0095-1137/83/040647-08$02.00/0
Copyright©1983, American SocietyforMicrobiology
Evaluation
of the Sensititre
System
for Identification of
Enterobacteriaceae
JOSEPHL.
STANECK,'*
JEANVINCELETTE,2 FRANCOIS LAMOTHE2AND ELIZABETHA.POLK3t
Departmentof Pathology and Laboratory Medicine, University of Cincinnati College of Medicine, Cincinnati,
Ohio 452671; Microbiologie, Hopital Saint-Luc, Montreal, Quebec, Canada H2X 3542;and Clinical
Microbiology, Tampa General Hospital, Tampa, Florida 336063
Received 29September 1982/Accepted 3 January 1983
The Sensititre identificationsystem(Seward Laboratory/GIBCO Laboratories) consists ofa microplate containing a pattern of 24 biochemicals repeated four timestogether withanautomatic inoculationdevice andamicrocomputer-assisted
data interpretation component. A total of1,415 isolates of Enterobacteriaceae plus 6 isolates of other glucose-fermentinggram-negative bacilli-were tested in threehospital laboratoriesinparallelwith API20E(Analytab Products). Discrep-ancies were resolved by conventional biochemical testing. Sensititre yielded
correct identifications at the species level with 94.6% of the isolates and at the
genuslevelwithanadditional 1.9%. API 20Eyieldedcorrectspecies identification with 91.1% andgenusonly identification withan additional 6.7% of the isolates.
For the routine identification of clinical Enterobacteriaceae isolates, the Sensi-titresystemcomparesfavorablywith API 20Eand offers clinical laboratories the
economyofa microtiter plate systemaswell asthe benefit ofa microcomputer
capable of other microbiological and data managementapplications.
A significant portion ofa routine clinical
mi-crobiology laboratory'sworkload consists of the
identification of aerobic gram-negative bacilli belonging to the family Enterobacteriaceae. Since specific identification within this group
often has clinical and epidemiological implica-tion,accurateand reproducible taxonomic
clas-sification of clinical isolates belonging to the
Enterobacteriaceae is mandatory. Within the past 10 years, the value of commercially
pre-pared,computer-assisted, multitestsystems has been well established in terms of quality of results, easeofuse, and economy. The microti-ter plate, which has been well received by clinical laboratories as aconvenient vehiclefor antimicrobialsusceptibility testing(7)anda
vari-etyofserological procedures, also lends itselfto
the miniature multitest approach to bacterial
identification. Severalauthors have commented
favorablyon the useof acommercial microtiter system with frozen reagents for the identifica-tion of Enterobacteriaceae (1, 4). The present
studyassesses theperformanceof a new micro-titer plate bacterial identification system
utiliz-ing dried test substrates (Sensititre; manufac-turedby Seward Laboratory, London, England, anddistributed in theUnitedStates and Canada
tPresent address: GIBCO Laboratories, Lawrence, MA 01843.
by GIBCO Laboratories, Lawrence, Mass.) in
comparisonwith thatofacommonlyused multi-test identification kit, the API 20E (Analytab Products, Plainview, N.Y.) (6). This study in-volved the identification of Enterobacteriaceae andwascarriedoutinthreeindependent clinical microbiology laboratories: University of Cincin-nati Hospital, Cincinnati, Ohio; Saint-Luc's Hospital, Montreal, Quebec, Canada;and Tam-pa GeneralHospital, Tampa, Fla.
MATERIALS ANDMETHODS
Clinical isolates. Gram-negative bacilli resembling
Enterobacteriaceae by virtue of colonial morphology
onsheep blood and MacConkeyagars were studied. With the exception ofAeromonas spp. and
Pleisio-monasspp.,all isolateswereoxidasenegative. Inthe
majority of instances, identical colonies growing from clinical specimens plated 1 day earlier on aprimary plating medium were used as inoculum for both the
API20Eand Sensititre identificationsystems. In those
instances when well-isolated colonies were
unavail-able, reisolation on appropriate solid medium was
carriedout before proceeding with an identification.
Consecutive clinical isolates were entered into the
evaluationthroughout the study period; however, to
avoid a disproportionate numberofEscherichia coli
isolates, eachstudycenter,afterrecognizing approxi-mately 100 E. coli isolates, excluded from the study additional isolates which gave characteristic E. coli morphology on MacConkey agar and were oxidase
negativeand spot indolepositive.
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648 STANECK ET AL.
Stockculture isolates. Each of the three study
cen-ters contributed less commonly encountered clinical
isolates to theevaluationfrom their respective stock
culture collections.Each isolate was given anumerical code by a laboratory member other than those
in-volved in the identification study so that each stock
isolate was run blindly. Each isolate was subcultured
daily onto sheep blood agar from the stock holding
medium for2consecutive daysbeforeinoculation ihto theidentification systems. Less than 5% of the study
wasconducted with isolates taken from stockcultures.
Interlaboratory reproducibility. An additional 20
stock culture isolates werenumerically coded by an
individual at the UniversityofCincinnati not
partici-pating in the identification study. Triplicates ofeach
isolate were prepared and distributed to the three
studycentersasunknowns. Each ofthecodedisolates
was thenentered into the study protocol at each of the
threecenters.Tabulation ofresponses was made later,
noting both individual biochemical reactionsand
taxo-nomicclassifications.
API 20Esystem. TheAPI 20E was usedaccordingto
the manufacturer's instructions. All test strips used
throughout thestudywereofthe same lot number. A
profile recognition system index (issued December
1979) was utilized for taxonomic designation. For
purposesofthisstudy, identificationswereconsidered
valid only if thequality of identification givenby the
profile index was"excellent," "verygood,"or
"ac-ceptable" without additional tests. Identification at
the genus level was considered valid if the report of
"good likelihood but low selectivity" (GLBLS) was
accompanied byafirst and second choicehaving the
same genus butdifferentspecies. Otherwise, GLBLS
was, in those instances where both the genus and
species ofthe first and second choice differed, not
consideredavalididentification andwasplacedin the
functionalcategory"noID." No attemptwasmadeto
clarifyinterpretations byusing thetelephone comput-erservicefromAnalytab Products.
Sensititre identificationsystem. The Sensititre
identi-fication system was used according to the package
insert instructions. Descriptions are given below
re-garding the system components, the method of use,
andthecomputer-assisted derivationofidentification.
(i) Components. The Sensititre identification plate
consists ofa96-well plasticmicrotiter plate packaged
in amoisture-proof aluminum foil pouch, containing 24
dried biochemical test substrates arranged in three
vertical columns repeated four times across the plate. Thefirst columnof eight wells contained the nitrate,
oxidase, indole,Voges-Proskauer, tryptophan
deami-nase, gelatin liquification, adonitol, and rhamnose
tests. The second column of eight wellscontained the
glucose,o-nitrophenyl-4-D-galactopyranoside, urease,
citrate, H,S, inositol, arabinose, and sorbitol tests.
The third column ofeight wells contained the
decar-boxylase brothbase, lysine decarboxylase, ornithine
decarboxylase, malonate, esculin, mannitol, maltose,
and sucrose tests. The top row ofsubstrates in each
quadrant (nitrate, glucose, decarboxylase broth base),
yielding positive, positive, and negative results,
re-spectively, isconfirmatoryfor members of thefamily Enterobacteriaceae (organisms producing other
pat-terns wereexcluded from this study). The remaining
seven rows are scored to yield aseven-digit biocode
whichcontainsthe informationnecessary for
identifi-cation.Reagents aresuppliedindropperbottlesbythe
manufacturer for those testsrequiringthe addition of
reagents afterincubation. Accessorymaterialsinclude
mineral oil, adhesive plate covers, and plastic
tem-plates for oil overlay andreagent addition. All
Sensi-titre plates used throughout the study were from the
same lot.
Inoculation and hydration of the test substrates were accomplished by using the Sensititre automatic inoculator, a device that delivers 100 ,ul of water
containingtheinoculum into each of the 24
biochemi-cal wells required foranidentification. The inoculum
isdelivereddirectlyfromaglasstube(16by120 mm)
with a standard threaded lip onto which is fitted an
individualdisposable plastic dosinghead that attaches totheinoculator. Each isolate isthereforeinoculated
from a self-contained, aerosol-free disposable dosing
FIG. 1. Sensititreautomaticinoculator.
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FIG. 2. Sensititre data interpretation system.
unit(Fig. 1). The inoculator transports the microtiter plate under thedosing head such that eachtest well
receives the appropriate inoculum volume. This se-quenceis repeated in each of the four testpatterns, enabling thetesting of four organisms per plate.
TheSensititre datainterpretation system consists of
a plate viewer, a microcomputer (Apple II), visual display unit,twofloppy disk drives, andaprinter (Fig. 2).Patientdemographics and isolate reference number maybe entered bykeyboard before reading the
reac-tions ofaparticular isolate. The plate viewer is de-signedtoallow the operatortoread theplate from the bottom via a mirror. A series of indicator lights illuminates in sequence the wellstoberead, the results of which are entered by means of the computer keyboard. The visual display unit displays the test
matrix for eachidentification, theinformation
neces-sary to read each reaction, each test result upon keyboard entry, and the calculation of theseven-digit octal number basedonthefinal21 testsof thepattern. This number is then utilized by the identification program togive ataxon designation together witha
statementofidentificationquality. The patient
demo-graphics, the entiretestresult record andsubsequent identification analysis, including mathematical
param-eters and testresults against each taxon choice, are
displayedonthe visualdisplay unit andcanbe gener-ated in hard copyvia the computer-driven printer.
(ii)Methodof use.Twoorthree identical colonies, 18 to 24 h old, were picked from a sheep blood or MacConkeyplate withawooden applicator stick and
emulsified in 7 ml of sterile demineralized water to
yield a cell concentration of approximately 107 CFU/ml. Delivery of inoculumtothemicrotiter plate
bymeansof theautomatic inoculator device occurred
within 30 min ofinoculum preparation. Mineral oil overlay was made to appropriate wells, transparent adhesive seals were applied, and perforations were
madeover allcitrate and malonate wells. Four isolates
could beinoculated per plate. Incubation of plates was carriedout at35°C under ambient atmosphere for 18 to
24 h.Before the plates were read, appropriate reagents
were added according to the manufacturer's
instruc-tions. The oxidase reaction was read immediately, whereas theremaining reactionswerereadafter10 to
15mintoallow for thedevelopment of positive Voges-Proskauer reactions.
To facilitate uniformreading of reactions between the threestudy centers, plateswerereadin the
follow-ingmanner ateachcenter.All reactionswerereadon
the plate viewer by using a fluorescent desk light situated above the viewer, diffused by a 1/8 in. (3.2 mm)thick white translucentPlexiglassheet(4by6in. [10.2by 15.2cm])suspendedapproximately6in.(15.2 cm)above theplate viewerplatform.This illumination providedbright reaction colorsagainstan even back-ground. For carbohydrate fermentation reactions, onlyyellowwasconsideredpositive,whereas blueor
any shade of green was considered negative. The decarboxylase reactions were read by comparison with the decarboxylase base broth negative control well.Tobe calledpositive, the decarboxylasetestwell
had to be purple and those wells which were not
purple, even if darker than the control well, were
callednegative.
(Uii) Computer-assisted derivation of identification. Thecomputer-assisted derivation of identification uti-lizedaseven-digit"biocode"generatedbyscoringthe
21reactions ofrows 2through7of theplate for each organism tested. Thefirst,second, and third reaction wellsofagivenrow werevaluedatfour,two,and one, respectively,ifpositive,andzeroifnegative.The sum
of these three valuesrepresented onenumberof the code. The biocode is referred by the computer toa
data basecompiled fromthetestingof over 40 taxa of
gram-negative organisms. The identification is
ob-tainedbyconsidering absolute likelihood (8)and mod-aldistance probability values (R. K. A. Felthamand
M. Stevens, in C. S. Gutteridge, ed., New Methods
for Detection and Characterization of
Microorga-nisms, inpress). Likelihood values dictatethe orderof
taxonchoices, whereasthequality oftheidentification
statement (excellent, good, etc.) is derived from the
modaldistancevalues.
Forpurposes of thisstudy,taxonomicdesignations
wereconsidered valid if the quality of Sensititre
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650
fication was excellent, good, acceptable, or "rare
biotype." Occasionally,whenthe mathematicalmodel indicated that an isolate was determined to be located
at the periphery of two overlapping modal distance
clusters, alisting of the choices would appear with the
comment "low selectivity." In these cases, the first
choice was not considered a valid identification for
purposes of this study.Occasionally, a low selectivity
commentwouldaccompany a listing in which choices
1 and 2 were of the same genus butseparation at the
species level could notbe made mathematically
with-outfurther testing. Such instances were accepted as
valididentificationat the genuslevel only.
Conventional identification of bacterial isolates.
Iso-lates for which the initial API 20E and Sensititre
identificationresponsesdiffered, eitheratthe genus or
species level, were subject torepeat testing on both
systems. Persistent discrepancies were resolved by
identification ofthe isolates with conventional tubed
biochemicaltests. Testresultsasreported by Edwards
and Ewing (3) and updatedby Brenner et al. (2) were
used to determine identification. Nomenclature as
described bySkermanetal. (5) wasutilized
through-out thestudy except forSalmonella arizonae, which is
referred to in this report as Arizona hinshawii. The
followingbiochemical tests or testmediawere routine-ly used: triple sugar iron agar, decarboxylation of
lysine or ornithine, motility, gas production, H2S
production, utilizationofcitrate, urease activity,
pro-ductionofindole, deamination of phenylalanine, and
cytochrome oxidase activity. Additional biochemical
tests were used when necessary. Incubation times
appropriate to each test were utilized. Serological
grouping with antiseradirected against somatic anti-gens was performed when indicated for suspected
isolatesofSalmonella and Shigella.
Study protocol. Isolates were examined
simulta-neously on API 20E andSensititre identification
sys-tems. Those isolates which yielded identical species
identification responses from the two systems were
subject to no further identification procedures. The
initial agreement response was considered to be the
correct taxonfor thatorganism.Isolates notin
agree-mentbetweenthe two systems, either at the genus or
species level, or isolates yielding a "genus only"
response on one or the other identification system
wererepeated on the two systems, andif disagreement
persisted, they were submitted to conventional
bio-chemicaltesting. Intheseinstances, the identification
response made by conventional biochemical testing
was accepted as the correct taxonomic classification.
Any time the genusorspeciesresponseofanisolate
wasidentical on both the API 20E and the Sensititre,
orwhen the response of eitherAPI20EorSensititre
wasidentical to that given by conventional
biochemi-caltesting,theresponse was considered fortabulation
purposes as"complete agreement."If theresponse of
either API 20E or Sensititre indicated the correct
genus but either the comment"species" orlow
selec-tivity (GLBLSon API 20E) at the species level, the
response was scored as a"partial agreement." Ano
ID category included those instances when the code
numbergenerated did not appear in either the API 20E
profile recognition index or the Sensititre computer
databaseorwhen theresponsebyeithersystemwas
low selectivity atthe genus level. Inthose instances
when a testsystem gaveavalid response thatdiffered
from that ofthe conventional biochemical testing at
the genus orspecieslevel,the responsewastabulated
as an "incorrect ID."
RESULTS
A total of 1,421 gram-negative bacilli repre-senting 35 different taxonomic groups were ex-amined in the study. There were 1,253 isolates whichyielded identical taxonomic responses on API 20E and Sensititre, including 47 isolates whose initial disagreement was resolved upon repeat testing. The remaining 168 persistently
discrepant isolates were studied further with
conventional biochemicals. The responses of each identification system were tabulated, and the distribution ofall identification responsesis
summarized in Table 1. Correctidentification at the species level was made for 91.1% of the isolates by API 20E, whereas correct identifica-tion wasmade atthe species levelby Sensititre for 94.6% of the isolates. Correct identification at the genus level, including instances where only the genuswasgivenastheresponse(partial
agreement columns, Table 1) and instances
where the response included the correct genus but incorrectspecies designation (Table 2), was made for an additional 6.7 and 1.9% of the
isolatesby API 20E andSensititre, respectively.
Thenatureof the individual incorrect respons-esby eitherAPI20EorSensititre is presented in Table 2. Results of conventional biochemical
testing were utilized to designate the correct
taxonomic classification. The API 20E gave 14 such incorrect responses involving six taxa,
includingsix instances in which Serratia liquefa-ciens was called Serratia marcescens. There were eight instances involving seven different taxa in which Sensititre yielded an incorrect identification.
Twenty coded stock isolates consisting of a variety of Enterobacteriaceaeweresubmittedto each study center to determine interlaboratory reproducibility. The numberof instances where onelaboratoryreada
specific
biochemical reac-tion for a particular isolate differently fromthe other two laboratories was only 18 and 16 for API 20E andSensititre,
respectively.
The total number of individual reactionscontributing
totheidentificationreadoneach systemforthe 20 isolates in thethreelaboratorieswas
1,260.
Innocases did this variation in individual reaction
reading influence final identification responses oneither system. Nordid any onetest reaction oneither system appeartobe
significantly
moresubject to variation in
reading
between labora-tories than any other reaction. Twenty identical identification responses were noted among the threelaboratorieswithin eachtestsystem. How-ever, the two systems differedslightly
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TABLE 1. Performance of theAPI 20E andSensititre systemsin theidentificationofEnterobacteriaceae
Total API 20E Sensititre
Taxon' no.of Complete Partial No Incorrect Complete Partial No Incorrect isolates agreementb agreement' IDd ID agreement agreement ID ID
Aeromonashydrophila Arizona hinshawii Citrobacter diversus Citrobacterfreundii Edwardsiella tarda Entericgroup17
Enterobacter aerogenes Enterobacteragglomerans Enterobactercloacae Enterobacter gergoviae Enterobactersakazakii Escherichia coli Hafnia alvei Klebsiella ozaenae Klebsiella oxytoca Klebsiella pneumoniae Klebsiella rhinoscleromatis Morganella morganii Plesiomonasshigelloides Proteusmirabilis Proteus vulgaris Providenciaalcalifaciens Providencia rettgeri Providenciastuartii Salmonella choleraesuis Salmonella enteritidis Serratia marcescens Serratialiquefaciens Serratiarubidaea Shigellaboydii Shigella dysenteriae Shigellaflexneri Shigellasonnei Yersiniaentercolitica Yersiniapseudotuberculosis 5 5 4 4 14 56 51 1 1 1 1 54 53 6 6 155 151 7 7 7 6 343 334 8 8 7 2 53 53 249 242 1 1 50 50 1 1 174 173 27 27 4 7 35 3 46 50 16 3 2 1 8 9 12 2 5 4
13 1 14
1 4 46
1
1 48
5
2 1 1 152
7
1 6
7 2 334
8
4 1 4
53
1 3 3 246
1 50 1 1 172 26 4 S 31 1 1 46 41
6 1 41
3 6 15
2 3 2 1 6 2 9 5 8 2 4 7 35 2 43 7 1 1 9 7 2
aTaxonomic classification determinedbyinitialagreementof API 20E andSensititreor,inthose instances of
initialdisagreement, byevaluation with conventional biochemicals.
bAgreementatbothgenusandspecies level.
cAgreementatgenuslevel; commercialsystemresponds species orlow selectivityatthe species level.
dCode numbernot indata base orlowselectivityresponse for which first and second choicesdifferatthe
genuslevel.
Whereas Sensititre reported C. diversus, API 20E reported Citrobacter species. In addition, both API and Sensititre reported identifications
atthegenuslevelonly foraShigella boydii anda Shigellaflexneri isolate, butagain each system
gave identical responses in each of the three
laboratories for these twoisolates.
DISCUSSION
The results of this study indicate that the Sensititresystemcomparesfavorablyto a well-accepted commercial multitest system, the API 20E, for routine identification of Enterobac-teriaceae. Correct identification to the species level was given by Sensititre for approximately
95% of the isolates tested, whereas API 20E yieldedacorrectspecies identification in 91% of the isolates. The difference between the two
systems at this level of performance can be
largely attributed to the ability of Sensititre to
yield acceptable species identification for Citro-bacter diversus andSalmonellaenteritidis,
orga-nisms routinelyidentified by API20E as
Citro-bacterspeciesandSalmonellaspecies. Of the 21
tests used for identification by Sensititre, the
API20E lacks malonate, esculin, adonitol, and maltose but has instead arginine dihydrolase, glucose (included in Sensititre control row 1),
melibiose, and amygdalin. The species identifi-cation of Citrobacter by Sensititre is facilitated
10 1 1 3 6 3 6 1 1 1 5 3 2 1 1 2 3 8 1 1 1 1 1 2 8 4 VOL.
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TABLE 2. Errorsof identification byAPI 20E orSensititre
Taxonby API Sensititre
conventionalNoofo.f
biochemicals Response error. Response eNrorosf
Entericgroup 17 Enteric group 17 Y. enterocolitica 1
E.cloacae E. agglomerans 1 E. cloacce
E. coli H. alvei 1 NMTb
E. coli Pasteurella multocida 1 S. sonnei 1
E. coli E. coli Shigella sp. 2
K. ozaenae K.rhinoscleromatis 1 K. ozaenae
K.pneumoniae E. agglomerans 1 NMT
K.pneumoniae S. rubidaea 1 K.pneumoniae
K.pneumoniae E. agglomerans 1 E. cloacae 1
P. mirabilis P. mirabilis M. morganii 1
S. choleraesuis NoID" Salmonellapullorum 1
S. marcescens S. marcescens S.liquefaciens 1
S. marcescens S.liquefaciens 1 S. marcescens
S. liquefaciens S. marcescens 5 S. liquefaciens
S. liquefaciens S. marcescens 1 NMT
aValididentificationresponsebyAPI orSensititredifferingfromtaxondefinedbyconventionalbiochemical
testing.
bNMT, Nomatchingtaxa.
'Code numbernotin
API
data base.by the malonate and adonitol. API 20E has chosen not toinclude a species designation for most strains of Salmonella, because of either test limitations or corporate philosophy and
in-stead strongly recommends, asdoes Sensititre, serological verification of both Salmonella and
Shigella species.
Correct identification at the genus level was made for approximately 97 and 98% of the isolates, respectively, by Sensititre and API 20E. For purposes of this study, numerical
codes with low
selectivity
responses at the ge-nuslevelweregrouped alongwithcode numbers notappearingin the data baseas noID respons-es. It could certainly be argued that the lowselectivity response is useful in that certain
taxonomic choices are given in order of likeli-hood probability, and the useris therefore free to either accept the most probable choice or
perform additional tests to
distinguish
between the choices. Among the 25 no ID responses of API 20E listed in Table 1, only 7 were actuallynotlisted in the data base.Of the45Sensititreno IDresponses, however, 30 responses were not found in the data base. This observation could
possiblybe attributedtotherelative maturityof theAPI 20E databaseversustheSensititre. The younger Sensititredatabasewould be expected
tohave agreater numberofnonmatchingcodes. It isinterestingto notethat exceptforacluster of six instances whenthe API system identified S. liquefaciensasS. marcescens, the number of incorrect responses(Table2) from either system wassimilar andrepresentedaverysmallportion
of the total number of isolates tested. The most
serious errors, considering the clinical
implica-tions, were the three instances in which E. coli was called Shigella by Sensititre. However, in each of these instances the computer display instructed the user to confirm this identification
by serology, andhad the serology component of the identification procedure been carried out, as
isthe customin most laboratories, an erroneous report would not have been made. Aside from these, the majority of responses other than com-plete agreement found in either system were scattered among the taxa studied.
Although the present study was confined to identification of Enterobacteriaceae, the Sensi-titre data base also contains codes for various
glucose-nonfermenting organisms. Casual
ob-servations throughout the study suggested that Pseudomonas aeruginosa is readilyidentified by
Sensititre; however, the performance of Sensi-titrefor otherglucose-nonfermenting organisms awaits evaluation. The API 20E has addressed the non-glucose-fermenting groupby providing an option for additional tests,increased incuba-tion times when necessary, and expansion of the octal code tonine digits.
The Sensititre system was reasonably simple and easytouse. Thedriedtestreagentsallowfor refrigerator storage ofthe plates and long shelf life. Theautomated inoculator was efficient and rapid in the delivery of inocula to the plate. Plastic templates supplied with the system sim-plified and reduced errors in oil overlay, cover
perforation, and later reagent addition to the plates. The testreactions were in generaleasily read with littlepractice, and the indicator lights of the plate viewer box greatly reduced the occasional confusion one encounters in reading
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amicrotiter plate containing 96 wells ofidentical size and shape.
During thecourseof the study, certain obser-vations were made regarding the reading proce-dure, which facilitated the recognition of true testresults. As with any new test system, strict guidelines needed to be followed in the reading of reactions to maximize performance and
re-producibility. It wasgratifying to notethe small
amount of variation in the reading of reactions between laboratories with Sensititre as well as API20E described in the interlaboratory repro-ducibility studies, which attests to the impor-tance of adherence to the reading guidelines. A white translucent Plexiglas sheet suspended above the Sensititreplateacted as alightdiffuser and provided awhite, evenly illuminated
back-ground against which the Sensititre test colors
could be read, greatly improving the ease and consistencyofreaction reading.The recommen-dation has been made to the manufacturer to incorporate this feature into the plate viewing box.
It was also noted that four rather than two drops ofKovacs reagent added to theSensititre indole well allowed the development color to spread throughout the entire well. Withonly two
drops,apositive reaction wouldappear as ared
ring, often difficult to detect when using the
plate viewer.TheSensititre oxidasetestwas the
only reactionthat required reading immediately
after reagent addition. Procedurally, therefore,
all reagents except the oxidase reagent were
added, and sufficient time was allowed for the slower reactions to develop. The oxidase rea-gent was then added immediately before plate
reading.
Mastery ofthe microcomputer componentof Sensititre was easy due to asoftware program
designed tobe "userfriendly." Simple instruc-tionsgivenin sequenceby the program allow the user to initiate and complete the identification
process instraightforward fashion. Althoughthe
edition of the user program employed in this
study allowed for test alteration only after the
completion of all reaction entries for a given
organism,a more recentprogram allows the user to change a reaction immediately should a key-board entry or reading error occur. Data
inter-pretation could either be displayed or printed after entryofeachorganism's test results or be stored andprinted afterthereading of all plates. Thelatter option allows for rapid,uninterrupted reading and lends itself well to use of the system in batch fashion.
Itshould be mentioned that although the mul-ticenterstudy utilized the automatedinoculator and data interpretation system, the Sensititre identification system can be utilized in a totally manual fashion by employing an eight-channel
repeating pipette inoculator, a simple mirrored plate viewer, and a printed bench code booklet.
Used as a manual system, the Sensititre offers considerable economic advantage over several
commercially available multitest kits, including API 20E.
Theeconomic impactof use of the automated system isdependentonuser testvolume and the
specific purchase arrangements offered by the vendor. The microplate itself, with a capacity foridentification of fourorganisms,offers poten-tial savingsonconsumable expenditures. Labor savings are realized via the automatic
inocula-tion deliveryversus manualinoculation, and the
computerizeddatainterpretationtakes less time
than the use ofprinted data code books. The timerequired for inoculumpreparation, reagent
addition, and reaction reading iscomparable to manual multitest systems.
Among the distinct advantages of the Sensi-titreequipment isits flexibility ofuse. Both the
inoculatorand thecomputer-assisteddata
inter-pretation modulecanbe usedwith the Sensititre antimicrobial susceptibility system, which
fea-tures various data management programs for testresults. TheSensititre systemalsopresents to microbiology laboratoriestheadvantage ofa
fully operational microcomputerin the AppleII component ofthe system. Themicrocomputer, utilizing floppy disks, allows foreasy program
update and modifications ofthe Sensititre sys-tem. The usefulness of the computer is not limited to Sensititre programs in that it allows for the use of nonmicrobiological, statistical, educational, and datamanagementprograms. In
addition,theoption exists foruserprogramming specificto themicrobiological, epidemiological,
oradministrative needs ofthelaboratory.
Among the possible limitations ofthe Sensi-titre Enterobacteriaceae identificationsystemis
the needfor 18 to 24h of incubation time. This would be problematic to laboratories desiringa
rapidorsame-day identification. Several identi-fication kits presently offer 4- to 5-h identifica-tions,includingAPI 20Ewhich offersa5-hdata profile index.Inaddition, the Sensititreprogram asitcurrently existsprovidesnofurther
sugges-tions regarding biochemical tests needed to re-solve lowselectivity choices, afeatureprovided by the API 20E index.
Insummary, the Sensititre system appears to be a reliable and convenient method for the
identification of Enterobacteriaceae. Sensititre canbeutilized either as a manualsystem provid-ingdistinct economic advantages or as a semiau-tomated microcomputer-assisted system
offer-ing economical running costs and the flexibility
of use with microtiter antimicrobial susceptibil-ity testing plus commercially available or user-written data management programs.
17,
on February 8, 2020 by guest
http://jcm.asm.org/
ACKNOWLEDGMENTS
We thankArthurStarrofTampa, Fla., for his interestand support throughout this study. The technical expertise and assistancegiven by Marie Gourdeauand Marielle Parent of Montreal and Marlene Willner, Sherene Mackos, Barbara Puthoff, William Bayer, and Barbara Laymon of Cincinnati aregratefully acknowledged. We also thank Sharon Vincent and StevenGlenn of Cincinnatifortheir technicalandeditorial contributions andJoyce Turnerof Cincinnati forher secretari-alassistance inthe preparationof thismanuscript.
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