Medical Microbiology

14 1 GeneralAspectsofMedicalMicrobiology

&Antigenvariation.Somebacteriaarecharacterizedbyapronounced

variabilityoftheirimmunogens(=immuneantigens)duetothegenetic

variabilityofthestructuralgenescodingtheantigenproteins.Thisresults

inproductionofaseriesofantigenvariantsinthecourseofaninfection

thatnolonger“match”withtheantibodiestothe“old”antigen.Examples:

gonococcicanmodifytheprimarystructureofthepilinoftheirattachment

MechanismofMolecularVariationofPilininGonococci

a b

6 5 4 3 2 1

silent pilin gene pilS3

active pilin gene (old) pilE

homologous

recombination

active pilin gene (new) pilE

P

P

variable region,

minicassettes 1–6

conserved

region

pilS3

pilS4

pilS5

pilS6

pilS7

pilS8

pilE

pilS1

pilS2

= conserved sequences

= variable sequences

Fig. 1. 2 Thestructuralelementoftheattachmentpiliofgonococciisthepoly-

peptidemonomerpilin.Mucosalimmunitytogonococcidependsonantibodiesin

thesecretionsoftheurogenitalmucosathatattachtotheimmunodominantseg-

mentofthepilin,thusblockingadhesionofgonococcitothetargetcells.

aModelofthegonococcalgenome.Theprimarystructureofthepilinisdeter-

minedbytheexpressedgenepilE.Thegonococcalgenomehasmanyotherpil

genesbesidesthepilEwithoutpromoters,i.e.,“silent”genesthatarenottran-

scribed(pilS1,pilS2,pilS3,etc.).

bpilgeneshavebothaconservedandavariableregion.Thevariableregionofall

pilgeneshasamosaicstructure,i.e.,itconsistsofminicassettes.Minicassette 2

codesforthemostimportantimmunodominantsegmentofthepilin.Intracellular

homologousrecombinationofconservedregionsofsilentpilgenesandcorre-

spondingsequencesoftheexpressedgeneresultsinpilEgeneswithchangedcas-

settes.Thesecodeforapilinwithachangedimmunodominantsegment.There-

fore,antibodiestothe“old”pilincannolongerbindtothe“new”pilin.

1

Kayser, Medical Microbiology © 2005 Thieme