host-parasite relationships

Lecture OneBIOL 533 1

Host-Parasite Relationships

Medical Microbiology

Lecture One

Agents That Cause Disease.


Characteristics of Parasitism

• Encounter: agent meets host• Entry: agent enters host• Spread: agent spreads• Multiplication: agent multiplies• Damage: agent, host response, or

both cause damage• Outcome: agent or host wins, or

coexist


Encounter

• In utero– Do not normally come in contact with

organisms• Protection of fetal membranes• Do not normally come in contact with

organisms from mother– Normally only present sporadically– Exceptions: sexual diseases, virus causes,

rubella


Encounter

• At moment of birth– Come in contact with organisms

present in vaginal canal and on skin• Previously, antibodies passed from

mother to fetus• Defenses are good for a period of time,

then they wane


Encounter

• Challenge between man and microbe wages many times during lifetime– Most disappear rapidly– Some become part of normal flora– Only a few cause disease


Later Encounters

• Exogenous: encountered in environment

• Endogenous: encountered in or on body– Organisms present on skin can cause

disease when they go into deeper tissues


Later Encounters

• Example:– Staphylococcus aureus enters cut and

forms boil– In this case, encounter took place long

before disease (at time skin was colonized)

• Encounter is not always sharply demarcated


Normal Flora

• What constitutes normal flora?– Some people possess Streptococcus

pyogenes in their throat for long periods, but rarely contract disease• Opportunistic pathogen existence (carrier

state)

– 95% of people never have this bacterium, and when they do, they get sick


Normal Flora Defined

• Constitutes normal flora if definition is “any organism present that is not causing disease”

• Not normal flora if used to mean organisms present in majority of population


Host-Parasite Interaction

• Exposure to virulent agents does not always lead to disease– Typhus and Black Plague epidemics:

only half of population became sick, even though most likely exposed


Host-Parasite Interaction

• Response of particular microbe to particular host– Depends on factors unique to each

interaction– Within a single individual– Changes with:

• Age• Nutritional state• Other factors


Entry

• Much of inside of body is connected to the outside; for example:– Lumen of intestine– Alveoli of lung– Tubules of kidney

• Almost all organs within thorax and abdomen are topologically connected to the outside


Entry

• Mechanisms to keep out invaders– Sphincters and valves– With exception of digestive and

genitourinary systems, these sites are normally sterile

– Organism that resides on lumen side of intestine or lung alveoli has not penetrated body


Entry Defined

• Ingress of microbes into body cavities contiguous with outside


Digestive System

• Enter through eating– Numbers of organisms are reduced

one million or more in stomach• Bacillary dysentery can result from only a

few hundred organisms

– Not many survive in intestine because of digestive enzymes and strong force of peristalsis


Digestive System

• More survive in ileum, but need mechanisms to prevent expulsion– Surface components serve as adhesins

to allow adherence to epithelial cells• Pili and surface polysaccharides

– Diseases such as cholera and “traveler’s diarrhea” are caused without penetrating epithelium• Toxins that affect epithelial cells


Respiratory System

• Enter through being inhaled– Air containing microbes goes through

air passages (nasal turbinates, oropharynx, larynx)

– Microbes reaching lower respiratory system face powerful epithelium sweeping action

– Colonization requires adhesion mechanisms


Other

• No term for urinary or genital entry

• By bypassing epithelial tissue, microbes can cause disease without penetrating deep into tissues– Cholera, whooping cough, infection of

urinary bladder


Penetration into Deeper Tissues

• Very few organisms can penetrate unbroken skin (worms are an exception)

• Some organisms can penetrate epithelial tissue; for example:– S. pneumoniae, Treponema pallidum

• Normally after some injury to tissue (many times caused by a virus)

– Viruses, by receptors


Carried in by Macrophage

• Alveolar macrophage trap organisms in lung– Normally carry upward on ciliary epithelium– Some cases, can carry deeper into tissues

• Some organisms can live, grow in macrophage:– Legionella– Bordetella pertussis– HIV (via virus-laden macrophage from semen)


Penetration by Other Means

• Insect bites: numerous viral and protozoan diseases

• Cuts and wounds: don’t normally lead to disease– Brushing teeth or defecating

vigorously causes minute abrasions of epithelium• Organisms quickly cleared from blood by

reticuloendothelial system



– Injury to internal tissue disrupts defense mechanisms and serious disease can result; for example• Subacute bacterial endocarditis

– Devastating before antibiotics– Caused by oral streptococci that became

implanted on heart valves damaged by rheumatic fever



• Organ transplants or blood transfusions– Jakob-Cruetzfeldt disease from

transplanted corneas– Cytomegalovirus from kidneys, probably

in donor kidney

• Because immunosuppressive drugs are used, virus may be endogenous

• Hepatitis B, HIV transmitted by blood


Disease Causation

• Why are organisms adapted to various locations?– Temperature optima; athletes foot

yeast cannot grow at 37°C– Oxygen requirements– Specialized factors important for

causing disease (i.e., virulence factors)– Virulence: degree of pathogenicity


Virulence Factor Examples

• Exotoxins• Endotoxins• Capsules• IgA proteases• Adhesins (pili)• Motility

• Invasive properties

• Ability to acquire iron

• Serum resistance• Ability to survive

inside phagocytes


Inoculum Size

• Inoculum size can determine whether organisms cause disease

• Normally, high number needed to cause disease/overcome defenses; e.g.– Baths in contaminated hot tubs (veritable

culture of bacteria—over one hundred million organisms per ml)


Inoculum Size

• Normally harmless organisms can overcome defenses; e.g.,– People get boils all over body

• If large number of organisms deposited in deeper tissues, infection usually results– Surgeon preps area to reduce numbers


Spread of Disease

• General: spread only if overcome host defenses

• Sometimes precedes, sometimes follows microbial multiplication– Precede: parasite causes malaria

disseminated before multiplication

– Follow: S. aureus multiplies locally before being disseminated


Spread of Disease

• Types:– Direct lateral propagation to

contiguous tissues– Dissemination to distant sites

• Characteristics:– Anatomical factors (e.g., ear infections)– Active participation by pathogens—

enzymes


Multiplication

• Factors that affect– Microbial nutrition: body is very

nutritious, but it also has antimicrobial substances

– Body contains very little free iron

• Physical factors: temperature, etc.– Narrow temperature optima—prudence of

lowering fever by “take two aspirin and call me in the morning”


Damage

• General: type and intensity depend on specific organism and tissue

• Types:– Mechanical: mostly result of

inflammation– Cell death: depends on:

• Which cells• How many infected• How fast infection proceeds


Damage

• Types, continued:– Pharmacological: toxins alter

metabolism– Damage due to host responses

• Inflammation can lead to destruction of neighboring cells

• Immune response

Lecture One

Agents That Cause Disease.

Lecture One

• Outcome: agent or host wins, or coexist

BIOL 533 35

host-parasite relationships

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