MLAB 2434 – CLINICAL MICROBIOLOGY SUMMER, 2005 CECILE SANDERS & KERI BROPHY

MLAB 2434 – CLINICAL MICROBIOLOGYSUMMER, 2005CECILE SANDERS & KERI BROPHY

Chapter 6 – Host-Parasite Interaction

Chapter 6 – Host-Parasite Interaction (cont’d) Pathogen – microbe that can

cause disease in a susceptible host

Opportunistic Pathogen – microbe that can cause disease only if a significant change occurs in host resistance or within the organism itself

Chapter 6 – Host-Parasite Interaction (cont’d) Opportunistic Infections -

infections caused by opportunistic pathogens

Iatrogenic Infections – infections resulting from medical treatment or procedures

Chapter 6 – Host-Parasite Interaction (cont’d) Virulence – relative ability of a

microorganism to cause disease, or the degree of pathogenicity

Virulence factors – factors such as capsules, toxins, enzymes, cell wall receptors, pili, etc. that allow pathogens to evade or overcome host defenses

Chapter 6 – Host-Parasite Interaction (cont’d) Host Resistance Factors

Physical barriers – skinCleansing mechanisms

• Desquamation• Fluids of the eye (IgA and

lysozyme)• Respiratory, digestive, urinary,

and genital tracts have fluids and movements to cleanse the surfaces

Chapter 6 – Host-Parasite Interaction (cont’d)

Antimicrobial Substances• Lysozymes• Antibodies• β-lysins• Interferon

Indigenous Microbial Flora Phagocytosis Chemotaxis Inflammation Immune Responses

Chapter 6 – Host-Parasite Interaction (cont’d) Infectious Agent Factors

Adherence – most infectious agents must attach to host cells before infection occurs

Proliferation – pathogens must be able to replicate after attachment to host cells (overcome host resistance factors)

Tissue Damage – makes the infection visible; results from toxins or from host inflammatory substances


Exotoxins• Most have two units, one for attachment

and the other is the toxin• Commonly coded by phages or plasmids

Endotoxins• G- bacteria• Toxicity caused by the lipid A portion of the

lipopolysaccharide in the cell wall• Cause dramatic host responses, including

inflammation, fever, hypotension, septic shock, death


Invasion – all pathogens have the ability to penetrate and grow in tissues

Dissemination• Spread of organisms to distant sites• Some pathogens stay at site (C.

diphtheriae); others spread (Salmonella ssp.)

Chapter 6 – Host-Parasite Interaction (cont’d) Routes of Transmission

Airborne• Coughing, sneezing, talking• Droplet nuclei• Airoborne pathogens must be

resistant to drying and inactivation by ultraviolet light

• Examples: Strep throat, otitis media, diphtheria, rhinoviruses (colds)


Transmission by Food and Water• Ingestion of contaminated food or

water• Sometimes oral-fecal route• Pathogens must be able to survive

stomach conditions and compete with normal flora of the gut

• Preformed toxins (Clostridium botulinum, S. aureus) vs. toxins produced after infection (C. difficile, V. cholerae)


Close Contact• Passage of organisms by salivary, skin,

and genital contact• Examples: Infectious mononucleosis,

STDsCuts and BitesArthropodsZoonoses – diseases of animals

accidentally transmitted to humans; examples: plague, rabies, tularemia

Chapter 6 – Host-Parasite Interaction (cont’d) Epidemiology – study of the

occurrence, distribution and causes of disease and injury

DefinitionsCarrier – person or animal who

harbors and spreads microorganisms that cause disease but does not become ill; examples: N. meningitidis, S. pyogenes


Endemic – Organism or disease is constantly present in a population; examples: Cholera is endemic in third world countries

Epidemic – Disease affects a significantly large number of people at the same time in a geographic area; examples: Influenza

Pandemic – Worldwide epidemic


Incidence Rate – number of times a new event occurs in a given period; usually given as cases per 1000 or 100,000 population

Incubation Period – time between exposure to a pathogen and the onset of symptoms

Index Case – first case of a disease which serves as source of infection


Morbidity Rate – rate at which an illness occurs

Mortality Rate – number of deaths caused by a disease in a population

Nosocomial Infection – infection acquired during hospitalization

Reservoir – source of infection, such as a person, animal or something in environment


Surveillance – collection of data pertaining to disease occurrence

Surveillance and ReportingCertain diseases are required by

law to be reported to public health authorities

http://www.tdh.state.tx.us/ideas/report/conditions/


Diseases that have significant effect on the population (STD) or have potential for grave consequences (anthrax, plague)

Centers for Disease Control and Prevention (CDC)

World Health Organization (WHO)

MLAB 2434 – CLINICAL MICROBIOLOGY SUMMER, 2005 CECILE SANDERS & KERI BROPHY

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