Host-Microbe Interactions Flashcards
(33 cards)
General concept
Only a few of the microbial species that exist in nature are found in association with humans. Interactions of microorganism with human or other animal host are determined both ybthe characteristics of microbes and those of the host.
Exposed surfaces of the body are…
colonized by a diverse population of microbes that develops in an orderly sequence (succession) after birth, leading to a relatively stable normal flora. Organisms in the normal flora may aid the host, harm the host, or be commensals (neither the microbe or the host is harmed or helped). Humans have about 10^14 BACTERIA in the normal flora (and about 10^13 cells
Areas of the body to which microbes can gain access from the environment
respiratory tract, urogenital tract, alimentary tract, mouth, conjunctiva, scratch injury, capillary, skin. Many become colonized with mircobial flora, but some do not. For example, can clear microbes from respiratory tract by cilia. CONSIDER ANATOMICAL REACTIONS
Infection
Process whereby a microbe enters into a relationship with the host. It may or may not cause a disease. So remember infection is not synomynous with infectious disease!!!!
Infectious disease
A disease caused by an infection with a microbe. Some infections are communicable, others are not and are not transmitted from patient to patient.
Microbes cause specific diseases (Koch’s postulate)
1) Specific microbes are present regularly in characteristic lesions of the disease.
2) The specific microbes can be isolated and grown in vitro.
3) Injection of the cultured microbes into animals reproduces the disease seen in humans.
4) The specific microbes can be re-isolated from lesions of the disease in animals.
(the serial dilutions - from animal to animal to animal - IDs the organism and doesn’t point to contaminants)
Pathogenicity
Defined as the ability (usually of a microbial species) to cause disease. Microbes that were able to cause disease readily in normal hosts were defined as FRANK pathogens. Microbes that caused disease primarily in compromised hosts but less often in normal hosts were defined as OPPORTUNISTIC pathogens
Virulence
Denoted the degree of pathogenicity (usually of a specific strain within a species). A highly virulent microbe was likely to cause disease when it was introduced into a host in small numbers. LIKE DOSE-AFFECT CURVES OF PHARM. Log(# of bacteria/animal)
More Contemporary views
recognize that both microbial and host factors contribute to the outcome of an infectious disease. From this “damage- response” framework, a pathogen is a microbe capable of causing host damage, virulence is the relative capacity of a microbe to cause damage in a susceptible host, and a virulence factor is a microbial component that can damage a susceptible host.
Stages of infection
Encounter, entry, spread, multiplication, damage, outcome.
Encounter: how the agent meets the host
endogenously v exogenously? contact with other ppls, animals, food, air? Route of infection? Infectious dose?
Entry: how the agent enters the host
Cross epithelial - but have to deal with tight junctions. Enter actively? 1) COLONIZATION of BODY SURFACES - an important first step in pathogenesis of many microbial infections. 2) ADHERENCE - mediated by specific binding of microbial surface components.
Spread: how the agent spreads from site of entry
Microbial products can promote or inhibit spread. Can use lymphatics, use vasculature, or can enter phagocytic cells and use them as a “bus” (if able to survive internal environment of phagosome)
Hyaluronidase, elastase, collagenase, etc.,
which facilitate the spread of microbes through tissues, are sometimes called “spreading factors”. Allows to get through tight junctions.
Coagulase
is an agent that inhibits spread of microbes by promoting the deposition of fibrin and helping to “wall off” and localize infections.
Multiplication: how the agent multiplies in the host
Normal flora and pathogens must REPLICATE in the host at rates that exceed their clearance by defense mechanisms. The growth rate of microbes in the host (in vivo) may be much slower than their maximal growth rate in laboratory cultures (in vitro). NOT there is a relationship btwn population size and onset of symptoms. Can also keep a certain level of microorganism in your body - for example TB and can get recurrent bouts of infection.
Damage: How tissue damage is caused by the agent and/or the host response
a) Some microbial products can cause direct damage to the host. b) Some microbial products contribute to host damage by blocking or interfering with host defense mechanisms c) Most of the microbial products that cause direct damage to the host or interfere with host defense mechanisms are either surface components of microbes, products that can be secreted into the microbial culture medium, or products that can be injected by the microbe directly into a target cell. Such microbial products can often be used as protective antigens for development of vaccines.
Outcome: does the microbial agent or the host win the battle or do they learn to coexist?
What are the consequences of failure to eliminate the agent? The survival of obligate parasites depends on a satisfactory portal of exit from the infected host as well as on a portal of entry.
Some factors that differ among individuals and affect the microbiome include
- Diet (breast-feeding, bottle feeding, solid food).
- Suppression of microbial flora by treatment with antibiotics.
- Anatomic abnormalities (e.g., blind loop syndrome).
- Genetic differences between individuals (e.g., specific ABO and Se genotypes).
Examples of physiologic importance of the microbiome includes
- Effects on tissue/organ differentiation (normal vs. germfree animals).
- Production of vitamins by gut flora.
- Biochemical conversions (e.g., bilirubin degradation, drug metabolism, production of potential carcinogens, etc.)
- Competition with pathogens for colonization of body surfaces. Source of agents for endogenously acquired infections.
Cholera
a toxin-mediated disease that alters the secretory function of the small intestine but does not cause histological damage. So structure remains the same, but function changes.
Pneumococcal pneumonia
acute inflammation caused by invasive, extracellular bacteria. has a slimy surface polysaccardie capsule and not easily ingested by neutrophils. immune response allows for phagositosis. ANTIBODY MEDIATED.
Tuberculosis
a chronic disease caused by a facultative intra-cellular bacterium and controlled by cell-mediated immunity. T cell mediated!
Acute Rheumatic Fever
a disease resulting from immunopathology triggered by the response to a prior group A streptococcal infection. CROSSREACTs. Like in the heart.
