Chapter 13Chapter 13

Microbe-Human Microbe-Human Interactions: Infection Interactions: Infection

and Diseaseand Disease

Contact-Infection-Disease: The Contact-Infection-Disease: The Host-Parasite RelationshipHost-Parasite Relationship

The human body is constantly in contact The human body is constantly in contact (contaminated) with microbes(contaminated) with microbes► Some are pathogens that may cause an Some are pathogens that may cause an

infection by circumventing the host defense infection by circumventing the host defense system, entering normally sterile tissues, system, entering normally sterile tissues, and multiplying thereand multiplying there

► When infections lead to a disruption in When infections lead to a disruption in tissues, infectious disease resultstissues, infectious disease results

► The outcome is highly variable, but most The outcome is highly variable, but most contacts do not result in infection, and most contacts do not result in infection, and most infections do not lead to diseaseinfections do not lead to disease

Associations between microbes Associations between microbes and humansand humans

The Body as a HabitatThe Body as a Habitat

► The resident flora or microflora is a The resident flora or microflora is a huge and rich mixed population of huge and rich mixed population of microorganisms residing on body microorganisms residing on body surfaces exposed to the surfaces exposed to the environment, including the skin, environment, including the skin, mucous membranes, parts of the mucous membranes, parts of the gastrointestinal tract, urinary tract, gastrointestinal tract, urinary tract, reproductive tract, and upper reproductive tract, and upper respiratory tractrespiratory tract

The Body as a HabitatThe Body as a Habitat

► Anatomical sites lying Anatomical sites lying within the body cavity within the body cavity (organs) and fluids (organs) and fluids (blood, urine) in those (blood, urine) in those sites do not harbor sites do not harbor floraflora

► Colonization: begins Colonization: begins just prior to birth and just prior to birth and continues over an continues over an individual’s life; individual’s life; variations occur in variations occur in response to individual response to individual differences in age, differences in age, diet, hygiene, and diet, hygiene, and healthhealth

Normal FloraNormal Flora

Bacteria may Bacteria may maintain a balance in maintain a balance in the normal the normal conditionsconditions

Studies with axenic Studies with axenic animals (free of any animals (free of any normal flora) show normal flora) show that flora contribute that flora contribute to the development to the development of the immune and of the immune and gastrointestinal gastrointestinal systems and also to systems and also to some diseases some diseases (dental caries)(dental caries)

Normal flora are Normal flora are sometimes agents of sometimes agents of infectioninfection

Landscape of the skinLandscape of the skin

►Microbes live only in Microbes live only in upper dead layers upper dead layers of epidermis glands, of epidermis glands, and follicles; dermis and follicles; dermis and layers below and layers below are sterileare sterile

Distribution of flora in the Distribution of flora in the gastrointestinal tractgastrointestinal tract

► Oral cavity: colonize Oral cavity: colonize the epidermal layer the epidermal layer of cheeks, gingiva, of cheeks, gingiva, pharynx; surface of pharynx; surface of teeth; found in saliva teeth; found in saliva in huge numbersin huge numbers

► Large intestine and Large intestine and rectum have huge rectum have huge numbers of floranumbers of flora

Colonized regions of the Colonized regions of the respiratory tractrespiratory tract

► The moist mucous The moist mucous blanket of the blanket of the nasopharynx has a nasopharynx has a well-entrenched well-entrenched flora.flora.

► Lower regions of Lower regions of bronchi, bronchi, bronchioles, and bronchioles, and lungs are free of lungs are free of resident microbes.resident microbes.

Flora of the reproductive Flora of the reproductive tracttract

► In females, the external In females, the external genitalia, vaginal, and genitalia, vaginal, and cervical surfaces have cervical surfaces have floraflora

► In females, the first In females, the first portion of the urethral portion of the urethral mucosa has flora.mucosa has flora.

► In males, the entire In males, the entire reproductive and urinary reproductive and urinary tract is sterile, except tract is sterile, except for a short portion of the for a short portion of the anterior urethraanterior urethra

Factors Affecting the Course of Factors Affecting the Course of Infection and DiseaseInfection and Disease

Pathogenicity and VirulencePathogenicity and Virulence Pathogenicity is the property of microorganisms to cause Pathogenicity is the property of microorganisms to cause

infection and diseaseinfection and disease Virulence is the precise factors used by the microbe to Virulence is the precise factors used by the microbe to

invade and damage host tissues; it helps define the degree invade and damage host tissues; it helps define the degree of pathogenicityof pathogenicity

Pathogenicity varies with a microbe’s ability to invade or Pathogenicity varies with a microbe’s ability to invade or harm host tissues and with the condition of host defensesharm host tissues and with the condition of host defenses

A true pathogen produces virulence factors that allow it to A true pathogen produces virulence factors that allow it to readily evade host defenses and to harm host tissuesreadily evade host defenses and to harm host tissues► True pathogens can infect normal, healthy hosts with True pathogens can infect normal, healthy hosts with

intact defensesintact defenses An opportunistic pathogen is not highly virulent but can An opportunistic pathogen is not highly virulent but can

cause disease in persons whose host defenses are cause disease in persons whose host defenses are compromised by predisposing conditions such as age, compromised by predisposing conditions such as age, genetic defects, medical procedures, and underlying genetic defects, medical procedures, and underlying organic diseaseorganic disease

Mechanisms of Infection and Mechanisms of Infection and DiseaseDisease

► The portal of entry is The portal of entry is the route by which the route by which microbes enter the microbes enter the tissues, primarily via tissues, primarily via skin, alimentary skin, alimentary tract, respiratory tract, respiratory tract (pneumonia), tract (pneumonia), urogenital tract urogenital tract (sexually transmitted (sexually transmitted diseases), or diseases), or placentaplacenta

Transplacental infection of the Transplacental infection of the fetusfetus

►Microbes Microbes penetrate the penetrate the maternal blood maternal blood vessels and enter vessels and enter the blood pool of the blood pool of the placenta.the placenta.

► They then invade They then invade the fetal the fetal circulation by circulation by way of the way of the umbilical veinumbilical vein

Mechanisms of Infection and Mechanisms of Infection and DiseaseDisease

Pathogens that come from outside the Pathogens that come from outside the body are exogenous; those that originate body are exogenous; those that originate from normal flora are endogenousfrom normal flora are endogenous

The size of the infectious dose is of great The size of the infectious dose is of great importanceimportance

Mechanisms of Infection and Mechanisms of Infection and DiseaseDisease

► In the process of In the process of adhesion, a microbe adhesion, a microbe attaches to the host attaches to the host cell by means of cell by means of fimbriae, flagella, fimbriae, flagella, capsules, or capsules, or receptors that receptors that position it for position it for invasion invasion

Virulence FactorsVirulence Factors

► Exoenzymes Exoenzymes digest digest epithelial epithelial tissues, tissues, disrupt disrupt tissues, and tissues, and permit permit invasioninvasion

Virulence FactorsVirulence Factors Toxigenicity is a Toxigenicity is a

microbe’s capacity to microbe’s capacity to produce toxins at site of produce toxins at site of multiplication which multiplication which affect cellular targetsaffect cellular targets► Toxinoses are Toxinoses are

diseases caused by diseases caused by toxins that damage toxins that damage structure or structure or function of host function of host cellscells

► Toxemia refers to Toxemia refers to toxins absorbed toxins absorbed into the bloodinto the blood

► Intoxication means Intoxication means ingestion of toxinsingestion of toxins

Virulence FactorsVirulence Factors► An exotoxin is a protein secreted by living bacteria with An exotoxin is a protein secreted by living bacteria with

powerful effects on a specific organpowerful effects on a specific organ► Examples are hemolysins and tetanus and diphtheria toxinsExamples are hemolysins and tetanus and diphtheria toxins► An endotoxin is the lipopolysaccharide portion of a gram-An endotoxin is the lipopolysaccharide portion of a gram-

negative cell wall released when a bacterial cell dies; causes negative cell wall released when a bacterial cell dies; causes generalized symptoms such as fevergeneralized symptoms such as fever

Virulence FactorsVirulence Factors

► Antiphagocytic Antiphagocytic factors include factors include leukocidins leukocidins (white blood cell (white blood cell poisons) and poisons) and capsulescapsules

Effects on Target Organ/Spread of Effects on Target Organ/Spread of InfectionInfection

► Patterns of Infection: Stages Patterns of Infection: Stages in Infection/Diseasein Infection/Disease Incubation period, the period Incubation period, the period

from contact with infectious from contact with infectious agent until appearance of agent until appearance of first symptomsfirst symptoms

Prodromium, a short period Prodromium, a short period of initial, vague symptomsof initial, vague symptoms

Period of invasion, a variable Period of invasion, a variable period during which microbe period during which microbe multiplies in high numbers multiplies in high numbers and causes severest and causes severest symptomssymptoms

Convalescent period, a Convalescent period, a period of recovery, with period of recovery, with decline of symptomsdecline of symptoms

Types of Infectious/DiseasesTypes of Infectious/Diseases Localized infection, microbe remains in isolated siteLocalized infection, microbe remains in isolated site Systemic infection, microbe is spread through the tissues Systemic infection, microbe is spread through the tissues

by circulationby circulation Focal infection, microbe spreads from local site to entire Focal infection, microbe spreads from local site to entire

body (systemic)body (systemic) Mixed infection, several microbes cause one type of Mixed infection, several microbes cause one type of

infection simultaneouslyinfection simultaneously Primary infection, the initial infection in a seriesPrimary infection, the initial infection in a series Secondary infection, a second infection that complicates a Secondary infection, a second infection that complicates a

primary infectionprimary infection Septicemia and bacteremia refer to microbes in the bloodSepticemia and bacteremia refer to microbes in the blood Acute infection appears suddenly, has a short course, and Acute infection appears suddenly, has a short course, and

is relatively severeis relatively severe Chronic infection persists over a long period of timeChronic infection persists over a long period of time Subacute infection has a pattern between acute and Subacute infection has a pattern between acute and

chronic chronic

Types of Infectious/DiseasesTypes of Infectious/Diseases

Signs and Symptoms: Manifestations of Signs and Symptoms: Manifestations of disease, indicators of pathogenic effects disease, indicators of pathogenic effects

on target organson target organs A sign is objective, measurable evidence noted by A sign is objective, measurable evidence noted by

an observeran observer► Examples include septicemia, change in Examples include septicemia, change in

number of white blood cells; skin lesions; number of white blood cells; skin lesions; inflammation; necrosis, lysis or death of tissuesinflammation; necrosis, lysis or death of tissues

► A symptom is a subjective effect of disease as A symptom is a subjective effect of disease as sensed by patientsensed by patient

► Examples are pain, fatigue, and nauseaExamples are pain, fatigue, and nausea► A syndrome is a disease that manifests as a A syndrome is a disease that manifests as a

predictable complex of symptoms; infections predictable complex of symptoms; infections that do not show symptoms are called that do not show symptoms are called asymptomatic, subclinical, or inapparentasymptomatic, subclinical, or inapparent

Major portals of exit of Major portals of exit of infectious diseasesinfectious diseases

► Through the portal of exit, Through the portal of exit, microbe is released with microbe is released with bodily secretions and bodily secretions and discharges to have access discharges to have access to new host; portals to new host; portals include respiratory include respiratory droplets from sneezing, droplets from sneezing, coughing, saliva, skin, coughing, saliva, skin, feces, urogenital tract feces, urogenital tract (urine, mucus, semen) and (urine, mucus, semen) and bloodblood

► A microbe may become A microbe may become dormant (latent) and dormant (latent) and cause recurrent infectionscause recurrent infections

► Damaging effects that Damaging effects that remain in organs and remain in organs and tissues after infection are tissues after infection are sequelaesequelae

EpidemiologyEpidemiology

Epidemiology is a science that determines Epidemiology is a science that determines the factors influencing causation, the factors influencing causation, frequency, and distribution of disease in a frequency, and distribution of disease in a communitycommunity► Epidemiologists are involved in surveillance of Epidemiologists are involved in surveillance of

reportable diseases in populations and consider reportable diseases in populations and consider measures to protect the public healthmeasures to protect the public health

► They are concerned with disease statistics such They are concerned with disease statistics such as prevalence (the total number of cases), as prevalence (the total number of cases), incidence (the number of new cases), morbidity incidence (the number of new cases), morbidity (general health of the population), and mortality (general health of the population), and mortality (death)(death)

Graphical representation of Graphical representation of epidemiological dataepidemiological data

Frequency of DiseaseFrequency of Disease► Endemic, a disease Endemic, a disease

constantly present constantly present in a certain in a certain geographic areageographic area

► Sporadic, a disease Sporadic, a disease that occurs that occurs occasionally with occasionally with no predictable no predictable patterpatter

► Epidemic, sudden Epidemic, sudden outbreak of disease outbreak of disease in which numbers in which numbers increase beyond increase beyond expected trendsexpected trends

► Pandemic, Pandemic, worldwide epidemicworldwide epidemic

Origin of PathogensOrigin of Pathogens

► The reservoir is a place where the pathogen The reservoir is a place where the pathogen ultimately originates (its habitat)ultimately originates (its habitat)

► Source of infection refers to the immediate Source of infection refers to the immediate origin of an infectious agentorigin of an infectious agent

► Carrier is an individual that inconspicuously Carrier is an individual that inconspicuously shelters a pathogen and spreads it to othersshelters a pathogen and spreads it to others

Carriers in transmission of Carriers in transmission of infectious agentsinfectious agents

Asymptomatic carrier is Asymptomatic carrier is infected without symptomsinfected without symptoms

Incubation carriers carry early Incubation carriers carry early in diseasein disease

Convalescent carriers carry in Convalescent carriers carry in last phases of recoverylast phases of recovery

Chronic carriers carry for long Chronic carriers carry for long periods after recoveryperiods after recovery

Passive carriers are uninfected Passive carriers are uninfected but convey infectious agents but convey infectious agents from infected persons to from infected persons to uninfected ones by hand and uninfected ones by hand and instrument contactinstrument contact

Vectors/ZoonosesVectors/Zoonoses

► A vector is an animal that transmits A vector is an animal that transmits pathogenspathogens

► A biological vector is an alternate animal A biological vector is an alternate animal host (mosquito, flea) that assists in host (mosquito, flea) that assists in completion of life cycle of microbecompletion of life cycle of microbe

► A mechanical vector is an animal that does A mechanical vector is an animal that does not host microbial life cycle, but is a short-not host microbial life cycle, but is a short-term transmitter (housefly)term transmitter (housefly)

► A zoonosis is an infection for which animals A zoonosis is an infection for which animals are the natural reservoir and host that can are the natural reservoir and host that can be transmitted to humansbe transmitted to humans

Acquisition of InfectionAcquisition of Infection

► Communicable Communicable infectious disease infectious disease occurs when occurs when pathogen is pathogen is transmitted from transmitted from host to host host to host directly or directly or indirectly; indirectly; contagious contagious diseases are diseases are readily readily transmissible transmissible through direct through direct contactcontact

Acquisition of InfectionAcquisition of Infection

► Non-communicable diseases are not Non-communicable diseases are not spread from host to host; acquired spread from host to host; acquired from one’s normal flora (pneumonia) from one’s normal flora (pneumonia) or from a nonliving environmental or from a nonliving environmental reservoir (tetanus)reservoir (tetanus)

Acquisition of InfectionAcquisition of Infection

► Direct transmissionDirect transmission Infectious agent is spread through direct contact of Infectious agent is spread through direct contact of

portal of exit with portal of entry (STDs, herpes portal of exit with portal of entry (STDs, herpes simplex)simplex)

► Indirect transmissionIndirect transmission A material (vehicle) contaminated with pathogens A material (vehicle) contaminated with pathogens

serves as intermediate source of infectionsserves as intermediate source of infections A fomite is an inanimate object contaminated with A fomite is an inanimate object contaminated with

pathogens (public facilities, personal items)pathogens (public facilities, personal items) Food serves as a vehicleFood serves as a vehicle Droplet nuclei are airborne dried particles Droplet nuclei are airborne dried particles

containing infectious agents, formed by sneezing containing infectious agents, formed by sneezing and coughingand coughing

Acquisition of InfectionAcquisition of Infection► Nosocomial infections Nosocomial infections

are infectious diseases are infectious diseases that originate in the that originate in the hospital or clinical hospital or clinical settingsetting They commonly occur They commonly occur

among surgical and among surgical and chronically ill patientschronically ill patients

Hospitals monitor Hospitals monitor various asepsis various asepsis procedures to help procedures to help reduce the number of reduce the number of infectionsinfections

Isolation of patients Isolation of patients and other universal and other universal precaution are precaution are necessary controlsnecessary controls

Koch’s PostulatesKoch’s Postulates

► Koch’s Koch’s postulates postulates defines a series defines a series of criteria that of criteria that must be must be followed to followed to determine the determine the etiologic etiologic (causative) (causative) agent of diseaseagent of disease