Microbe-Human Interactions: Infection and Disease Chapter 13 Microbe-Human Interactions: Infection and Disease

Contact-Infection-Disease: The Host-Parasite Relationship The human body is constantly in contact (contaminated) with microbes Some are pathogens that may cause an infection by circumventing the host defense system, entering normally sterile tissues, and multiplying there When infections lead to a disruption in tissues, infectious disease results The outcome is highly variable, but most contacts do not result in infection, and most infections do not lead to disease

Associations between microbes and humans

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

The Body as a Habitat Anatomical sites lying within the body cavity (organs) and fluids (blood, urine) in those sites do not harbor flora Colonization: begins just prior to birth and continues over an individual’s life; variations occur in response to individual differences in age, diet, hygiene, and health

Normal Flora Bacteria may maintain a balance in the normal conditions Studies with axenic animals (free of any normal flora) show that flora contribute to the development of the immune and gastrointestinal systems and also to some diseases (dental caries) Normal flora are sometimes agents of infection

Landscape of the skin Microbes live only in upper dead layers of epidermis glands, and follicles; dermis and layers below are sterile

Distribution of flora in the gastrointestinal tract Oral cavity: colonize the epidermal layer of cheeks, gingiva, pharynx; surface of teeth; found in saliva in huge numbers Large intestine and rectum have huge numbers of flora

Colonized regions of the respiratory tract The moist mucous blanket of the nasopharynx has a well-entrenched flora. Lower regions of bronchi, bronchioles, and lungs are free of resident microbes.

Flora of the reproductive tract In females, the external genitalia, vaginal, and cervical surfaces have flora In females, the first portion of the urethral mucosa has flora. In males, the entire reproductive and urinary tract is sterile, except for a short portion of the anterior urethra

Factors Affecting the Course of Infection and Disease

Pathogenicity and Virulence Pathogenicity is the property of microorganisms to cause infection and disease Virulence is the precise factors used by the microbe to invade and damage host tissues; it helps define the degree of pathogenicity Pathogenicity varies with a microbe’s ability to invade or harm host tissues and with the condition of host defenses A true pathogen produces virulence factors that allow it to readily evade host defenses and to harm host tissues True pathogens can infect normal, healthy hosts with intact defenses An opportunistic pathogen is not highly virulent but can cause disease in persons whose host defenses are compromised by predisposing conditions such as age, genetic defects, medical procedures, and underlying organic disease

Mechanisms of Infection and Disease The portal of entry is the route by which microbes enter the tissues, primarily via skin, alimentary tract, respiratory tract (pneumonia), urogenital tract (sexually transmitted diseases), or placenta

Transplacental infection of the fetus Microbes penetrate the maternal blood vessels and enter the blood pool of the placenta. They then invade the fetal circulation by way of the umbilical vein

Mechanisms of Infection and Disease Pathogens that come from outside the body are exogenous; those that originate from normal flora are endogenous The size of the infectious dose is of great importance

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

Virulence Factors Exoenzymes digest epithelial tissues, disrupt tissues, and permit invasion

Virulence Factors Toxigenicity is a microbe’s capacity to produce toxins at site of multiplication which affect cellular targets Toxinoses are diseases caused by toxins that damage structure or function of host cells Toxemia refers to toxins absorbed into the blood Intoxication means ingestion of toxins

Virulence Factors An exotoxin is a protein secreted by living bacteria with powerful effects on a specific organ Examples are hemolysins and tetanus and diphtheria toxins An endotoxin is the lipopolysaccharide portion of a gram-negative cell wall released when a bacterial cell dies; causes generalized symptoms such as fever

Virulence Factors Antiphagocytic factors include leukocidins (white blood cell poisons) and capsules

Effects on Target Organ/Spread of Infection Patterns of Infection: Stages in Infection/Disease Incubation period, the period from contact with infectious agent until appearance of first symptoms Prodromium, a short period of initial, vague symptoms Period of invasion, a variable period during which microbe multiplies in high numbers and causes severest symptoms Convalescent period, a period of recovery, with decline of symptoms

Types of Infectious/Diseases Localized infection, microbe remains in isolated site Systemic infection, microbe is spread through the tissues by circulation Focal infection, microbe spreads from local site to entire body (systemic) Mixed infection, several microbes cause one type of infection simultaneously Primary infection, the initial infection in a series Secondary infection, a second infection that complicates a primary infection Septicemia and bacteremia refer to microbes in the blood Acute infection appears suddenly, has a short course, and is relatively severe Chronic infection persists over a long period of time Subacute infection has a pattern between acute and chronic

Types of Infectious/Diseases

Signs and Symptoms: Manifestations of disease, indicators of pathogenic effects on target organs A sign is objective, measurable evidence noted by an observer Examples include septicemia, change in number of white blood cells; skin lesions; inflammation; necrosis, lysis or death of tissues A symptom is a subjective effect of disease as sensed by patient Examples are pain, fatigue, and nausea A syndrome is a disease that manifests as a predictable complex of symptoms; infections that do not show symptoms are called asymptomatic, subclinical, or inapparent

Major portals of exit of infectious diseases Through the portal of exit, microbe is released with bodily secretions and discharges to have access to new host; portals include respiratory droplets from sneezing, coughing, saliva, skin, feces, urogenital tract (urine, mucus, semen) and blood A microbe may become dormant (latent) and cause recurrent infections Damaging effects that remain in organs and tissues after infection are sequelae

Epidemiology Epidemiology is a science that determines the factors influencing causation, frequency, and distribution of disease in a community Epidemiologists are involved in surveillance of reportable diseases in populations and consider measures to protect the public health They are concerned with disease statistics such as prevalence (the total number of cases), incidence (the number of new cases), morbidity (general health of the population), and mortality (death)

Graphical representation of epidemiological data

Frequency of Disease Endemic, a disease constantly present in a certain geographic area Sporadic, a disease that occurs occasionally with no predictable patter Epidemic, sudden outbreak of disease in which numbers increase beyond expected trends Pandemic, worldwide epidemic

Origin of Pathogens The reservoir is a place where the pathogen ultimately originates (its habitat) Source of infection refers to the immediate origin of an infectious agent Carrier is an individual that inconspicuously shelters a pathogen and spreads it to others

Carriers in transmission of infectious agents Asymptomatic carrier is infected without symptoms Incubation carriers carry early in disease Convalescent carriers carry in last phases of recovery Chronic carriers carry for long periods after recovery Passive carriers are uninfected but convey infectious agents from infected persons to uninfected ones by hand and instrument contact

Vectors/Zoonoses A vector is an animal that transmits pathogens A biological vector is an alternate animal host (mosquito, flea) that assists in completion of life cycle of microbe A mechanical vector is an animal that does not host microbial life cycle, but is a short-term transmitter (housefly) A zoonosis is an infection for which animals are the natural reservoir and host that can be transmitted to humans

Acquisition of Infection Communicable infectious disease occurs when pathogen is transmitted from host to host directly or indirectly; contagious diseases are readily transmissible through direct contact

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

Acquisition of Infection Direct transmission Infectious agent is spread through direct contact of portal of exit with portal of entry (STDs, herpes simplex) Indirect transmission A material (vehicle) contaminated with pathogens serves as intermediate source of infections A fomite is an inanimate object contaminated with pathogens (public facilities, personal items) Food serves as a vehicle Droplet nuclei are airborne dried particles containing infectious agents, formed by sneezing and coughing

Acquisition of Infection Nosocomial infections are infectious diseases that originate in the hospital or clinical setting They commonly occur among surgical and chronically ill patients Hospitals monitor various asepsis procedures to help reduce the number of infections Isolation of patients and other universal precaution are necessary controls

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