Cheese Flavor and the Genomicsof Lactic Acid BacteriaGenomics and molecular biology are valuable in helping to dene howthese bacteria contribute to the avor and texture of cheesesJeffery R. Broadbent and James L. SteeleCheesemilks leap toward immortality-Clifton Paul FadimanHumans place great value on technol-ogies to improve the keeping quali-tiesoffoods,andoneofthemostancient of these practices depends onlactic acid bacteria (LAB) to fermentmilk. Becausethesebacteriaareconstituentsofraw milk, cheese and other fermented milk foodshave likely been part of the diet since humans rstcollectedmilkandheldit incrudecontainers.Over the centuries, these accidental fermenta-tions were controlled and molded into the morethan 1,000 unique cheeses, yogurts, and fer-mented milks that are available today.Because fermented dairy foods developed be-fore the emergence of microbiology as a science,manufacturingprocessesforall varietieslongrelied upon naturally occurring LAB to acidifymilk. It was not until discovery of the lactic acidfermentation by Pasteur in 1857, and develop-ment of pure LABstarter cultures later thatcentury,thatthedoortoindustrializedcheeseand milk fermentations opened. Since then, pro-duction of fermented milk and especially cheesehave undergone dramatic, sustained growth. Inthe UnitedStates alone, for example, cheeseproductionincreasedmorethan200%inthelast quarter century, and total worldwide pro-duction nowruns approximately 13 milliontons per year.To sustain such growth and productivity, thedairy industry has evolved into a leader instarter microbiology and fermentation technol-ogy. Decades of experiencehaveprovedthatlarge-scale production of uniform, high-qualitycheeseis facilitatedbytheuseof thoroughlycharacterized starter bacteria. Thus, eventhoughsometraditional cheesefermentationsrely on the natural souring of rawmilk, the greatmajorityofindustrializedprocessesusestartercultures. Since future growth and economic vi-tality of the cheese industry depends on startercultures withknown, predictable, andstablecharacteristics, fundamental understandingofLABgeneticsandphysiologyholdsenormousvalue globally.Genome Studies in DairyLactic Acid BacteriaLABare arelativelyheterogeneous groupofgram-positive cocci, coccobacilli, and bacillithat inhabit a broad range of ecological niches,yet share several dening characteristics, includ-ing: (i) low(55 mol%) GCcontent; (ii) highacid tolerance; (iii) non-spore forming; (iv) nu-tritionallyfastidious; (v) aerotolerant but notaerobic; (vi) unabletosynthesizeporphyrins;and (vii) strictly fermentative metabolism withlactic acid as the major metabolic end product.Included within this group are several species ofLactobacillus, Lactococcus, Leuconostoc, andStreptococcusthatserveasstarterculturesforthe commercial manufacture of cheese and fer-mented milks.Genetics research in food-grade LAB beganabout35yearsago,duringwhichperiodfourbasic types of genetic elements were character-ized in dairy LAB: plasmid DNA, transposableelements, bacteriophages, andcompletechro-mosomes. Representatives from all four of thesegenetic elements affect milk fermentation. How-ever,detailedknowledgeofLABchromosomeJeffery R. Broad-bent is a professorof food science inthe Department ofNutrition and FoodSciences and West-ern Dairy Center,Utah State Univer-sity, Logan, andJames L. Steele isa professor of foodscience in the De-partment of FoodScience, Universityof Wisconsin,Madison.Volume 71, Number 3, 2005 / ASM News Y 121structure and organization is of particular valuebecause genes for all the essential housekeeping,catabolic, and biosynthetic activities of the cellare located in the chromosome.As with many microorganisms, efforts tocharacterizeLABchromosomesbeganinear-nest with the advent of pulsed eld gel electro-phoresis (PFGE) technology during the early1980s. Researchers quicklylearnedthat LABhave a single, relatively small (1.8 to 3.4 Mbp),circular chromosome, and that genome size andorganization differ among individual speciesandstrains. AlthoughPFGEis still useful inchromosome studies, the most exciting and in-novative research in this realm of microbiologyisnowfueledbygenomicnucleotidesequenceanalysis.Genome sequence information for the rst ofseveral industrially important LAB starter spe-ciesappearedin2001, whenSorokinandco-workers releasedthegenomicDNAsequencefor Lactococcus lactis IL1403(Table1). Ge-nome sequence informationfor several otherimportant dairy LAB is also now available, andadditional sequencingprojectsareunderwayand, indeed, sequence information is being gath-eredformorethanonestrainof aparticularspecies (Table 1). The latter development couldprovide insight to the molecular basis for com-mercially signicant strain-dependent proper-ties, such as the ability to produce specic a-vors, propensity for autolysis, acidicationrates, andcell vitalityinfrozenorlyophilizedstarterconcentrates, whicharecommonlyen-countered in dairy LAB.Because of their economic relevance, many ofthese sequences are being mined for intellectualproperty and are not yet available to the generalscientic community. Nonetheless, nucleotidesequence data is publicly available for more thanTable 1. Genome sequencing projects for dairy-related lactic acid bacteria and other speciesSpecies StrainGenomesize(MBp) Project sponsoraPublicLactobacillus acidophilus ATCC700396 2.0 Dairy Management, Inc. and Rhodia, Inc. (U.S.) NoL. brevis ATCC 367 2.0 JGI-LABGCb(U.S.) YesL. casei ATCC 334 2.9 JGI-LABGC (U.S.) YesL. casei BL23 2.6 INRA (France) NoL. delbrueckii subsp. bulgaricus ATCCBAA-365 2.3 JGI-LABGC (U.S.) YesL. delbrueckii subsp. bulgaricus ATCC11842 2.3 INRA and Genoscope (France) NoL. delbrueckii subsp. bulgaricus DN-100107 2.1 Danone Vitapole (France) NoL.gasseri ATCC 33323 2.0 JGI-LABGC (U.S.) YesL. helveticus CNRZ32 2.4 Dairy Management, Inc. and Chr. Hansen, Inc.(U.S.) NoL.helveticus DPC 4571 NRcTeagasc and University College, Cork (Ireland) NoL. johnsonii NCC533 2.0 Nestle (Switzerland) YesL. plantarum WCFS1 3.3 Wageningen Centre for Food Sciences (Netherlands) YesL. rhamnosus HN001 2.4 Fonterra Research Center (New Zealand) NoLactococcus lactis subsp.cremorisSK11 2.3 JGI-LABGC (U.S.) YesL.lactis subsp. cremoris MG1363 2.6 Univ. Groningen (Ne); INRA (France) NoL. lactis subsp. lactis IL1403 2.3 INRA and Genoscope (France) YesLeuconostoc mesenteroides ATCC 8293 2.0 JGI-LABGC (U.S.) YesPediococcus pentosaceus ATCC 25745 2.0 JGI-LABGC (U.S.) YesStreptococcus thermophilus LMD-9 1.8 JGI-LABGC (U.S.) YesS. thermophilus LMG18311 1.9 Univ. Catholique de Louvain (Belgium) NoS. thermophilus CNRZ1066 1.8 INRA (France) NoBidobacterium longum NCC2705 2.3 Nestle (Switzerland) YesB. longum DJ010A 2.1 JGI-LABGC (U.S.) YesB. breve NCIMB8807 2.4 University College, Cork (Ireland) NoBrevibacterium linens ATCC9174 3.0 JGI-LABGC (U.S.) YesPropionibacterium freundenreichii ATCC6207 2.6 DSM Food Specialties (Netherlands) NoaAs of 1 January 2005.bJGI-LABGC, Department of Energy Joint Genome Institute and Lactic Acid Bacteria Genomics ConsortiumcNR, not reported.122 Y ASM News / Volume 71, Number 3, 2005Broadbent Finds Pleasure in Studying Lactobacilli, Watching RaptorsJefferyBroadbent nds pleasurestudying and teaching about bac-teria that, instead of being harm-ful, do much good. In particular,lactic acid bacteria (LAB) providea central service worldwide to thefermentation and bioprocessingindustrieshelping,forinstance,to produce 13 million tons ofcheese annually. The many foodsproduced with these bacteria re-mainsteepedinartisantraditionand thus provide fertile opportu-nity to researchers like myselfwhoare intriguedby microbialecologyandphysiologyincom-plex environments, he says.How many other foods do weknowinglyconsumethatcontainmillions of live bacteria? Broad-bent continues. One of the mostbasic goals of science is to teach ustoviewseeminglycommonob-jectswithrenewedcuriosityandappreciation. I hope the next timepeople nd themselves enjoying agoodpieceofcheese,theypauseto savor the microbiological mar-vel it represents.Broadbent, 43, isprofessorofdairy microbiology in the depart-ment of nutritionandfoodsci-encesatUtahStateUniversityinLogan City, where he focuses onthisdiversegroupof gram-posi-tive cocci, coccobacilli, andba-cilli. The future of LAB researchisbright withpromise; withge-nome sequences and molecular bi-ology tools now available for sev-eral key species, opportunities toinvestigate LAB evolution, genet-ics, physiology, andmetabolismhave never been greater, he says.His interest in science, how-ever, did not originate withcheese-making bacteria, but in-steadourishedfromalifelongenchantment with wild fauna andora, particularly birds, he says.Some of myearliest memoriesare rooted in these experiencescatching frogs in the Utah moun-tains with my grandfather, beingspellbound as a young boy by thedazzling cacophony of color andsound in cage-loads of exoticbirds at an open air market.Anative of Utah, Broadbentmoved with his family as a youngchild to places that further fueledtheseinterests. Forexample, hisfamily spent ve years in SaoPaulo, Brazil, where my fascina-tion with birds and other wildlifebecame indeliblyimprintedintomy psyche, and also where Ilearned to feed my curiositythrough a fairly voracious readinghabit, he says.Bythe time Broadbent beganhighschool, inTempe, Ariz., hehadblossomedintoacompetentamateur ornithologist, and wasparticularly interested in birds ofprey. Determined then to pursue acareerinwildlifebiology,hebe-came involved in several researchprojectsinhisbiologyandzool-ogy courses, and eventually spentthe summer before his senior yearas a volunteer ornithologist doingeldresearchonaperegrinefal-conprojectintheGilaNationalForest of New Mexico.Broadbent enrolled at UtahState University in the fall of1979, but his undergraduate stud-ies weredisruptedseveral timesduetonancial problems, keep-ing him from completing hisbachelorofsciencesdegreeuntil1987. By then, his coursework fo-cushadshiftedfromwildlifere-search to microbial biotechnol-ogy, a eld he found equallyfascinating.After graduation, I had the re-markable good fortune of landinga research tech position in the lab-oratoryof Dr. JeffreyKondo, aworld-renowned geneticist in thelacticacidbacteriacommunity,Broadbentsays.Jeffsthoroughand articulate research opened myeyes to the elegance of lactic acidbacteriaandintroducedmetoaeldthat, tothis day, is brightwith opportunity.Hebecameagraduatefellowand, while still under Kondos su-pervision, receivedhis Ph.D. innutrition and food sciences in1992. Later, heacceptedaposi-tion on the faculty at Utah State.While Broadbents researchand academic interests shiftedfrom wildlife to lactic acid bacte-ria, he continues to pursue thoseearly passions. He and his wifeahigh school special educationteacherand their two daugh-ters,12and11,maintainwhatsometimes feels like a small zooin their home, he says. It includesa falcon, four dogs, a cat, a hedge-hog, sh, a loft of homing pi-geons, and three egg-laying chick-ens. As you might expect,leaving town for a family vacationrequires some planning, he says.Broadbent alsoisapracticingfalconer, havingengagedinthesport since high school. With hisfalcon and two of the dogs, bothEnglish Setters, he pursues ducks,pheasant, grouse, andpartridge.I am still enchanted by raptors,and falconry provides the connec-tion to wildlife I have soughtthroughout my life, he says.Marlene CimonsMarlene Cimons is a freelance writerwho lives in Bethesda, Md.Volume 71, Number 3, 2005 / ASM News Y 123half of the sequenced LAB strains (Table 1), and10 of the 14 publicly accessible sequences werecontributedaspartofajointventurebetweenthe Department of Energy Joint Genome Insti-tute andthe U.S.-basedLactic AcidBacteriaGenomics Consortium (LABGC). The LABGCmissionistoadvanceacademicandindustrialresearchonLABthroughreleaseofgenomese-quenceinformationformicroorganismspromi-nently associated with the fermented foods indus-try.(ForadditionalinformationontheLABGCeffort, see http://wineserver.ucdavis.edu/people/Faculty/mills/LABGC/lab.htm).Cheese Flavor Basics: AddLactic Acid BacteriaConverting bland and rubbery fresh curds into adeliciousmaturecheeseisacomplexanddy-namic process whose intricacies are dictated bythe type and composition of milk being used, theculturesandenzymesthatareadded, andthespecicmanufacturingandripeningregimensthat are applied. Many cheese types are stored atlow temperature for months or years to attaintheircharacteristicavorandbodyattributes.During storage, the microorganisms anden-zymes that are trapped in the cheese matrix acton carbohydrates, citrate, proteins, and lipids ina manner that is heavily inuenced by the curdmicroenvironment and which ultimately yieldsdistinct types of cheeses.Although a link between LAB and cheeseavor was rst postulatedmorethan100years ago, complexities in microbiology, en-zymology, and cheese microenvironmentsconfounded early efforts to establish a den-itive role for these bacteria affecting avors.However, in the late 1950s, Elisabeth Sharpeand coworkers at the Institute for Food Re-search in Shineld, England, developed tech-nologies to manufacture cheeses aseptically,enabling researchers to prove LAB are essen-tial for avor development in Cheddar andother cheeses.LABthatcontributetothisprocessmayincludedeliberatelyaddedstarters, adjunctbacteria (select strains that intensify or accel-erate avor development), and adventitiousspecies, called nonstarter lactic acid bacteria(NSLAB), that enter curd from the process-ing environment. While many different LABspecies may affect cheese avor, research inthis area mainly focuses on Lactococcus lac-tis, whichserves as thestarter bacteriumforCheddar, Gouda, and many other cheeses, andondairy-relatedspeciesof Lactobacillus(Fig.1). Interest in lactobacilli such as Lactobacillushelveticus and Lactobacillus delbrueckii subsp.bulgaricusstemsfromtheirwidespreaduseasboth starter and adjunct cultures, and becauseNSLABpopulationsarealmost alwaysdomi-nated by facultatively heterofermentative Lacto-bacillus sp. such as Lactobacillus casei.Because of the role of LABindevelopingavor, efforts to dene its biochemical basis incheese focus on the physiology of these micro-organisms. The numbers of starter bacteriacommonly exceed 109CFU per gram of cheesewhen ripening begins (Fig. 2), but the microen-vironmentofripeningcheeseisharsh. Forin-stance, itistypiedbyanabsenceofresiduallactose,highlevelsofNaCl,lowpH,andlowtemperature. Those conditions extract a toll onstarter viability, and, typically, a sizable fractionof the starter cells undergo autolysis, which re-leasesintracellularenzymesandothercellularcomponents into the cheese matrix where they,too, can inuence ripening.Meanwhile, NSLAB populations, whose ini-tial numbers are typicallybelow102CFU/g,begintogrowandeventuallyplateauat celldensities of 107-109CFU/g after 39 months ofaging (Fig. 2). Depending on the species that isused and whether a particular strain can growinTABLE 2. Components of the Lactobacillus helveticus CNRZ32proteolytic enzyme system isolated before and after genomesequence determinationGenes isolated prior tosequencing project(19902001)New genes from genome annotation(20012004)Proteinases:prtH prtH2 plus 9 additional proteasesEndopeptidases:pepE, pepO, pepO2 pepE2, pepF, pepO3, plus 2glycoprotein endopeptidasesAminopeptidases:pepC, pepN, pepX pepC2 plus 7 additionalaminopeptidasesDi-Tripeptidases:pepD, pepI, pepQ, pepR pepD2, pepD3, pepD4, pepQ2, pepT1,and pepT2Oligo- and di-tripeptide transportsystems:None oppA,oppA2, oppB-D, oppF, and dtpA,dtpA2, and dtpTMultiple amino acid transporters124 Y ASM News / Volume 71, Number 3, 2005ripening cheese, populations of adjunct bacteriamay mirror those of the starter or NSLAB frac-tions.Key Modes of Microbial Actionin Cheese RipeningStarter, adjunct, and NSLAB collectively inu-enceavordevelopmentthroughseveralbasicmechanisms that include fermenting lactose,converting milk proteins (primarily caseins) intopeptides andfree aminoacids, andbreakingdown citrate, lipids, esters, and amino acids intovolatile aroma compounds.Fermenting lactose into L-lactic acid is a pri-maryfunctionofanystartercultureincheesemanufacture. Acidproductivityis critical forcontrollingcheesequalitybecausetheculturedetermines the nal pH and mineral content ofthe curd, which affects the protein structure andamount of residual coagulant in the curd, and,thus, texture and avor properties. Lactate itselfis alsoacomponent of cheese avor andinSwiss-typecheesesservesasakeynutrientforpropionibacteria. They convert it into propionicacid, which is another important avor compo-nent, and carbon dioxide, which gives the cheeseits eyes.If starter bacteria rapidly depleteresidual milk sugar in the curd, they can help toprevent its useas asubstratefor undesirableadventitiousbacteria, suchasheterofermenta-tive Lactobacillus brevis, that can produce seri-ous avor and texture defects.Proteolysisanditssecondaryreactionsalsoplay a major role in bacterially ripened cheeses,making casein hydrolysis and its relationship toavor development an area of intense researchinterest for decades. Thehydrolysis of intactcaseins is almost exclusivelycatalyzedbythecoagulant and endogenous milk proteinases(e.g., plasmin), while LAB proteinases and pep-F I G U R E 1Colored scanning electron micrographs of representative cheese starter (A-C) and nonstarter (D-F) lactic acid bacteria (LAB). Species showninclude Lactobacillus helveticus (A), Lb. delbrueckii subsp. bulgaricus (B), Lactococcus lactis (C), Lb. casei (D), Pediococcus pentosaceus (E),and Lb. brevis (F). Images provided by B. McManus and J. Broadbent.Volume 71, Number 3, 2005 / ASM News Y 125tidases are responsible for producing water-sol-uble peptides and free amino acids.Together, primary and secondary proteolysisof caseins inuences cheeseavor inat leastthree signicant ways. First, casein networkbreakdown softens cheese texture, which facili-tates the release of avor compounds when thecheeseisconsumed.Second,someofthelow-molecular-weight peptides produced in these re-actions directly affect avor, but this conse-quence is generally negative since these peptidesimpart bitterness. Third, thefreeaminoacidsthat are liberated can also directly affect avor.Forinstance,glutamateandaspartateresiduesenhance avors.More commonly, releasedaminoacids areprecursorsforabroadrangeofpotentaromacompounds. Thesereactionsareof particularinterest because agrowingbodyof evidenceindicatesthatLABsconvertingoffreeaminoacids into aroma compounds is the rate-limitingstepinthedevelopmentofmaturecheesearo-mas. Theproductsof aminoacidcatabolism,which may arise via decarboxylation, deamina-tion, transamination, desulfuration, or sidechain removal, can impart desirable or undesir-able avor attributes.Much of the research on amino acid catabo-lism by LAB has been directed toward the fatesof aromatic, sulfur-containing, andbranched-chain classes of amino acids because of their keyrole in aroma. For example, convert-ingmethionine intovolatile sulfurcompounds such as methanethiol,hydrogensulde, dimethyl sulde,and dimethyl trisulde is thought tocontribute desirable sulfur avorsto many cheese types, whereasbreakingdownleucineisthelikelysource of a desirable nutty avornote in Cheddar cheese. In contrast,breakingdownaromaticaminoac-ids contributes several undesirableoff-avors to cheese, including de-rivatives such as indole, skatole,[para]-cresol, andphenyl acetalde-hyde.Freefattyacidsformedbylipaseoresteraseactivityonmilkfatalsodirectly affect cheese avor, and canhave further effects by serving as pre-cursors for esters andother avorcompounds. Moreover, esterases and lipasescatalyzethehydrolysis or synthesis of esters,depending on cheese water activity and levels ofother available fatty acids andalcohols. En-zymes involvedinthese reactions maycomefromrennetpastes,frommilkitself,andfromstarter and nonstarter LAB.It is well established, for example, that pregas-triclipases andesterases fromruminants areresponsibleforthesharp,fattyacid-baseda-vorsthatcharacterizesomeItaliancheeses. Incheeses such as Parmesan that do not use pre-gastric lipases andesterases, however, avornotes associated with lipolysis are probably duetoindigenousmilkenzymesandmicrobialen-zymes. Most LABlacklipolytic activity andhave very low esterolytic activity, but in cheesewith long ripening times these cells can generateenoughfree fattyacids andesters toimpactavor.Finally, LAB use citrate to produce succinateor diacetyl. Succinate, a compound withmonosodiumglutamate-like avor-enhancingproperties, can be isolated from several cheesevarieties, and sensory studies suggest it contrib-utes savory avor to Swiss-type cheese and to afull, aged avor in Cheddar. In Swiss and othercheeses where Propionibacterium freudenreichiisubsp. shermanii attain high numbers, succinateproduction is attributed to aspartic acid catabo-lismbythepropionibacteria. InCheddarandF I G U R E 20Log 10 CFU/g cheese10Ripening time8642Non-starter lacticsLactococcal starterMicrobiology of ripening Cheddar cheese.126 Y ASM News / Volume 71, Number 3, 2005other varieties, however, NSLAB produce succi-nate from citrate via the reductive tricarboxylicacid pathway.The other important citrate-derived avorcomponent, diacetyl, imparts a buttery notewhose importance inbutter, buttermilk, andsome cheese types has been recognized for dec-ades. Diacetyl is formed by oxidative decompo-sition of-acetolactate, an intermediate in thepathwaysforpyruvatemetabolismandaminoacid biosynthesis. In recent years, detailedknowledgeof citratemetabolismanddiacetylproductionhasyieldedeffectivestrategiesforengineering L. lactis strains to enhance diacetylproduction.Genomics Will Propel FurtherAdvancementsThoughgreatprogresshasbeenmadetowardunderstanding LABphysiology and the pro-cesses that drive cheese avor development,muchremainstobelearnedaboutthesereac-tions. Currently, signicant researchadvancesdepend on recombinant DNA technology. ThecomplexityofthepeptidaseenzymesysteminLAB, for example, confounded earlier efforts toestablish the role of individual enzymes in caseinhydrolysis and cheese ripening. However, toolsfor constructing isogenic strains that differ in theactivity of only single peptidases now are pro-viding researchers with an effective approach todetermine howindividual enzymes contribute tocell growth and cheese properties.By combining molecular tools with genomics,researchers in industry and academia are creat-ing even greater opportunities to investigate themeans by which LAB act within and respond tocheese and milk microenvironments. Hence, re-search to better dene the relationship betweenLAB physiology and avor development should,whenever possible, focus on strains that: (i) pos-sess established avor-producing capabilities;(ii) are amenable to genetic manipulation; and(iii) are analyzed at the genome sequence level.Onesuchcandidatestrainfor forthcomingstudyis Lactobacillus helveticus CNRZ32, acommercial cheeseavoradjunct that canre-duce bitterness and intensify avor develop-ment.Werecentlyassembledadraft(fourfoldcoverage)genomesequenceforCNRZ32,andare currently using that sequence to investigatemechanisms by which this strain affects cheeseavor.For example, because proteolysis plays such acritical part in cheese ripening, one of us (Steeleand his collaborators) spent more than a decadecloning and characterizing CNRZ32 genes thatencode proteolytic enzymes (Table 1), develop-inggenetransfer systems, andconstructingaseriesof singleandmultipledeletionmutantslacking functional genes for many of those en-zymes. Despite suchconcertedefforts, initialannotationofthegenomesequencerevealedalargenumberofadditional genesinCNRZ32whoseproductsarepredictedtocontributetothe proteolytic enzyme systemof this bacterium.Fromourperspective,suchdataunderscoreboth the power of genome sequence informationfor applied bacteriology, and the challenges onemust face in interpreting and applying that in-formation. Although sequencing efforts ex-pandedthegeneticdatabasefortheCNRZ32proteolytic enzyme systembyabout vefold,efforts to conrm and characterize all the newgeneassignments will requiremoretimeandresources. Nonetheless, functional analysis ofthenewlydiscoveredendopeptidasegeneshasalready identied enzymes with important rolesin the hydrolysis of bitter peptides in cheese.Functional genomics is alsobeing usedtoinvestigate pathways for amino acid biosynthe-sisandcatabolisminLactobacillushelveticusCNRZ32. Incheese, converting aminoacidsinto volatile cheese avor compounds may oc-cur directly or through interactions amongstarter, adjunct, and NSLAB components. Somestrainscanindependentlyconvertaminoacidsinto aroma compounds, while others may pro-duceor degradeonlyoneor moremetabolicintermediates.The basis for this phenomenon has not beendetermined, but all LAB are auxotrophic for oneormoreaminoacids,andtheprimarymecha-nismfor aminoacidbreakdownbyLABin-volves the reversible action of enzymes involvedinbiosyntheticpathways. Thus, muchof theinterplaythat occurs betweenLABinaminoacid catabolism probably reects the nature ofaminoacidauxotrophiesamongthedifferentbacteria in cheese. Since the primary sequencesof most enzymes involved in these reactions arerelatively well-conserved, access to genome se-quence information should dramatically en-hance our ability to predictand testhow in-Volume 71, Number 3, 2005 / ASM News Y 127dividual organisms contribute toaminoacidcatabolism in cheese.Dairy technologists and microbiologists haveidentied many of the fundamental mechanismsby which LAB affect avor, and this knowledgeis facilitating industry efforts to accelerate andintensify avors. There is still a great deal to belearned,however,andthecombinedstrengthsof genomics andmolecular biologytools arecertaintoplayaleadingrole inresearchtodenethemoleculardynamicsofLABinpro-ducing ne cheeses.SUGGESTED READINGBeresford, T. P., N. A. Fitzsimons, N. L. Brennan, and T. M. Cogan. 2001. Recent advances in cheese microbiology. Int. DairyJ. 11:259274.Broadbent, J. R. 2001. Genetics of lactic acid bacteria, pp. 243299. In J. L. Steele and E. H. Marth (eds.), Applied DairyMicrobiology, 2nd ed. Marcel Dekker, Inc., New York.Davidson, B. E., N. Kordias, M. Dobos, and A. J. Hillier. 1996. Genomic organization in lactic acid bacteria. Antonie vanLeeuwenhoek 70:161183.Klaenhammer,T.,E.Altermann,F.Arigoni,A.Bolotin,F.Breidt,J.Broadbent,R.Cano,S.Chaillou,J.Deutscher,M.Gasson, M. van de Guchte, J. Guzzo, T. Hawkins, P. Hols, R. Hutkins, M. Kleerebezem, J. Kok, O. Kuipers, M. Lubbers, E.Maguin, L. McKay, D. Mills, A. Nauta, R. Overbeek, H. Pel, D. Pridmore, M. Saier, D. van Sinderen, A. Sorokin, J. Steele, D.OSullivan, W. de Vos, B. Weimer, M. Zagorec, and R. Siezen. 2002. Discovering lactic acid bacteria by genomics. Antonievan Leeuwenhoek 82:2958.Olson, N. F. 1990. The impact of lactic acid bacteria in cheese avor. FEMS Microbiol. Rev. 87:131148.Yvon, M., and L. Rijnen. 2001. Cheese avour formation by amino acid catabolism. Int. Dairy J. 11:185202.128 Y ASM News / Volume 71, Number 3, 2005