Chapter 13: Microbe- Human Interaction

What state does the human body exist?

in a state of dynamic equilibrium

What do the interactions between the human body and microorganisms involve?

the development of biofilms

What does the colonization of the body involve?

involves a constant “give and take”

Most significant interactions

• Microbes provide a protective and stabilization effect on body surfaces

• Microbes are involved in maturation of host defenses and development of the immune system

• Microbes can invade and grow in sterile tissues, causing disease

Normal resident microbiota

Microbes that engage in mutual or commensal association with humans

• other terms: indigenous, microflora, normal flora, and commensals

Infection

a condition in which pathogenic microbes penetrate host defenses, enter tissues, and multiply

Pathogens

microbes acting as infectious agent

Infectious disease

an infection that causes damage or disruption to tissues and organs

Resident Microbiota: The human as a Habitat

Most areas of the body in contact with the outside environment harbor resident microbes

• Transients

• Residents

Transients

microbes that occupy the body for only short periods

Residents

microbes that become established

Sites that harbor Normal Resident Microbes

• Skin and its contiguous mucous membranes

• Upper respiratory tract (oral cavity, pharynx, nasal mucosa)

• Gastrointestinal tract (mouth, colon, rectum, anus)

• Outer opening of urethra

• External genitalia

• Vagina

• External ear and canal

• External eye (lids, lash follicles)

stability of Resident Microbiota

generally stable, but fluctuates with general health, age, diet, hygiene, hormones, and drug therapy. Dysbiosis

Microbial antagonism

microbiota benefits host by preventing overgrowth of harmful microbes

Endogenous infections

occur when normal flora is introduced to a site that was previously sterile

Initial Colonization of fetus and Newborn

Uterus and contents are normally sterile until just before birth

• Breaking of fetal membrane exposes the infant

Subsequent handling and feeding of newborn continue to introduce what will be normal flora

• Nature of the microbiota initially colonizing large intestine is influenced by whether the baby receives breast milk or formula

Normal Flora

is essential to the health of humans

What does flora create?

an environment that may prevent infections and can enhance host defenses

What can alter flora?

Antibiotics, dietary changes, and diseases

Probiotics

introducing known microbes back into the body

Major factors in development of infections

• True pathogens

• Opportun

True pathogens

capable of causing disease in healthy persons with normal immune defenses

• Influenza virus, plague bacillus, malarial protozoan

Opportunistic pathogens

cause disease when the host’s defenses are compromised or when they grow in part of the body that is not natural to them

• Pseudomonas sp. and Candida albicans

Factors that weaken host defenses and increase susceptibility to infection

• Old age and extreme youth (infancy, prematurity)

• Genetic defects in immunity and acquired defects in immunity

• Surgery and organ transplants

• Organic disease: cancer, liver malfunction, diabetes

• Chemotherapy/immunosuppressive drugs

• Physical and mental stress

• Other infections

*These conditions compromise defense barriers or immune responses.

What does the severity of the disease depends on?

the virulence of the pathogen

Virulence

is a term for describing the degree of pathogenicity

Virulence factor

characteristic or structure that contributes to the ability of a microbe to cause disease

Infectious Dose (ID)

Minimum number of microbes required for infection to proceed

• Microbes with small IDs have greater virulence

• Lack of ID will not result in infection

Toxins

specific chemical product of microbes, plants, and some animals that have poisonous effects on other organ

toxinoses

adverse effects of toxins

• toxemias

• Intoxications

Toxemias

when the toxin is spread by the blood from the site of infection (tetanus, diphtheria)

Intoxications

caused by ingestion of toxins (botulism)

Types of bacterial toxins

• Endotoxin

• Exotoxin

Endotoxin

not secreted, but released after the host cell is damaged

• composed of part of the outer membrane of gram-negative cell walls (LPS)

Exotoxin

secreted by a living bacterial cell into the infected tissue

• strong specificity for a target cell, disrupting its membrane

• hemolysins, disrupt membrane of red blood cells

• A-B toxins, becomes internalized and interrupts an essential pathway (A-active, B-binding)

neurotoxin, enterotoxin

Four stages of clinical infections

1. Incubation period

2. Prodromal stage

3. Period of invasion

4. Convalescent period

Incubation period

time from initial contact with the infectious agent to the appearance of first symptoms; agent is multiplying but damage is insufficient to cause symptoms; several hours to several years

Prodromal Stage

vague feelings of discomfort; nonspecific complaints

Period of invasion

multiplies at high levels, becomes well-established; more specific signs and symptoms

Convalescent period

as a person begins to respond to infection, symptoms decline

Localized infection

microbes enter the body and remains confined to a specific tissue

Systemic infection

infection spreads to several sites and tissue fluids usually in the bloodstream

Focal infection

when infectious agent breaks loose from a local infection and is carried to other tissues

Mixed infection

several microbes grow simultaneously at the infection site- polymicrobial

Sequence of occurrence for infection

• primary infection

• secondary infection

Primary infection

initial infection

Secondary infection

subsequent infection by a different microbe

Acute infection

comes on rapidly, severe but short-lived effects

Chronic infections

progress and persist over a long period of time

Portals of exit

• Respiratory and salivary portals – mucus, sputum, nasal drainage, saliva

• Epithelial cells – skin and scalp

• Fecal exit

• Urogenital tract

• Removal of blood or bleeding

Epidemiology

study of the frequency and distribution of disease and other health-related factors in defined human populations

considers many diseases other than infectious ones, including heart disease, cancer, drug addiction, and mental illness

what does Epidemiology involve

it involves many disciplines-- not only microbiology but also anatomy, physiology, immunology, medicine, psychology, sociology, ecology, and statistics

Vector

A live animal (other than human) that transmits an infectious agent from one host to another

types of vectors

majority are arthropods; fleas, mosquitoes, flies, and ticks

Some larger animals can also spread infection; mammals, birds, lower vertebrates

Biological vectors

actively participate in a pathogen’s life cycle

Mechanical vector

not necessary to the life cycle of an infectious agent and merely transports it without being infected

Zoonosis

Infection indigenous to animals naturally transmissible to humans (dead-end host)

How many zoonoses exist?

At least 150 zoonoses worldwide (70% of all new emerging diseases worldwide)

Eradicating disease

impossible to eradicate the disease without eradicating the animal reservoir

Patterns of Transmission

• Direct contact

• Indirect contact

Direct contact

physical contact or fine aerosol droplets

Indirect contact

passes from infected host to intermediate conveyor and then to another host

• Vehicle

• Airborne

• Oral-fecal route

Vehicle

inanimate object (formite), food, water, biological products

Airborne

droplets, aerosols

Oral-fecal route

vehicle first becomes contaminated through contact with fecal material and is then transported to someone’s mouth

Epidemiologist

consider virulence, portals of entry and exit, and the course of the disease

They also preform surveillance

Surveillance

collecting, analyzing, and reporting data on rates of occurrence, mortality, morbidity and transmission of infections

Reportable diseases must be reported to authorities

Prevalence

Total number of existing cases with respect to the entire population usually represented by a percentage of the population

Incidence

measures the number of new cases over a certain time period, as compared with the general healthy population

Mortality rate

the total number of deaths in a population due to a certain disease

Morbidity rate

number of people afflicted with a certain disease

Endemic occurrence

disease the exhibits a relatively steady frequency over a long period of time in a particular geographic locale

Sporadic Occurence

when occasional cases are reported at irregular intervals in widely dispersed locations

Epidemic Occurrence

when prevalence of a disease is increasing beyond what is expected

Pandemic Occurrence

epidemic across continents

Nosocomial infections (Health-care-associated infections (HAIs)

are diseases that are acquired or developed during hospital stays

• From surgical procedures, equipment, personnel, and exposure to drug-resistant microorganisms

• Gram-negative intestinal flora (Escherichia coli, Klebsiella, Pseudomonas)

• Gram-positive bacteria (staphylococci and streptococci) and yeasts

• True pathogens such as the tubercle bacillus, Salmonella, hepatitis B, and influenza virus

How many cases are there of nosocomial infections per year?

2 to 4 million cases/year in U.S. with approximately 90,000 deaths