Evaluation of the updated MS 2 Bacterial Identification system in comparison with the API 20E system

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0095-1137/83/070128-08$02.00/0

CopyrightC 1983,AmericanSocietyforMicrobiology

Evaluation of the Updated MS-2 Bacterial Identification

System

in

Comparison with the API 20E System

JOSEPHR. DIPERSIO,l*JOHN W. DYKE,2ANDRICHARD D. VANNEST3

DepartmentofClinicalMicrobiology, TheChristHospital, Cincinnati, Ohio452191; Department of Clinical

Microbiology,EdwardW.SparrowHospital, Lansing, Michigan 489092; and Abbott Laboratories, Irving,

Texas760213

Received 5November1982/Accepted6April1983

The recently updated MS-2 Bacterial Identification system software (Abbott

Laboratories, Diagnostic

Division, Irving, Tex.)was compared with the original

MS-2 Bacterial Identification software and the API 20E, using 968 strains of

Enterobacteriaceae. The updated MS-2 software correctly identified 94.4% of the

isolates tested. API 20E and theoriginal MS-2 software correctly identified 91 and

85.3% of the strains, respectively.

MS-2

responses were considered to be

equivocal (needing

additionaltestsforverification) if the percent likelihood values

wereless than 80%. The percentage ofequivocal responses was reduced from

6.5% with the original software to 2.2% with the updated software, and the

percentage of incorrect identifications was reduced from 8.2 to 3.4% with the

original

and

updated software,

respectively. Organisms belonging to 25 taxonomic

groups weretested. Direct

comparison

of thetwoMS-2programsshowed that the

updated

MS-2 software increased the identificationaccuracyof Salmonella spp.,

Enterobacter

cloacae,

Providencia stuartii, Escherichia coli, Shigella spp.,

Klebsiella pneumoniae, Serratia marcescens, Proteus mirabilis, and

Acineto-bacter calcoaceticus. A decrease in accuracy was seen with CitroAcineto-bacterfreundii,

Hafnia alvei, Enterobacteragglomerans, and Yersiniapseudotuberculosis when

the

updated

software was used. The remaining 12 taxonomic groups were not

affected by the software

changes.

The

updated

MS-2 software appears to

significantly

improve

the identificationaccuracyof the MS-2 Bacterial

Identifica-tion system.

Inthe pastseveralyears, increasedemphasis

has been placed on the rapid identification of

microorganisms recovered from clinical

speci-mens. A number of manual conventional and

manual multitest systems are now available for

identifying various bacterial species faster than

was previously possible. In addition, several

automated instrumentshave been developed for

determining both organism identifications and

antimicrobial susceptibilities within 4 to 5 h.

Vannestand his colleagues (R. D. Vannest, D.

Brunson,M.Cornell, H. Terk, B. Perry, and R.

Wilbom, Abstr.Annu.Meet. Am. Soc.

Microbi-ol. 1979, C(H)86, p. 360) reported preliminary

data on the use of Abbott MS-2 (Abbott

Diag-nostics Division, Abbott Laboratories, Irving,

Tex.) for rapid automated identification of

En-terobacteriaceae. This preliminary report was

subsequently followed by a more detailed

col-laborativestudy byMcCracken et al. (8), which

evaluated the MS-2 Bacterial Identification

(BID) system by using 150 coded unknown

organisms

and 1,154recent clinical isolates. The

MS-2 identified 86% of the isolates correctly

compared with conventional manual tube

meth-ods. Anadditional 8to

9%

of the organismswere

correctly identified, but with a lower percent

likelihood (less than 80%). These strains

re-quired additional biochemical testing to confirm

thefirst-choice identification.

Thepresentcollaborativereportevaluates the

impact of an expanded data base and related

softwareimprovementson the identification

ac-curacy ofthe MS-2 BID system. The

perform-ance of the MS-2 BID system was also

com-pared with thatofthe API 20E,using the same

organisms.

MATERIALSANDMETHODS

Organisms. Atotal of931 organisms isolated from theMicrobiology Sections of The Christ Hospital and

E. W.Sparrow Hospital were used. The isolates were collected inasequential manner, and minimal attempt

was made to selectively include or exclude specific strains.Inaddition, 37 stockcultures representing less

frequently encountered species were included.

Ap-proximately one-half of the isolates were tested in eachlaboratory, using original MS-2 software, updat-edMS-2software, andAPI20E.

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API 20E. The API 20E strips were inoculated as recommended by the manufacturer and then were incubated at 35°C for 20 to 24 h. Organisms were identified by using the current profile index and were placed in the following categories:

(ij

correct identifi-cation (high assurance); (ii) correct identifiidentifi-cation (low selectivity); (iii) no identification (biotype notincluded in profile index);and(iv) misidentification.

MS-2 system. The MS-2 BID system was used in accordance with the manufacturer's operationmanual. Briefly, four to five morphologically similarcolonies of a freshclinicalisolate were inoculated into a tube containing 5 ml of sterile deionized water. The tube was mixed to provide a uniform suspension and was adjusted to equal a 0.5 McFarland standard. Then 200 gLlof the adjusted suspension was inoculated into each of the reaction chambers of the BID cartridge. The

cartridges weregently rocked to disperse evenly the

freeze-driedbiochemical substrates. Initial light

trans-mission readings were taken (within 30 min of

car-tridge inoculation) by inserting each carcar-tridge into an MS-2 analysis module equipped with original BID software. Initiallighttransmission readings were also taken in a second analysis module equipped with updated BIDsoftware. After5h ofincubation at 35°C,

cartridgeswereremovedfrom the incubator, and 50 ,ul

of Kovacs reagent was added to the indole test reac-tion chambers.Interpretation of the biochemical tests was obtained by reinserting each cartridge into both

MS-2 analysis modules for final readings. Organism

identification choices and percent likelihood values

were printed by each MS-2 system after finalreadings.

Organismswereplacedin thefollowing categories: (i)

correctidentification (likelihood greater than or equal

to

80%);

(ii)correctidentification(likelihoodless than

80o);and(iii) misidentification.

Reference Idendfication. Results obtained with the updated MS-2 software and with the API 20E strips were compared.If the two identifications agreed, then

theresultwastakenasthe referenceidentification. If

the two identifications disagreed or low assurance

identificationswereobtained,the isolateswerefurther

evaluated by using conventional tube biochemical

testing. The final reference identification of these

organisms wasbased upon the evaluation of 32

con-ventionalbiochemical tests and the use ofexpanded

biochemicalchartsfor theidentification of the

Entero-bacteriaceae(2-6).

RESULTS

The identification accuracy of the MS-2 BID

system when tested with either the

original

or

the

updated

database

software

against

968

bac-terial

isolates

isshown in Tables1and 2. A total

of25different bacterial

species

wereincluded in

the

study. Except

for 37 stock cultures ofrare

isolates, all test

organisms

were obtained from

fresh clinical sources and tested

immediately.

Thedistribution of

species

reflects the

frequen-cy ofisolation

experienced

inmostclinical

mi-crobiology

laboratories.

The

updated

MS-2 software was more

accu-rate than the

original

software in

identifying

several

species (Tables

1 and

2).

Notable

im-provements in the percentageofcorrect

identifi-cation were seen with Salmonella spp. (77 to

95%),

Enterobacter cloacae (69 to 90%), and

Providencia stuartii (68 to

100%o).

Improved

accuracy was also observed with Escherichia

coli (96 to 99%), Shigella spp. (92 to 100%),

Klebsiella pneumoniae (94 to 100%), Serratia

marcescens (83 to89%), and Proteus mirabilis

(96 to 98%). Decreased accuracy, however, was

seen with Citrobacter freundii(95 to87%),

En-terobacter agglomerans (88 to 75%), Hafnia

alvei(92 to62%),and Yersinia

pseudotuberculo-sis(100 to67%). The results obtained with API

20E using the same 968 isolates are shown in

Table3.

Incorrect identifications obtained when the

updated MS-2 BID software was used are

de-tailed in Table 4. The biochemical reactions

responsible for misidentificationarealso shown.

A positive interpretation of arabinose caused

four isolatesof S. marcescens to be identified as

Serratia

liquefaciens,

and

negative

ornithine

re-actions resulted in three strains of E. cloacae

being misidentifiedasE.agglomerans.Three H.

alveiwerecalledSalmonella enteritidis dueto a

combination of citrate-negative and

malonate-negative

reactions,

and three C.

freundii

strains

were incorrectly identified as E. agglomerans

dueto

false-negative

lactose reactions.

A comparisonof the overall accuracy of the

MS-2 BID system, using either the original or

theupdated data base softwareversus API20E,

is shown in Table 5. The original and updated

MS-2 programs identified 85.3 and 94.4%,

re-spectively, of

the

isolates.

The percentage of

equivocal

(low

likelihood

or low assurance)

identificationswas 6.5% when the original

soft-ware was used and

2.2%

when the updated

software wasused. Thepercentageof incorrect

identifications with the original and updated

programs was 8.2 and 3.4%, respectively. The

API20Esystemcorrectlyidentified 91.0% of the

isolates. Four percent of the strains were

cor-rectly identified butrequiredconfirmatory

test-ing. No identification could be made for 1.6% of

theorganisms, and 3.4%were misidentified.

DISCUSSION

Automatedinstrumentation suchasthe

Auto-Microbic system (Vitek Systems, Inc.,

Hazel-wood, Mo.),theAutobacsystem

(General

Diag-nostics,

Warner-Lambert

Co.,

Morris

Plains,

N.J.), andtheAbbott MS-2can

identify

various

gram-negative

bacilli within 3 to 8 h

(1, 7, 8).

Besides

rapid

identification,

another

important

goal

of

companies developing

automated sys-tems is to

provide

an identification accuracy

comparable

tothatof conventional

procedures.

The Abbott MS-2 BID system was first

intro-duced in1980. In the

original

collaborative

study

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130 DiPERSIO, DYKE, AND VANNEST

TABLE 1. Accuracy of theoriginalMS-2 BIDsoftwarewith 968isolates'

Referenceidentificationb

MS-2identification E. Shi- E. C. C. Ari- Salmo- K. pneu- K. oza- K.rhino- E.aer- E. clo-coli gella_Sp. tarda freun-_dii diver- zona nella moniae enae sclero-aca-ogenes

sus Sp. Sp. matis oensae

Escherichiacoli 360. . ._ ._ . . .

Shigellasp. 1 12 _ _ _

Edwardsiellatarda _ _ 2 - - -

-Citrobacterfreundii 2 - - 22 - 1 1 - 3

Citrobacterdiversus - _ _ _ 13 - - _ _ 1

Arizona sp. . . 2 1 _-

-Salmonellasp. 4 - .- _ _ 17 _-

-Klebsiella _ _ _ _ _ _ _ 178 - _ - 1

pneumoniae

Klebsiellaozaenae 1 . . . 1 4 - - 1

Klebsiella . . . .._. _2 - -rhinoscleromatis Enterobacter 1 _ . . . . 4 28 -aerogenes Enterobacter 1 1 _ _ - 36 cloacae Hafniaalvei 5. . . . .-4 _ _ _ -Enterobacter 1 3 _ _ _ 6 agglomerans Serratialiquefaciens 1._ _ 3 Serratiamarcescens - _ -Proteusvulgaris _ _ _ _ Proteusmirabilis _ _ _ _ Morganellamorganii - _ _ _ Proteus retigeri _ _ _ _ Providencia _ _ _ _ alcalifaciens Providenciastuartii _ _ _ _ _ Yersinia 1 _ _ _ _ enterocolitica Yersinia _ _ _ _ pseudotuberculosis Acinetobacter _ 1._ _ _ _ calcoaceticus Serratiarubidaea _ _ _. 3 2 - - 1

%Correct 96 92 100 95 93 100 77 94 57 100 100 69 aTistableand Tables 2 and 3 showcomparisonsof each of the three systems tested toconventional results byspecies. Deviationsfrom thediagonallineofcorrelatingresults represent disagreement between the system identificationand theconventionalidentification. bFor adefinition,seethe text. TABLE 2. AccuracyoftheupdatedMS-2BIDsoftware with 968isolatesa Referenceidentification MS-2identification E Shi- E. C. C. Ani- Salmo- Kpneu- K.oza- K. rhino- E.aer- E. clo co geltarda freun- diver- zona nella moniae enae sclero-Slsp tarda dii sus sp. sp. matis ogenes acae E.coli 374 - - - -Shigellasp. - 13. . . - - - - -E.tarda 2 - - - -C.freundii 1- - 20 - - - 1

C.diversus - - - - 13 - - -

-Arizonasp.. . . ..- - 2 -1 - -Salmonellasp. 1 - - - 21

K.pneumoniae - - - 191 - - - 1

K.ozaenae 1 - - - 4 -

-K.rhinoscleromatis - - - -- 2 -

-E.aerogenes - - - 28

-E.cloacae - - - - 1 - - - 47

H.alvei. . .. . .- -- - -E.agglomerans - - - 3 - - - - 2 - - 3

S.liquefaciens. . . -

-S.marcescens.... - - -

-P.

vulgaris....-

- -

-P.

mirabilis....-

- -

-M.

morganii....-

- -

-P.

rettgeri....-

- -

-P.

alcalifaciens....

- - -

-P.stuartii....- - -

-Y.

enterocolitica....

- -

-Y.pseudotuberculosis. .. . - -

-A.

calcoaceticus....

- -

-S.rubidaea - .-.-.---- -

-%Coffect 99 100 100 87 93 100 95 100 57 100 100 90 a See footnote to Table1.

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MS-2 BACTERIAL IDENTIFICATION SYSTEM 131 TABLE1-Continued

TABLE2-Continued

Reference identification

H E.ag- S. li- S.mar- P P.Mir- M. P P.P.al-P.im Y.en- Y.pseudo- A.cal- No

glom- quefa- ces- mor- reft-

califa-

teroco- tubercu- coace-

identifi-'erans ciens cens rganii geri ciens litica losis ticus cation

2 - - - _ _ _ _ _ _

1 41 - _ _ _ _ _

3 - - - 1 -

-- - 1 1 - - - _ _

8 - - - _ _ _ _

- 6 - - _ _ _ _ _ _

-- 7 4 - - - _ _ _ _ _ _

- - - 41 - - - _ _ _ _ _ _ _

----7 -- 2 - _ _

---81 - - -

-- - - 1 200 - - -

-_ _ _ _ - 8 - - -

-_ _ _ - _ - - - 6 - - - -

-- - - 1 - 2 - 19 - - -

-_ _ _ _ _ _ _ _ _ _ 5 _ _ _

-~ ~ _ _ _ _ - - 2 -

-_ _ _ _ _ _ - 8

-- 1 - - - _ _ _ _ _

62 75 88 89 100 98 100 80 100 100 100 67 100

-VOL.

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TABLE 3. Accuracyof the 24-h API 20E with968 isolatesa

APIidentification E.coli Shigella sp. E. tarda C.freundii C. diversus Arizona sp. Salmonellasp. K.pneumoniae K.ozaenae K.rhinoscleromatis E.aerogenes E. cloacae H.alvei E. agglomerans S. liquefaciens S.marcescens P.vulgaris P.mirabilis M.morganii P. rettgeri P.alcalifaciens P.stuartii Y.enterocolitica Y.pseudotuberculosis A. calcoaceticus Citrobacter amalonaticus Entericgroup19

Yersiniaruckeri Noidentification %Correct E. coli 371 1 4 99

aSee footnotetoTable1.

Shi-gella sp. 12 1 92 E. tarda 2 100 C. freun-dii 23 100 C. diver-sus 2 9 3 64 Referenceidentification Ari-zona sp. 1 1 50 22 100 1 1 3 2 96

to evaluate the system, McCracken et al. (8)

reported an overall identification accuracy of

94%. Itwasconcluded that the accuracy of the

MS-2systemwassimilartotheaccuracy

report-ed for other commercial identification systems

which are read visually after incubation for 4 h

orovernight. Asignificantpercentage (8.6%) of

the MS-2 responses, however, were equivocal

and therefore required additional testing. Five percent ofthe isolates were misidentified, and

another1% ofstrains couldnotbeidentified.

Continuedroutinelaboratoryuseofthe MS-2

system has allowed the recognition of certain

strains whicharemorelikelytobemisidentified

or to lead toequivocal responses. Some of the

identificationerrors wereduetoalack of

impor-tantbiotypesintheoriginal organism data base.

Inanefforttoimprove theaccuracyof the MS-2

andtoreduce thenumberofequivocal

respons-es, Abbott Laboratories significantly enlarged

itsorganism data base and developed new

soft-warefor the MS-2. The newer software is

cur-rently replacing the older software, which has

been in use since the BID system was first introduced.

The original MS-2 BID software identified

85.3% of the strainscorrectly. Thispercentageis

the same as the percentage reported in the

McCrackenetal. study (8)andprobably reflects

the similar distribution ofspecies included in the

twoinvestigations. The updated MS-2 software

increased the percentage of strains correctly

identified to94.4%. Onlyan additional 2.2% of

thestrainsrequired additional testingtoconfirm

the first-choice MS-2 identification. The

im-proved analytic software also significantly

re-duced thepercentageofincorrectidentifications

(8.2to3.4%). Ithas beensuggested that

equivo-cal responses be excluded when estimating the

accuracy of identificationtest systems, because

equivocal responses are neither accurate nor

inaccurate (1). If this had been done in the

presentstudy, then theaccuracyof theupdated

MS-2 BID system and ofthe API 20E would

3 4 43 0 2 0 28 100 51 1 98

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MS-2 BACTERIAL IDENTIFICATION SYSTEM 133

TABLE 3-Continued Referenceidentification

H. E.ag- S.lique- S. mar- P.vul-

P'

M. P. P.al- P.Stu- Y. en- Y.pseudo- A. cal- No

alvet glom-_erans facicns cescens gans mira- mor- rett-

calUfa-

arti teroco- tubercu- coace-

identi-bilis ganii geri ciens litica losis ticus fication

- - ~~~64 - - -

--- - ~~~139 - - -

--- - - ~~~~~-77 - - - - -- -

--- - - - ~~~~~520 - - -- -

-62 100 75-- 10 - 9 1 - 1 _

--- - 1 - - - ~~~~~-~~~18 -- -

-62 100 75 85 86 93 100100 100 95 100 100 100

-have been 95.6 and 95.5%, respectively. The

API 20E, however, had almost twice as many

equivocal

responses, and an additional 15

bio-types

(1.6%)

could notbe found in thecurrent

profile

index.

Theidentification accuracy wasincreased for

9of the 25 taxonomic groups studied with the

updated

MS-2

software.

Most of the species

found in these ninegroups are

commonly.recov-ered from

clinical

specimens.

Theidentification

accuracy decreased for only four taxonomic

groups when the new software was used. Inthe

caseofY.

pseudotuberculosis,

onlythreestrains

were included in the study. The identification

accuracy ofthe

remaining

12taxonomicgroups

wasunaffectedby the software update.

Misidentifications were still noted for some

species with the updated MS-2 software. Three

lactose-negative strains of C. freundiiwere

mis-identified as E.

agglomerans,

and three strains

ofH.alveiwereidentifiedasS. enteritidis dueto

negative citrate and malonate tests. The

inclu-sion of an

H2S

test in the MS-2 battery may

permit

the correct identification of these

orga-nisms. Four isolates

of

S. marcescens were

identified

asS.

liquefaciens

duetofalse-positive

arabinose reactions. Arabinose is a

relatively

unstable carbohydrate, and degradation of the

sugar can produce false reactions in most test

systems. In

rapid identification

systemssuchas

the MS-2 BID system,

false-negative reactions

are a

major

concern. The three strains ofE.

cloacae identifiedasE.

agglomerans

were

mis-identified due tofalse-negative ornithine

decar-boxylasereactions, and the two strains of

Pro-teus rettgeri identified as P. stuartii were

misidentified due to

false-negative

adonitol and

mannitol tests. The other

discrepancies

ob-served did not produce consistent error

pat-terns.

In

conclusion,

this collaborative

study

showed thatthe

updated

MS-2 software

signifi-cantly

improved

the identification accuracy of manyofthe

commonly

encountered

gram-nega-VOL.18,1983

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TABLE 4. Summaryof MS-2discrepancieswithupdatedMS-2BIDsoftware

Referenceidentification Speciesidentification Biochemical test

(no.tested) withMS-2 responsible

E.coli (376) C.freundii(1)

Lysine-S.enteritidis(1)

E.agglomerans (3)

C.freundii(23)

C.diversus (14) E.cloacae (1)

Indole-

Lactose-

Indole-Salmonella sp. (22)

K.ozaenae(7)

E.cloacae (52)

Arizonahinshawii (1)

E.agglomerans (2) S. rubidea (1)

E.agglomerans (3)

K.pneumoniae(1)

C.freundii(1)

E. coli (2) Salmonella sp. (3)

C.freundii(1)

S.rubidaea (1)

K.ozaenae(1)

Lactose'

Citrate-/malonate-Lactose' Adonitol+

Adonitol/ornithine-S.marcescens(46)

P. mirabilis(83)

P.rettgeri(8)

S. liquefaciens(4)

K. ozaenae(1)

M.morganii (1)

P. stuartii (1)

P. stuartiiurea+ (2)

Y.pseudotuberculosis (3) Salmonella sp. (1)

Urea-TABLE 5. Comparativeaccuracyof bacterial identification with 968 isolates

MS-2original MS-2updated API-20E2/82

Identification software software codebook

No. % of total No. % of total No. %of total CorrectID,highassurance 826 85.3 914 94.4 881 91.0 CorrectID,lowselectivitya 63 6.5 21 2.2 39 4.0

IncorrectID 79 8.2 33 3.4 33 3.4

Noidentification 0 0 0 0 15 1.6

aConfirmatory testing suggested.

tive bacilli. Theupdated MS-2 BIDsystem

pro-duced an overalllevel ofaccuracy comparable,

if not slightly superior, to the overnight API

20E, which is currently used in many clinical

laboratories. Ahigh level ofaccuracy, coupled

witha4-to5-hidentification, makes the newer

MS-2 BID system an attractive alternative for

routineuseinclinicalmicrobiology laboratories.

ACKNOWLEDGMENTS

We thank George Maierand Ted Krafczykfor their excel-lent technical assistance.

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Adonitol-Lactose'

Ornithine-Lysine+

Malonate-E. agglomerans (8)

S.liquefaciens (8)

Arabinose+ Arabinose+

Indole+ Indole+

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