a) Healthy individuals are INFECTED and are being infected anew constantly.
b) Some of these organisms maybe PATHOGENS (more frequently among
the transient flora group).
Some among the normal flora may be OPPORTUNISTS.
c) Our relationship with microbes is very dynamic:
THERE IS A BALANCE BETWEEN:
The disease causing properties of the microbes <<--->> and the antimicrobial defenses of
d) INFECTIOUS DISEASE is disease caused by pathogenic microorganisms.
Pathogenic = disease causing}
Virulence = degree or intensity of pathogenicity
Disease = abnormal state, deviation from a state of wellness or health
Contamination means that microorganisms are present.
Infection, when parasitic microorganisms increase in number either within or on the body of the
e) Whether or not you will catch a disease depends on:
(1) YOU: your health, nutrition, immune status.
(2) The pathogen's VIRULENCE
(A) How TOXIC the organism is
(B) How INVASIVE the organism is
II. THE NORMAL FLORA
* The human adult is estimated to have 1013 human cells and 1014 bacterial cells.
* SEE your book for diagram of human body sites and chart of normal human flora organisms and their
* In all honesty, we are a cylinder with a hole through the center. Just about every surface is colonized.
There are aparse areas:
- THE STOMACH - was thought to be free of organisms
-- but recently shown that some
organisms can survive --- Helicobacter pylori---- This is probably not considered normal flora
since it is now known to cause ulcers.
- THE BLADDER and the LOWER REACHES OF THE RESPIRATORY TRACT. These
areas can be transiently infected but normal clearance mechanisms exist to get rid of the
- We are colonized at birth with Lactobacillus.
- Over time we acquire and establish populations of:
* COLIFORMS - (Escherichia coli and other enteric gram negative rods) - intestines
* Staphylococcus aureus, Staphylococcus epidermidis, Proprionibacterium acnes and
other diptheroids - skin
* Streptococci (viridans and pneumoniae) as well as some anaerobes - mouth
* Lactobacilli - vagina
III. BENEFICIAL EFFECTS OF THE NORMAL FLORA
* Competition and the production of inhibitors (bacteriocins)
* If normal flora is destroyed ex.:
- Sthaphilococcal enteritis - S. aureus
- Pseudomembrane colitis - Clostridium difficile
- Candida infections - Thrush, vaginitis caused by antibiotics, douches and deodorants;
2) Production of nutrients- Vitamins B and K in the intestines
by E. coli
3) Stimulate maturation of the immune system
Note: The above are examples of mutualism
IV. REVIEW SYMBIOSIS, COMMENSALISM, MUTUALISM, PARASITISM
* Imposed on the the idea of opportunism
OPPORTUNISTS - These are organisms that normally
don't cause disease but will if given an
- As in secondary infections
- If resistance is low: - Pneumocystis carinii - pneumonia in immunocompromised
individuals. Neisseria miningitidis - meningitis in children, 5-15% carriage rate in
apparently healthy people.
- If they get into the wrong place. Ex: E. coli - Normally in the intestine, but if in the bladder
or peritoneal cavity - Big problem!. Susch as when one sees peritonitis after a ruptured
appendix. Also with E. coli one has different pathogenic variants such as O157:H7 - This
is EHEC or enterohemorrhagic E. coli which produces powerful toxins responsible for
hemolytic uremic syndrome.
pneumoniae - Pneumonia after influenza or other respiratory tract
particularly in the elderly people.
- Candida albicans - Vaginitis and thrush
- Pseudomonas aeruginosa - Infections of burns and infections of the lungs in cystic fibrosis
- Staphilococcus aureus - Skin infection and toxic shock syndrome
HOW TO DETERMINE IF AN ORGANISM IS THE ETIOLOGIC AGENT OF DISEASE
REVIEW KOCH'S POSTULATES:
1. The agent must be observed in every case of the disease.
2. The agent must be isolated from a diseased host and grown in pure culture.
3. When purified agent is inoculated into a healthy but susceptible host, it must cause the same
4. The agent must be reisolated from the newly infected, diseased host, and be identical to the
previously identified causative agent.
Note: Identification of disease agents, according to the postulates requires growing the organism; this can be difficult or impossible for some: Treponema pallidum, Mycobacterium leprae, and so this cannot be an inflexible approach.
CLASSIFYING INFECTIOUS DISEASES.
How they behave within a population?
I. Spread within a population
1) COMMUNICABLE DISEASES = CONTAGIOUS, easily spread from host to another either
directly or indirectly, ex.: influenza, common cold, tuberculosis, chicken pox, measles, genital
Herpes. Note: NOT all communicable diseases are equally contagious). Contagiousness depends
on several factors.
2) NON-COMMUNICABLE DISEASES a disease that is not spread from one host
ex.: tetanus, food poisoning, subacute bacterial endocarditis, etc.
II) Frequency in a population:
a) Epidemiology: study of factors and mechanisms governing spread of disease within a population,
incidence, spread, control, prevention. (what, how, when).
Diseases in a population:
a) Sporadic disease: outbreak occurs in population occasionally and at irregular intervals eg, many
gastrointestinal diseases from contaminated food. (Typhoid fever)
b) Endemic disease: constantly present at low level eg, colds
c) Epidemic disease: outbreak occurs in unusually high numbers in a population eg, mumps in
elementary schools, influenza.
d) Pandemic disease: epidemic over a large geographical area (such as the world). eg, worldwide
influenza outbreak in 60s; AIDS.
TERMS TO DEFINE DISEASE IN A POPULATION.
a) Incidence: # of people in a population with new cases over a specific period of time. This term
reflects the spread of the disease. (1992 AIDS had a 8% incidence)
b) Prevalence: Total # of cases of disease in a population at any given time. This term reflects how
sick the population is. (1992 AIDS prevalence was 19%).
c) Morbidity: Occurrence of a disease in a population over a defined period of time. Expressed as a
rate # new cases in population in a period / total population.600 in 10,000 = 6%.
SEVERETY OR DURATION OF DISEASE:
1) Acute disease – develops rapidly but ends shortly (influenza, herpes)
2) Chronic disease – slow development but continuous for long period of time (TB, infectious
mononucleosis [EBV], hepatitis B
3) Latent disease – agent remains inactive then reactivates (TB, toxoplasmosis, shingles [VZV])
1) Local VS systemic – infectious agent remain in one area in the body (abscesses in teeth, skin
infections) or they can be spread through the body by the blood (measles, chicken pox).
2) Primary vs secondary – acute infection that causes initial illness or caused by an opportunistic
pathogen after body weakened by initial infection.
3) Bacteremia vs septicemia – bacteria are present in the blood. When bacteria are actually growing
and dividing in the blood.
a) Symptoms – (subjective) – pain, dizziness, aches, etc. Commonly felt by the patient.
b) Signs – (objective) – fever, swelling, paralysis, vomiting, etc. Commonly observed by a physician.
c) Syndrome – a combination of both symptoms and signs.
HOSPITAL ACQUIRED INFECTIONS
* 5-15% of all patients will suffer these
* Most common organisms: E. coli, Klebsiella, Proteus, and S. aureus.
Nosocomial infections -- Hospital acquired infections are especially
- Many hospital organisms are resistant to antibiotics
- Patients are immuno compromised (By disease, injury, or chemotherapy)
- There are many invasive procedures
- Chain of transmission
DEVELOPMENT OF DISEASE:
1. The incubation period - initial infection and the first appearance of signs or symptoms.
2. The prodromal period -short duration. Period of initial mild sign or symptoms.
3. The period of illness -period of maximum presentation of signs and symptoms.
4. The period of decline -signs and symptoms start to decline.
5. The period of convalescence - regain strength
SPREAD OF INFECTION IN A POPULATION
* Need to know the reservoir and mode of trasnmission
a) RESERVOIR - Can think of the reservoir as the source of the
1) Human source:
Sick human -- Acute illness (rapidly developing and often rapid resolution - get better or die.
Ex: influenza, chicken pox, cholera, strep throat.
Carrier human -- Inapparent
infections, subclinical infection, chronic infection. Ex: tuberculosis,
Epstein-Barr infection (mononucleosis), syphilis, HIV infection, Hepatitis B
2) Animal source: wild animal (sylvatic)?
Zoonosis -- primarily an animal disease and then spread to humans. Examples of diseases with
a significant animal reservoir: rabies, plague, leptospirosis, lyme disease,
Toxoplasmosis, psittacosis, salmonella food poisoning
3) Non-living source:
- a) Soil harbors organisms that cause a variety of infectious diseases. Ex: tetanus, botulism,
anthrax, Pseudomonas infections, a variety of fungi.
- Water is often contaminated and can serve as a vehicle of infection. Ex. giardiasis, typhoid
fever, amebiasis, leptospirosis -- note that water is easily contaminated by human and animal
MODE OF TRANSMISSION
1. CONTACT TRANSMISSION:
a) Direct contact -- Touch, kiss, sex, animals
b) Indirect: - Fomites (Inanimate objects that serve as means of spread) -- doorknobs, towels,
phones, needles and other medical equipment.
c) Droplet transmission: droplet nuclei - travel short distances - Important in many
respiratory infections (see picture Tortora pg. 407)
2. VEHICLE TRANSMISSION:
a) Waterborne -- oral-fecal transmission
b) Foodborne -- unproperly cooked, fecal contamination
c) Airborne -- droplet nuclei in dust -- must travel more than 1 meter.
3. Vector -- and infected animal (usually an insect) that
transmits to humans
- Biological transmission -- The animal host is needed by the microbe for some metabolic or
other essential process. Ex.: malaria protozoan- mosquito; Lyme disease spirochete -- tick;
sleeping sickness protozoan -- tsetse fly; the plague bacterium -- flea and rodent (rat)
- Mechanical transmission -- the animal host host picks up the microbes and moves them
around. Ex.: flies landing on feces or dead animals. cockroaches and lizards rats can do the
THE VIRULENCE FACTORS OF MICROORGANISMS
Generally organisms referred to as pathogens have a high probability of causing an infection (and more reasonably should be called frank pathogens!), but under unusual circumstances any microorganism, even a non-pathogen, might be capable of causing an infection.
Virulence - The degree of pathogenicity. An attribute generally ascribed to a strain.
* Organisms have to get in first (How?) -- The
portals of entry:
- Mucous membranes - mouth, nose, eyes, respiratory tract, GI, GU.
- Breaks in the skin
- Breaks into the tissues below the skin into the tissues, veins, or arteries --PARENTERAL
2) ORGANISMS HAVE TO ADHERE -- THEY HAVE ADHESINS
CAPSULE -- HELPS TO ATTACH BUT ALSO IMPEDES PHAGOCYTOSIS.
ex.:Streptococcus pneumoniae, Klebsiella pneumoniae, Streptococcus mutans - dextran and
FIMBRIAE (PILI) -- ARE OFTEN TISSUE SPECIFIC. ex.: E.
coli strain differences in tissue
specificity due to different types of fimbriae - urinary tract strains differ from enteropathogenic
strains in the type of pili they make. Whooping cough bacterium - Bordetella pertusis attaches
specifically to repiratory epithelium cilia.
Neisseria gonorrhea - has pili and outer membrane proteins
that allow it to stick to urethral and
vaginal epithelium, non-ciliated fallopian tube cells, sperm cells and neutrophils; Streptococcus
pyogenes - protein F attaches to pharyngeal epithelium, protein M attaches to keratinocytes in the
skin both proteins are are part of the fimbriae of these organisms.
THE CELL WALLS OF SOME BACTERIA HAVE VIRULENCE PROMOTING
PROPERTIES. ex.: Streptococcus pyogenes - protein G binds to the back end of antibodies
preventing their normal function - this is a CLOAKING DEVICE for the bacterial cell and it helps
the cell avoid phagocytosis. Staphylococcus aureus has protein A - which works the same way.
Mycobacterium tuberculosis has wax D and sulfolipids which inhibit the killing mechanisms of
The outer membrane of Gram negative organisms (endotoxin, LPS, contains
lipid A). The structure
is shed periodically but in large scale when the organism dies. Profound effect on humans: fever,
weakness, aches, schock, hemorrhage,etc. These effects are caused by the release of interleukin
(IL)-1 and tumor necrosic factor (TNF) by macrophages.
BACTERIAL ENZYMES ASSOCIATED WITH VIRULENCE - many are exoenxymes
a) BACTERIAL TOXINS -- EXOTOXINS (many are enzymes)
The landmark of endotoxemia are: 1) Fever and 2) Toxic shock.