MICROBE-HUMAN INTERACTIONS, INFECTION

AND DISEASE

Melvin FailagaoBSED-Bio. Sci

PROGRESS OF AN INFECTION

TYPES OF PATHOGENS

TRUE PATHOGENS are capable of causing infection and disease in healthy persons with normal immune defenses.  OPPORTUNISTIC PATHOGENS can cause disease when the host's defenses are compromised or when they become established in a part of the body that is not natural to them.

Stages in Infection/Disease a. Incubation period, the period from contact with

infectious agent until appearance of first symptoms. b. Prodromium, a short period of initial, vague

symptoms. c. Period of invasion, a variable period during

which microbe multiplies in high numbers and causes severest symptoms.

d. Convalescent period, a period of recovery, with decline of symptoms

Type of Infections/Diseases a. Localized infection, microbe remains in isolated

site. b. Systemic infection, microbe is spread through

the tissues by circulation. c. Focal infection, microbe spreads from local site

to entire body (systemic). d. Mixed infection, several microbes cause one

type of infection simultaneously. e. Primary infection, the initial infection in a series.

Type of Infections/Diseases f. Secondary infection, a second infection that

complicates a primary infection. g. Septicemia and bacteremia refer to

microbes in the blood. h. Acute infection appears suddenly, has a

short course, and is relatively severe. i. Chronic infection persists over a long period

of time. j. Sub-acute infection has a pattern between

acute and chronic.

Levels and Classes of Microbes

Class 1: not known to cause disease. Class 2: moderate risk agents. Class 3: readily transmitted and virulent agents.

Class 4: highest risk, deadly pathogens.

Virulence factors are the properties of microbes to invade a host and produce toxins (toxigenicity).

Virulence or the degree of disease can be due to single or multiple factors.

Pathogenicity is the ability to cause disease in host.

Virulence Factors a. Exoenzymes digest epithelial tissues, disrupt

tissues, and permit invasion. b. Toxigenicity is a microbe’s capacity to produce

toxins at site of multiplication which affect cellular targets

(1) Toxinoses are diseases caused by toxins that damage structure or function of host cells.

(2) Toxemia refers to toxins absorbed into the blood. (3) Intoxication means ingestion of toxins.

Virulence Factors (4) An exotoxin is a protein secreted by living

bacteria with powerful effects on a specific organ. Examples are hemolysins and tetanus and diphtheria toxins.

(5) Endotoxin is the lipopolysaccharide portion of a gram-negative cell wall released when a bacterial cell dies; causes generalized symptoms such as fever

c. Antiphagocytic factors include leukocidins (white blood cell poisons) and capsules.

Portal of Entry: Gateway to Infection

Exogenousoriginating from source outside the body (environment, human, or animal). Endogenousalready existing on or in the body. Storchcommon infections of fetus and neonate.

Signs and Symptoms: Manifestations of disease, indicators of pathologic

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 tissue.

b. Symptom is a subjective effect of disease as sensed by patient. Examples are pain, fatigue, and nausea.

c. Syndrome is a disease that manifests as a predictable complex of symptoms; infections that do not show symptoms are called asymptomatic, subclinical, or in apparent.

Size of Inoculum 

Infection doseminimum number for infection to occur. Microorganisms with smaller infectious

dose have greater virulence. Lack of infectious dose will generally not result in infection. Once in excess of ID, disease can be extremely rapid

Mechanisms of Invasion and Establishment of the Pathogen

Adhesionprocess by which microbes gain more stable foothold at the portal of entry. a.Fimbriae b.Glycocalyx  c.Adhesins (ligands) d.Biofilm

Fimbriae Adhesion

Glycocalyx and Biofilm

EPIDEMIOLOGY: THE STUDY OF DISEASE IN POPULATIONS

Notifiable- agencies required by law to collect data Communicable- host to host transmission. Contagious- extremely efficient host to host

transmission. Non-Communicable- no host to host transmission. Correlation- one factor assumed to be related to

another. Cause and Effect- one factor is proven to caused

a measurable change in another.

EPIDEMIOLOGY science that determines the factors influencing

causation, frequency, and distribution of disease in a community

Epidemiologist involved in surveillance of reportable diseases

in populations and consider measures to protect the public health.

involved in surveillance of reportable diseases in populations and consider measures to protect the public health.

Frequency of Disease Endemic, a disease constantly present in a

certain geographic area. Sporadic, a disease that occurs

occasionally with no predictable pattern. 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.

CLASSIFICATION OF CARRIERS 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 vector is an animal that transmits pathogens. biological vector is an alternate animal host

(mosquito, flea) that assists in completion of life cycle of microbe.

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

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

Acquisition of Infection 1. Communicable infectious disease occurs when

pathogen is transmitted from host to host directly or indirectly; contagious diseases are readily transmissible through direct contact.

2. Non-communicable diseases are not spread from host to host; acquired from one’s own flora (pneumonia) or from a non-living environmental reservoir (tetanus).

3. Direct Transmission Infectious agent is spread through direct contact of portal of exit with portal of entry (STDs, herpes simplex)

Acquisition of Infection

4. Indirect Transmission

a.material (vehicle) contaminated with pathogens serves as intermediate source of infections.

b.fomite is an inanimate object contaminated with pathogens (public facilities, personal items).

c.Food serves as a vehicle.d.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. a. They commonly occur among surgical and

chronically ill patients. b. Hospitals monitor various asepsis procedures

to help reduce the number of infections. c. Isolation of patients and other universal

precautions are necessary controls.

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

1. Microorganisms are isolated from a dead animal.

2. Microorganisms are grown in pure culture. The microorganisms are identified. 

Koch's Postulates

3. The microorganisms are injected into a healthy animal.

4. The disease is reproduced in the second animal; microorganisms are isolated from this animal.

5. Pathogenic microorganisms are grown in pure culture. Identical microorganism identified.

DEFENSE MECHANISMS OF THE HOST IN PERSPECTIVE

DEFENSE MECHANISMS OF THE HOST IN PERSPECTIVE

The First Line of Defense

Nonspecific Chemical Defenses Sebaceous secretions and specialized glands-

antimicrobial Lysozyme in tears Lactic acid and electrolyte concentrations of

sweat Skin’s acidic pH and fatty acid content

DEFENSE MECHANISMS OF THE HOST IN PERSPECTIVE

The First Line of Defense

Nonspecific Chemical Defenses HCl in the stomach Digestive juices and bile in the intestine Semen- antimicrobial chemical Acidic pH in the vagina

TYPES OF NON-SPECIFIC MECHANISMS

First Line of Defenses MECHANICAL AND PHYSICAL BARRIERS CHEMICAL BARRIERS COMPLEX BIOLOGICAL MECHANISMS

AND REPONSES

evaluation