918

Journal of Food Protection, Vol. 53, Pages 918-923 (November 1990) Copyright© International Association of Milk, Food and Environmental Sanitarians

Behavior of Listeria monocytogenes in the Presence of Streptococcus Iactis in a Medium with Internal pH Control

JANE M. WENZEL and ELMER H. MARTH*

Department of Food Science and The Food Research Institute, University of Wisconsin-Madison, Madison, Wisconsin 53706 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/53/11/918/1658778/0362-028x-53_11_918.pdf by guest on 30 September 2021

(Received for publication April 9,1990)

ABSTRACT mented dairy products, the amount used may need to be increased so the quantity of lactic acid formed in a given time Growth of Listeria monocytogenes strains V7, Scott A, and California (initial inoculum 103/ml) at 21 or 30°C in the presence is comparable to that produced by healthy cells (23). of Streptococcus Iactis (initial inoculum 0.25 or 1.0%) was deter­ Control of pH when preparing bulk starter is not a new mined using a medium with internal pH control (IPCM-1). The pH concept. Over the past 10 years, it has become a standard of the uninoculated medium (control) was 7.0 before and after procedure in the industry for growing bulk starter (23). There incubation. Populations of L. monocytogenes in IPCM-1 without are two methods to control pH of bulk starter when it is being 7 S. Iactis after 30 h at 21°C were ca. 10 /ml for strains V7 and Scott prepared: external and internal pH control. External control 6 A and ca. 10 /ml for strain California, and at 30°C they were ca. methods maintain pH by automatic or manual addition of 8 10 /ml for all three strains. When data were plotted, areas of graphs neutralizer during the fermentation process (14). The present between curves representing controls and treatments were calcu­ study focused on the internal method for pH control of bulk lated to quantitate the extent of inhibition of L. monocytogenes caused by S. Iactis. Each such area is called the "area of inhibition" starter cultures. This type of medium is usually a blend of (AI). Growth of the pathogen was inhibited by S. Iactis; the degree whey, autolyzed yeast, and chemical buffers that provide was dependent on temperature and concentration of lactic culture internal control of acidity (75). The buffer salts are insoluble and, in some instances, strain of Listeria. Greatest inhibition of at pH values near 7.0, but as the pH drops, buffers are each strain occurred with the largest inoculum of S. Iactis and at gradually and continuously solubilized to neutralize the acid. the highest temperature. No significant difference (p>0.05) in AI This type of control maintains the pH of the medium or pH among the three strains was observed at 21°C. At 30°C, strain between 5.1-5.2, thus allowing maximum starter growth California was inhibited significantly more (p<0.05) than V7 or with minimal acid damage to cells. Scott A by both concentrations of S. Iactis at 24 and 30 h of Bulk starter prepared with this type of medium contains incubation. No significant difference (p>0.05) in pH was found at this temperature regardless of concentration of S. Iactis or strain approximately 10 times more cells per unit volume than does of Listeria. IPCM-1 inoculated with a lactic starter culture is ready the conventional culture (23). Consequently, inoculum rates for use at pH 5.5 after 15-18 h of incubation. Inhibition of Listeria can be reduced because maintenance of optimum pH results was not complete at this pH under any of the experimental in a very active culture. It is not uncommon for inoculum conditions. Substantial numbers of L. monocytogenes (103-105 rates to be reduced by one-half compared to the amount of CFU/ml) were present when this medium was ready for use to conventional culture that is used. produce cultured dairy foods. The primary function of starters is to ferment lactose of the milk into lactic acid. The decreased pH promotes syneresis of cheese curd while inhibiting growth of spoilage or pathogenic microorganisms (77). It is well known that Conventional bulk starter preparation methods involve certain starter cultures are markedly antagonistic to various addition of lactic starter bacteria to milk or milk-based foodborne pathogens and food spoilage organisms (22). Thus, media which are incubated (6). Milk buffers some of the acid many fermented dairy products have commonly been con­ produced by the fermenting organisms, but as their numbers sidered as free of pathogens. Some starter cultures were increase and they continue to produce acid, the buffering inhibitory to a limited number of pathogens in studies where capacity is overcome. As the pH drops, acidic conditions the pH of the growth medium was controlled. Salmonella become unfavorable for continued growth of the starter and Staphylococcus aureus grew in neutralized media, but bacteria, acid production stops, and cells of the starter may inhibition of the pathogens occurred in the presence of lactic be damaged. For Streptococcus Iactis, two mechanisms may starter cultures (8,16). To date no research has examined the cause the damage (9). The low pH may directly inactivate behavior of Listeria monocytogenes in media in which the several enzymes, or it may cause loss of control of the rates pH is controlled. at which individual enzymes are synthesized. When bulk The overall objective of this project is to develop starter containing damaged cells is used to prepare fer­ additional information which will be useful in improving the

JOURNAL OF FOOD PROTECTION, VOL. 53, NOVEMBER 1990 LISTERIA MONOCYTOGENES AND STREPTOCOCCUS LACTIS 919 safety of cheese and other cultured dairy foods. The present merate S. lactis, samples were surface-plated in duplicate on APT study sought to answer the question, "how will Listeria agar (Difco). These plates were incubated at 25°C for 48 h and then monocytogenes behave if present when a starter culture is colonies were counted using a Darkfield Quebec Colony Counter prepared using a bulk starter medium with internal pH (American Optical Corp., Buffalo, NY). control?" Calculation of area of inhibition (AI)

MATERIALS AND METHODS The following formula was used to calculate the area of inhibition at each hour of incubation (Fig. 1):

Preparation of cultures (t2 - t,)/ 2 x [(d + c) - (b + a)J Three strains of L. monocytogenes were used in this study: where: t2 = hour at time 2 V7 (milk isolate, serotype 1), Scott A (SA) (clinical isolate, tj = hour at time 1 serotype 4b), and California (CA) (serotype 4b, isolated from and a, b, c, and d are log populations of Listeria in IPCM-1 Mexican-style cheese implicated in a 1985 outbreak of listeriosis containing: in California). All cultures were maintained in tryptose agar (TA) a = Listeria + S. lactis at t^ (Difco Laboratories, Detroit, MI), stored at 7°C and transferred b = Listeria + S. lactis at t2 bimonthly. To begin an experiment, inoculum from the stock c = Listeria alone at t, Downloaded from http://meridian.allenpress.com/jfp/article-pdf/53/11/918/1658778/0362-028x-53_11_918.pdf by guest on 30 September 2021 culture was transferred to duplicate tubes of tryptose broth (TB) d = Listeria alone at t^ (Difco) and incubated in air at 35°C for 24 h. A second transfer Log populations of Listeria in the presence of S. lactis (a, b) are was made to duplicate tubes of TB which were incubated under means of four plate counts (duplicate platings of two samples). Log the same conditions. Inocula of 0.05 ml each from the second set populations of Listeria alone (c, d) are means of two plate counts of TB tubes were added to duplicate 500-ml screw cap Erlenmeyer (duplicate plating of one sample). flasks each containing 200 ml of sterile skim milk (autoclaved at 121°C for 17 min) which were then incubated at 35°C for 24 h, 10 Area of Inhibition = Each of a second set of sterile skim milk samples was inoculated 9H with 0.05 ml of the first milk culture and incubated at 35°C for 48 E (t2-t1)/2x[(d + c)-(b + a)] h. These transfers were made to obtain a consistent L. *"^3 8 8 u monocytogenes population of ca. 10 CFU/ml in milk which was 7 used to inoculate test samples. c Frozen S. lactis culture was obtained from Chr. Hansen's o 6- Laboratories (Milwaukee, WI). The culture was thawed and a _ns 3 5- series of 0.01 ml transfers was made into sterile freezer vials each Q. O 4- containing 1.0 ml of sterile skim milk. Vials containing the culture a. were stored at -84°C until time of use. A flask containing 200 ml 3 of sterile skim milk was inoculated with 0.1 ml of the thawed o culture and incubated for 14 h at 21°C. This served as inoculum 2 for test samples. 1 i • • • 0 10 2 0 3 0 Preparation of internal pH control medium (IPCM-1) Hours IPCM-1 (Galloway West, Fond du Lac, WI) (15.0 g) was Figure I: Growth curves of L. monocytogenes control and L. added to a 500-ml Erlenmeyer flask containing 200 ml of cold tap monocytogenes in the presence of S. lactis in IPCM-I illustrating water (21°C) and stirred for approximately 5 min. The pH of each use of the area of inhibition (AI) calculation. Key: L represents the sample was measured with a pH meter (Fisher Accumet model population ofL. monocytogenes grown in IPCM-1 alone; L (+ SL) 825MP, Springfield, NJ) equipped with an Orion gel-filled com­ represents the population of L. monocytogenes when grown in bination electrode. Flasks containing the medium were steamed for IPCM-1 with S. lactis; t2 represents the hour at time 2, t} represents 45 min and then placed in a cold water bath (15°C) for 20 min. the hour at time 1, a, b, c, and d are log population of L. After cooling, the medium was tempered at 21 or 30°C for ap­ monocytogenes in IPCM-1 containing: L. monocytogenes and S. proximately 18 h. lactis at tr L. monocytogenes and S. lactis at t, L. monocytogenes alone at tr and L. monocytogenes alone at f, respectively. Inoculation, sampling, and pH determination Duplicate samples of IPCM-1 were inoculated with L. monocytogenes (strains V7, SA, or CA) to yield an initial popu­ 3 Statistical analysis lation of ca. 10 CFU/ml and with 0.25 or 1.0% (v/v) S. lactis. All data on areas of inhibition and pH were analyzed using Controls were included which contained Listeria alone, S. lactis the SAS statistical computer package (1988 SAS Institute Inc., SAS alone, or were uninoculated. Samples were incubated at 21 or 30°C Circle, P.O. Box 8000, Cary, N.C. 27512-8000). A one-way for 30 h. Those incubated at 21°C were sampled at 0,3,6,12,18,24, analysis of variance (ANOVA) compared areas of inhibition for and 30 h, whereas those incubated at 30°C were sampled at the three strains and pH at 6, 24, and 30 h of incubation for a 0,3,6,9,12,18,24, and 30 h. Five ml from each test flask were specific set of conditions; temperature and concentration of S. lactis. removed aseptically and pH was determined at 6-h intervals. Fisher's LSD (least significant difference) test aided in quantifying Enumeration of L. monocytogenes and S. lactis significant (p<0.05) differences between means of each analysis. One-ml samples were diluted in sterile aqueous 0.5% peptone Each mean was derived from at least two trials, each consisting of (Difco) solution, if necessary. Samples were surface-plated onto an AI calculation and pH for that hour and set of conditions. McBride's listeria agar (MLA) (Difco) with 0.5% added lithium chloride for enumeration of L. monocytogenes in the presence of S. lactis. For controls containing Listeria alone, samples were RESULTS surface-plated in duplicate onto TA. Both MLA and TA plates were incubated in air at 35°C for 48 h. To enumerate S. lactis, samples IPCM-1 supported growth of each of the three strains were surface-plated in duplicate in air at 35°C for 48 h. To enu­ of L. monocytogenes when the medium contained the JOURNAL OF FOOD PROTECTION, VOL. 53, NOVEMBER 1990 920 WENZEL AND MARTH pathogen alone (controls) and when it contained Listeria plus lated with 0.25 or 1.0% S. lactis. Inhibition of growth by the the lactic culture (treatment samples). Final L. monocytogenes pathogen began after 18 h of incubation when the initial populations in control after 30 h at 21°C were ca. 107 CFU/ inoculum was 0.25% S. lactis. At the 1.0% level of added ml for stains V7 and SA and ca. 106 CFU/ml for CA. At 30°C, lactic culture, inhibition began after 12 h of incubation. At the final population for all three strains in controls was ca. the end of the 30-h incubation, populations of L. 108 CFU/ml. S. lactis populations in samples initially con­ monocytogenes in samples inoculated with 0.25% S. lactis taining 0.25 or 1.0% lactic culture were ca. 3.0 x 105 and 1.3 were 1.5-2.0 orders of magnitude less than populations of x 106 CFU/ml, respectively. After the 30-h incubation, S. lactis the pathogen in controls. At the 1.0% inoculum level of 5. populations were ca. 4.0 x 109 regardless of temperature or lactis, populations of L. monocytogenes were 2.5-3.0 orders amount of initial inoculum. The initial pH of all samples was of magnitude less than in controls. After the 30-h incubation, 7.0 and did not change in uninoculated or Listeria controls the pH of treatment samples was 5.5-5.7 regardless of during the 30-h incubation. percentage of lactic culture added.

Incubation at 30°C Incubation at 21"C Downloaded from http://meridian.allenpress.com/jfp/article-pdf/53/11/918/1658778/0362-028x-53_11_918.pdf by guest on 30 September 2021 The three strains of L. monocytogenes behaved similarly At this temperature, strains V7 and SA behaved simi­ at this temperature. Thus to avoid repetition, only results larly, but behavior of these two strains differed markedly obtained with strain V7 are given. Data in Fig. 2 show the from that of strain CA. Thus, results from strains V7 and CA behavior of strain V7 when the medium was initially inocu- but not SA are presented in Fig. 3 and 4. In general, growth of the pathogen was inhibited earlier at 30 than 21°C. After 30 h of incubation, the pH of all treatment samples was (a) between 5.0-5.2, regardless of strain of the pathogen or X initial percentage of added lactic culture. Q.

E (a) "5 »*- X o D. c o o

3 I Q. 3" O »*- 0.° O c o O

3 Q. O Hours Q. (b) O X Q. u - 9- Hours o I 8- (b) ~3 7-; »*- o 6i "***".*• Q. c o O 1 - 3 V7 I Q. 3" V7 (+SL) »•- O 3- • O Q. SL 2- *••-• • PH c o O 1 - 1 T | 1-I-T-I-l 1 0 2 0 3 0 3 Q. Hours O a. Figure 2. Behavior of L. monocytogenes V7 in IPCM-l in the presence of(a) 0.25% and(b) 1.0% S. lactis at 21°C. Key: V7 (+ o SL) represents the population of L. monocytogenes strain V7'grown in the presence ofS. lactis in IPCM-l, pH represents the pH in Hours IPCM-l containing V7 and S. lactis at the time of sampling, SL represents the population of S. lactis when grown with V7, V7 Figure 3. Behavior of L. monocytogenes V7 in IPCM-l in the pres­ represents the population of L. monocytogenes strain V7 grown ence of (a) 0.25% and(b) 1.0% S. lactis at30"C. Key to abbreviations alone inlPCM-1. is in title of Fig. 2. JOURNAL OF FOOD PROTECTION. VOL. 53, NOVEMBER 1990 LISTERIA MONOCYTOGENES AND STREPTOCOCCUS LACTIS 921 noted with strain CA. When the initial lactic inoculum was (a) 0.25%, L. monocytogenes populations in treatment samples were 4.75 orders of magnitude less than those in controls X Q. after 30 h of incubation. Populations of the pathogen in o samples inoculated with 1.0% S. lactis were 6.25 orders of I magnitude less than populations in controls. O Listeria populations at pH 5.5 According to the manufacturers, IPCM-1 is ready for c o use when a pH of 5.5 has been reached after 16-18 h of incubation. Table 1 gives the numbers of L. monocytogenes 3 Q. present in the medium at this pH value. Data for growth of O 0. L. monocytogenes at 21°C in IPCM-1 inoculated with 0.25% S. lactis are not included because pH 5.5 was not reached or Downloaded from http://meridian.allenpress.com/jfp/article-pdf/53/11/918/1658778/0362-028x-53_11_918.pdf by guest on 30 September 2021 o in most instances even after the 30 h of incubation.

TABLE 1. Listeria monocytogenes populations" present in IPCM- Hours 1 after fermentation to pH 5.5 by S. lactis. (b) Strain of L. monocytogenes

Condition V7 Scott A California 1.00% S lactis, 30°Cb 2.0 x 105 4.0 x 104 1.2 x 10' 0.25% S. lactis, 30°C» 1.7 x 105 3.1 x 105 8.7 x 10' E C 4 4 1 1.00% S. lactis, 21°C 2.4 x 10 4.9 x 10 6.9 x 10' a Populations represent trial means; trials consisted of four plate c o counts each (duplicate platings of two samples for each strain, temperature, and concentration of S. lactis. 3 b Q. 24-h incubation. O c Q. 30-h incubation.

DISCUSSION

i i i i i i | i i i i i i—i i i | i i i i i i i i i | • 0 10 2 0 3 0 IPCM-1 supported growth of each of the three strains Hours of L. monocytogenes. Growth of the pathogen was inhibited Figure 4. Behavior of L. monocytogenes CaliforniainIPCM-1 in the when S. lactis was present; the degree of inhibition was presence of (a) 0.25% and(b) 1.0% S. lactis at30°C. Key: CA (+ SL) dependent on temperature, percentage of S. lactis added to represents the population of L. monocytogenes strain California in the medium, and, in some instances, strain of the pathogen. the presence ofS. lactis,/)// represents the pH inIPCM-I containing The calculated "area of inhibition" was useful for strain California and S. lactis at the time of sampling, SL represents expressing the degree to which L. monocytogenes was in­ the population of'S. lactis in the presence of strain California, CA hibited by the presence of S. lactis. The question could be represents the population of strain California when grown alone in asked, "why not simply express the results as populations IPCM-1. of L. monocytogenes found after incubation together with S. lactisT Results expressed as populations are useful but are limited in that they do not take into account the difference between Listeria populations in treatment samples and con­ Strain V7 and SA. Growth of L. monocytogenes began trols, i.e., inhibition. For example, this would occur when to be inhibited after 12 h when the initial inoculum was comparing growth of two different strains of Listeria where 0.25% S. lactis (Fig. 3a). After 30 h of incubation, numbers treatment samples behaved similarly yet growth of controls of the pathogen in treatment samples were 3.0-3.5 orders of was different. Expressing results simply as populations of magnitude less than those in controls. When 1.0% S. lactis the pathogen in treatment samples may not give a complete was added initially (Fig. 3b), inhibition of the pathogen description of the inhibition that occurred. In the present started after 9 h of incubation. The populations of L. study, the AI calculation was used primarily for statistical monocytogenes in treatment samples were about 4.0 orders analysis. The purpose of the analyses was to determine if any of magnitude less than in controls after 30 h of incubation. significant differences existed among trials of the same Strain CA. Of the three strains examined, CA was most strain and among strains under the same conditions. sensitive to the effects of S. lactis. Inhibition of growth began Inhibition as measured by AI was directly related to the after 9 h of incubation when 0.25% S. lactis was added (Fig. percentage of S. lactis added and temperature of incubation. 4a) and 6 h when 1.0% S. lactis was added to the medium Thus, for any given strain of L. monocytogenes, greatest (Fig. 4b). The greatest difference in populations of L. inhibition occurred at 30°C when the initial inoculum was monocytogenes between treatment and control samples was 1.0% S. lactis. Our results are consistent with those of

JOURNAL OF FOOD PROTECTION, VOL. 53, NOVEMBER 1990 922 WENZEL AND MARTH Schaack and Marth (20) who studied behavior of L. of the pathogen were present in the medium at pH 5.5, when monocytogenes when grown in skim milk together with it normally would be ready for use in making products. mesophilic lactic starter cultures. Their data indicate that L. Inhibition of the pathogen was not complete and depending monocytogenes grew better at 30 than at 21°C. Not only did on percentage of added lactic culture and temperature of the pathogen grow better at the higher temperature, but lactic incubation, 103 to as many as 105 CFU of L. monocytogenes! cultures did as well. Schaack and Marth (20) suggested the ml were present when the medium with its lactic culture greater inhibition of Listeria seen at 30 than at 21°C could could be used. Manufacturers currently using internal pH be explained by the increase rate of acid production by the control media to prepare bulk starter culture need to be aware lactic culture at the higher temperature. of the importance of strict sanitation practices in preventing Statistical analysis using a one-way ANOVA examined contamination of bulk starter with L. monocytogenes. significant differences in AI and pH among the three strains L. monocytogenes has been found in food processing of L. monocytogenes for each temperature and initial con­ environments (4,5) and in some finished products resulting centration of S. lactis at 6, 24, and 30 h of incubation. At in product recalls. Recalls have involved fermented dairy

21 °C, no significant difference in AI or pH was found among products such as cheese (1,2) and buttermilk (3). Our re­ Downloaded from http://meridian.allenpress.com/jfp/article-pdf/53/11/918/1658778/0362-028x-53_11_918.pdf by guest on 30 September 2021 the three strains regardless of other test conditions. Growth search supports the findings of others (18,19,20,21) that L. of the pathogen at this temperature was minimal as was the monocytogenes can survive the fermentation involved in the degree of inhibition. At 30°C, strains did not differ in AI or manufacture of cultured dairy products. pH at 6 h of incubation. Any significant differences at 6 h The World Health Organization (WHO) (24) has made would indicate inconsistency in treatment of the three recommendations to the food industry regarding control of strains. At this same temperature, the AI values for strains Listeria. Stressed was the importance of gathering informa­ V7 and SA after 24 and 30 h of incubation were significantly tion about the ability of various foods to support growth of different from that of CA. Strain CA was inhibited signifi­ L. monocytogenes. Use of Hazard Analysis Critical Control cantly more than were strains V7 and SA regardless of the Points (HACCP) was recommended as the best way to assure initial concentration of S. lactis. This supports recent safety and quality of foods. These recommendations need to findings (17, 18) that strain CA is generally less hardy than be extended to manufacturers currently using internal pH are strains SA and V7 and therefore may differ physiologi­ control media in the preparation of bulk starter. The WHO cally from the other two. Listeria was inhibited to a greater concluded that because Listeria spp. are probably more degree when the initial inoculum of S. lactis was 1.0% rather common and more tenacious than most pathogens, it is than 0.25%. It is noteworthy that regardless of the initial probable that measures for their control will have to be concentration of lactic culture, the pH of the growth medium applied rigorously with much attention to detail. in which the three strains grew did not differ significantly. ACKNOWLEDGMENTS This indicates that pH may have had a role in the degree of inhibition but was not the sole factor responsible for it. This research was supported by the College of Agricul­ Gilliand and Speck (8) found intensity of antagonistic tural and Life Sciences, University of Wisconsin-Madison, action cannot be predicted by the rate at which acid is the Wisconsin Milk Marketing Board, Madison, WI and Chr. produced by streptococci. They studied the effectiveness of Hansen's Laboratories, Milwaukee, WI. We thank Dr. R. L. streptococci in repressing growth of salmonellae and staphy­ Sellars for helpful suggestions regarding this study. lococci with and without pH control of milk. Even though pH was maintained at that of uninoculated milk, the antago­ REFERENCES nistic action of the streptococci toward Salmonella was still evident. Similar results were found with S. aureus and a 1. Anonymous. 1985. FDA finds Listeria in three General Foods soft Cheddar cheese starter culture. The authors concluded the cheese products. Food Chem. News 27(25):33. 2. Anonymous. 1986. Class I recall: Jean Lincet Brand Brie cheese. FDA antagonism exhibited by streptococci did not result entirely Enforcement Rep., April 9, pp. 1-3. from low pH. 3. Anonymous. 1986. Class I recall: Milk products. FDA Enforcement It has been recognized that lactic acid bacteria can Rep., June 25, pp. 1-3. produce inhibitory substances, other than organic acids, that 4. Anonymous. 1987. Listeria continues to be found by FDA in dairy plants. Food Chem News 29(l):47-48. are antagonistic toward other microorganisms (7,10, 12,13, 5. Cox, L. J„ R. Klciss, J. L. Cordicr, C. Cordellana. P. Konkel, C. 16). These substances are produced in much smaller amounts Pedrazzini, R. Beumer, and A. Sicbenga. 1989. Listeria spp. in food than is acid and include hydrogen peroxide, diacetyl, bac- processing, non-food and domestic environments. Food Microbiol. teriocins and secondary reaction products such as 6:49-61. 6. Cox, W. A., and G. Stanley. 1978. Starters: Purpose, production and hypthiocyanate generated by the action of lactoperoxidase problems, pp. 279-295. In F. A. Skinner and L. B. Quesnel (eds.), on hydrogen peroxide and thiocyanate. Whatever factor or Streptococci. Academic Press, London. combination of factors is responsible for inhibition of L. 7. Daeschcl, M. A. 1989. Antimicrobial substances from lactic acid monocytogenes, our results show it was enhanced by use of bacteria for use as food preservatives. Food Technol. 43(1): 164-167. 8. Gilliand, S. E., and M. L. Speck. 1972. Interaction of food and starter a larger rather than smaller percentage of lactic culture cultures and foodborne pathogens: Lactic streptococci versus staphy­ inoculum and by a higher rather than lower temperature of lococci and salmonellae. J. Milk Food Technol. 35:307-310. incubation. 9. Harvey, R. J. 1965. Damage to Streptococcus lactis resulting from growth By far the most obvious and most important result of at low pH. J. Bacteriol. 90:1330-1336. 10. Hurst, A. 1978. Nisin: Its preservative effect and function in the growth this study is that L. monocytogenes reproduced and survived cycles of the producer organism, pp. 207-315. In F. A. Skinner and L. during fermentation in IPCM-1. Thus, significant numbers B. Quesnel (eds.), Streptococci. Academic Press, London.

JOURNAL OF FOOD PROTECTION, VOL. 53, NOVEMBER 1990 LISTERIA MONOCYTOGENES AND STREPTOCOCCUS LACT1S 923

11. International Dairy Federation. 1980. Starters in the manufacture of 18. Ryscr, E. T., and E. H. Marth. 1987. Fate of Listeria monocytogenes cheese. Doc. 129. IDF, Brussels, p. 4. during manufacture and ripening of Camembert cheese. J. Food Prot. 12. Klaenhammer, T. R. 1988. Bacteriocins of lactic acid bacteria. Biochimie 50:372-378. 70:337-349. 19. Ryser, E. T., E. H. Marth, and M. P. Doyle. 1985. Survival of Listeria 13. Kozak, W., J. Bardowski, and W. T. Dobrzanski. 1978. Lactostrepcins- monocytogenes during manufacture and storage of cottage cheese. J. Bacteriocins produced by lactic streptococci. J. Dairy Res. 45:247-257. Food Prot. 50:372-378. 14. Limsowtin, G. K. Y., H. A. Heap, and R. C. Lawerence. 1980. A new 20. Schaack, M. M., and E. H. Marth. 1988. Behavior of Listeria approach to the preparation of bulk starter in commercial cheese plants. monocytogenes in skim milk during fermentation with mesophilic New Zeal. J. Dairy Sci. Tcchnol. 15:219-224. lactic starter cultures. J. Food Prot. 51:600-606. 15. Mcrmelstein, N. H. 1982. Advanced bulk starter medium improves 21. Schaak, M. M., and E. H, Marth. 1988. Survival of Listeria fermentation processes. Food Technol. 36(8):69-76. monocytogenes in refrigerated cultured milks and yogurt. J. Food Prot. 16. Richardson, G. H., and M. A. Divata. 1973. Lactic culture inocula 51:848-852. required to inhibit staphylococci in sterile milk. J. Dairy Sci. 56:706- 22. Speck, M. L. 1972. Control of food-borne pathogens by starter cultures. 709. J. Dairy Sci. 55:1019-1022. 17. Rosenow, E. M., and E. H. Marth. 1987. Growth of Listeria 23. Thunell, R. K. 1988. pH-controlIed starter: A decade reviewed. Cult. monocytogenes in skim, whole, and chocolate milk, and in whipping Dairy Prod. J. 23(3): 10, 12-14, 16. cream during incubation at 4, 8, 13, 21, and 35°C. J. Food Prot. 50:452- 24. WHO. 1988. Foodborne listeriosis. Report of WHO informal working

459. group. Geneva, Switzerland, Feb. 15-19. Downloaded from http://meridian.allenpress.com/jfp/article-pdf/53/11/918/1658778/0362-028x-53_11_918.pdf by guest on 30 September 2021

JOURNAL OF FOOD PROTECTION, VOL. 53, NOVEMBER 1990