• For example, enveloped viruses can steal a membrane directly from their host

Cytopathic Effects (CPE):

• Visible effects of viral infection
• Disruption of cell processes
• For example, Herpes simplex virus stops host cell division
• Destruction of intracellular structures
• Formation of inclusion bodies from viral parts (capsomeres)
• Giant cell formation where several infected cells fuse to form one giant cell
• For example, colds
• Viral infection often results in host cell death