Host-Microbe Interactions

Chapter 14

• Disease

• Etiology

• Pathogen– Primary vs. opportunistic

• Virulence

• Contamination

• Infection vs. Infestation

• Pathogenesis

• Pathology

• Symptoms

• Signs

• Syndrome

• Sequelae

Anatomical Barriers as Ecosystems

• Skin and mucous membranes are physical barriers to infection– May supply foundation

for microbial ecosystem

• Humans are usually sterile in utero • Exposed to microbes during and immediately

after birth – microbial populations begin to establish

• Normal flora – Resident flora – Transient flora

• Probiotics

• Isolated colonies in specific body regions

•Dominant type of organism may change with age and situation

• Factors that influence distribution of Flora:– Nutrient availability, salinity, oxygen

availability, host defenses and mechanical factors

– Normal flora may offer protection from disease-causing organisms

– microbial antagonism– Competitive exclusion– Bacteriocins

• Symbiotic relationships form between microorganism and host– Relationships may change depending on

state of host and attributes of microbes

– Mutualism (++)• both partners benefit

– Intestinal bacteria

– Probiotics

– Commensalisms (+ neutral) • one partner benefits and other is unharmed

– Flora on skin and conjunctiva

– Parasitism (+-) • microbe benefits at expense of host

– Pathogens

– State of host resistance usually determines extent of infection

• primary infection

• secondary infection

• Sub-clinical or in-apparent infection

Pathogenicity

• Many people are carriers of pathogens– Viruses; Neisseria; Salmonella;

Streptococcus

• Why are they not affected?

• Predisposing factors:– gender– genetic background– climate and weather– inadequate nutrition– age– habits and lifestyle– chemotherapy– emotional disturbances

• Characteristics of infectious disease:– communicable

• contagious

• Often reflects ID50

– non-communicable disease

Etiology of Infectious Diseases

• Robert Koch demonstrated that specific microbes caused specific diseases– experimented with grazing animals infected

with anthrax – Later work with TB got more interest

• Koch developed a series of steps (Koch’s postulates): – Same pathogen must be present in each

case of disease– Pathogen is isolated from diseased host

and grown in pure culture– Pure culture must cause disease when

inoculated into healthy animal– Pathogen must be re-isolated from

inoculated animal

• Exceptions to Koch’s postulates– some bacteria have unique culture

requirements – some diseases are caused by multiple

pathogens• Polymicrobial diseases (mixed infections)

– Ethical considerations

Stages of an Infectious Disease

Duration of a Disease

– acute diseases

– chronic diseases

– latent disease

Distribution of pathogens

– Local infections

– Systemic infections• Bacteremia• Toxemia• Viremia

– Septicemia