Host-Microbe Interactions

بسم الله الرحمن الرحيم

Host-Microbe Interactions
The pathogenesis of bacterial infection includes initiation of the infectious process and the mechanisms that lead to the development of signs and symptoms of disease. Characteristics of bacteria that are pathogens include transmissibility, adherence to host cells, invasion of host cells and tissues, toxigenicity, and ability to evade the host's immune system.
Many infections caused by bacteria that are commonly considered to be pathogens are asymptomatic. Terms used in of infectious diseases are defined in table 1.

Term

Definition
Adherence (attachment)
The process by which bacteria stick to the surfaces of host cells. After bacteria have entered the body, adherence is a major initial step in the infection process.
Bacteremia
Bacteria circulating in the bloodstream
Carrier
A person or animal with asymptomatic infection that can be transmitted to another susceptible person or animal.
Colonization
Establishment and growth of a microorganism on a body surface
Infection
Multiplication of an infectious agent within the body. Multiplication of the bacteria that are part of the normal flora of the gastrointestinal tract, skin.
Invasion
The process whereby bacteria, animal parasites, fungi, and viruses enter host cells or tissues and spread in the body.
Immunocompromised
A host with weaknesses or defects in the innate or adaptive defenses
Infectious disease
Disease caused by an infecting microorganism or virus


Opportunistic pathogens
Organisms that cause disease only when introduced into an unusual location or into an immunocompromized host
Non-pathogen
A microorganism that does not cause disease; may be part of the normal flora.
Pathogen
Any disease-causing microorganism or virus
Pathogenicity
The ability of an infectious agent to cause disease.
Primary infection
Infection in a previously healthy person
Secondary infection
An additional infection that occurs as a result of a primary infection and that occurs during or immediately following the primary infection
Septicemia
Acute illness caused by infectious agents or their products circulating in the bloodstream

Superantigens

Protein toxins that activate the immune system by binding to major histocompatibility complex (MHC) molecules and T-cell receptors (TCR) and stimulate large numbers of T cells to produce massive quantities of cytokines.
Systemic infection
Widespread infection through blood or lymph

Toxemia
Toxin circulating in the bloodstream
Toxigenicity
The ability of a microorganism to produce a toxin that contributes to the development of disease.
Viremia
Viruses circulating in the bloodstream

Virulence determinants
Attributes of a microorganism or virus that promote pathogenicity



Normal Microbial Flora of the Human Body
The term "normal microbial flora" means the population of microorganisms that inhabit the skin and mucous membranes of healthy normal persons. It is doubtful whether a normal viral flora exists in humans.
The skin and mucous membranes always harbor a diversity of microorganisms that can be arranged into two groups:

(1) The resident flora consists of relatively fixed types of microorganisms regularly found in a given area at a given age; if disturbed, it reestablishes itself.
(2) The transient flora consists of nonpathogenic or potentially pathogenic microorganisms that inhabit the skin or mucous membranes for hours, days, or weeks; it is derived from the environment, does not produce disease, and does not establish itself permanently on the surface.
However, if the resident flora is disturbed, transient microorganisms may colonize, proliferate, and produce disease.

Nose

Staphylococcus aureus
Staphylococcus epidermidis
Corynebacterium species
Throat
Streptococcus species
Branhamella catarrhalis
Corynebacterium species
Haemophilus species
Neisseria species
Mycoplasma species
Large intestine
Bacteroides fragilis
Escherichia coli
Proteus mirabilis
Klebsiella species
Lactobacillus species
Streptococcus species
Candida albicans
Clostridium species
Pseudomonas species
Enterococcus species
Mouth
Streptococcus species
Fusobacterium species
Actinomyces species
Leptotrichia species
Veillonella species
Skin
Staphylococcus epidermidis
Propionibacterium acnes
Pityrosporum ovale
Vagina
Lactobacillus species
Streptococcus species
Candida albicans
Gardnerella vaginalis
Urethra
Streptococcus species
Mycobacterium species
Escherichia coli
Bacteroides species


The Protective Role of the Normal Flora
The most significant contributions of the normal flora to the overall health of the human host include protection against potentially harmful microorganisms, and stimulation of the immune system.
When organisms of the normal flora are killed or their growth suppressed, as can happen during treatment of the host with antibiotics, harmful organisms may colonize and cause disease. For example, oral administration of antibiotics can suppress normal intestinal flora, allowing the overgrowth of toxin-producing strains of Clostridium difficile, causing the disease antibiotic associated colitis.

The presence and multiplication of normal flora competitively excludes pathogens by several different mechanisms. These include
1.covering binding sites that might then be used for attachment,
2. consuming available nutrients, and
3.producing compounds that are toxic to other bacteria.

Transmission of Infection:

*Some bacteria that commonly cause disease in humans exist primarily in animals and incidentally infect humans. For example, Salmonella and Campylobacter species typically infect animals and are transmitted in food products to humans.

For example, Y pestis (plague) has a well-established life cycle in rodents and rodent fleas, and transmission by the fleas to humans is inadvertent;
Bacillus anthracis (anthrax) lives in the environment, also infects animals, and is transmitted to humans by products such as hair from infected animals.
*Other bacteria produce infection of humans that is inadvertent, a mistake in the normal life cycle of the organism; the organisms have not adapted to humans, and the disease they produce may be severe.

*The Clostridium species are generally in the environment and are transmitted to humans by
*ingestion (ex: Clostridium perfringens gastroenteritis and Clostridium botulinum [botulism])
or when wounds are contaminated by soil (eg, C perfringens [gas gangrene] and Clostridium tetani [tetanus]).

*Vibrio cholerae can cause diarrhea, which may contaminate salt and fresh water; ingestion of contaminated water or seafood such as oysters and crabs can produce infection and disease.


Similarly, contamination of food products with sewage containing E coli that causes diarrhea results in transmission of the bacteria.
M tuberculosis (tuberculosis) naturally infects only humans; it produces respiratory disease with cough and production of aerosols, resulting in transmission of the bacteria from one person to another.
*Many bacteria are transmitted from one person to another on hands. A person with S aureus carriage in the anterior nares may rub his nose, pick up the staphylococci on the hands, and spread the bacteria to other parts of the body or to another person, where infection results.

*The most frequent portals of entry of pathogenic bacteria into the body are the sites where mucous membranes meet with the skin, which are the respiratory (upper and lower airways), gastrointestinal (primarily mouth), genital, and urinary tracts.
Normal skin and mucous membranes provide the primary defense against infection.
To cause disease, pathogens must overcome these barriers.

The Infectious Process:

In the body, bacteria must attach or adhere to host cell, usually epithelial cells. After the bacteria have established a primary site of infection, they multiply and spread directly through tissues or via the lymphatic system to the bloodstream. This infection (bacteremia) can be transient or persistent. Bacteremia allows bacteria to spread widely in the body and permits them to reach tissues particularly suitable for their multiplication.

Pneumococcal pneumonia is an example of the infectious process. S pneumoniae can be cultured from the nasopharynx of 5–40% of healthy people. Occasionally, pneumococci from the nasopharynx are aspirated into the lungs.