chap 14

Infectious disease

disease caused by a microbe

pathogens

microbes that cause infectious diseases are collectively referred to as

Infection

commonly used as a synonym for infectious disease (e.g. an ear infection is an infectious disease of the ear)

What do microbiologists reserve the word infection to mean?

colonization by a pathogen; the pathogen may or may not go on to cause disease

What happens when a person is infected by a pathogen?

they could be infected but not have an infectious disease

Why infection doesn’t always occur

• microbe lands on anatomic site —> unable to multiply

• pathogens aren’t able to attach to specific receptor sites before they are able to multiply and cause damage

• indigenous microflora may inhibit growth of the foreign microbe

• indigenous microflora may produce antibacterial factors (i.e., bacteriocins) that destroy the pathogen

• the individuals nutritional and overall health status often influences the outcome of the pathogen-host encounter

• the person may be immune to that particular pathogen

• phagocytes present in the blood may destroy the pathogen

Four periods or Phases in the course of an infectious disease

1. the incubation period

2. the prodromal period (“maybe”) (i think, i might be sick)

3. the period of illness

4. the convalescent period

Localized Infection

once an infectious process is initiated, the disease may remain localized or it may spread

Examples of localized infections

pimples, boils, abscesses (skin)

Systemic or generalized infections

when the infection spreads throughout the body

Acute disease

rapid onset, and is usually followed by a relatively rapid recovery

Acute disease examples

measles, mumps, and influenza

Chronic disease

slow onset and lasts a long time

Chronic disease examples

tuberculosis, leprosy, syphilis

Subacute disease

comes on more suddenly than a chronic disease, but less suddenly than an acute disease

Subacute disease examples

bacterial endocarditis

Symptom of a disease

some evidence of a disease that is experienced by the patient; subjective

• more difficult to assess —> further symptoms

• “I saw the sign”

• the symptomatic and asymptomatic

Example of symptoms

aches or pains, ringing in the ears, blurred vision, nausea, dizziness

Sign of a disease

some type of objective evidence of a disease

Examples of signs of a disease

elevated blood pressure, abnormal heart sounds, abnormal pulse rate abnormal laboratory results

Latent Infections

infectious diseases that go from being symptomatic to asymptomatic, and then later, go back to being symptomatic

Examples of latent infections

syphilis and herpes virus infections such as cold sores, genital herpes, and shingles

Primary infection

the first disease

Secondary infection

the second disease

Example of one infectious disease may commonly follow another

serious cases of bacterial pneumonia frequently follow mild viral respiratory infections

What happens during the primary infection?

the virus causes damage to the ciliated epithelial cells of the respiratory tract; these cells are then unable to clear opportunistic bacterial pathogens from the respiratory tract, leading to the secondary infection (easier to develop because already susceptible)(pneumonia)

Steps in the Pathogenesis of Infectious Diseases - EAM SED

1. entry of the pathogen into the body

2. attachment of the pathogen to some tissues within the body

3. multiplication of the pathogen

4. invasion or spread of the pathogen

5. evasion of host defenses

6. damage to the host tissue

EAM SED - “ eam sed (said) these are the steps”

1-5 is just infection, 6 damage

Virulent

synonym for pathogenic

Strain species

virulent (pathogenic) strains and avirulent (nonpathogenic) strains

Virulent strains VS Avirulent strains

virulent strains are capable of causing disease; avirulent strains are not

Example of virulent and avirulent strains

toxigenic strains of Corynebacterium diphtheriae can cause diphtheria, but nontoxigenic strains of C. diphtheriae cannot. Thus, the toxigenic strains are virulent, but the nontoxigenic strains are not

Virulence occasionally used to…

express the measure or degree of pathogenicity - how virulent something is

Example of virulence (degree of pathogenicity)

• it only takes 10 shigella cells to cause shigellosis, but it takes between 100 and 1000 salmonella cells to cause salmonellosis. Thus, shigella is more virulent than Salmonella

• some strains of streptococcus pyogenes (flesh eating strains) are more virulent than other strains of S. pyogenes

Virulence factors

attributes that enable pathogens to attach, escape, destruction, and cause disease

Examples of ways virulence factors attach

adhesins and pili

adhesins (ligands)

special molecules on the surface of pathogens - enable pathogens to recognize and bind to particular host cell receptors

pili (bacterial fimbriae)

enable bacteria to attach to surfaces, such as tissues within the human body

What are considered to be virulence factors

capsules and flagella

Examples of encapsulated bacteria

Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influencae, and Neisseria meningitidis

Why are flagella are virulence factors?

they enable flagellated bacteria to invade aqueous areas of the body; many also help the bacterium to escape phagocytosis

Toxins

poisonous substances released by various pathogens

2 types of toxins

endotoxins and exotoxins

endotoxins

• parts of the cell wall structure of Gram-negative bacteria

• can cause serious, adverse physiologic effects such as fever and shock

exotoxins

• poisonous proteins secreted by a variety of pathogens

Examples of exotoxins

neurotoxins, enterotoxins, exfoliative toxin, erythrogenic toxin, leucocidins

Exfoliative toxin example

causes skin to peel off

• scalded skin syndrome

Leukocidins example

destroy white blood cells —> evade immune system

Neurotoxin example

botulism

Exoenzymes released by bacteria include

necrotizing enzymes, kinases, collagenase, lecithinase, coagulase, hyaluronidase, hemolysins

Mechanisms by which pathogens escape immune response

antigenic variation, camouflage and molecular mimicry, destruction of antibodies

antigenic variation

changing their surface antigens

camouflage and molecular mimicry

some organisms conceal their foreign nature by coating themselves with host proteins

destruction of antibodies

some pathogens produce IgA protease an enzyme that destroys some of the hosts antibodies, ex. Haemophilus influenzae

Why do you need yearly vaccines sometimes?

influenza and covid very good at constantly changing