Evaluation of the AutoMicrobic system for susceptibility testing of aminoglycosides and gram negative bacilli

0095-1137/87/030546-05$02.00/0

CopyrightC 1987, American Society forMicrobiology

Evaluation of the AutoMicrobic

System

for

Susceptibility Testing of

Aminoglycosides

and

Gram-Negative Bacilli

JANET A. HINDLER* ANDDAVIDA. BRUCKNER

ClinicalMicrobiology Section, Department ofPathology, University of California atLosAngelesMedicalCenter, Los Angeles, California 90024

Received 8 September 1986/Accepted 3 December 1986

The AutoMicrobic system (AMS; Vitek Systems, Inc., Hazelwood, Mo.) was compared with a reference

broth microdilution MIC methodtodetermine the accuracy and reproducibility ofaminoglycoside suscepti-bilitytestingofgram-negativebacilli. Stock clinical isolates(n= 176)which demonstratedresistanceto atleast oneaminoglycoside, extended-spectrum penicillin, orbroad-spectrum cephalosporin (oracombination)were

selected forthisstudy.Isolates with moderatesusceptibilitytotheaminoglycosideswerealso included. Ofthese isolates, 116wereeither resistantormoderately susceptibletoone or moreofamikacin, gentamicin,netilmicin, andtobramycin. When AMS MIC results for 704 antimicrobial agent-organismcombinationswerecompared withparallel microdilutionMIC results,exactagreement(AMSMIC= referenceMIC)rateswere:amikacin,

71.6%; gentamicin, 71.6%; netilmicin, 83.0%; and tobramycin, 69.3%. Agreement rates within ±+1 log2

dilution were: amikacin, 96.0%; gentamicin, 93.8%; netilmicin, 97.2%; and tobramycin, 96.0%. When National Committee for Clinical Laboratory Standards criteria wereused toqualitatively evaluate

perform-ance, the overall agreement rates were: amikacin, 100.0%; gentamicin, 99.4%; netilmicin, 98.9%; and tobramycin, 99.4%. There were only four very major discrepancies, which represented 0.6% of the tests performed, and therewere nomajordiscrepancies. Thepercentages of minordiscrepancies were: amikacin,

9.6%; gentamicin, 14.2%; netilmicin, 11.9%;andtobramycin, 10.8%.Oftheoverallaverageof 11.6% minor discrepancies, 9.7% occurredeventhoughthe AMS MICwaswithin ±1 log2dilution of thereferenceMIC.

The intralaboratory reproducibility ranged from 93.3 to 100% for the four drugs examined. With this challenge group of gram-negative bacilli, the AMS generated aminoglycoside MIC results that were

comparable tothose obtained byareference broth microdilution method. One of several features of the AutoMicrobic system

(AMS;Vitek Systems, Inc.,Hazelwood, Mo.)is antimicro-bialsusceptibility testing. Asingle Gram-Positive Suscepti-bilityCard and severalGram-NegativeSusceptibility (GNS) cardsare availablefortestinggroupB streptococci,groupD streptococci, and staphylococci;and aerobic and facultative anaerobic gram negative-bacilli, respectively. Each card

containsagrowthcontrol well andupto29 wellscontaining antimicrobial agents. The drug concentrations vary and

depend on the antimicrobial agent being tested. The differ-encebetween the various GNS cards relates to the antimi-crobialagentsincluded, withmostof thecardscontainingat least one of the aminoglycosides amikacin, gentamicin, netilmicin, or tobramycin. Results from this automated

susceptibility testsystemaregenerally available within6to

10 h. Inadditiontogeneratinglog2dilution MICresults, the

AMS affords the userthe flexibility of generating National CommitteeforClinicalLaboratory Standardscategory inter-pretations orinteger MICs.

In1984, Woolfrey et al. (7) reported that AMS MICs for Pseudomonasaeruginosaandaminoglycosides(gentamicin, tobramycin, and amikacin) were higher than those obtained with a reference microdilution method with cation-supplemented (calcium,5.5+ 0.5mg/dl,and magnesium,2.5

± 0.2 mg/dl) Mueller-Hinton broth (Difco Laboratories,

Detroit, Mich.). That study made useoftheGram-Negative General Susceptibility Urinary Card, which incorporated a broth containing a higher concentration of calcium and

magnesiumthanin thebroth used incurrentmodifications of theAMS antimicrobial susceptibility cards.

*Corresponding author.

Otherinvestigators (1, 4, 6) haverecentlynoted satisfac-tory performance of the AMS for the testing ofamikacin, gentamicin, and tobramycin with gram-negative bacilli. Nadler et al. reported that the AMS produced acceptable results for thetesting of amikacin, gentamicin, and tobramy-cin with members of the family Enterobacteriaceae;

how-ever, some problems were observed with amikacin and

gentamicin for P. aeruginosa (2). Of 25 P. aeruginosa isolates included in that study,5were resistantto amikacin and3 wereresistanttogentamicin bythereferencemethod, but these isolates were susceptible to these drugs when testedbytheAMS.

Some of the studies mentioned above made use of

now-obsolete versions of AMS software, and none of them included anevaluationoftheperformance of the AMS with

netilmicin.

Ourstudywasdonetoassessthereliability of the AMSas

compared with a reference microdilution MIC method for

determining aminoglycoside susceptibility in a select group ofgram-negative bacilli.

MATERIALS AND METHODS

Bacterial isolates. The 176 isolates were stock isolates obtainedfrompatientsatthe UniversityofCaliforniaatLos Angeles Medical Center. The isolates were selected based ontheirresistanceto atleastoneaminoglycoside,

extended-spectrumpenicillin, broad-spectrumcephalosporin, or com-bination thereof. Isolates demonstratingmoderate suscepti-bilitytoaminoglycosideswere alsoincluded.Oftheisolates

tested, 41 wereP. aeruginosa, 11 were Pseudomonas

mal-tophilia, and 10were Acinetobacteranitratus. Members of the Enterobacteriaceae tested included 2 Citrobacter 546

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TABLE 1. Aminoglycoside susceptibility of test isolates as determined bymicrodilution MIC testing

No.of

Susceptibility"

to:

Organism isolates Amikacin Gentamicin Netilmicin Tobramycin

Acinetobacteranitratus 10 5, 5, 0 0, 0, 10 1, 2, 7 4, 6, 0

Citrobacterdi'ersus 2 2, 0,0 1, 0, 1 2, 0, 0 1, 1,0

Citrobacterfreundii 18 18, 0,0 11, 2, 5 13, 2, 3 13, 1,4

Enterobacteraerogenes 7 7, 0, 0 7, 0, 0 7, 0, 0 7, 0, 0

Enterobacter cloacae 23 23, 0, 0 18,0,5 22,0, 1 18, 0, 5

Esc-herichiacoli 23 22, 1, 0 10, 4, 9 21, 0, 2 10, 4, 9

Klebsiella pneumoniae 15 15,0,0 2, 6,7 8, 4, 3 2, 4, 9

Morganella morganii 6 6, 0,0 4, 0, 2 6, 0, 0 4, 0, 2

Proteus mirabilis 4 3, 1,0 1, 1,2 3, 1, 0 1, 1, 2

Prov'idencia rettgeri 2 2,0,0 0,0, 2 1,0, 1 0, 0, 2

Providencia stuartii 3 3, 0,0 0, 1,2 1, 1, 1 0, 1, 2

Pseudomnonas aeruginosa 41 27, 8,6 11, 7, 23 5, 13, 23 24, 3, 14

Pseudomonas maltophilia il 2, 4, 5 0, 6, 5 1, 5, 5 2, 4, 5

Serratia marcescens il 10, 1,0 8, 0, 3 9, 0, 2 6, 1,4

aResultsareexpressed as numbers of isolatessusceptible. moderately susceptible, and resistant to eachdrug.

diversus, 18Citrobacterfreundii,7Enterobacteraerogenes,

23 Enterobacter cloacae, 23 Escherichia coli, 15Klebsiella pneumoniae, 6 Morganella morganii, 4 Proteus mirabilis, 2 Providencia rettgeri, 3Providencia stuartii, and 11Serratia marcescensisolates. No duplicate isolates ofagiven species

from the same patient weretested.

All isolates were stored at -70°C in brucella broth-15% glycerol and were subcultured on Trypticase soy agar

con-taining 5% sheep blood (BBL Microbiology Systems, Cockeysville, Md.) before testing. The AMS and reference MIC tests were performed simultaneously with isolated colonies fromthe same 18-to 24-h subculture plate.

Microdilution MIC testing. Reference (REF) MICs were

determined according to the procedure described by the National Committee for Clinical Laboratory Standards (3). MIC trays were prepared in-house with a Quick Spense

dispenser (Bellco Glass, Inc., Vineland, N.J.), dispensing 0.1-mlvolumesof eachantimicrobialagentdilutionperwell.

Mueller-Hinton broth (Difco) supplemented with 50 mg of calcium and 25 mg of magnesium per liter was used.

Anti-microbial reference standard powderswerekindly provided

byBristol Laboratories, Syracuse, N.Y. (amikacin), Scher-ing Corp., Kenilworth, N.J.(gentamicinandnetilmicin), and Eli Lilly & Co., Indianapolis, Ind. (tobramycin). Theranges

of log2 concentrations tested were 0.5 to 32.0 Fig/ml for amikacin and 0.5 to 16.0 ,utg/ml for netilmicin. Gentamicin andtobramycinweretestedat0.5, 1.0, 2.0, 4.0, 6.0, 8.0,and 10.0 ptg/ml; however,forcomparativepurposes,REF micro-dilutionresults of6.0,ug/mlwereconsideredat8.0 p.g/mland results of 10.0 ,ug/ml were considered at >8.0 ,ug/ml to correspond to the log2 dilution schema reported with the

AMS.

MICtrayswereinoculatedbythe direct inoculummethod

by suspending fresh colonies in Mueller-Hinton broth and adjusting the turbidity to match that of a McFarland no. 3

turbiditystandard. AnMIC 2000 inoculator (Dynatech Lab-oratories, Inc., Alexandria, Va.) was used, and the final

concentration oforganisms was5 x 105CFU/ml. After 16to 18 h of incubation at 35°C, MIC endpoints were read

manuallyas thelowestconcentration of antimicrobial agent that completely inhibited growth as determined visually. Extensive quality control was done at the time of tray preparation andwasdonethereafter by testingE.coli ATCC 25922 and P. aeruginosa ATCC 27853 on adaily basis.

AMS. The AMSGNS-C card wasused for testing

amika-cin, gentamiamika-cin, and tobramycin, andthe GNS-R card was usedfor netilmicin. Therangesoflog2dilution MICs(,ug/ml)

reported are: amikacin, s2.0 to >32.0; gentamicin and tobramycin, c0.5 to >8.0; and netilmicin, <4.0 to >16.0. Tests were done accordingtothe instructions of the

manu-facturer, and an AMS Gram Negative Identification Card was runsimultaneously withall GNS cards. Quality control

wasdone accordingtothe instructions of the manufacturer. MIC results and category interpretations were automati-callyprinted attheend of theincubationperiod,whichwas 6to 10 h formost isolates. Software versionR2.04 (Vitek)

wasused.

Evaluation of results. Each AMS log2 MIC result was

compared with the corresponding REF MIC result. For quantitative comparisons, exact agreement occurred when theAMS MICwasidenticaltothe REFMIC. AnAMSMIC within +1log2dilutionofthe REF MICwasconsideredtobe in agreement. An AMS MIC greater than ±+1 log2 dilution

from the REF MICwas considered to be in disagreement. Current National CommitteeforClinicalLaboratory Stan-dardscategoryinterpretationsof MICs(jig/ml)for

suscepti-TABLE 2. Comparisonof AMS and REF MICs forallisolates"

Antimicrobial No. of isolates withAMS/REFMIC ratios of: Exact % Overall %

agent <0.12 0.25 0.5 ib 2 -4 agreement" agreement"

Amikacin 7 20 126 23 71.6 96.0

Gentamicin 1 9 29 126 10 1 71.6 93.8

Netilmicin 2 2 14 146 il 1 83.0 97.2

Tobramycin 7 17 122 30 69.3 96.0

Atotalof 176 isolatesweretested, including41 P.

aeruginosa,

111

Enterobacteriaceae.

and 24additionalisolates. bAMSMIC identicaltoREFMIC.

AMS MIC/REFMICratio between 0.5 and 2(AMS MIC within ±1 10g2 dilution of the REF MIC).

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TABLE 3. Comparison of AMS and REF MICs for P. aeiruginosa"

Antimicrobial No. of isolates with AMS/REF MIC ratios of: Exact% Overall%

agent -0.12 0.25 0.5 1" 2

24

agreement" agreement"

Amikacin 8 23 10 56.0 100

Gentamicin 1 9 27 3 1 65.9 95.1

Netilmicin 2 10 24 5 58.5 95.1

Tobramycin 1 7 30 3 73.2 97.6

Atotalof41 P. aerugilnosaisolatesweretested. '>AMSMIC identicaltoREFMIC.

SeeTable 2,footnotec.

ble, moderately susceptible, and resistant results are,

re-spectively, -<16, 32,and >32 foramikacin, -<4, 8,and >8for gentamicin and tobramycin, and -4, 8 to 16, and >16 for netilmicin (3). By these criteria for qualitative comparisons, the AMS and REF methods were considered to be in

agreement when the category results were identical. Cate-gory results that differed were counted as discrepancies.

Very major discrepancies included AMS-susceptible and REF-resistant results, and major discrepancies included AMS-resistant and REF-susceptible results. Minor discrep-ancies included susceptible or resistant category results by

one method, with moderately susceptible results by the other.

Both AMS MIC and REFMIC tests were repeated when

disagreements occurred; however, the initial AMS MIC result was used for all comparisons. The initial REF MIC resultwasused when therepeatwaswithin +±1log1 dilution, and the organism was eliminated from thestudy if the REF

MIC resultdid not reproduce within +1 log2 dilution.

Reproducibility studies. To determine the reproducibility of eithertestsystem,30 isolates weretestedbyeach method on3 consecutivedays. The isolates examined wereselected

torepresent various species and variousdegrees of

suscep-tibility to all fouraminoglycosides. Two isolates (a mucoid P. aeruginosa and a clumpy S. marcescens) were tested becausethey showed peculiar growthcharacteristicsonagar media.

RESULTS

Thenumbersof isolates of eachspecies that were

suscep-tible, moderately suscepsuscep-tible, and resistant to amikacin, gentamicin, netilmicin, and tobramycin are indicated in Table 1.

A comparison of AMS MICs with REF MICs for all isolates tested is shown in Table 2. Results for the 41 P.

aeruginosa isolates only are shown in Table 3. When all

isolates were examined, the percentages oftests forwhich the AMS MIC was identical tothe REF MIC were: amika-cin, 71.6%; gentamiamika-cin, 71.6%; netilmiamika-cin, 83.0%;and tobra-mycin, 69.3%. For P. aceruginosa only, the percentages were: amikacin, 56.0%; gentamicin, 65.9%; netilmicin,

58.5%; and tobramycin, 73.2%. The overall agreement, in which theAMSMICwaswithin 1log2dilution of the REF

MIC forall isolates as well as for the subset of P.

aerugi-nosa, wasabove 93% for the fouraminoglycosides. Alisting of isolates that demonstrated AMS MICsthatdisagreedwith REF MICs is shown in Table 4. There were 30

disagree-ments, representing22 of the 176 isolates. Disagreement for 28 ofthe 30 occurrences involved a lower AMS MIC than REFMIC. Most of thedisagreements involvedgentamicin, and these weremostcommonwith K.pneumoniae. Fiveof the sevendisagreementsfor amikacin involvedalower AMS MIC than REFMIC forE. coli.

A comparison of AMS and REF system interpretive categoryresults is shown in Table5for all isolates testedand in Table 6 for P. aeruginosa only. There were no major discrepancy errors and 0.6% very major discrepancies

among the 704 antimicrobial agent-organism combinations examined. One E. coliand twoP. aeruginosa isolateswere involved in the four very major discrepancies which

per-sisted afterrepeattesting. Thediscrepancy with E. coliwas

with tobramycin. AMS MIC results from four repeat tests

were susceptible at 4, 2, 2, and 2 fig/ml. Repeat reference MIC results were >10, 10, 8, and >10,ug/ml. One of the P. aeruginosaisolates demonstrated AMSresults of 2,8, 4,and 8 p.g/ml with netilmicin upon repeat testing. Four repeat reference tests showed MICs of >16 kg/ml. The other P. aeruginosa isolatewas discrepant withgentamicin and

net-ilmicin. AMS results for gentamicin from four repeat tests

were susceptible at s4 1tg/ml, and reference results indi-cated resistance at -10 .tg/ml. All four netilmicin results TABLE 4. Isolates demonstrating AMS MICsthat disagreed with REFMICs"

Amikacin Gentamicin Netilmicin Tobramycin

Organism il

AMS-H' AMS-L' AMS-H AMS-L AMS-H AMS-L AMS-H AMS-L

Acinetobaccter 3 (10) 2 2 2

arniratus

Citirobacter

freuindii

2(18) 1 1

Escherichia coli 7(23) 5 2

Klebsiellapneuimoniae 6 (15) 6 2

Proteus mirabilis 1 (4) 1

Pseudoinonas 3 (41) 1 2 1

aeruginosa

AMSMIC greater than±11og2dilution from REF MIC (six isolatesdemonstrateddisagreement with more than one drug).

Numberofisolatesthat demonstrated disagreement. The total numberof isolates of respective species tested during thisevaluationisshown inparentheses.

<H.For eachdrug, number of isolates for which AMS MIC was higher than REF MIC.

"L,For eachdrug,numberof isolates for which AMS MIC was lowerthan REF MIC.

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TABLE 5. Comparison ofinterpretive category results forAMS and REFsystems for allisolatesa

No.(%)ofdiscrepancies No. (%)of isolates in: Antimicrobial

agentVery

~~~~~~~~~~Overaîl

agent Very Major Minor Agreement'

agreement

major areet

Amikacin 0 0 17(9.6) 159 (90.3) 176(100) Gentamicin 1(0.6) 0 25 (14.2) 150 (85.2) 175(99.4) Netilmicin 2 (1.1) 0 21 (11.9) 153 (86.9) 174(98.9) Tobramycin 1(0.6) 0 19(10.8) 156(88.6) 175(99.4)

aA total of 176 isolates were tested including 41 P. aeruginosa, 111

Enterobacteriaceae, and 24 additional isolates.

b Very major discrepancies include AMS-susceptible and REF-resistant results; major discrepancies include AMS-resistant and REF-susceptible results; minor discrepancies includemoderately susceptible results for one systemwith susceptibleorresistantresults for the other.

cCategory results identical.

dMinorerrorsconsidered inagreement.

with the AMS were c4,ug/ml,and those with thereference

testresults were -16

,ig/ml.

Overall, there were

11.6%

minorerrors: amikacin, 9.6%; gentamicin, 14.2%; netilmicin, 11.9%; and tobramycin,

10.8%. Ofthese errors, most (amikacin, 9.0%; gentamicin,

9.6%; netilmicin, 11.4%; and tobramycin, 8.5%) occurred eventhough the AMSMIC was within±1log2dilution of the REFMIC. For P. aeruginosa, allminorerrorswith amika-cin, gentamicin, and tobramycin involved 1 log2 dilution

difference between the AMS MIC and the REF MIC.

For

every minor error for netilmicin, the REF MIC was 8.0

,ug/mland the AMS MIC was c4.0 ,ug/ml.

All fouraminoglycosidesdemonstratedcomparable

repro-ducibility in either test system. For the 120 antimicrobial

agent-organism combinations(30isolates tested), 80% ofthe

AMS MICs and 84.2% of theREF MICs were identical on

the 3consecutive days.More than

97%

ofresultsreproduced within 1 log2 dilution for eithersystem.

DISCUSSION

TheAMS generallyprovides susceptibilityresultswithin6 to10h.This offerssignificant advantagesoverconventional susceptibilitytest systems,which require overnight incuba-tion.

It is well knownthat the results of antimicrobial

suscep-tibility testing of aminoglycosides and P. aeruginosa are

influenced by the test medium. An earlier study indicated

that the AMS didnot

perform

adequately as

compared

with areference system(7).Atthetime of

Woolfrey's study,

the

divalent cation concentration of the base medium in the

AMS

susceptibility

cards variedfromthat

generally

recom-mended. However, since then, Vitekhas modified the base

medium

and,

as shown in the present

study,

with the TABLE 6. Comparisonofinterpretive category results for AMS

andREFsystemsforP.aeruginosaa

No.(%)ofdiscrepancies No.(%)of isolates in:

Antimicrobial

agent Very Major Minor Agreement' agreement

major areet

Amikacin 0 0 7(17.1) 34(82.9) 41(100) Gentamicin 1(2.4) 0 7(17.1) 33(80.5) 40 (97.6) Netilmicin 2(4.9) 0 11(26.8) 28(68.3) 39(95.1) Tobramycin 0 0 3 (7.3) 38(92.7) 41(100)

aAtotalof 41 P.aeruginosaisolatesweretested.

b SeeTable5,footnoteb.

Categoryresultsidentical.

dMinorerrorsconsideredinagreement.

exception

oftwo

isolates,

results obtained with the amino-glycosides and P. aeruginosa are comparable to those

ob-tained witha reference method.

The isolates examined in the present study included iso-lates thatwere more resistant than those generally encoun-tered. Thesearerepresentative of problem isolatesthat may be associated with nosocomialinfections and oftenpresent

challenges

for in vitro

testing

and challenges for effective

therapy.

This

study

showed that MICresultsobtainedfrom

testing

the

aminoglycosides

with the AMSare

comparable

to

those obtained by use ofa reference method when

testing

such a select group of

organisms.

However, it should be noted that when

discrepancies

occur, the AMS MIC is

generally

lower than the REF MIC.

Category

interpretation

ofMICs demonstrated no

major

discrepancies

and a very low percentage of very

major

discrepancies.

Twoofthe threeisolates

showing

very

major

discrepancies

were P. aeruginosa. In addition to the 41 P.

aeruginosa isolates documented in this

study,

there were three isolates that did not grow in the AMS cards but did growin the REFMICsystem. Itisconceivablethat thisfalse

susceptibility

is due to

inadequate growth,

and a modifica-tion of the

growth

threshold forP. aeruginosa in the AMS

susceptibility

cards may eliminate such

problems.

For the 704 antimicrobial

agent-organism

combinations,

therewere11.6%minorerrors. Ithas been

suggested

thatfor

testing

of random clinical

isolates,

aminor

discrepancy

error percentage ofnogreaterthan 10%has beendesirable

(5).

In the present

study,

isolates were selected with diverse

sus-ceptibilities

to the four

aminoglycosides

evaluated inorder

to

challenge

the AMS with a

variety

of

susceptibility

endpoints.

This selection bias may have contributed to the

high

percentage of minor errors noted.

Upon

close

exami-nation,

of the 11.6% minor errors, 9.7% occurred even

though

the AMS MIC was within

±+1

log2

dilution of the

reference MIC. Since it is

generally agreed

that in many

situations in which

aminoglycosides might

be

administered,

MIC results offer

significantly

more useful

susceptibility

informationas

compared

with

qualitative

results,

theoverall clinical

significance

of this incidence of minor errors is

unknown.

Concerning

the current status of

aminoglycoside

testing

with the

AMS,

itshouldbe noted that the sectionoftheGNS

software

package

used in this

study

for

analysis

of

amikacin,

gentamicin,

netilmicin,

and

tobramycin

was introduced in

1984with software version P14XOA and has been included without modificationin all software versions to date.

Simi-larly,

the medium as describedherein for

testing

amikacin,

gentamicin,

netilmicin,

and

tobramycin

was

incorporated

intothe GNScards in 1983 and hasnotbeenmodifiedsince. Both theAMS and REF MIC resultswere

reproducible.

In

light

ofthese

findings,

the AMS

generated

aminoglycoside

MIC results that were

comparable

to those obtained

by

a

REFmicrodilutionmethod when

testing

a

challenge

groupof

gram-negative

bacilli.

LITERATURE CITED

1. Backes,B.A.,S.J.Cavalieri,J.T.Rudrik,and E. M. Britt.1984. Rapidantimicrobial

susceptibility

testingof

gram-negative

clini-cal isolates with the AutoMicrobic system. J. Clin. Microbiol. 19:744-747.

2. Nadler, H. L., C. Dolan, L. Mele, and S. R. Kurtz. 1985. Accuracyand

reproducibility

oftheAutoMicrobicsystem

Gram-Negative General

Susceptibility-Plus

Card for testing selected

challengeorganisms. J. Clin. Microbiol.22:355-360.

3. National Committee for Clinical Laboratory Standards. 1985. Methods for dilution antimicrobial

susceptibility

tests for

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bacteria that grow aerobically. Approved Standard M7-A. National Committee for Clinical Laboratory Standards, Vil-lanova, Pa.

4. Rçiber,N. E., M. T. Kelly, J. M.Latimer, D. L. Tison, and R. M. Hysmith. 1985. Comparison of the Cathra Repliscan II, the AutoMicrobic system Gram-Negative General Susceptibility-Plus Card, and the Micro-MediaSystem Fox panel for dilution susceptibility testing of gram-negativebacilli. J. Clin. Microbiol. 21:959-962.

5. Thornsberry, C. 1985. Automated procedures for antimicrobial susceptibilitytests, p. 1015-1018.In E. H. Lennette, A. Balows,

W. J. Hausler, Jr., and H. J. Shadomy (ed.), Manual ofclinical microbiology, 4th ed.American Society for Microbiology, Wash-ington, D.C.

6. Van Horn, K. G., A. M. Vandernoot, E. W. Burke, and J. C. McKitrick. 1984. Evaluation of the AutoMicrobic system Gram-Negative General Susceptibility-Plus Card. J. Clin. Microbiol. 20:630-635.

7. Woolfrey, B. F., R. T.Lally,M. N.Ederer, and C. O. Quall.1984. Evaluation of theAutoMicrobic system for susceptibilitytesting of Pseudomonas aeruginosa to gentamicin, tobramycin, and amikacin. J. Clin. Microbiol. 19:502-505.

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