Microbiology - The Development of Infectious Disease | Biology

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Study GuideMicrobiology–The Development of Infectious Disease1.Contributing FactorsInfectious disease develops as acompetition between the parasite (pathogen)and thehost (thebody). If the pathogen successfully overcomes the body’s defenses, the normal state of health isdisturbed anddisease occurs.Whether a microorganism is able to cause disease depends on severalcontributing factors. Thesefactors influence the ability of the pathogen toenter the body, survive, multiply, and causedamage.1.1Portals of EntryFor a pathogen to cause infection, it must first enter the body through aportal of entry.The most common portals of entry are:•Mucous membranes, especially those of the:oRespiratory tractoGastrointestinal tractoUrogenital tractAnother important portal of entry is theskin.•Microorganisms can enter through:oCuts or woundsoHair folliclesWhen pathogens enter the bodybeneath the skin, the route is called theparenteral route.1.2Dose of the PathogenThedoserefers to thenumber of microorganismsrequired to establish an infection.•Some diseases require only asmall numberof organisms

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Study GuideoExample: typhoid fever (a few hundred bacteria)•Other diseases require avery large numberoExample: cholera (millions of bacteria)The dose is sometimes expressed asLD₅₀(lethal dose 50):•This is the number of microorganisms required tokill 50% of the hostsexposed1.3InvasivenessInvasivenessis the ability of a pathogen topenetrate and spread through body tissues.•Highly invasive organisms are more likely to cause disease•For example, organisms that causeintestinal ulcerscan penetrate deeply into tissueTissue invasion usually begins withadherence, which is the ability of a microorganism toattach tohost cells.•Adherence is often aided by structures such aspili•Capsules or glycocalyx layersalso help because they aresticky, allowing strongerattachment to tissues1.4Capsules and Resistance to Host DefensesSome microorganisms are surrounded by acapsule, which greatly increases their ability to causedisease.Capsules help pathogens by:•Preventing phagocytosis•Interfering with the action ofwhite blood cellsNormally, white blood cells engulf and destroy invading microorganisms. However:•Toxic substances in the capsule candamage or kill white blood cells•This allows the pathogen to survive and multiplyA well-known example is the organism that causespneumonia, which has a capsule containing toxicmaterials. Many other disease-causing microorganisms also possess capsules.

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Study GuideKey Points to Remember•Disease depends on thebalance between host defenses and pathogen strength•Pathogens must enter through aportal of entry•Thedoseof microorganisms affects disease development•Invasivenessallows pathogens to penetrate tissues•Capsuleshelp pathogens avoid destruction by the immune system2.Enzymes and ToxinsPathogenic microorganisms usespecial enzymes and toxinsto damage body tissues, avoidimmune defenses, and establish infection. These substances increase the organism’s ability to causedisease.2.1Enzymes–Tools That Help Pathogens SpreadMany pathogens produceenzymesthat weaken the body’s defenses and help them survive insidethe host.Leukocidins•Leukocidins are enzymes thatdestroy white blood cells•Since white blood cells performphagocytosis, their destruction reduces the body’s ability tofight infectionHemolysins•Hemolysins are enzymes thatdestroy red blood cells•They are produced by organisms such as:oStreptococcioStaphylococcioSomeClostridiumspecies

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