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Copyright 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins

Burton's Microbiologyfor the Health Sciences

Section VII. Pathogenesisand Host Defense Mechanisms

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Burton's Microbiologyfor the Health Sciences

Chapter 14.Pathogenesis of Infectious Diseases

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Chapter 14 Outline

Introduction

Infection Versus Infectious

Disease Why Infection Does Not

Always Occur

Four Periods or Phases in theCourse of an Infectious

Disease Localized Versus Systemic

Infections

Acute, Subacute, andChronic Diseases

Symptoms of a DiseaseVersus Signs of a Disease

Latent Infections Primary Versus Secondary

Infections

Steps in the Pathogenesis ofInfectious Diseases

Virulence

Virulence Factors (AttributesThat Enable Pathogens toAttach, Escape Destruction,and Cause Disease)

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Introduction

The prefix path refers to disease.

Pathogenicitymeans the ability to causedisease.

Pathogenesis refers to the steps or

mechanisms involved in the development of adisease.

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Infection Versus Infectious Disease

An infectious disease is a disease caused by a microbe,and the microbes that cause infectious diseases are

collectively referred to aspathogens. Infection is commonly used as a synonym for infectious

disease (e.g., an ear infection is an infectious disease ofthe ear).

Microbiologists reserve the word infection to meancolonization by a pathogen; the pathogen may or maynot go on to cause disease.

A person can be infected with a pathogen, but nothavean infectious disease.

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Why Infection Does Not Always Occur

The microbe may land at ananatomic site where it isunable to multiply.

Many pathogens must attachto specific receptor sitesbefore they are able tomultiply and cause damage.

Antibacterial factors may be

present at the site where thepathogen lands.

Indigenous microflora of thatsite may inhibit growth ofthe foreign microbe (i.e.,microbial antagonism).

The indigenous microfloramay produce antibacterialfactors (i.e., bacteriocins)

that destroy the pathogen.

The individuals nutritionaland overall health statusoften influences the outcomeof the pathogen-hostencounter.

The person may be immuneto that particular pathogen.

Phagocytes present in theblood may destroy thepathogen.

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7/24Copyright 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins

Four Periods or Phases in the Course of anInfectious Disease

The incubation period

The prodromal period

The period of illness

The convalescent period

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Localized Versus Systemic Infections

Localized Infections

Once an infectious process is initiated, the diseasemay remain localized or it may spread; examples oflocalized infections are pimples, boils and abscesses.

Systemic Infections

When the infection spreads throughout the body it issaid to have become a systemicor generalizedinfection; an example is miliary tuberculosis causedby Mycobacterium tuberculosis.

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Acute, Subacute, and Chronic Diseases

An acute disease is one that has a rapid onset, and isusually followed by a relatively rapid recovery; examples

are measles, mumps, and influenza.

A chronic disease has a slow onset and lasts a long time;examples are tuberculosis, leprosy, and syphilis.

A subacute disease is one that comes on more suddenlythan a chronic disease, but less suddenly than an acutedisease; an example would be bacterial endocarditis.

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Symptoms of a Disease Versus Signs of aDisease

A symptom of a disease is defined as some evidence of adisease that is experienced by the patient; something

that is subjective; for example, aches or pains, ringing inthe ears, blurred vision, nausea, dizziness, etc.

There are symptomaticand asymptomatic diseases.In a symptomatic disease, the patient is experiencingsymptoms. In an asymptomatic disease, the patient

is not experiencing any symptoms.

A sign of a disease is defined as some type of objectiveevidence of a disease; for example, elevated bloodpressure, abnormal heart sounds, abnormal pulse rate,abnormal laboratory results, etc.

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Latent Infections

Latent infections areinfectious diseases that gofrom being symptomatic to

asymptomatic, and then,later, go back to beingsymptomatic.

Examples includesyphilis and herpes virus

infections such as coldsores, genital herpes,and shingles.

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Primary Versus Secondary Infections

One infectious disease may commonly follow another; insuch cases, the first disease is referred to as aprimary

infection and the second disease is referred to as asecondary infection.

Example: serious cases of bacterial pneumoniafrequently follow mild viral respiratory infections.

During the primary infection, the virus causesdamage to the ciliated epithelial cells of therespiratory tract; these cells are then unable toclear opportunistic bacterial pathogens from therespiratory tract, leading to the secondary

infection (pneumonia).

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Steps in the Pathogenesis of InfectiousDiseases

A common sequence of steps in the pathogenesis ofinfectious diseases is:

1. Entry of the pathogen into the body.

2. Attachment of the pathogen to some tissue(s) withinthe body

3. Multiplication of the pathogen.4. Invasion or spread of the pathogen.

5. Evasion of host defenses.

6. Damage to host tissue(s).

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Virulence

The term virulent is sometimes used as a synonym forpathogenic.

There may be virulent (pathogenic) strains and avirulent(nonpathogenic) strains of a particular species.

Virulent strains are capable of causing disease;avirulent strains are not.

For example, toxigenic (toxin-producing) strains ofCorynebacterium diphtheriae can cause diphtheria,but nontoxigenic strains ofC. diphtheriae cannot.Thus, the toxigenic strains are virulent, but thenontoxigenic strains are not.

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Virulence, cont.

Sometimes, the term virulence is used to express themeasure or degree of pathogenicity.

Example: It only takes 10 Shigella cells to causeshigellosis, but it takes between 100 and 1,000Salmonella cells to cause salmonellosis. Thus,Shigella is more virulent than Salmonella.

Example: Some strains ofStreptococcus pyogenes

(e.g., the flesh-eating strains) are more virulentthan other strains ofS. pyogenes.

Example: Some strains ofS. aureus produce toxicshock syndrome, but other strains ofS. aureus donot. Those that do are considered more virulent.

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Virulence Factors

Virulence factors are attributes that enable pathogens toattach, escape destruction, and cause disease.

Virulence factors are phenotypic characteristics that aredictated by the organisms genotype. Examples:

Adhesins (ligands) - special molecules on the surfaceof pathogens are considered to be virulence factors

because they enable pathogens to recognize andbind to particular host cell receptors.

Pili (bacterial fimbriae) are considered to be virulencefactors because they enable bacteria to attach tosurfaces, such as tissues within the human body.

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Adhesins and Receptors

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Examples of Virulence Factors

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Obligate Intracellular Pathogens

Pathogens that must live within host cells in order tosurvive and multiply are referred to as obligate

intracellular pathogens (examples: Rickettsia andChlamydia spp.).

Intraleukocytic pathogens (e.g., Ehrlichia spp.andAnaplasma phagocytophilum) live withinwhite blood cells, causing diseases known as

ehrlichiosis and anaplasmosis.

Plasmodium spp. (which cause malaria) andBabesia spp. (which cause babesiosis) areexamples ofintraerythrocytic pathogens; they livewithin red blood cells.

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Facultative Intracellular Pathogens

Facultative intracellular pathogens are capable ofboth anintracellular and extracellular existence.

Intracellular Survival Mechanisms

Possess a cell wall composition that resists digestion(e.g., Mycobacterium tuberculosis)

Fusion of lysosomes with phagosomes is prevented

Production of phospholipases that destroy thephagosome membrane, thereby preventinglysosome-phagosome fusion

Other unknown mechanisms

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Capsules and Flagella Capsules and flagella are

considered to be virulencefactors.

Examples of encapsulatedbacteria: Streptococcuspneumoniae, Klebsiellapneumoniae, Haemophilusinfluenzae and Neisseriameningitidis.

Flagella are virulence factorsbecause they enableflagellated bacteria to invadeaqueous areas of the body;may also help the bacterium

to escape phagocytosis.

Photomicrograph of stained Streptococcuspneumoniaeshowing capsules (theunstained halos that surround the bacteria).


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Toxins Toxins are poisonous substances released by various

pathogens. There are 2 general types:

Endotoxins

Part of the cell wall structure of Gram-negative bacteria

Can cause serious, adverse physiologic effects such asfever and shock

Exotoxins

Poisonous proteins secreted by a variety ofpathogens

Examples: neurotoxins, enterotoxins, exfoliativetoxin, erythrogenic toxin, and leukocidins

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Mechanisms by Which Pathogens EscapeImmune Responses

Antigenic Variation

Some pathogens evade the immune system by changingtheir surface antigens antigenic variation; examples,Neisseria gonorrhoeae and Borrelia recurrentis.

Camouflage and Molecular Mimicry

Some organisms conceal their foreign nature by coatingthemselves with host proteins like camouflage (e.g.,adult schistosomes).

Destruction of Antibodies

Some pathogens produce IgA protease, an enzyme thatdestroys some of the hosts antibodies; example,Haemophilus influenzae.

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