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

2. 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

3. Associations between microbes and humans

4. 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

5. 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

6. 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

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

8. 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

9. 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.

10. 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

11. Factors Affecting the Course of Infection and Disease

12. 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

13. 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

14. 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

15. 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

16. 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

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

18. 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

19. 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

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

21. 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

22. 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

23. Types of Infectious/Diseases

24. 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

26. 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

27. 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)

28. Graphical representation of epidemiological data

29. 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

30. 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

31. 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

32. 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

34. 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

35. 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)

36. 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

37. 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

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