evaluation of electronic colony counters

Evaluation of Electronic Colony CountersAuthor(s): M. G. Fleming and F. O'ConnorSource: Irish Journal of Agricultural Research, Vol. 14, No. 1 (Apr., 1975), pp. 21-26Published by: TEAGASC-Agriculture and Food Development AuthorityStable URL: http://www.jstor.org/stable/25555750 .

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Ir. /. agric. Res. 14: 21-26, 1975

EVALUATION OF ELECTRONIC COLONY COUNTERS

M. G. Fleming and F. O'Connor

An Foras Taluntais, Dairy Microbiology Department, Moorepark Research Centre, Fermoy, Co. Cork

ABSTRACT

Two electronic colony counters (Foss Biomatic and Fisher Automatic) were evaluated for precision and accuracy in counting colonies on petri dishes prepared from raw milk samples. The results indicate that the instruments are suitable for this purpose and as such are important labour-saving

devices in the routine bacteriological analyses of milk.

INTRODUCTION

A bacterial colony count is a reliable indicator of the bacteriological quality of refrig erated farm milk supplies. However, the application of this method to large-scale routine testing is impractical without automation. Hence, the recent development of automated procedures for milk sample plating (1, 2, 3, 4, 5) and electronic colony counting (6, 7, 8) is not surprising. Malligo (6) reported on the evaluation of a colony counter, the technology of which was described by Mansberg (9), Mansberg, Yama

gami and Berkley (10) and Alexander and Glick (11). Working with pure cultures he found that the precision of the instrument was good, with a stable bias when compared to manual counting. He also concluded that counting time was reduced by 40 % using the instrument.

Goss, Michaud and McGrath (7) found that an automated counter could readily detect surface and sub-surface colonies of 0.3-mm size or greater, with a high degree of precision and with a uniform bias when compared to manual count. Packard and Ginn (8) compared colony counts on raw milk in the range 30 to 300 obtained by electronic and manual counting. Results indicated that the electronic count was sub

ject to greater variability than manual count and the accuracy was good except in

plates with low counts.

A number of instruments similar to those described above are now available. The

21



22 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 14, NO. 1, 1974

study reported here assessed the suitability of two such instruments for routine colony counting on plastic petri dishes prepared from farm milk supplies.

Description of instruments

Foss Biomatic 169001: This instrument consists of three units. These include a meas

uring unit consisting of camera, power supply and electronic circuits, a monitor to

provide a visual display of the colonies on the petri dish, and a printer incorporating a dilution factor. A white marker displayed on the monitor beside each colony counted

by the instrument enables the operator to control the accuracy of each count. The

instrument is designed with a resolution of 0.2 mm in horizontal and vertical direc

tions. A size discriminator enables the operator to select the particle (colony) threshold size (range 0.2 to 2.8 mm). The total colony area on the petri dish can also be de

termined.

Fisher Automatic Bacterial Colony Counter2: This counter consists of the three basic units of the Biomatic. The resolution is similar to that of the Biomatic but accessory lenses are available for counting particles of less than 0.2 mm. This instrument also

possesses a top as well as a bottom illumination system for counting opaque particles on a non- or semi-transparent medium. A binary coded decimal output for interfacing

with computer, teletype or printout system is also fitted.

EXPERIMENTAL

Before use, certain routine operations were carried out on both instruments. These

included selection of the appropriate correction-factor setting to compensate for the area of the petri dish not scanned by the instrument, sharp lens focussing, correct iris

diaphragm aperture setting and correct threshold sensitivity setting. Both instruments were evaluated for precision and accuracy in determining the

colony counts on petri dishes prepared from raw milk samples and incubated at 30?C for 3 days. The precision of each instrument was determined by making duplicate counts on each petri dish, the second count being made after rotation of the dish

through 90? approximately. Accuracy was determined by counting the number of colonies in the range 30 to 500 on a large number of petri dishes, manually and on each instrument. A small percentage of dishes containing tiny colonies of size less than the resolution of the instrument or containing spreading colonies or moulds could not be counted accurately and was not included in the study.

1 A/S N. Foss Electric, 39, Roskildevej, DK-3400 Hiller0d, Denmark

2 Fisher Scientific Co., 711 Forbes Avenue, Pittsburg, Pennsylvania 15219, U.S.A.



FLEMING AND O'CONNOR: COLONY COUNTERS 23

RESULTS

The precision of both instruments and manual count by operator is expressed (Table 1) as standard deviation of differences between duplicate counts (all counts transformed

logio^.

The precision of both instruments was better than that of an experienced operator counting the dishes manually. Similarly, the precision of the Fisher Automatic Counter

exceeded that of the Foss Biomatic. The lower precision of the Foss Biomatic was due to a sensitivity differential across the scanning field. This resulted in failure by the

instrument to detect very small colonies (near the limit of resolution) in a particular area of the field. Hence, the precision of the instrument depended on the distribution of the smaller colonies in the petri dish.

The accuracy of both electronic counters is expressed as standard deviation of

differences between the colony counts obtained by the instrument and those obtained

by manual count. The results of these experiments are shown in Table 2 (all counts transformed log10).

The agreement of both instruments with the standard plate count was satisfactory.

However, the accuracy of the Fisher Automatic was better than that of the Foss

Biomatic even over a wider range of colony counts (30 to 500 compared with 30 to 300 for the Foss Biomatic). The colony count obtained by each instrument was higher (Foss 1.6%, Fisher 5%) than that obtained by manual counting. These differences between the means could be attributed to one or both of the following factors: correc tion factor setting and colony distribution on petri dishes. The graduation of the

TABLE 1: Precision of colony counting method

sd of differences between Method n duplicate counts

Manual 369 ?0.035 Foss Biomatic 229 ?0.024 Fisher Automatic 140 ?0.016

TABLE 2: Accuracy of electronic colony counters

Instrument n sd X Y Y-X

goss Biomatic 229 ?0.072 1.9389 1.9322 -0.007 Fisher Automatic 140 ?0.053 2.0562 2.0350 -0.021

m standard deviation of differences between instrument and manual counts X m mean of instrument counts * mean of manual counts




3.0

"o i-i 0) 0

M c 3

f 2.5 . / ? *?

* >r;

2.0- '?/*

: s/f

^ x i ?

,#? * s

/Y Y = 1.069 X - 0.140 (?0.069)

^/ r = 0.97

m/ ,_,_,_fc 1.0 1.5 2.0 2.5 3.0

Automated count (log10)

Fig. 1: Relationship of automated (Foss) to manual count

correction-factor setting control is not continuous and the distribution of colonies over the plate surface may be irregular. However, the value of the differences was

acceptable and could possibly be further reduced by recalibration and greater care in plate preparation.

The relationship of the count obtained by the Foss Biomatic to that obtained

manually is expressed by the formula Y = 1.069JT - 0.140 (?0.069) with r = 0.97

(Fig. 1). Similarly, the relationship of the Fisher Automatic count to the manual count is Y = 1.023X - 0.069 (?0.052) with r = 0.99 (Fig. 2). Hence, the best fit line ex



FLEMING AND O'CONNOR: COLONY COUNTERS 25

3.0 *

*^ _#

i ** o .. *7*

? 2'5 ' * _5r

2.0 *

'^

>/ '/' 15- ^

^ // Y= 1.023 X- 0.069 (?0.052)

'X r = 0.99

i.oLel^--? _ 1.0 1.5 2.0 2.5 3.0

Automated count ( log10J

Fig. 2: Relationship of automated (Fisher) to manual count

pressing the relationship of instrument to manual counts is very close to the line of ideal relationship (broken line) for both instruments and the bias is not significant

within the count ranges used in evaluation.

With experience, an operator having prepared the instruments for use can count

at least 200 plates per hour, manually recording results, and up to double this number if printout facilities are available.




CONCLUSION

Both instruments are precise and accurate in determining the bacterial colony counts on plastic petri dishes containing transparent media and with clearly defined colonies of diameter greater than 0.2 mm and which are separated from each other by at least 0.2 mm. The application of these instruments to colony counts on dishes with opaque

media or on dishes containing debris particles in the medium was not examined. The

durability or stability of the instruments during prolonged use was not determined.

ACKNOWLEDGMENTS

We thank Mr. Liam Bermingham and Mr. Sean Donnellan for skilled laboratory assistance; and the equipment manufacturers for their keen interest and help during the study.

REFERENCES

1. Sharpe, A. N., Biggs, D. R. and Oliver, R. J? Appl. Microbiol. 24: 70, 1972. 2. Bradshaw, T. G., Francis, D. W. and Read, R. B. Jr., J. Dairy Sc. 54: 755, 1971. 3. Gilchrist, J. F., Campbell, J. E., Donnelly, C. B? Peeler, J. T. and Delaney, J. M., Appl.

Microbiol. 25: 244, 1973. 4. Posthumus, G., Klijn, C. J. and Giesen, Th. J. J., Neth. Milk Dairy J. 28: 79, 1974. 5. Jaartsmeld, F. H. and Swinkels, R., Neth. Milk Dairy J. 28: 93, 1974. 6. Malligo, J. E., Appl. Microbiol. 13: 931, 1965. 7. Goss, W. A., Michaud, R. N. and McGrath, M. B., Appl. Microbiol. 27: 264, 1974. 8. Packard, V. S. and Ginn, R. E., J. Milk Fd Technol. 37: 435, 1974. 9. Mansberg, H. P., Science, N.Y. 126: 823, 1957.

10. Mansberg, H. P., Yamagami, Y. and Berkley, C, Electronics 30: 142, 1957. 11. Alexander, N. E. and Glick, K. P., I.R.E. Trans, med. Electron. 12: 89, 1958.

Received December 30, 1974



evaluation of electronic colony counters

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