Medical Microbiology

148 3 GeneralBacteriology

&Simplestaining.Inthistechnique,a singlestainingsubstance,e.g.,

methyleneblue,isused.

&Differentialstaining.Twostainswithdifferingaffinitiestodifferentbac-

teriaareusedindifferentialstainingtechniques,themostimportantofwhich

isgramstaining.Gram-positivebacteriastainblue-violet,Gram-negative

bacteriastainred(seep. 211 formethod).

Threebasicformsareobservedinbacteria:spherical,straightrods,and

curvedrods(seeFigs.3. 1 –3. 4 ).

FineStructuresofBacteria................................

Nucleoid(NucleusEquivalent)andPlasmids.........

The“cellularnucleus”inprokaryotesconsistsofatangleofdouble-stranded

DNA,notsurroundedbyamembraneandlocalizedinthecytoplasm(Fig.3. 5 ).

InE.coli(andprobablyinallbacteria),ittakestheformofasinglecircular

moleculeofDNA.ThegenomeofE.colicomprises4.63! 106 basepairs(bp)

thatcodefor 4288 differentproteins.Thegenomicsequenceofmanybacteria

isknown.

Theplasmidsarenonessentialgeneticstructures.Thesecircular,twisted

DNAmoleculesare 100 – 1000 !smallerthanthenucleoidgenomestructure

andreproduceautonomously(Fig.3. 6 ).Theplasmidsofhumanpathogen

bacteriaoftenbearimportantgenesdeterminingthephenotypeoftheircells

(resistancegenes,virulencegenes).

Table3. 1 MorphologicalCharacteristicsofBacteria(seeFig.3. 1 forexamples)

Bacterialform Remarks

Cocci Occurinclusters(Fig.3. 2 ),chains,pairs(diplococci),packets

Straightrods Uniformthickness,roundedends(Fig.3. 3 ),pointedends,

clubform

Curvedrods Commashaped,spiral(Fig.3. 4 ),screwshaped

Mycoplasmas Bacteriawithoutarigidcellwall;coccoidcells,longthreads

Chlamydiae Twoforms:spherical/ovalelementarybodies(300nm);

spherical/ovalreticulatebodies(1 000 nm)

Rickettsiae Shortcoccoidrods(0.3– 1 lm)

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Kayser, Medical Microbiology © 2005 Thieme