Ecology of Listeria monocytogenes in the environment of raw poultrymeat and raw pork meat processing plants

Elise Chasseignaux a, Pascale Ge¤rault b, Marie-The¤re'se Toquin a, Gilles Salvat a,Pierre Colin a, Gwennola Ermel a;�

a AFSSA Ploufragan, Research Unit of Hygiene and Quality of Poultry and Pork Products, Zoopole Beaucemaine, BP 53, 22 440 Ploufragan, Franceb AFSSA Ploufragan, Research Unit of Swine Epidemiology and Quality Assurance, Zoopole Les Croix, BP 53, 22 440 Ploufragan, France

Received 10 September 2001; received in revised form 15 March 2002; accepted 19 March 2002

First published online 25 April 2002

Abstract

The zoonotic Listeria monocytogenes is mainly transmitted to humans by the food-borne route. This bacterium was often found in theenvironment of food processing plants. Therefore the aims of this study were (i) the identification of environmental factors associated withL. monocytogenes contamination on working and non-working surfaces in poultry or pork processing plants and (ii) the understanding ofits survival in such environments. The physicochemical risk profiles showed that a surface in resin or plastic, rather than uneven, withorganic residues, with a neutral pH, a low temperature and a high hygrometry was associated with L. monocytogenescontamination. 2 2002 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.

Keywords: Ecology; Poultry; Pork; Processing plant environment; Listeria monocytogenes

1. Introduction

Listeria monocytogenes has been described as one ofthe major human food-borne pathogens. Listeriosis canoccur as a sporadic disease or as an outbreak and is oftenrelated to the consumption of contaminated food. InFrance, during the last 10 years, di¡erent outbreakswere associated with either delicatessen (pork tonguein jelly in 1992 [1], pork ‘rillettes’ in 1993 [2] and 1999^2000 [3]) or soft cheeses (‘Brie de Meaux’ in 1995 [4],‘Pont l’Eve“que’ and ‘Livarot’ in 1997, ‘Epoisses’ in 1999[3]). Di¡erent studies on L. monocytogenes prevalenceshowed that 16% of raw pork meat and 17% of rawpoultry meat were contaminated [5]. The plant environ-ment can also be contaminated: about 8% of samples inpoultry slaughterhouses [6], 26% of samples in raw poul-try meat plants [7] and 68% of samples in raw pork meatplants [8].

Therefore, the understanding of the survival ofL. monocytogenes isolates is essential to prevent conta-mination in food plant environments. Actually, despitesome studies on the tracing of L. monocytogenes inprocessing plants [9^12], to our knowledge the associatedenvironmental risk factors of this microorganism arenot completely analysed. Generally, ecological and phys-iological data came from laboratory experiments:L. monocytogenes could generally grow from 1 to 45‡C[13], even if some strains can develop at 0.5‡C [14] or at30.2‡C [15]. L. monocytogenes isolates could grow frompH 5.0 to 9.6; however, the optimal pH is neutral toslightly alkaline [16]. All these assays were realised witharti¢cial media, therefore some nutriments or inhibitorysubstances found in natural environments could be miss-ing.The aim of this study was to identify the environ-

mental risk criteria associated with L. monocytogenes col-onisation on working and non-working surfaces in ¢veprocessing plants: two of raw poultry (A and B) and threeof raw pork meat (C, D and E). These surfaces wereconsidered as the main source of meat contamination.The link between risk factors and physicochemical char-acteristics of the surfaces was studied by statistical anal-ysis.

0378-1097 / 02 / $22.00 2 2002 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.PII: S 0 3 7 8 - 1 0 9 7 ( 0 2 ) 0 0 6 3 7 - 7

* Corresponding author. Tel. : +33(2)96 016 287;Fax: +33(2)96 016 283.

E-mail address: g.ermel@ploufragan.afssa.fr (G. Ermel).

FEMSLE 10457 23-5-02

FEMS Microbiology Letters 210 (2002) 271^275

www.fems-microbiology.org

2. Materials and methods

2.1. Plants studied

Two poultry (A and B) and three pork (C, D and E)meat processing plants were studied. Plants A, B, C and Dare in the North West of France whereas plant E is in theSouth East. The plants were divided into three di¡erentareas: reception of raw materials, meat processing andproduct processing. The working rooms were studied dur-ing the processing or after the cleaning operations at dif-ferent times: 1 year (plant A), over 4 months (plant C),2 months (plants B and D) and only one visit (plant E).

2.2. Swabbing

The sampling surfaces were grouped into two classes:(i) the environment for the surfaces without any contactwith the raw meat (£oor, wall, sewer) and (ii) the equip-ment for the surfaces in direct contact with the raw meat(working table, transport belt, knives, etc.). The swabbingwas delimited with a sterile stainless steel frame (a surfaceof 162.5 cm2) using a tissue swab moistened with 5 mltryptone salt, 10% (v/v) Isobio0 (Laboratoire LCB,Lugny, France). Isobio0 is used to neutralise cleaningproducts and disinfectants.For each swabbing, di¡erent data were collected con-

cerning the surface: composition (plastic, resin, stainlesssteel, metal, painted metal, tiling, cement or painted ce-ment), cleanness (clean, presence of organic residues, pres-ence of dust, presence of organic grease or encrustation),visual status (smooth, granular, stripped or damaged),moisture (dry surface, moisture or presence of water), tem-perature and pH of the surface. Surface temperature wasmeasured using a thermocouple thermometer and the sur-face pH was determined using pH indicator paper. Roomtemperature and hygrometry were also collected using adigital hygrometer.

2.3. L. monocytogenes detection procedure

The tissue swab was resuspended in 90 ml half Fraserbroth (bioMe¤rieux, Marcy l’Etoile, France) and incubatedat 30‡C. After 24 h, each sample was streaked on Palcamagar (bioMe¤rieux, Marcy l’Etoile, France) and incubatedfor 48 h at 37‡C. The Vidas test (bioMe¤rieux, Marcyl’Etoile, France) was then performed with typical colonieson Palcam agar plates. The plates were soaked with 3 mltryptone salt broth and scraped. The obtained bacterialsuspension was used to perform the Vidas test accordingto the manufacturer’s recommendations and to our ownvalidated protocol [17].

2.4. Statistical analysis

For each plant, an identical data analysis was per-

formed. The dependent binary variable LIST was coded1 if the swab was positive for L. monocytogenes ; otherwiseit was coded 0. Eleven independent environmental andphysicochemical variables were listed. They were dividedinto two groups. The ¢rst group was composed of thevariables for the room conditions (room, sampling surface,activity or cleaning operation) and the second group cor-responded to the physicochemical and environmental con-ditions (composition, cleanness, visual state, moisture,temperature and pH of the surface, temperature and hy-grometry of the room).Frequencies were calculated for all the qualitative vari-

ables, and minima and maxima were noted. Histogramswere drawn for all the quantitative variables and themean, standard deviation, median and quartiles were cal-culated. Each quantitative variable was split into separateclasses depending on our biological and microbiologicalknowledge. The class numbers were not under 10% ofthe swabs. Relationships between LIST and each of theother variables were then assessed in two-by-two tablesand tested with the chi-square method. During this ¢rstscreening, variables statistically associated with LIST(P9 0.15) were retained. Tests between independent vari-ables were also performed so that redundant variableswere eliminated.Multiple correspondence analyses (MCA) were then

performed using the software SPAD-N. The data of group2 retained after the chi-square tests were active whereasthe variables of group 1 retained after the second phasewere illustrative. Successive MCA were performed and thevariables with the highest inertia on the ¢rst axes wereretained. Then the proximity between LIST+, LIST3and the modalities of the other variables were studied.

3. Results and discussion

3.1. Detection of L. monocytogenes

The examination of the 497 samples, of which 263 wererealised during activity and 234 after the cleaning opera-tions, showed that 23.7% of the samples were contaminat-ed by L. monocytogenes. Table 1 indicates the contamina-tion in the di¡erent plants either in the environment or onthe equipment of the di¡erent studied rooms.During processing, 38% of the samples contained

L. monocytogenes, 38.9% in the poultry processing plantsand 37% in the pork processing plants. This contamina-tion was higher than that observed by Lawrence et al. [7]in a raw poultry meat processing plant (26%) but lowerthan that noticed by Salvat et al. [8] in a raw pork plant(55%). Nevertheless it was heterogeneous in the di¡erentplants. Two cases were beheld: an overall contaminationeither in the environment (plants A and B) or on theequipment (plants C, D and E). However, di¡erenceswere observed when the di¡erent rooms were considered.

FEMSLE 10457 23-5-02

E. Chasseignaux et al. / FEMS Microbiology Letters 210 (2002) 271^275272

In plant A, the environments of the reception and theproduct processing areas were the most contaminated(mostly on £oors) whereas the contamination was presenton the equipment of the meat processing area. In plant B,only the environments of all areas were contaminated(only on £oors). In plants C and D, the environment ofthe reception area was colonised, and during meat andproduct processing the contamination was more importanton the equipment (working tables, transport belts, etc.).However, in plant E, L. monocytogenes was detected onthe equipment of the reception and meat processing areasand in the environment of the product processing work-room.After the cleaning operations, the contamination was

subsequently lowered (7.7%), with 13.1% in the poultryplants. This result is similar to those obtained by Salvatet al. [8]. In the pork plants, the contamination wasslightly lower (2.5%). The residual contamination wasmainly observed in the environments of plants A, B, andE: 27.5%, 25% and 13.3%, respectively, of contaminatedswabs were found. On the contrary, in plant C, no con-tamination remained in the environment whereas oneequipment was still contaminated. In plant D, no remain-ing contamination was detected in either kind of swab-bing. This could be due to distinct cleaning habits : alter-native utilisation of two di¡erent cleaning productswhereas the other plants always used the same one. There-fore, in these other plants, some bacteria could develop aresistance against the used cleaning product, survive andeventually grow [18].

3.2. Pro¢les of risk factors for the environmental andphysicochemical variables

Each plant was studied separately. Table 2 presents thecharacteristics of the surfaces and workrooms in the pres-

ence or absence of L. monocytogenes contamination. Forsome parameters, the results were not conclusive, meaningthat no real di¡erence was observed for the variable ineither the presence or the absence of a contamination.However, the global analysis of the di¡erent parametersshowed a convergence for some of them between the dif-ferent plants whereas only inclinations or even divergenceswere observed in other cases.Convergence was observed for the cleanness of the sur-

face and the hygrometry of the room. A clean surface wasassociated with the absence of a contamination in almostall plants (A, C, D and E) even if the presence of dust wasalso recorded in plants A and E. However, in each plantorganic residues were noticed on the surface in case of aL. monocytogenes detection.In the absence of L. monocytogenes contamination, the

hygrometry of the workrooms was below 70%, whereas inthe presence of a contamination a higher hygrometry wasobserved (from 70 to 80% in plants B, C, E and evenabove 80% in plant A). However, no conclusion couldbe drawn for plant D, as during activity the hygrometrywas always below 70% and conversely after the cleaningoperations it was always above 80%. Helke et al. [19]observed a higher survival of L. monocytogenes in bio¢lmat 75.5% of relative humidity than at 32.5%.Inclinations were observed for the status, the pH and

the temperature of the surface and for the temperature ofthe workrooms. A smooth surface was clearly linked withthe absence of L. monocytogenes detection. However,when L. monocytogenes was detected, the surface was un-even: granular (plants A and B), stripped (plant D) ordamaged (plant C).A pH under 6 was frequently associated with the ab-

sence of L. monocytogenes. However, when L. monocyto-genes was detected, di¡erences in pH were noted butnevertheless close to neutral (a pH from 6 to 6.5 was

Table 1Distribution of the 497 swabbing samples regarding L. monocytogenes contamination in the ¢ve plants (A^E) in the environment or on the equipmentof the di¡erent workrooms during activity and after cleaning

Plant Type of swabbing Number of samples contaminated by L. monocytogenes/total number of samples

Reception Meat processing Product processing Total

Act. Cle. Act. Cle. Act. Cle. Act. Cle.

A Environ. 9/17 (53%)a 11/27 (41%) 2/5 (40%) 0/7 (0%) 7/14 (50%) 0/6 (0%) 18/36 (50%) 11/40 (27.5%)Equip. 1/3 (33%) 0/3 (0%) 15/18 (83%) 0/32 (0%) 7/51 (13.7%) 0/24 (0%) 23/72 (31.9%) 0/50 (0%)

B Environ. 4/4 (100%) ND 8/12 (66.5%) 2/8 (25%) 3/8 (37.5%) 2/8 (25%) 15/24 (62.5%) 4/16 (25%)Equip. 0/2 (0%) ND 0/4 (0%) 0/2 (0%) 0/6 (0%) 0/6 (0%) 0/12 (0%) 0/8 (0%)

C Environ. 1/6 (16.5%) 0/5 (0%) 0/11 (0%) 0/11 (0%) 0/6 (0%) 0/5 (0%) 1/23 (4.4%) 0/21 (0%)Equip. 0/3 (0%) 0/5 (0%) 18/23 (78.2%) 0/19 (0%) 1/9 (11%) (0%) (7.1%) 19/35 (54.3%) 1/38 (2.6%)

D Environ. 0/4 (0%) 0/4 (0%) 1/3 (0%) 0/3 (0%) 2/4 (50%) 0/4 (0%) 2/11 (18.2%) 0/11 (0%)Equip. Xb X 4/11 (36.3%) 0/11 (0%) 5/8 (62.5%) 0/9 (0%) 10/20 (50%) 0/20 (0%)

E Environ. 0/3 (0%) 0/3 (0%) 1/4 (25%) 1/4 (25%) 3/8 (37.5%) 1/8 (12.5%) 4/15 (26.5%) 2/15 (13.3%)Equip. 1/1 (100%) 0/1 (0%) 7/7 (100%) 0/6 (0%) 0/7 (0%) 0/7 (0%) 8/15 (53.3%) 0/15 (0%)

Act., during activity; Cle., after cleaning; Environ., environment (£oor, wall, sewer) ; Equip., equipment (for example, working table, transport belt,etc.) ; ND, not determined.aPercentage of positive samples.bX, no equipment present in this room.

FEMSLE 10457 23-5-02

E. Chasseignaux et al. / FEMS Microbiology Letters 210 (2002) 271^275 273

linked with the contamination in plants C and E, whereasfor plant A it was from 6.5 to 8, and for plant B above6.5). This is in agreement with the growth characteristicsof L. monocytogenes, whose optimal pH is neutral toslightly alkaline [16].Regarding the temperature of the surface, di¡erences

were observed. In the absence of contamination, the sur-face temperature seemed rather high, above 10‡C (above4‡C in plant D, between 10‡C and 13‡C in plant A, above12‡C in plant E and above 20‡C in plant B). The onlyexception was observed in plant C where the temperaturewas either below 8‡C or above 13‡C. On the contrary,when L. monocytogenes was detected, the temperaturewas low, below 10‡C. However, in plant C the temper-ature was between 10‡C and 13‡C but the contaminationwas still associated with the refrigerated temperature. Thesame results were obtained for the room temperatures : thetemperature was rather high in the absence of L. mono-cytogenes contamination and inversely low in the presenceof a contamination. This result was expected as the surfacetemperature was partly in£uenced by the room tempera-ture. This was in agreement with the psychrotrophic prop-erties of L. monocytogenes : this ability could selectL. monocytogenes while other competitive micro£ora couldnot grow [16,20]. On the contrary, at high temperature,the micro£ora could compete with L. monocytogenes, out-number it, and therefore not allow its implantation. More-over, our study con¢rmed the results of Helke et al. [19],which showed a higher survival when the bio¢lm was at6‡C compared to 25‡C with a relative humidity of 75.5%.

In our study, L. monocytogenes contamination was foundwith low temperature and high hygrometry and, con-versely, no contamination with high temperature andlow hygrometry.Finally, for the nature and the moisture of the surface,

divergences were observed. When no L. monocytogeneswas detected, the surface was of stainless steel (plants A,B and C), tiles (plants C and E) or cement (plant E). Whena contamination was observed, the surface was composedof either resin (plants A and B) or plastic (plants C and E).Nevertheless, this was in agreement with the surface state:smooth in the absence of L. monocytogenes detection andgranular, stripped or damaged with L. monocytogenes con-tamination. Moreover, in the di¡erent studied plants, theresin was used on the £oor as a non-skid surface andtherefore it was granular. On plastics, pits and cracks re-sulted of their use. Thus, for both surface kinds, micro-spaces existed, unreached by disinfectants, where soil andbacteria could persist. Wong [21] also supported that hy-pothesis. Moreover, di¡erent studies related to bio¢lms onstainless steel showed that nutrients could either enhanceor inhibit a bio¢lm of L. monocytogenes [19,22,23]. In ourstudy, this surface was found smooth or lightly strippedand associated with the absence of contamination byL. monocytogenes. Therefore, organic meat residues couldlimit the establishment of L. monocytogenes bio¢lm onstainless steel.Concerning the moisture of the surface, a dry surface

was found in three plants (A, B and E) whereas a moistsurface was found in two plants (C and D) with no detec-

Table 2Characteristics of the surfaces and workrooms in the absence or presence of L. monocytogenes contamination in the ¢ve plants studied

Absence of L. monocytogenes contamination

Plant A B C D E

Surface Nature stainless steel stainless steel stainless steel ; tiles NC cement; tilesCleanness clean or PD NC clean clean clean or PDState smooth smooth smooth smooth NCMoisture dry dry moist moist drypH 6 6 6 6.5 6 6 NC 6 6Temperature 10^13‡C s 20‡C 6 7‡C s 8‡C s 12‡C

Workroom Temperature 8^12‡C s 10‡C 6 8‡C s 4‡C s 10‡CHygrometry 6 80% 6 70% 6 70% NC 6 70%

Presence of L. monocytogenes contamination

Plant A B C D E

Surface Nature resin resin plastic NC plasticCleanness POR NC PORD PORG PORState granulous granulous damaged stripped NCMoisture moist moist dry dry moistpH 6.5^8 s 6.5 6^6.5 NC 6^6.5Temperature 6 7‡C 6 10‡C 10^13‡C 6 8‡C 6 12‡C

Workroom Temperature 6 8‡C 6 10‡C 8^12‡C 6 4‡C 5^10‡CHygrometry s 80% 70^80% 70^80% NC 70^75%

NC, not conclusive ; PD, presence of dust; POR, presence of organic residues; PORD, presence of organic residues and dust; PORG, presence of or-ganic residues and grease. Results of the MCA: plant A (axis 1 (20.14%), axis 2 (16.49%), axis 3 (12.27%)), plant B (axis 1 (19.86%), axis 2 (17.94%),axis 3 (15.94%)), plant C (axis 1 (27.62%), axis 2 (15.32%), axis 3 (13.89%)), plant D (axis 1 (26.55%), axis 2 (21.64%), axis 3 (16.58%)), plant E (axis 1(22.85%), axis 2 (19.06%), axis 3 (15.34%)).

FEMSLE 10457 23-5-02

E. Chasseignaux et al. / FEMS Microbiology Letters 210 (2002) 271^275274

tion of L. monocytogenes. Opposite results were noticedwhen L. monocytogenes was detected. Therefore, no con-clusion can be expressed for that criterion.As a conclusion, the physicochemical risk pro¢les

showed that a surface in resin or plastic, therefore ratheruneven, with organic residues, with a neutral pH, a lowtemperature and a high hygrometry was associated withL. monocytogenes contamination. Now, it could be inter-esting to consider the accompanying £ora found in eitherthe presence or the absence of L. monocytogenes contam-ination. Indeed, as well as physicochemical factors, £oracould also in£uence the surface colonisation by L. mono-cytogenes.

Acknowledgements

The authors acknowledge the Ultra-propre NutritionIndustrie Recherche group for ¢nancial support for theinvestigations, the French Ministe're de l’Education Natio-nale, de la Recherche et de la Technologie and the AgenceNationale pour la Recherche et la Technologie (ANRT)for its grant. The authors also acknowledge Y. Le No“tre-Michel, F. Eono and S. Gorin for their technical helpduring the study.

References

[1] Jacquet, C., Catimel, B., Brosch, R., Buchrieser, C., Dehaumont, P.,Goulet, V., Lepoutre, A., Veit, P. and Rocourt, J. (1995) Investiga-tions related to the epidemic strain involved in the French listeriosisoutbreak in 1992. Appl. Environ. Microbiol. 61, 2242^2246.

[2] Goulet, V., Rocourt, J., Rebiere, I., Jacquet, C., Moyse, C., Dehau-mont, P., Salvat, G. and Veit, P. (1998) Listeriosis outbreak associ-ated with the consumption of rillettes in France in 1993. J. Infect.Dis. 177, 155^160.

[3] de Valk, H., Vaillant, V. and Goulet, V. (2000) Epidemiology ofhuman Listeria infections in France. Bull. Acad. Natl. Med. 184,267^274.

[4] Jacquet, C., Catimel, B., Goulet, V., Lepoutre, V., Veit, P., Dehau-mont, P. and Rocourt, J. (1995) Typing of Listeria monocytogenesduring epidemiological investigations of the French listeriosis out-breaks in 1992, 1993 and 1995. In: XII International Symposiumon Problems of Listeriosis, Perth, Western Australia, 2^6 October1995, Promoco Conventions Pty Ltd, pp. 161^176.

[5] Jay, J. (1996) Prevalence of Listeria spp. in meat and poultry prod-ucts. Food Control 7, 209^214.

[6] Ojeniyi, B., Wegener, H., Jensen, N. and Bisgaard, M. (1996) Listeriamonocytogenes in poultry and poultry products: epidemiological in-

vestigations in seven Danish abattoirs. J. Appl. Bacteriol. 80, 395^401.

[7] Lawrence, L. and Gilmour, A. (1994) Incidence of Listeria spp. andListeria monocytogenes in poultry processing environment and inpoultry products and their rapid con¢rmation by multiplex PCR.Appl. Environ. Microbiol. 60, 4600^4604.

[8] Salvat, G., Toquin, M., Michel, Y. and Colin, P. (1995) Control ofListeria monocytogenes in the delicatessen industries: the lessons of alisteriosis outbreak in France. Int. J. Food Microbiol. 25, 75^81.

[9] Destro, M., Leitao, M. and Farber, J. (1996) Use of molecular typingmethods to trace the dissemination of Listeria monocytogenes in ashrimp processing plant. Appl. Environ. Microbiol. 62, 705^711.

[10] Dauphin, G., Ragimbeau, C. and Malle, P. (2001) Use of PFGEtyping for tracing contamination with Listeria monocytogenes in threecold-smoked salmon processing plants. Int. J. Food Microbiol. 64,51^61.

[11] Giovannacci, I., Ragimbeau, C., Queguiner, S., Salvat, G., Vendeu-vre, J.L., Carlier, V. and Ermel, G. (1999) Listeria monocytogenes inpork slaughtering and cutting plants. Use of RAPD, PFGE andPCR-REA for tracing and molecular epidemiology. Int. J. Food Mi-crobiol. 53, 127^140.

[12] Unnerstad, H., Bannerman, E., Bille, J., Danielsson Tham, M.,Waak, E. and Tham, W. (1996) Prolonged contamination of a dairywith Listeria monocytogenes. Neth. Milk Dairy J. 50, 493^499.

[13] Larpent, J.P. (1995) Les Listeria. Lavoisier, Paris.[14] Junttila, J., Niemela, S. and Hirn, J. (1988) Minimum growth temper-

atures of Listeria monocytogenes and non-haemolytic Listeria. J. Appl.Bacteriol. 65, 321^327.

[15] Walker, S., Archer, P. and Banks, J. (1990) Growth of Listeria mono-cytogenes at refrigeration temperatures. J. Appl. Bacteriol. 68, 157^162.

[16] Pearson, L. and Marth, E. (1990) Listeria monocytogenes ^ threat to asafe food supply: a review. J. Dairy Sci. 73, 912^928.

[17] Chasseignaux, E., Michel, Y., Toquin, M.T., Ermel, G., Salvat, G.and Colin, P. (1999) Comparison of two rapid methods for the de-tection of Listeria monocytogenes with a standard procedure in nat-urally contaminated environments of raw poultry processing. J. Rap-id Methods Autom. Microbiol. 7, 147^154.

[18] Mereghetti, L., Quentin, R., Marquet-Van Der Mee, N. and Audu-rier, A. (2000) Low sensitivity of Listeria monocytogenes to quater-nary ammonium compounds. Appl. Environ. Microbiol. 66, 5083^5086.

[19] Helke, D., Somers, E. and Wong, A. (1993) Attachment of Listeriamonocytogenes and Salmonella typhimurium to stainless steel andbuna-N in the presence of milk and individual milk components.J. Food Protect. 56, 479^484.

[20] Maurice, J. (1994) The rise and rise of food poisoning. New Sci. 144,28^33.

[21] Wong, A.C.L. (1998) Bio¢lms in food processing environments.J. Dairy Sci. 81, 2765^2770.

[22] Kim, K. and Kranck, J. (1994) E¡ect of growth nutrients on attach-ment of Listeria monocytogenes to stainless steel. J. Food Prot. 57,720^726.

[23] Kim, K., Murano, E. and Olson, D. (1994) E¡ect of heat shock onproduction of listeriolysin O by Listeria monocytogenes. J. Food Saf.14, 273^279.

FEMSLE 10457 23-5-02

E. Chasseignaux et al. / FEMS Microbiology Letters 210 (2002) 271^275 275