JOURNAL OF CLINICAL MICROBIOLOGY, Mar. 1978, p. 265-2720095-1 137/78/0007-0265$02.00/0Copyright © 1978 American Society for Microbiology

Vol. 7, No. 3

Printed in U.S.A.

Electrical Impedance Measurements: Rapid Method forDetecting and Monitoring Microorganisms

P. CADY,* S. W. DUFOUR, J. SHAW, AND S. J. KRAEGERBactomatic, Inc., Palo Alto, California 94303

Received for publication 16 May 1977

A conceptually simple and easy-to-use technique is described that uses contin-uous impedance measurements for automated monitoring of microbial growthand metabolism. The method has been applied to a wide range of microorganisms.Optical clarity is not required. The sensitivity and reproducibility of the methodare demonstrated. The mechanism whereby microbial growth alters the imped-ance of the medium is discussed, as well as potential applications of the methodto clinical microbiology.

Impedimetric methods in microbiology havereceived increasing attention since the presen-tation of two impedance-monitoring systems atthe First International Symposium on RapidMethods and Automation in Microbiology inJune of 1973 (2, 11), and the description of athird system at the 75th Annual Meeting of TheAmerican Society for Microbiology, 1973 (13).

Since that time, impedance-monitoring instru-mentation has been used in blood (4-6) andurine cultures (3, 5).Although very sensitive and highly automated

impedance monitoring is a relatively new pro-ce lure, the concept dates back to the last cen-tury (9). It has long been known that the me-dium supporting the growth of microorganismschanges its chemical composition with time asnutrients are consumed and metabolic end prod-ucts are produced. These changes in mediumcomposition are associated with changes in theimpedance-the resistance to the flow of analternating current conducted through the me-dium. The use ofa reference chamber containinguninoculated medium allows one to compensatefor changes in impedance brought about byphysical and chemical factors not related tomicrobial growth such as fluctuations in temper-ature, absorbtion of gases, enzymatic and spon-taneous changes in chemical composition of themedium, etc.This paper describes some of the basic fea-

tures of microorganism impedance changesmonitored by a BACTOMETER 32 microbialmonitoring system (Bactomatic, Inc., Palo Alto,Calif.), a 32-channel automated impedancebridge designed especially for use with microbialcultures. The magnitude, shape, and reproduci-bility of microbial impedance responses for awide variety of microorganisms and media werestudied. Also discussed are the organism concen-

trations necessary to produce impedancechanges and the factors that influence the re-sponse. Finally, the potential for using imped-ance measurements to automate various micro-biological procedures of importance to the clin-ical microbiologist is reviewed.

MATERIALS AND METHODSInstrumentation. The impedance-measuring in-

strument (and accessories) used in all experiments wasthe BACTOMETER 32 microbial monitoring system(Bactomatic) (Fig. 1). This instrument has been de-scribed by Hadley and Senyk (5). It has two chambersfor each measurement, a sample chamber and a ref-erence chamber, both of which are filled with mediabut only one of which is inoculated with microorga-nisms. The instrument measures the ratio, Zi,/(Z1t +Zs), where Zs and Z,, are the impedances in the sampleand reference chambers, respectively. By measuringthis ratio, the instrument cancels out many factors,such as fluctuations in temperature, which affect theimpedance of both sample and reference chamberssimultaneously.A universal serial interface was constructed to en-

able data measured by the instrument to be processedor stored by a Data General Nova 1220 minicomputer(Data General, Southboro, Mass.) equipped with aBall Computer Products (Sunnyvale, Calif.) dual-diskdrive-memory capable of 5.0-Mbytes storage. This lat-ter feature greatly facilitated the accumulation andinterpretation of very large quantities of data. In ad-dition, the instrument was modfified so that the fre-quency of the applied sinusoidal signal could be variedto certain set values between 200 Hz and 30 kHz oncommand from the computer.

Electrodes. Unless otherwise noted, experimentswere performed at 2 kHz, using clusters of impedancechambers known as modules (Fig. 2). These containeda pair of vertical stainless-steel electrodes arising froma stainless-steel lead frame embedded in plastic andprojecting into each chamber. A module consists ofeight sample and eight reference chambers made ofstyrene acrylonitrile copolymer plastic. The modules

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266 CADY ET AL.

FIG. 1. Impedance-monitoring instrument (BAC-TOMETER 32 microbial monitoring system) andstrip chart recorder.

FIG. 2. Drawing of impedance sample chambers(module) with cut-away showing vertical stainless-steel electrodes arising from embedded stainless-steellead frame.

were sterilized by either ethylene oxide or by radiation.The chambers of the modules were sealed with tape.Impedance measurements. Sterile modules were

aseptically filled with 1.0 ml of sterile medium in thereference chamber and an equal amount of inoculatedmedium in the sample chamber. All chambers were

sealed, and the module was inserted into the connectorwithin the incubator portion of the instrument. Imped-ance ratios were automatically recorded every 96 s on

the strip chart recorder or sent in digital form to thecomputer by means of the interface.

Changes in impedance ratio have been converted tochanges in the impedance of the sample for all datapresented in this paper.

Detection of microbial growth. Impedancechanges due to microbial growth accelerate with timeand thus can be distinguished from noise and driftarising from nonmicrobial factors such as a fluctua-tions in temperature. The impedance change due tomicrobial growth is defined as an accelerating changein the impedance that is equal to or greater than 0.8%of the base line impedance when the measuring signalis at 2,000 Hz. This relatively large change in imped-ance was chosen to clearly distinguish microbial re-

sponses from noise and drift.Cultures and media. Escherichia coli and Pro-

teus vulgaris cultures were derived from AmericanType Culture Collection cultures 12015 and 6380, re-spectively. All other cultures were clinical isolates.Commercially dehydrated or concentrated media wereused and reconstituted according to the manufac-turer's directions. Brain heart infusion broth (BHI),Trypticase soy broth (TSB), Trypticase soy agar, andColumbia broth were obtained from Baltimore Biolog-ical Laboratory, Cockeysville, Md. Medium 199, withHanks balanced salts solution 10-times concentratedwithout NaHCO3 (Microbiological Associates) was re-constituted according to the manufacturer's directions.

Plate counts. Plate counts were taken from dupli-cate samples incubated in identical modules in thesame incubator from which no measurements werebeing recorded. The duplicate modules avoided thebrief disturbance in impedance measurements thatfollow sample taking and allowed more frequent andlarger portions of the sample to be taken. All platecounts were done in triplicate on Trypticase soy agarplates incubated at 35°C.

RESULTS AND DISCUSSION

(i) Spectrum of microorganisms causingimpedance change. The basis of impedancemonitoring is assumed to be changes in chemicalcomposition of the medium brought about bymetabolic processes occurring within the micro-bial cell, or its surface, or by exogenous microbialenzymes in the medium. These processes bringabout a corresponding change in the conductiv-ity of the medium and, to a smaller degree, achange in the capacitive reactance of the me-dium. In addition, there is a change in the com-position of the double layer of charged materialsadsorbed onto the electrodes that has a verystrong effect upon the surface capacitive react-ance of the electrode. Since impedance is a com-plex entity made up of a resistive or conductivecomponent and a reactive component, all ofthese processes could contribute to the imped-ance change measured during microbial growth(3, 8). Any organism that alters medium com-position by its growth or metabolism shouldproduce a corresponding change in the imped-ance measured by electrodes in the medium.To test this hypothesis, a wide range of micro-

organisms was tested for the ability to produceimpedance change when growing in a variety ofmedia. These organisms are shown in Table 1,where it can be seen that aerobic and anaerobicbacteria, yeasts, molds, and mycoplasma are rep-resented. To date, every microorganism thatshowed macroscopic evidence of growththroughout a broth culture has been shown toproduce an impedance change as well. Orga-nisms that produced microcolonies or grew onlyon the medium surface may have produced weakor undetectable impedance changes even though

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IMPEDANCE MONITORING OF MICROORGANISMS 267

TABLE 1. Microorganisms capable ofproducing animpedance change in a variety ofmedia

BacteriaAlcaligenes faecalisAcinetobacter cal-

coaceticusBacillus stearother-mophilus

B. subtilisBacteroides clostridi-

iformisB. fragilisB. melaninogenicusB. ochraceus

Citrobacter diversusC. freundiiClostridium bifermen-

tansC. perfringensC. sporogenes

C. tyrobutyricumEnterobacter aero-genes

E. cloacae

Escherichia coli

Eubacterium limosumFusobacterium nu-

cleatumF. symbiosumHaemophilus influ-enzae

Klebsiella ozaenaeK. pneumoniaeLactobacillus brevisL. caseiMycobacterium phleiPediococcus cerevi-

siaePeptococcus prevotiiPeptostreptococcus

anaerobiusProteus mirabilisP. vulgarisPseudomonas aerugi-nosa

Bacteria cont.P. diminutaSalmonella enteriti-

disS. typhimurium

Serratia marcescensShigella sonnei

S. flexneriS. dysenteriaeStaphylococcus au-

reusS. epidermidisStreptococcus faecalisS. pyogenes

Streptomyces griseusZymomonas anaero-

bia

Mycoplasma

Acholeplasma laid-lawii

Mycoplasma fermen-tans

M. hominisM. pneumoniae

M. pulmonisUreaplasma urealyti-cum

YeastsCandida albicansC. parapsilosisSaccharomyces cere-

visiaeTorulopsis glabrata

MoldAspergillus niger

macroscopic growth was evident. The list inTable 1 is not intended to be inclusive, butrather illustrative.

Typical impedance changes recorded for var-ious microorganisms are shown in Fig. 3, whichshows the percent decrease in impedance pro-duced by cultures of Klebsiella pneumoniae, E.coli, and Staphylococcus aureus growing inTSB.

(ii) Threshold of response. A certain con-centration of microorganisms, known as thethreshold concentration, was necessary before

there was a detectable impedance change. Con-centrations of organisms greater than thresholdproduced an immediate impedance change.However, low initial concentrations oforganismscould be detected by allowing them to replicateto threshold concentrations. The threshold con-centration is a function of the microorganism,the medium in which the organism was growing,and the electrodes used. Furthermore, thresh-olds were altered by the frequency of the signalbeing used-lower thresholds were observed atlower frequencies. Finally, the threshold is verymuch a function ofhow much impedance changemust take place to satisfy the definition of de-tection. For most practical purposes, we defineddetection as an accelerating change in imped-ance of 0.8%. This is a generous measure, readilyseen and unlikely to be caused by nonmicrobialcauses. By this definition, thresholds generallyrange between 106 and 107 organisms per ml.Some representative thresholds defined in thisway are shown for various organisms in Table 2.

(iii) Detection times. Microorganism detec-tion times, i.e., the times required for the orga-nisms to grow to threshold concentrations, area function of the initial concentration of micro-organisms, the generation time of the organismsor population of organisms, and the lag phase ina particular medium. For a given organism orpopulation in a given medium, detection timesincrease as the initial concentration becomessmaller. Detection times graphed against the logof the initial concentration generally fall along astraight line (Fig. 4) due to the exponentialgrowth of the organisms. This relationship be-tween detection time and initial concentrationenables one to classify samples according tobacterial levels (early detection times indicatinghigh bacterial concentrations and late detection

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500 r0 4 6

TIME (HOURS)FIG. 3. Representative impedance curves for three

common microorganisms. Percent inpedance de-crease is graphed against time in hours for K. pneu-moniae (solid curve), E. coli (long dashed curve), andS. aureus (short dashed curve).

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TABLE 2. Typical generation times and thresholds for a variety of microorganismsa

Organism Medium' Generation Approx. thresholdOrganism ~~~~~~~~~~~~~~time(minY~ (per MI)d

BacteriaEscherichia coli TSB 20 2 x 107E. coli (SS) BHI 20 2 x 107Proteus vulgaris TSB 20 2 x 107P. vulgaris (SS) BHI 25 4 x 107Salmonella enteritidis BHI 20 107Klebsiellapneumoniae (SS) BHI 25 4 x 107Pseudomonas aeruginosa BHI 25 5 x 106P. aeruginosa (SS) BHI 25 3 x 107Staphylococcus aureus BHI 35 4 x 106Streptococcus pyogenes Todd-Hewitt broth 60 4 x 106Lactobacillus brevis (320C) Universal beer broth 90 5 x 107Pediococcus cerevisiae (32°C) Universal beer broth 240 5 x 107Neisseria gonorrhoeae Thayer-Martin broth 45 2 x 107

FungiSaccharomyces cerevisiae (320C) GPYE broth 90 106Candida albicans (320C) TSB + 2 % glucose 45 4 x 106Aspergillus niger (320C) GPYE broth 70 107

MycoplasmaAcholeplasma laidlawii PPLO broth 210 107Ureaplasma urealyticum Urea medium 150 2 x 106a Unless otherwise indicated, all organisms were grown without agitation at 350C. All organisms were

monitored with gold-plated printed-circuit board electrodes except organisms noted with (SS) where verticalstainless-steel electrodes were used. This table is from A. N. Sharpe and P. S. Clarke (ed.), Mechanizingmicrobiology, 1978. Courtesy of Charles C Thomas, Publisher, Springfield, Ill. (3).

b GPYE, Glucose-peptone-yeast extract.c Determined by plate count.' Thresholds correspond to detection defined as 0.8% impedance change and have a standard deviation of

roughly 0.2 log when the same electrodes, organism strains, and media are used.

times signaling low concentrations), or to obtainrough estimates of the microorganisms' genera-

0~\\° tion time (by dividing the delay between detec-U. k \\tions of two different dilutions by the number of

times the population would have to double toincrease from the smaller to the larger initial

£wt;o \ concentration).(iv) Replicate agreement. Figure 5 shows

,, < Q the change in impedance with time when two9 oa \ \ concentrations (1 x 103 and 1 x lOr organismsU) 1 2 0 0 per ml) of E. coli are grown in TSB at 35°C. The

2 \ \ curve represents the mean impedance change ofzo\*\0 15 replicates of each dilution. The vertical bars

\ \0 represent + 1 standard deviation from the meanof the impedance change from base line at the

DETECTION TIME (HOURS) times indicated. The base line is establishedafter the first 0.5 h of measurement, since there

FIG. 4. Detection times in hours graphed against is considerable nonmicrobial impedance changeinitial concentration of microorganisms for E. coli onsiderale nonmicroduledce cher-(A) and S. aureus (0) both growing in TSB at 35°C. occurring initially as the module comes to ther-The solid lines represent least-squares linear fits to mal equilibration with the incubator and thethe data points for each organism. The slope of the electrodes equilibrate with the medium. Theline reflects the organisms' generation times (26 min arrows denote the mean detection times, and thefor E. coli and 38 min for S. aureus). The correlation horizontal bars represent 1 standard deviationbetween log initial concentration and detection time before and after the detection time. These stan-was 0.95 for E. coli and 0.94 for S. aureus. dard deviations (approximately 10% of the mean

268 CADY ET AL. J. CLIN. MICROBIOL.

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IMPEDANCE MONITORING OF MICROORGANISMS

10

105/ML 10 3/ML

0~~~~~~~~~~~~~W0

2 4 6 8

TIME (HouRs)

FIG. 5. Percent impedance decrease with time when E. coli in two concentrations (K10; and 105 organismsper ml) is grown in TSB at 35°C. The curve represents the mean of 15 replicates. The vertical bars represent1 standard deviation above and below the mean. Detection times, defined as the time when 0.8% acceleratingchange in impedance has takeen place, are depicted by the vertical arrows. The horizontal bars represent thedetection time standard deviation before and after the mean.

response and 0.3 h for the detection times) in-dicate the high degree to which the impedanceresponses from these two dilutions of E. coli canbe discriminated from each other and from thebase line.

(v) Relationship of impedance change tochange in microorganism concentration.Figure 6 shows a comparison of impedancechange with viable plate counts for E. coli grow-ing in medium 199 and in Columbia broth. Thesemedia were selected to represent a completelydefined although complex medium on the onehand, and a medium rich in peptones but ill-defined chemically on the other hand. For bothmedia, the cumulative impedance change (uppergraphs) increases roughly exponentially withabout the same doubling time as the viable count(lower graph). In fact, the correlation coeffi-cients between the log of the cumulative imped-ance change and the log of the viable countsranged between 0.96 and 0.99 for the four pairsof curves (two dilutions in two media) shown inFig. 6.Upon closer examination, however, it can be

seen that there are some significant differencesbetween the viable count and impedance curves.In the Columbia broth, for example, the cumu-lative impedance change shows a small plateau,

flattening out for about an hour when the viablecounts reach 108 colony-forming units per ml,and then resuming their previous rate of in-crease. There is no evidence of such a plateau inthe viable count curves.

It should also be noted that the impedancestops changing before the microorganisms reachstationary phase. This, together with data fromother organism/medium combinations in whichthe impedance continues to change long afterthe organisms are in stationary phase (5), servesto remind one that it is most probable that it ismetabolic activity rather than cell numbers thatis being recorded by impedance changes. To theextent that the rate of microbial metabolismparallels microbial growth, impedance changemay serve to estimate growth rates. However,this relationship must be established for eachorganism, medium, and type of electrode underconsideration.

Finally, observe that in Fig. 6 the onset ofimpedance change coincides with concentrationsof microorganisms well below the threshold lev-els (corresponding to 0.8% impedance change).The practice of defining detection in terms of asubstantial (0.8%) change reduces the chance ofconfusing drift and noise with a microbial re-sponse at the expense of a higher threshold.

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270 CADY ET AL.

uLJLV)

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108, 10 :O 5U..

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TIME (HOURS)FIG. 6. Comparison in two media, Columbia broth

(COL) and medium 199 (M 199), of impedance de-crease with time and microorganism growth withtime. The upper two graphs depict the percent imped-ance change with time in hours. The lower two graphsdepict the number of organisms determined by platecount (0) using the same time axis. E. coli was grownat two initial concentrations. The higher is shown bya solid line. The lower by a dashed line. Both percentimpedance change and the number of organisms are

graphed along logarithmic axes of the same scale toallow comparison of the slopes.

(vi) Characteristic impedance responses.There is some evidence that the shape of theimpedance response may be, to some degree,characteristic of the organism, medium, andelectrodes used to produce it. For example, Fig.7 shows impedance responses from two strainseach of E. coli, K. pneumoniae, P. vulgaris, andPseudomonas aeruginosa, all growing at 35°Cin BHI.There are clearly some features common to

all these organisms and some features whichsuggest that there may be characteristic profilesfor each species (especially P. aeruginosa) as

has been suggested for microcalorimetric ther-mograms (1, 7). More data is needed with agreater number of strains of each species toconfirm this point.

(vii) Mechanisms of impedance change.Which chemical events taking place during mi-crobial growth are responsible for the impedancechange measured in these experiments? Changesin pH, although a frequent concomitant of mi-crobial growth, cannot be evoked, since strongimpedance decreases are noted with acid pro-ducers such as E. coli growing in TSB andammonia producers such as P. mirabilis growingin urea broth. In the former case, the pH isdecreasing, in the latter, it is increasing. Yet theimpedance decreases in both cases. Changes inthe conductivity of the medium may be a morelikely contributor to impedance change, how-ever. Conductivity changes have been frequentlyreported to accompany microbial growth (3, 5),since uncharged nutrients become converted bymetabolic processes into charged end products.Changes in conductivity are changes in the re-sistive component of impedance. The relation-ship is given by:

Z2 = R2 + X 2

where R is the resistance and Xc is the capacitivereactance; and Xc = 1/(2 7, fC), where f is thefrequency and C denotes the capacitance. Bymeasuring the impedance at different frequen-cies, the relative contributions of the capacitivereactance and resistance can be obtained. Totest the hypothesis that it is the conductance (orresistance) which changes in the medium, E.coli was grown in TSB at 35°C and the fre-quency of the applied signal varied from 400 Hzto 30 kHz. The impedance response at thesefrequencies for a single culture is shown in Fig.8. There is a considerable increase in the imped-ance change at lower frequencies and a corre-sponding reduction in threshold as well. Fur-thermore, these data show that although thereare measurable changes in conductivity, it isprimarily the reactive component which is al-tered during microbial growth. These data areconfirmed as well by variable cell-length meas-urements (as discussed by Schwan [8]). We spec-ulate that the observed change in reactance re-sults from the charged double layer on the sur-face of the electrodes which is altered duringmicrobial growth. The exact mechanism remainsto be elucidated.

(viii) Applications. Impedance measure-ments provide a convenient way of monitoringmicroorganism growth. In addition, they providea method for the automation of tests of sterility,such as blood and spinal fluid cultures, and for

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IMPEDANCE MONITORING OF MICROORGANISMS

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P.vugais_ndP._LP.AUorganis K. PNEUMONIAEZ 4

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FIG. 7. Rate of impedance decrease graphed against time for two strains each of E. coli, K. pneumoniae,P. vulgaris, and P. aeruginosa. All organisms were grown at 350C in BHI and monitored with stainless-steelelectrodes and a fr-equency of 2,000~Hz.

40

400 Hz

2 KHz

30 KHz

0 5 10 15TIME (HOURS)

FIG. 8. Percent impedance decrease graphed against time for E. coli growing in TSB at 350C andmonitored at frequencies of 400, 2,000, and 30,000 Hz.

rapid screening for high bacterial concentra- ance measurements lend themselves readily totions, such as in urine cultures or food-quality automation; many samples can be processed atassurance. By the use of multiple samples, rapid one time without mechanical movement of sam-automated antibiotic susceptibility testing and ple holder or sensor. Optical clarity is not abacterial identification may be possible. Imped- requirement, and thus opaque samples are mon-

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272 CADY ET AL. J. CLIN. MICROBIOL.

itored as easily as optically clear samples. Fi-nally, there is little or no sample preparationrequired, and the equipment is simple to use.

LITERATURE CITED

1. Boling, E. A., G. C. Blanchard, and W. J. Russell.1973. Bacterial identification by microcalorimetry. Na-ture (London) 241:472-473.

2. Cady, P. 1975. Rapid automated bacterial identificationby impedance measurement, p. 73-99. In C. G. Hedenand T. Illeni (ed.), New approaches to the identificationof microorganisms. John Wiley and Sons, Inc., NewYork.

3. Cady, P. 1978. Progress in impedance measurements inmicrobiology, p. 199-239. In A. N. Sharpe and P. S.Clarke (ed.), Mechanizing microbiology. Charles CThomas, Publisher, Springfield, Ill.

4. Hadley, W. K., W. Kazinka, and G. Senyk. 1975. Earlydetection of microbial growth in blood cultures by elec-trical impedance measurements. International Confer-ence on Mechanized Microbiology, Ottawa.

5. Hadley, W. K., and G. Senyk. 1975. Early detection ofmicrobial metabolism and growth by measurement ofelectrical impedance, p. 12-21. In D. Schlessinger (ed.),Microbiology-1975. American Society for Microbiology,Washington, D.C.

6. Kagan, R. L., W. H. Schuette, C. H. Zierdt, and J. D.

MacLowry. 1977. Rapid automated diagnosis of bac-teremia by impedance detection. J. Clin. Microbiol.5:51-57.

7. Russell, W. J., J. F. Zettler, G. C. Blanchard, and E.A. Boling. 1975. Bacterial identification by microcalor-imetry, p. 101-121. In C. G. Heden and T. Illeni (ed.),New approaches to the identification of microorga-nisms. John Wiley and Sons, Inc., New York.

8. Schwan, H. P. 1963. Determination of biological imped-ances, p. 323-407. In W. L. Nastuk (ed.), Physical tech-niques in biological research, vol. 6. Academic Press,New York.

9. Stewart, G. N. 1899. The changes produced by the growthof bacteria in the molecular concentration and electricalconductivity of culture media. J. Exp. Med. 4:235-243.

10. Ur, A., and D. F. J. Brown. 1974. Rapid detection ofbacterial activity using impedance measurements.Biomed. Eng. 9:18-20.

11. Ur, A., and D. F. J. Brown. 1975. Monitoring of bacterialactivity by impedance measurements, p. 61-72. In C. G.Heden and T. Illeni (ed.), New approaches to the iden-tification of microorganisms. John Wiley and Sons, Inc.,New York.

12. Ur, A., and D. F. J. Brown. 1975. Impedance monitoringof bacterial activity. J. Med. Microbiol. 8:19-28.

13. Wheeler, T. G., and M. C. Goldschmidt. 1975. Deter-mination of bacterial cell concentrations by electricalmeasurements. J. Clin. Microbiol. 1:25-29.

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