lnternattonal Journal ofFoodMtcroblology, 7 (1988) 31-40 31 Elsevier

JFM00219

A medium for the detection of yeasts using a conductimetric method

Peter Connolly, Stephanie J. Lewis and Janet E.L. Corry Food Science Dlt,tston. Ministry of Agriculture, Ftsherles and Food, London, S I4:1P 3RD. U K

(Received 4 March 1988, accepted 12 April 1988)

A new medium is described for the detection of yeasts usmg a Malthus nucrobml growth analyser whtch measures conductl~aty Thas medmm ~s superior to prewous medm dewsed for use m the Malthus, and allows the detecuon of a wxde range of yeasts including those ~mportant m food spotlage The new medium was inferior to exxstmg medm used to detect yeasts m the Bactometer 32 instrument which measures impedance

Key words ConductLmetrac method, Yeasts, Impedance

Introduction

Although it was dxscovered nearly a century ago that the growth of rmcrobes will b n n g about changes m their med ium winch can be electrically momtored (Stewart, 1899), ~t is only w~thln the last decade that tins proper ty has been exploited commercmlly as a rapid method for the detect ion and enumera t ion of rmcro- organisms - mostly m food microbiology.

Two makes of ins t rument are current ly commercmlly avadable. The Bactometer measures Impedance (Bactometer 32) or impedance, conduc tance or capaci tance (Bactometer 123) winle the Mal thus microbml growth analyser measures conduc- tance only. The ins t ruments were first used as rapid methods for es t imat ing vmble counts of micro-orgamsms, since the t ime taken to give an electrical response correlated w~th the init ial numbers of rmcro-organisms inoculated. Esumates of viable counts could be ob ta ined m a few hours rather than the day or more reqmred for convent ional plate counts. Various medm were found suitable for the ins t ru- ments. Brain heart infusion was found to be satisfactory for both the Bactometer and Malthus ins t ruments , e.g. for counts m fish using the Bactometer (van Spree- kens and Stekelenburg, 1986) and for meat in the Mal thus (Bialte and Reuter, 1984) and Bactometer (Fl rs tenberg-Eden, 1983). Modif ied plate count agar was developed

Correspondence address J E L Cort2¢, Food Science Division, Mlmstry of Agriculture, Fisheries and Food, 65 Romney Street, London SW1P 3RD, U K

0168-1605/88/$03 50 ,Yd 1988 Elsevier Science Pubhshers B.V (Btomedlcal Division)

32

for counts in milk in the Bactometer (Flrstenberg-Eden and Tricarlco, 1983). Other media such as nument broth and seawater broth for counts in fish (Gibson et al., 1984), tryptone soya broth, MRS (De Man. Rogosa and Sharpe medium: De Man et al., 1960) and lactobacilh broth for counts in rmlk (Malthus Instruments), were suitable for the Malthus Later, selective media were developed for enumerating particular groups of bacteria Coliform media were developed for use in the Bactometer (Flrstenberg-Eden and Klein, 1983) and Malthus instruments (M20 and Malthus Instruments cohform medium). A salmonella isolation medium was devised for rise in the Malthus and was also found suitable for Bactometer instruments (Easter and Gibson, 1985; Gibson, 1988). Since the precise mechanism by which electronic methods of measuring microbial growth have not been elucidated, new media have, with few exceptions (e g., Owens et al., 1985), been developed on an empirical basis. However best results are obtained using media where microbial activity significantly changes the electrical properties of the medium. Bacteria tend to increase medium conductance whereas fungi tend to reduce conductance, increas- ing impedance. Fungi are generally slower growing than bacteria.

A number of media have been found to be satisfactory for detecting fungi in the Bactolneter, e.g, malt extract glucose (Shapton and Cooper, 1984) and )'east carbon base ammonium sulphate (Zlnduhs, 1984) No media have been found to be completely satisfactory in the Malthus. We found that a wort broth manufactured by Malthus instruments was adequate for Saccharomvces cereL,tstae but gave poor results for other strains of yeasts of more importance with respect to food spoilage (unpublished results)

This paper describes a medium for use with the Malthus instrument in which a wide variety of yeasts can grow and produce a significant and reproducible change in conductance.

Materials and Methods

Yeasts

Yeasts initially tested against all media were Hansenula subpelhculosa CBS 115 (Dr J A. Barnett, University of East Angha, Norwich, U.K.), Ptchta burtonu NCYC 439, Rhodotorula rubra NCYC 63, Saccharomvces cerevtslae NCYC 87, Schlzo~ac- charomyces pombe NCYC 132 (National Collection of Yeast Cultures, Norwich, U.K.) and Hansenula fablanu CBS 5640 (Centraal Bureau yon Sch~mmelcultures, Delft, Holland). Additional yeasts tested in CBAT (carbon base ammonium tartrate: see media section below) are listed in Table IV.

Preparation of yeast mocula

Yeast lnocula were prepared by incubating the strains at 30 °C for 48 h in 9-ml volumes of the medium to be tested in the growth analyser. Cell concentrations (between 10" and 2 × l0 s cells per ml) were determined microscopically using

33

haemocytometer slides and then adjusted to 10 v cells per ml by dilution with quarter strength Rangers and peptone (Oxold) Cell concentrations were then adjusted to 10 6 cells per ml m the different test media

Medta

Media used were: (i) Malt extract glucose (MEG" Shapton and Cooper, 1984)" malt extract 20 g / l ;

glucose 20 g/ l : pH 5.5, autoclave at 121°C for 15 mm. (li) Malthus wort medium (MWB: purchased from Malthus Instruments): 33.4

g / l dehydrated medium; glycerol 2.35 ml/ l : adjust pH to 5.0 with citric acid, autoclave at 115°C for 10 nun.

(16) Modtfied wort broth (WB" recommended by Malthus Instruments)' malt extract 15.8 g/1, fructose 7.8 g/ l : glucose 7.8 g/1, glycerol 2.4 ml / l : sodmm chloride 2.5 g/ l : ammonium chloride 1.0 g / l , dlpotassium hydrogen phosphate 1 0 g / l , pH 6.5; autoclave at 115°C for 10 nun.

(iv) Eleven )'east carbon base (CB) medta Based on carbon base ammonium sulphate (Zmdulis, 1984). These contained 22 g/1 yeast carbon base (Dlfco) and 3 g/1 of one of the 11 following different nitrogen sources: D-alamne (CBA); ammonium acetate (CBAA); ammonmm chloride (CBAC): ammonmm sulphate (CBAS): ammonium tartrate (CBAT); ammonium sodium hydrogen orphophosphate (CBAH): dibasic ammonium phosphate (CBAP): ammonium carbonate (CBACA); special peptone (CBSP), tnammonmm citrate (CBTAC) and urea (CBU). CBTAC was prepared as either 3 g/1 tr iammonmm citrate (CBTAC.3) or 1 g / l tnammomum citrate (CBTAC.1). The pH of each medium was adjusted to 5.5 using 0.1 M HC1 and autoclaved at 115°C for 15 nun.

Growth analysis

Yeast growth was monitored using a Malthus microbial growth analyser AT 128 (Malthus Instruments, Stoke-on-Trent, U.K.) and a Bactometer 32 (Bactomanc Ltd., Henley-on-Thames, Oxfordshire, U.K.).

(0 Measurement parameters Detectton Ttme (Td): the time at which rmcroblal metabohsm leads to a slgmfi-

cant change in electncal signal. Significance ~s determined by visual inspection w~th the Bactometer 32 and by preset detection parameters when using the Malthus AT 128.

Peak Response Rate (PRR): the maximum rate of change of the electrical signal. Peak Response Ttme (PRT): the time at which the PRR occurs.

(u) Growth studtes m the Malthus A T 128 Triplicate 1.6 ml volumes of the test media were dispensed into sterile 2 ml

Malthus cells and 0.4 ml of the appropriate yeast suspension was added to give an mmal cell concentration of 2 × 105.

34

The change in conductance was measured and recorded by the instrument at 30 °C for 48 h with the exception of Debaromyces hanseml NCYC 767 (65 h).

(m) Growth studies it1 the Bactometer 32 Duplicate 0.4 ml volumes of test media were dtspensed into sterile Bactometer

growth modules and 0.1 ml of the appropriate yeast suspension was added in order to give an imtml cell concentration of 2 × 105 cells per ml. Reference wells were filled with 0.5 ml portions of umnoculated sterile test media. Impedance changes were measured and recorded by the instrument at 30 o C for 48 h. The rate of change of ~mpedance ~s proportional to the number of flips m a g~ven time as displayed on the Bactometer chart recorder (Adak et al., 1987).

Results

lmtml experiments (data not shown) using the Malthus AT 128 and MEG, MWB, WB and CBAS demonstrated that CBAS was the only medmm where all six yeasts test stratus (listed m Materials and Methods) grew and gave s~gnificant and reproduoble conductance changes. S. pombe d~d not grow in Malthus wort (MWB).

Further modifications of carbon base media were therefore investigated. Table I shows the maximum change in conductance achieved as a result of the growth of the

T A B L E 1

Maximum negatwe conductance value (rmcros~emens) for test yeasts (mean and range of 3 deterrmna- tton,,) using the Malthus A T 128 instrument

Medium a H t t P R Sacch S fablamt subpelhc- burtoml rubra cerec'tstae pombe

ulosa

C B A 1.8_+ 0 8

C B A A 138 1 _+ 10 5

C B A C 1 3 9 _ + 1 0 5 ~

C B A S 7 0 0 _ + 5 3 . 2 ~

C B A T 323 5 + 22 9

C B A H 44 2_+ 7

C B A P 91 7 + 14 9

C B A C A

C B S P 160_+ 3 3

C B T A C 3 1 7 6 8 + 1 7 9

C B T A C 1 1 0 2 2 - + 1 0 6 C B U 0 9 - + 0 6 b

07_+ 0 3 n d n d n d 892_+ 6 4 n d 1478_+ 9 5 a 3114_+ 1 5 8 9 7 _ + 7 2 0 8 2 3 + 1 7 . 9 ~ 13.1-+ 6 3 ~ 3 1 9 - + 2 5 8 c

2 5 8 + 3 0 ~ 2 6 6 + 1 4 4 c 1 0 1 _ + 3 1 0 c 307_+ 8 7

2 0 3 4 _ + 1 0 6 "~ 1 9 2 8 - + 7 0 "~ 2 6 3 7 _ + 1 4 2 " 3831_+ 1 9

463__+215 n d 9 6 7 + 1 4 8 "~ 238_+ 1 2 0

7 8 5 2 7 7 ~ n d 5 1 4 - + 1 6 a 1 2 6 1 - + 1 0 6 2

N o growth 2 5 5 - + 1 3 2 2 6 6 ± 3 2 n d 4 8 6 - + 4 4

1075_+ 8 1 " n d 1 6 7 3 _ + 2 9 3 "~ 2 7 3 4 + 7.5

7 4 0 _ + 1 0 2 a n d 5 4 5 - + 7 2 a 1330_+ 1 9

141_+ 2 9 a 22_+ I 1 ~ n d 4 2 8 - + 2 7

37_+ 0.8

2239_+ 5 2

287_+ 6 0

52 1 +_41 5 ~ 2676___ 4 2

4 8 5 - + 7 7 ~

95 2 - + 4 2 1

218_+ 11 1907_+ 2 2

934_+ 1 8 21_+ 0 5 bt

n d not done a Yeast stdl producing considerable negative conductance change after 48 h

~' Large positive change ~ Change m polarity observed (negative to p o s t u r e ) J See Mate r i a l s and Methods for key to media

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TABLE II

Peak response rate negauve rmcroslemens/h, mean and range (h) of 3 deterrmnauons using Malthus AT

128 instrument

Media b H fabtanu Sacch ceretnstae S pombe

CBAA 1 3 3 ± 3 0 1 0 5 5 ± 1 4 9 a 1 3 3 ± 0 5

CBAT 2 7 . 4 ± 3 4 5 7 3 ± 275 1 8 2 ± 1 7

CBAP 9 6 ± 2 9 4 2 6 ± 2 6 8 a 8 8 ± 2 0

CBTAC.3 1 2 3 ± 1 9 6 4 8 ± 2 5 8 a 1 4 8 ± 1 0

CBTAC1 6 3 ± 0 . 9 2 6 6 ± 4 4 ~ 8 8 ± 1 7

a Electncal no~se due to CO 2 evoluuon b See Materials and Methods for key to medm.

test organisms m the 11 carbon base media. The six yeasts failed to grow an CBACA and grew only poorly m CBU. The faster growing yeasts, H fablanlt, Sacch, cerevts~ae and S. pombe in general gave larger maxamum conductance changes than the slower growmg yeasts, R. rubra, P. burtonu and H. subpelhculosa.

Table II shows the peak response rate (PRR) for the faster growing yeasts dunng the exponential phase of growth when grown in those medm considered to gwe significant conductance changes. Tables I and II show that CBAT produced both the largest rate of conductance change and the largest overall conductance changes.

TABLE III

Comparison of CBAT (carbon base ammomum tartrate) and MEG (malt extract glucose) m the Bactometer 32

Yeast Medtum

CBAT MEG

T a PRT PRR Ta PRT PRR

H fabtanu run 1 16 6 29 2 1 6 9 3 15 1 5 3 run 2 16 3 28 3 1 9 11 2 17 2 4 8

H subpelhculosa run 1 19 6 32 1 0 8 13 7 18 9 4 0 run 2 22.3 32 9 0 7 14 4 18 3 4 5

P burtonu run 1 18 5 34 2 1 4 11 3 19 2 3 0

r u n 2 188 341 1 l 119 193 28

Sacch ceremstae run 1 12 5 19 5 6 0 7 6 11 1 8 0

run 2 13 9 200 5 5 8 5 12 l 7 5

S pombe run 1 28 6 37 9 2 5 24 3 31 9 4 0 run 2 31 0 41 7 2 8 22 3 28 9 4 8

T a Detection time (h) PRT Peak response ume (h) PRR Peak response rate (flxps per hour) CBAT carbon base ammomum tartrate MEG malt extract glucose

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TABLE IV

Yeasts stud~ed using carbon base ammonmm tartrate (CBAT) m the Malthus AT 128, mean detectmn nme (h) and range of 3 deterrmnatmns

Yeast

CandMa albtcans WH C albwans NCYC 1363 C boldmu NCYC 1513 C bomblcota CBS 6009 C gmllermondt NCYC 973 C kefvr NCYC 744 C kruset NCYC 1398 C mv~oderma (lab strata) C parapsdosls NCYC 926 C rugosa NCYC 1560 C troplcahs NCYC 1393 C tropt~ahs (lab strain) C l,ahda NCYC 178 Debaromyce~ hansenu CBS 767 Hansemaspora nodmtgru NCYC 1382 Hansenula anomala CBS 5759 H anomala NCYC 682 H anomala NCYC 1509 H canadensts NCYC 497 H fabmnn CBS 5640 H peter~onu CBS 5555 H pohmorpha NCYC 1457 H subpelhculo~a CBS 115 H subpelhculo~a (lab strata) KIm'cerom~ces dobhansku NCYC 538 K mar~:tanus v lac NCYC 752 K marxtanus v mar,rt NCYC 1429 K marxtanus v mar~:t NCYC 587 K polysporus NCYC 523 K thermotolerans CBS 6340 K thermotolerans NCYC 701 Lodderomvces elongtsporus CBS 2605 Metschmkowta lunata CBS 5946 M pulcherrma NCYC 166 Ptchta ambrostae CBS 6003 P burtonn NCYC 439 P burtonu NCYC 1588 P ~apsuhzta CBS 1993 P carsontt NCYC 724 P ~ellobtosa NCYC 1381 P dtspora NCYC 1532 P membranefactens NCYC 938 P on)~hrts NCYC 1394 P sttpms NCYC 1540 Rhodosportdtum dtobot,atum NCYC 778 Rhodotorula rubra NCYC 63 R rubra NCYC 1645 R rubra NCYC 1647 R rubra NCYC 1650 R glutmls NCYC 377

7 7 ± 0 4 15.1±24 2 0 7 ± 0 5 3 0 2 ± 3 5 2 4 0 ± 0 6 6 0 ± 0 0 4 8 ± 0 3

1 9 7 ± 0 1 4 6 2 ± 0 2 2 8 5 ~ 1 9

5 8 ± 0 3 6 4 ± 0 2

183±0.3 6 2 1 ± 5 4

7 6 ± 0 2 1 1 7 ± 0 4 1 2 9 ± 0 8 9 9 ± 0 8 6 0 ± 0 4

1 2 0 ± 0 9 1 6 9 ± 0 9 2 4 9 ± 1 3 1 7 2 ± 0 2 1 6 9 ± 0 9 9 7 ± 0 4 7 7 ± 0 2 3 8 ± 0 2 7 1 ± 0 2

1 7 3 ± 0 7 7 1 Z 0 2 7 2 ± 0 4

2 0 1 ± 0 8 nogrowth 1 3 1 ± 0 2 no growth 2 5 2 ± 0 6 1 8 0 ± 0 9 nogro~th 1 7 7 ± 0 1 2 2 6 ± 1 4

8 4 ± 1 2 1 7 9 ± 0 7 1 0 5 ± 0 7 1 0 3 ± 0 2 nogrowth 3 7 5 ± 0 9 2 3 5 ± 1 1 nogro~th nogrowth 2 1 1 ± 0 8

TABLE IV (continued)

37

Yeast

Saccharomyees cereotstae NCYC 70 Sacch cerevtslae NCYC 74 Sac6h cerevlstae NCYC 87 Sacch ceret,tstae NCYC 89 Sacch cerevlstae NCYC 177 Sacch ceret'tstae NCYC 240 Sacch ceret,~stae NCYC 244 Sacch ceret,tsme NCYC 344 Sacch cerevzstae NCYC 625 Sacch ceree, tstae NCYC 694 Sacch cereolstae NCYC 853 Sacch cereetsme NCYC 975 Sacch ceret, zstae NCYC 1026 Sacch ceret,tstae NCYC 1324 Sacch cerevtslae NCYC 1593 Sacch datriensts CBS 421 Sac~h Sacch

datrlensls NCYC 1474 exlguus NCYC 1476

Sacch kluvt, erJ NCYC 543 Saccharomycodes ludw~gn CBS 821 Schzzosaccharomyces pombe NCYC 132 S pombe NCYC 380 S pombe NCYC 535 S pombe NCYC 936 S pombe NCYC 1354 S pornbe NCYC 1355 S pombe (lab strata) Schwanmomyces occldentahs (lab strata) Torulosporus delbruektt NCYC 566 T delbruekn NCYC 766 T pretorlensls NCYC 524 Trwhosporon betgeht (lab strata) Wtlhopsls saturnus NCYC 23 Yarrowta hpolytlca NCYC 825 Zvgosaccharornyces batht NCYC 1427 Z batht NCYC 580 Z btsporus NCYC 1515 Z cldrl NCYC 1567 Z rouxu NCYC 1522

4 .7±02 6 3 ± 0 5 4 9 ± 0 6 6 6 ± 0 2 7 1 ± 0 4 6 8 ± 0 4 7 6 ± 0 0 4 9 ± 0 2 4 6 ± 0 2 43±0 .1 7 .4±03 4 3 ± 0 2 5 9 ± 0 4 69±0 .0

1 7 5 ± 0 2 1 2 8 ± 0 4 1 5 4 ± 0 8 nogrowth

5 3 ± 0 2 8 7 ± 0 2

107±0.5 1 0 9 ± 0 5 1 1 7 ± 0 5

9 9 ± 0 2 109±0.5 10.3±02 1 6 8 ± 0 0 1 2 4 ± 0 4

8 8 ± 0 4 8 9 ~ 0 2 9 4 ± 0 2

2 4 1 ± 0 7 1 0 7 ± 0 8 2 4 1 ± 0 3 1 4 7 ± 0 3 1 1 8 ± 0 2 1 7 8 ± 0 7 1 4 4 ± 0 2 2 3 5 ± 0 1

WH Westnunster Hospital CBS Central Bureau von Schammelcultures NCYC National Collectton of Yeast cultures

Fig. 1 c o m p a r e s t h e c o n d u c t a n c e r e s p o n s e s o b t a i n e d fo r g r o w t h o f S a c c h

cereots tae i n C B A T a n d M a l t h u s wor t . I t c a n b e s e e n t h a t in C B A T t h e d e t e c t i o n

t i m e a n d p e a k r e s p o n s e t i m e a re r e d u c e d , w h i l s t p e a k r e s p o n s e r a t e ( P R R ) a n d

o v e r a l l c o n d u c t a n c e c h a n g e a re c o n s i d e r a b l y i n c r e a s e d .

38

2~ 200

c

,~) - 4 0 0

--t

'x 16 0. . - - , . ._~ 32 0 48 0 I]

(aj

TiPne (h i

I 20 I

16-0 32C, 4 8 0

\ ~"- 200

2 I ~ - 3 0 0 / .~ | (b) " ~ - ------,I

L I

T)r'ne ( hi

F ig 1 C h a n g e o f c o n d u c t a n c e vs t i m e for Saccharomvces cerevtstae N C Y C 87 tn (a) M a l t h u s w o r t a n d

(b) CBAT (carbon base ammonmm tartrate) in the Malthus AT 128

Table III indicates that CBAT is inferior to MEG when impedance changes are monitored on the Bactometer 32. MEG was formulated for use in the Bactometer and when compared to CBAT in this instrument, it gives faster detections and peak response times and higher peak response rates (quantitative results are difficult to determine in the Bactometer 32). Table IV indicates that the majority (83) of the 90 yeasts tested could be detected using the Malthus instrument's standard detection parameters. The seven yeasts that were not detected produced little or no growth during the 48 h of the test. All yeasts which produced significant growth also produced significant conductance changes so that they could be detected by adjusting the detection parameters. Results indicate that the largest conductance changes and the shortest detection times occur with the faster growing yeasts such as Saccharomvces, Candlda and Kluyveromyces spp. (Tables I, II and IV). Fig. 2 shows that the important food spoilage yeasts such as Zygosaccharomyces balm and Sacch ludwlgu give large reproducible conductance changes in CBAT.

D i s c u s s i o n

In view of early results, the Wort media and MEG were abandoned and the carbon base media were further investigated. When choosing nitrogen sources to

39

( a )

O, ,.,-,".

- 2 0 0

o _ 4 0 0

\

I 20 i ~ I ,

,{

-I00~- •

-2oo c

° I "C

o (b)

] i

32 0

%-_

I'I'P 4 8 0

T,me(h)

32 0 , ~ l , I J__

48 0

Tmne ( h )

Ftg 2. Change of conductance vs time for (a) Zygosaccharomyces balht NCYC 1427 and (b) Sac- charomycotdes ludwzgtt CBS 821 m CBAT (carbon base arnmonmm tartrate) m the Malthus AT 128

add to carbon base media, mtrites and nitrates were omitted because not all of the test yeasts metabolise them (Barnett et al., 1983). The poor results obtained in CBACA and CBU were probably due to ammonium carbonate and urea decompos- ing dunng autoclaving.

Carbon dioxide production by Sacch. cerevtsiae in some media was a problem, reducing effective electrode area and resulting in distorted conductance changes m the majority of carbon based media tested (see Table II). No ewdence of gas interference was seen with either CBAT or Malthus wort.

Impedance comprises a combination of capacitance and conductance. It was previously beheved that yeasts affected the capacitance component more than the conductance signal (Firstenberg-Eden and Zindulis, 1984; Zmdulis, 1984), the latter being unrehable as a means of detection of fungal growth. In the present study modification of CBAS medium by replacing ammonium sulphate w~th a m m o m u m tartrate (to give CBAT) resulted m an increased conductance change during growth, such that conductance can now be used as a means of monitoring growth of a wide variety of yeasts in the Malthus. Conversely our results with CBAT in the ~mped- ance measuring Bactometer 32 (where capacitance forms the majonty of the electncal signal), suggest that CBAT is poorly suited to this instrument when

40

compared to MEG, even though the similar CBAS medium was originally designed for use m Bactometer instruments. It may be, however, that CBAT would perform better in a newer model of Bactometer, m the conductance mode

The results indicate that the anions m the medium rather than the nitrogen source affect the performance of yeast carbon base media when used to monitor conductance changes. The reason why tartrate gives the best conductance changes is not clear CBAT is a prolruslng medium for enumerating yeasts in the Malthus system, in particular for the faster growmg yeats.

The results indicate that of the media used m this study, CBAT is overall the most effectwe yeast medium for the Malthus instrument However as w~th the bacteria, there may be scope for the development of media for the detection of specific groups of yeasts. Further work needs to be done to assess CBAT for the detection of pure cultures of moulds as well as spoilage moulds and yeasts from naturally contaminated substrates. It might be necessary to add ant~bloncs or other substances to intub~t bacteria in samples with rmxed populations.

References

Adak G , Corr , , J E L and Moss, M O (1987) Use of lmpedtmetr~ to detect trtchothecene m'motoxans l Screen for susceplble micro-organisms Int J Food Mlcroblol 5, 1-13

Barnett. J A , Payne. R W and Yarrow. D (1983) Yeasts charactenshcs and tdennficatlon Cambridge Umverslty Press

Bulte, M and Reuter. G (1984) Impedance measurement as a rapid method for the deterrmnanon ot microbial contarmnation of meat surfaces using two different instruments lnt J Food Mtcroblol 1. 113-125

De Man, J C , Rogosa, M and Sharpe, M E (1960) A medium for the cultl,,atlon of lactobacdh J Appl Bactenol 23, 130-135

Easter, M.C and Gibson, D M (1985) Rapid and automated detection of Salrnonella by electrical measurements J Hyg Camb 94. 245-262

Flrstenberg-Eden. R (1983) Rapid estimation of numbers of rmcro-orgamsms m raw n-ulk by impedance measurements Food Technol. 37, 64-70

Flrstenberg-Eden. R and Klein, C.S (1983) Evaluation of a rapid lmpe&metnc procedure for the quant :a t lve estimation of cohforms. J Food Scl 48, 1307-1311

Flrstenberg-Eden. R and Tncanco, M K (1983) Impedlmetnc determination of total, mesoph*llc and psychotrophlc counts m raw rmlk, J Food Scl 48. 1750-1754

Ftrstenberg-Eden, R and Zmduhs , J (1984) Electrical changes in media due to nncroblal grov, th J Mlcrobiol Methods 2, 103-115

Gibson, D M , Ogden, I D and Hobbs. G (1984) Estimation of the bacterial quality of fish by automated conductance measurements lnt J Food Mlcroblol 1 127-134

Gibson. D M (1988) Some modifications to the media for rapid automated detection of Salmonellas by conductance measurements J Appl Bactenol In press

Owens. J D., Mlskm, D R , Wacker-Viveros, M C. and Benge, L.C (1985) Sources of conductance changes d u n n g bacterial reduction of mmethylarmne oxide to tnmethylamlne m phosphate buffer J Gen Mlcroblol 131, 1357-1361

Shapton. N and Cooper, P J (1984) Rapid deterrnlnatlon of yeast numbers by impedance measurements using a Bactometer 32 J Soc Food Technol 37, 60-62

Stewart. G N (1899) The changes produced by the growth of bacteria in the molecular concentration and electrical conductwlty of growth media J Exp Med 4, 235-244

van Spreekens, K J A and Stekelenberg, F K (1986) Rapid estimation of the bacterial quality of fresh fish b) impedance methods Appl Microblol Blotechnol 24. 95-96

Zinduhs, J (1984) A medium for the lmpedimetrac detection of yeasts in foods Food MlcroNol 1. 159-167