Infection and Disease
The Host-Microbe Relationship
Human body maintains a symbiosis with microbes Infection refers to:
relationship between the host and microbe competition for supremacy between them
If the host loses the competition, disease occurs
Contact, Colonization, Infection, Disease
Infectioncondition where pathogenic microbes
penetrate host defensesenter tissues and multiply
Disease any deviation from health, disruption of a
tissue or organ
infectious disease Caused by microbes or their products
**Not all contacts lead to infections…….**Not all infections lead to disease!!!!!
Resident Flora Microbiota, transient,
resident, indigenous
Microbes that engage in mutual or commensal associations
most areas of the body in contact with the outside environment
internal organs & tissues & fluids are microbe-free
bacterial flora benefit host by preventing overgrowth of harmful microbes
Initial Colonization of the Newborn
Uterus and contents normally sterile remain so until just before
birth
Breaking of fetal membrane exposes the infant all subsequent handling and
feeding continue to introduce normal flora
Pathogens….
True pathogens capable of causing disease
in healthy persons normal immune defenses
influenza virus, plague bacillus, malarial protozoan
Opportunistic pathogens cause disease when hosts
defenses are compromised Pseudomonas sp & Candida
albicans
Pathogenicity Ability of a microorganism to
cause disease
Severity of disease depends on the virulence of the pathogen Degree of pathogenicity characteristic or structure that
contributes to the ability of a microbe to cause disease
virulence factor Adhesion factors Biofilms Extracellular enzymes Toxins Antiphagocytic factors
Pathogens…..
In order to become established: Portal of entry Attaching Surviving host defenses
1. Portals of entry characteristic route a
microbe follows to enter the tissues of the body skin
nicks, abrasions, punctures, incisions
gastrointestinal tract food, drink, and other
ingested materials Mucous membranes urogenital tract
Sexual transplacental
Infectious Dose (ID)
minimum number of microbes required for infection to proceed
microbes with small IDs have greater virulence 1 rickettsial cell in Q fever 10 bacteria in TB, giardiasis 109 bacteria in cholera
2. Mechanisms of Adhesion
fimbrae
flagella
adhesive slimes or capsules
cilia
scolex
hooks
barbs
3. Surviving Host Defenses
Initial response of host defenses comes from phagocytes Ability to survive
intracellular phagocytosis
Antiphagocytic factors
Antiphagocytic factorsFactors prevent phagocytosis by the host’s
phagocytic cells Bacterial capsule
Composed of chemicals not recognized as foreignSlippery; difficult for phagocytes to engulf bacteria
Antiphagocytic chemicalsPrevent fusion of lysosome and phagocytic vesiclesLeukocidins directly destroy phagocytic white blood cells
Enzymatic Destruction
Extracellular enzymes (exoenzymes) Secreted by the pathogen Dissolve structural chemicals in the body Help pathogen maintain infection, invade, and avoid body
defenses
Invasiveness
Coagulase form a blood clot that protects them from phagocytosis
Streptokinase dissolves fibrin clots and allows dissemination of the bacteria
Hyaluronidase enhances pathogen penetration through tissues
Leukocidins disintegrate neutrophils and macrophages
Hemolysiins dissolve red blood cells
Virulence can be enhanced in biofilms immune cells cannot reach bacterial cells
Toxins
toxigenicity capacity to produce toxins at the site of multiplication endotoxins
lipid A of LPS of gram-negative bacteria exotoxins
proteins secreted by gram-positive and gram-negative bacteria
Toxins Toxinoses
Disease caused by toxin
Toxemias Spread by blood
Intoxication Ingested
Enterotoxins act on the gastrointestinal tract
Antitoxins produced by the host body and
neutralize toxins
Toxoids toxins whose toxicity has been
destroyed but still elicit an immune response
Portals of Exit
Respiratory, saliva
Skin
Fecal exit
Urogenital tract
Removal of blood
Establishment of Infection and Disease
incubation period time between entry of the
microbe and symptom appearance
prodromal phase I time of mild signs or
symptoms
acme period (climax) signs and symptoms are most
intense
period of decline As sign and symptoms subside
period of convalescence body systems return to normal
Patterns of Infection localized infection
remains confined to a specific tissue
focal infection when infectious agent breaks loose from a local infection carried to other tissues
systemic infection infection spreads to several sites and tissue fluids usually in the bloodstream
Patterns of InfectionMixed infection
several microbes grow simultaneously at the infection site
Primary infection initial infection
Secondary infectionanother infection by a
different microbe
Warning Signals of Disease
Sign objective evidence of
disease as noted by an observer
Symptom subjective evidence of
disease as sensed by the patient
Syndrome Disease is defined by
complex of signs and symptoms
Signs and Symptoms of Inflammation
Earliest symptoms of disease as a result of the activation of the body defenses fever, pain, soreness, swelling
Signs of inflammation: edema
accumulation of fluid granulomas and abscesses
walled-off collections of inflammatory cells and microbes
LesionGeneral term for site of infections or
disease
Signs of Infection in the Blood
Changes in the number of circulating white blood cells leukocytosis
increase in white blood cells
leukopenia decrease in white
blood cells
Signs of Infection in the Blood
septicemia microorganisms multiplying in the blood and
present in large numbers
bacteremia small numbers of bacteria present in blood not necessarily multiplying
viremia small number of viruses present not necessarily multiplying
Infections That Go Unnoticed
Asymptomatic subclinical infections although infected, the host doesn’t show any signs
of disease
Inapparent infectionperson doesn’t seek medical attention
Persistence of Microbes and Pathologic Conditions
Latency microbe can periodically become active
produce a recurrent disease person may or may not shed it during the latent stage
Sequelae long-term or permanent damage to tissues or organs
Acute diseases develop rapidly cause severe symptoms fade quickly
Chronic diseases linger for long periods of time slower to develop and recede
Reservoirs: Where Pathogens Persist
Reservoir primary habitat of pathogen in
the natural world human or animal carrier, soil,
water, plants
Source individual or object from which
an infection is actually acquired
Animals as Reservoirs and Sources
vector live animal that transmits an
infectious agent from one host to another
Arthropods fleas, mosquitoes, flies, and ticks
larger animals mammals, birds, lower vertebrates
Animals as Reservoirs and Sources
zoonosis infection indigenous to
animals but naturally transmissible to humans
humans do not transmit the disease to others
Acquisition and Transmission of Infectious Agents
Communicable disease an infected host can transmit the infectious agent to
another host
Highly communicable disease is contagious
Non-communicable infectious disease does not arise through transmission from host to host occurs primarily when a compromised person is
invaded by his or her own normal microflora contact with organism in natural, non-living reservoir
Patterns of Transmission
Direct contact physical contact, droplet
nuclei, aerosols
Indirect contact passes from infected host to
intermediate conveyor and then to another host
vehicle food, water, biological
products, fomites
Nosocomial InfectionsHealth care-associated
infections (HAIs) occur as a result of
receiving treatment for another condition
Nosocomial infections are HAIs associated with hospitalsThey occur as a result of
chains of transmission
Nosocomial InfectionsFrom surgical procedures,
equipment, personnel, and exposure to drug-resistant microorganisms
More than 1/3rd of nosocomial infections could be prevented
2 to 4 million cases/year in U.S. approximately 90,000 deaths
Most common organisms involved Gram-negative intestinal flora E. coli, Pseudomonas,
Staphylococcus
Epidemiologystudy of the frequency and distribution of
disease & health-related factors in human populations
Surveillance collecting, analyzing, & reporting data on
rates of occurrence, mortality, morbidity and transmission of infections
Reportable, notifiable diseases must be reported to authorities
Epidemiology
Centers for Disease Control and Prevention (CDC) in Atlanta, GAprincipal government agency responsible for
keeping track of infectious diseases nationwide
http://www.cdc.gov
Epidemiology
Prevalence total number of existing cases with respect to the
entire population usually represented by a percentage of the population
Prevalence= Total # of cases in pop X 100 Total # of persons in pop
Epidemiology
Incidence measures the number of new cases over a certain
time period, as compared with the general healthy population
Incidence = # of new cases = ratio # of healthy persons
The incidence and estimated prevalence of AIDS
Epidemiology
Mortality rate total number of deaths in a population due to a
certain disease
Morbidity rate number of people afflicted with a certain disease
EpidemiologyEndemic
disease that exhibits a relatively steady frequency over a long period of time in a particular geographic locale
Sporadic when occasional cases are
reported at irregular intervals
Epidemic when prevalence of a disease is
increasing beyond what is expected
Pandemic epidemic across continents
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