Incidence of Listeria monocytogenes in two milk processing environments, and assessment of Listeria monocytogenes blood agar for isolation

A year-long survey of two Northern Ireland milk processing plants for Listeria monocytogenes was carried out. Sample sites included the milk processing environment (walls, floors, drains, and steps), processing equipment, raw and pasteurised milk. The FDA listeria-selective enrichment procedure was used to process samples and an additional agar medium, L. monocytogenes Blood Agar (LMBA), was utilized as part of the isolation procedure in order to compare its performance to that of the recommended Oxford and Palcam agars. LMBA proved to be a very useful tool and was able to detect L. monocytogenes from 94.1% of sites compared to the 76.5% and 79.4% detection rate displayed by Oxford and Palcam agars, respectively. The overall incidence of listeria on equipment was 18.8% (6.3% L. monocytogenes), in the environment was 54.7% (40.6% L. monocytogenes) and in raw milk 44.4% (22.2% L. monocytogenes). On one occasion, L. welshimeri was isolated from pasteurised milk, probably demonstrating post-pasteurisation contamination of product. The main environmental sources of L. monocytogenes were considered to be a floor drain and stainless steel steps.     

Evaluation and interlaboratory validation of a selective agar for phosphatidylinositol-specific phospholipase C activity using a chromogenic substrate to detect Listeria monocytogenes from foods

Phosphatidylinositol-specific phospholipase C (PI-PLC) activity is a potential virulence factor and is exhibited only by the Listeria species Listeria monocytogenes and Listeria ivanovii. A chromogenic substrate for the direct detection of PI-PLC activity is available in a new medium (BCM L. monocytogenes plating agar). The use of a chromogenic substrate offers a mechanism with which to directly screen for L. monocytogenes and L. ivanovii other than the esculin used in Oxford (OXF) and Palcam (PAL) agars, which screen for all Listeria species. The specificity levels of BCM plating agar and of BCM confirmation and rhamnose agars were evaluated with 107 Listeria and 10 Bacillus species isolates. In addition, BCM L. monocytogenes plating agar was compared with standard Listeria selective agars (OXF and PAL agars) with regard to the recovery of L. monocytogenes from 2,000 food and environmental samples obtained from eight participating laboratories. A Listeria species was isolated from at least one of the agars in 209 analyses, and L. monocytogenes was isolated in 135 of these analyses. In 27 of the analyses in which L. monocytogenes was isolated, one or more of the selective differential agars used failed to isolate L. monocytogenes, and therefore the results of these analyses were discrepant. Relative to a reference method involving the use of all three agars (OXF, PAL, and BCM agars), the OXF-BCM, PAL-BCM, and OXF-PAL combinations had sensitivities of 99.3, 99.2, and 90.2%, respectively. In statistical analyses of the different combinations of agars, the OXF-BCM and BCM-PAL combinations were found to be superior to the OXF-PAL combination for the detection of L. monocytogenes.     

Thin agar layer method for recovery of heat-injured Listeria monocytogenes

A thin agar layer (TAL) method was developed to recover heat-injured Listeria monocytogenes. Modified Oxford medium (MOX), a selective plating medium, inhibits heat-injured L. monocytogenes from growing, whereas tryptic soy agar (TSA), a nonselective medium, does not. In order to facilitate recovery of heat-injured L. monocytogenes cells while providing selectivity of isolation of L. monocytogenes from other bacteria in the sample, a unique TAL procedure was developed by overlaying 5 ml of nonselective medium (TSA) onto prepoured and solidified MOX medium in an 8.5-cm-diameter petri dish. The injured L. monocytogenes repaired and started to grow in the TSA during the first few hours after incubation of the plate. During the resuscitation of injured cells, the selective agents from MOX diffused to the TSA top layer to inhibit other microorganisms. L. monocytogenes showed a typical reaction (black colonies) on TAL after 24 h of incubation at 37 degrees C. The recovery rate for heat-injured L. monocytogenes with the TAL method was compared with those rates associated with TSA, MOX, and the traditional overlay method (OV; pouring selective agar on top of resuscitated cells on TSA agar after 3 h incubation). Milk and 0.1% peptone water that were inoculated with L. monocytogenes (4 to 5 log CFU/ml) were heated for 15 min at 55 degrees C. L. monocytogenes was enumerated on TSA, MOX, OV, and TAL media and procedures. No significant difference occurred among TSA, OV, and TAL (P > 0.05) in terms of enumeration of heat-injured L. monocytogenes, but these media recovered significantly higher numbers than did MOX agar (P < 0.05)-in both samples. The TAL method involves only one step, whereas OV is a more cumbersome two-step procedure.     

Prevalence of Listeria spp. in feces and carcasses at a lamb packing plant in Brazil

The objective of this work was to study the occurrence of Listeria species in feces and on dressed and cooled carcasses of lambs at a packing plant in Brazil. Listeria spp. were recovered on Oxford and Palcam agars. The 35 fecal samples yielded Listeria welshimeri (20%) and Listeria innocua (8.6%). The 69 carcass samples yielded L. innocua (34.8%), Listeria monocytogenes (4.3%), and Listeria ivanovii (1.5%). More Listeria spp. were recovered with two selective agars than with either agar alone.     

Evaluation of the international reference methods NF EN ISO 11290-1 and 11290-2 and an in-house method for the isolation of Listeria monocytogenes from retail seafood products in france

Retail seafood products were analyzed on their use-by date using the international reference methods NF EN ISO 11290-1 and 11290-2 (collectively method R) or an in-house method (method B) for the isolation of Listeria monocytogenes. The sensitivity of the methods was about 78%. Method R detected more positive samples of smoked salmon and herb-flavored slices of smoked salmon than did method B, whereas the reverse was true for samples of carpaccio-like salmon, herb-flavored slices of raw salmon, and smoked trout. Most products produced a positive result after the first of two enrichments, and little difference was observed after changing the isolation medium (Listeria selective agar, L. monocytogenes blood agar, agar for Listeria according to Ottaviani and Agosti, Oxford agar, and Palcam agar). L. monocytogenes was isolated from 151 (27.8%) of the 543 samples, with concentrations mostly below 100 CFU/g. The pathogen prevalence and concentration in these seafood products varied greatly depending on the producer and the nature of the product. In certain cases, these differences could be explained by problems in cleaning and disinfection operations in the food-processing environment. The identities of L. monocytogenes isolates were confirmed by PCR, and isolates were characterized by random amplification of polymorphic DNA and pulsed-field gel electrophoresis (PFGE). PFGE patterns obtained with the enzymes Apal and AscI produced 26 different pulsotypes. In general, different pulsotypes were present in the different categories of seafood products and were not specific to one producer. The genetic diversity observed in the products was not related to the prevalence found at the manufacturing site. It is therefore important for producers to determine the source(s) of contamination of their product so the risks linked to the presence of L. monocytogenes can be reduced.     

Recovery of heat-injured Listeria innocua

Listeria innocua was subjected to thermal inactivation and the extent of heat-injured cells was quantified. Cultures were heated in liquid medium for different times, using temperatures in the range of 52.5 to 65.0 degrees C, and plated on Tryptic Soy Agar with 0.6% yeast extract (TSAYE) used as non-selective medium and on TSAYE plus 5% NaCl (TSAYE+NaCl) and Palcam agar with selective supplement (Palcam agar) as selective media. The difference observed in counts in non-selective and in selective media gave an indication of cell injury during the heat treatment. D- and z- values were calculated for all conditions considered. For each temperature, D-values obtained using non-selective recovery procedures were higher than the ones obtained using the two selective media. When comparing the selective media, it can be concluded that Palcam agar allowed recovery and growth of thermally injured cells and so it was less inhibitor than TSAYE+NaCl. Another important result was the influence of temperature on the degree of cellular injury. As temperature increases, the degree of heat-injured cells also increases, and consequently concern has to be taken with the temperature and the counting medium used in food processing studies. The results of this work clearly demonstrated that selective media used for Listeria monocytogenes enumeration/detection might not be suitable for the recovery of heat-injured cells, which can dangerously underestimate the presence of this foodborne pathogen.     

Edible chitosan films on ready-to-eat roast beef for the control of Listeria monocytogenes

The use of chitosan as an edible film was evaluated for its antimicrobial activity against Listeria monocytogenes (LM) on the surface of ready-to-eat (RTE) roast beef. L. monocytogenes, decimally diluted to give an initial inoculation of >6.50logCFU/g, was inoculated onto the surface of RTE roast beef cubes, and air-dried. The samples were dipped into chitosan (high or low molecular weights) solutions dissolved with acetic or lactic acid at 0.5% (w/v) or 1% (w/v) then bagged and refrigerated at 4 degrees C. The bacterial counts were determined on days 0, 7, 14, 21, and 28. The samples were spread plated onto modified Oxford agar plates and incubated at 37 degrees C for 48h. An initial 6.50logCFU/g of L. monocytogenes inoculated onto the surface of the non-coated RTE roast beef increased too >10logCFU/g by day 28. On day 14, L. monocytogenes counts were significantly different for all the chitosan-coated samples from the control counts by 2-3logCFU/g and remained significantly different on day 28. Our results have shown that the acetic acid chitosan coating were more effective in reducing L. monocytogenes counts than the lactic acid chitosan coating. Our study indicated that chitosan coatings could be used to control L. monocytogenes on the surface of RTE roast beef.     

Comparison of media and sampling locations for isolation of Listeria monocytogenes in queso fresco cheese

Listeriosis associated with Hispanic-style soft cheese is an ongoing public health concern. Although rapid detection methods based on molecular and immunological technologies have been applied successfully for detecting Listeria monocytogenes in foods, obtaining isolates of the pathogen is a critical procedure for epidemiologic studies and regulatory analysis. Oxford agar, a medium recommended by the U.S. Food and Drug Administration Bacteriological Analytical Manual (BAM) to isolate L. monocytogenes from cheese, is unable to differentiate L. monocytogenes from other Listeria species. Hence, two selective isolation media, L. monocytogenes blood agar (LMBA) and Rapid 'L. mono agar (RLMA), were compared with Oxford agar for isolating L. monocytogenes from cheese. Queso fresco cheese was inoculated at 10(0) or 10(1) CFU/g with a five-strain mixture of L. monocytogenes or with the five-strain L. monocytogenes mixture and Listeria innocua. Cheese samples were stored at 21, 12, and 4 degrees C and Listeria counts were determined at 3, 7, and 10 days; 7, 10, 14, 21 days; and 2, 4, 8, and 12 weeks postinoculation, respectively. Surface and interior cheese samples as well as liquid exudate produced during storage were assayed individually to determine differences in Listeria contamination at different sampling locations. L. monocytogenes was more easily differentiated from L. innocua on RLMA than LMBA and Oxford agar. Similar L. monocytogenes counts (ca. 10(4) CFU/g) were obtained on the last sampling day on the surface and interior of cheese samples (P > 0.05) for all storage temperatures and both initial inoculation levels, but smaller cell numbers were detected in the exudate produced during storage. In addition, simultaneous inoculation of L. innocua with L. monocytogenes did not affect the final L. monocytogenes counts in the cheese. The amount of exudate released from the cheese and decrease of pH correlated with storage temperature. More exudate was produced and a greater decrease of pH occurred at 21 degrees C than at 12 or 4 degrees C. Our results indicate that RLMA is a suitable medium for isolating L. monocytogenes from queso fresco cheese. Higher counts of L. monocytogenes were obtained from surface and interior samples of cheese than from the exudate of the cheese during storage. In addition, pH may be a useful indicator of improperly stored queso fresco cheese.     

Acidified sodium chlorite treatment for inhibition of Listeria monocytogenes growth on the surface of cooked roast beef

The effects of acidified sodium chlorite (ASC) against Listeria monocytogenes on the surface of cooked roast beef were investigated. L. monocytogenes, strain V7, serotype 1/2a, was inoculated at numbers of 6.0 log CFU/g onto 5-g cubes of cooked regular or spicy roast beef. The samples were allowed to air dry for 1 h. The cooked roast beef samples were dipped into ASC or sprayed with ASC solutions of 250, 500, 750, or 1,000 ppm, then placed in bags with or without a vacuum and refrigerated at 4 degrees C. L. monocytogenes counts were determined after 0, 7, 14, 21, and 28 days of storage by spread plating roast beef samples onto Oxford agar plates that were incubated at 37 degrees C for 48 h. At day 28, the number of L. monocytogenes on the > or = 500 ppm ASC-treated spicy roast beef samples had count reductions that were >4.0 log CFU/g, whereas the same concentrations of ASC-treated regular roast beef samples had approximately a 2.5 log CFU/g reduction in L. monocytogenes counts when compared with the untreated samples. No significant differences (P > 0.05) were observed in L. monocytogenes counts between the vacuum- or nonvacuum-packaged ASC-treated cooked roast beef samples. Sensory evaluation showed no significant differences (P > 0.05) between ASC-treated and untreated roast beef. ASC can be used as a processing aid in the form of a dip or spray treatment to control L. monocytogenes on the surface of cooked roast beef.     

The development of an efficient and rapid enzyme linked fluorescent assay method for the detection of Listeria spp. from foods

Conventional isolation methods, including the Health Products and Food Branch (HPFB), Health Canada method used for the isolation and identification of Listeria species and Listeria monocytogenes from foods and environmental samples, can take a week or more to complete and are usually labor-intensive. This has led to the development of various rapid methods which attempt to generate results comparable to standard methods but in a reduced time-frame with less hands-on operation. Our previous work with rapid detection systems indicated that the recommended enrichment protocols failed to grow the Listeria to detectable levels in a reliable and consistent manner.In the present study, a novel enrichment protocol is described and consists of samples being pre-enriched in Palcam broth (incubated at 35 degrees C for 26 h), enriched in UVM 2 (30 degrees C for 26 h) and then plated and analysed by a rapid detection kit, with results being generated after 52 h of incubation.In total, 200 naturally contaminated samples were analysed by both the HPFB standard method and the Palcam method. The results showed that the Palcam method is comparable to the HPFB method. Further analysis involved a rapid detection system, which applies ELISA techniques and automation in an enzyme-linked fluorescent assay (ELFA) system. This system, referred to as the Vitek Immuno Diagnostic Assay System or VIDAS, can identify Listeria to the genus or species (L. monocytogenes) level.In this comparison, an additional 324 naturally contaminated samples were analysed by both the Palcam and ELFA methods. The sensitivity and specificity of the ELFA method were 98.1% and 97.0%, respectively, while the efficiency was 97.5%. False-negative and false-positive rates were 1.9% and 3.0%, respectively. These results show that the ELFA method (when using the Palcam method for pre- and secondary enrichment) was efficient and gave reliable results after 52 h of incubation, and met Health Canada's criteria for approval as a rapid method.     

Growth control of Listeria monocytogenes on cold-smoked salmon using a competitive lactic acid bacteria flora.

A Lactobacillus sake strain LKE5 and four strains of Carnobacterium piscicola were evaluated as biopreservation cultures to control the growth of Listeria monocytogenes on vacuum-packed, cold-smoked salmon stored at 5 degrees C. All five strains were antilisterial as live cultures in an agar diffusion assay. Cell-free supernatants of two strains of C. piscicola and L. sake LKE5 were also antilisterial because of the production of bacteriocins. The presence of high cell numbers of strains of C. piscicola had no influence on the sensory quality of cold-smoked salmon stored at 5 degrees C, but L. sake LKE5 caused strong sulfurous off-flavors and was rejected as a culture for biopreservation of cold-smoked salmon. A bacteriocin-producing strain of C. piscicola (A9b) initially caused a 7-day lag phase of L. monocytogenes, followed by a reduction in numbers of L. monocytogenes from 10(3) CFU/ml to below 10 CFU/ml after 32 days of incubation, coinciding with the detection of antilisterial compounds. The presence of a nonbacteriocin-producing strain of C. piscicola (A10a) prevented the growth of L. monocytogenes during the 32-day incubation. The growth of L. monocytogenes was strongly repressed on cold-smoked salmon in the presence of C. piscicola A9b and A 10a, respectively. The initial cell numbers of L. monocytogenes that were found on Oxford plates incubated at 25 degrees C reached low maximum cell counts of 10(4) and 2 x 10(3) after 14 and 20 days of storage in mixed culture with C. piscicola A9b and A10a.     

Survival and recovery of viable but nonculturable Listeria monocytogenes cells in a nutritionally depleted medium

Survival of a desiccated five-strain Listeria monocytogenes mixture during storage in sand at 4 degrees C for 2 months was determined using the acridine orange direct count method with novobiocin and plate counts. Samples of inoculated sand were taken every 2 weeks, incubated at 37 degrees C for 6 h, stained with acridine orange, and then examined with a fluorescence microscope. Elongated viable but nonculturable cells were most frequently observed during weeks 2 and 4. At weeks 6 and 8, most of the cells either remained viable or were dead. In each microscopic field, only one or two viable but nonculturable cells were observed among hundreds of other viable culturable cells, indicating that L. monocytogenes does not generally become viable but nonculturable. Therefore, viable but nonculturable cells are not a concern when plating environmental samples or desiccated L. monocytogenes cells on nonselective media. Tryptic soy agar with 0.6% (wt/vol) yeast extract (TSAYE) and Columbia agar were used as nonselective plate count media. Modified Oxford agar and TSAYE + 5% (wt/vol) sodium chloride were used as the selective plate count media. The effects of aerobic or anaerobic incubation and media supplementation with 0.1% or 1% (wt/vol) sodium pyruvate were tested to optimize recovery of desiccated cells. Nonselective media showed better recovery when TSAYE and Columbia agar contained 0.1% (wt/vol) pyruvate and were incubated aerobically. These two culture methods were equally effective (P > 0.05) for recovering desiccated L. monocytogenes cells.     

RECOVERY LIMITS OF HEAT-INJURED LISTERIA MONOCYTOGENES FROM ENRICHMENT BROTH AND ENRICHED CULTURES OF INOCULATED FOODS

Recovery of heat-injured Listeria monocytogenes strain LM82 was evaluated quantitatively in Listeria enrichment broth (LEB) and in enriched cultures of cooked shrimp and Brie cheese. LM82 cells [10⁸ colony forming units (CFU)/ml] were heated for 60 min at 52C in phosphate-buffered saline. After 24 and 48 h enrichment, injured LM82 (6 replicates at each of 5 inoculation levels) were isolated on 3 selective media: lithium chloride-phenylethanol-moxalactam agar (LPMA), modified McBride agar (MMA) and Oxford agar (OXA). The recovery limit was expressed as a 50% end point value (RL₅₀), which is the calculated inoculation value necessary to recover LM82 on half of the replicates of each type of isolation agar plate after streaking from the enrichment of measured inoculum. The RL₅₀ values for injured cells were comparable to those of uninjured cells after 48 h enrichment in LEB without food. The type of isolation agar did not affect the RL₅₀ value, although with food, MMA gave consistently but not significantly higher values, i.e., recovery inferior to that of LPMA and OXA. RL₅₀ values were higher in Brie and cooked shrimp, presumably because of the competitive microflora in those foods. Addition of lactose or pyruvate to LEB improved recovery but had little or no effect when foods were present .     

A solid agar overlay method for recovery of heat-injured Listeria monocytogenes

A solid agar overlay method was developed for recovery of heat-injured Listeria monocytogenes. Presolidified nonselective tryptic soy agar with 0.6% yeast extract (TSAYE, 2% agar) was overlaid on top of solidified modified Oxford agar (MOX). Heat injury of L. monocytogenes was conducted at 58 degrees C for 6 min in a jacketed flask filled with tryptic soy broth. Both noninjured and heat-treated L. monocytogenes cells were plated onto TSAYE, MOX, and TSAYE-MOX plates. No significant differences (P > 0.05) in recovery were found among the three media for noninjured bacterial cells. Recovery of heat-injured L. monocytogenes cells on TSAYE-MOX overlay plates was equivalent to that on the nonselective TSAYE medium, whereas recovery on the selective MOX medium was significantly lower (P < 0.05) compared with both TSAYE and the overlay plates. There were no significant differences (P > 0.05) among the overlay plates prepared 0, 2, 4, 6, 8, 16, and 24 h prior to plating heat-injured bacterial cells. The TSAYE-MOX overlay also allowed differentiation of L. monocytogenes from a mixture of four other types of foodborne pathogens. This solid agar overlay method for recovery of heat-injured L. monocytogenes cells is less time-consuming and less complicated than the conventional overlay-underlay technique and the double overlay modification of the thin agar layer method and may allow for greater laboratory plating efficiencies.     

Comparison of dry culture medium and conventional plating techniques for enumeration of bacteria in pasteurized fluid milk

Standard plate counts, psychrotrophic bacterial counts, and coliforms were determined by conventional plating techniques and by Petrifilm TM plates, a dry culture medium, for 48 commercially processed milk samples (24 whole milk and 24 skim milk). The Petrifilm SM plate counts were compared with counts on standard methods agar for the standard plate count, psychrotrophic bacterial count, and rapid psychrotrophic bacterial count. The Petrifilm violet red bile plate counts were compared with counts on violet red bile agar for coliform test with a solid medium and the preliminary incubation method for detection of coliforms. Standard plate counts were determined within 24 h of packaging and after 7, 10, and 14 d of storage at 6.1 degrees C. Psychrotrophic bacterial counts and coliform counts were determined with 24 h of packaging and after 7 d storage. There was a strong linear relationship between Petrifilm SM and standard methods agar plates (excluding counts on samples plated within 24 h of packaging) and for the psychrotrophic bacterial count method. Petrifilm SM had a weak linear relationship with Standard Methods Agar plates for the rapid psychrotrophic bacterial count. Coliform counts determined on Petrifilm violet red bile plates were generally within the same range as counts on violet red bile agar plates. The positive predictive values for the Petrifilm violet red bile plates and violet red bile agar plates were essentially the same for samples plated within 24 h of packaging.     

Comparative evaluation of adhesion and biofilm formation of different Listeria monocytogenes strains

Thirteen Listeria monocytogenes strains were used to grow biofilms on glass surfaces in static conditions at 37 degrees C for up to 4 days. After the initial 3-h adhesion and in subsequent 1-day intervals, cell numbers were determined using standard plate count after swabbing the cells from the glass surface. The three-dimensional structure of in situ biofilms was determined by confocal scanning laser microscopy (CSLM). After 3 h incubation, bacterial cells for all 13 strains of L. monocytogenes were found attached to glass slides and all strains formed biofilms within 24 h. The strains varied significantly in their ability to adhere to the surface and significant differences for cell numbers after 24 h biofilm growth were found. Cell counts in biofilms formed by five L. monocytogenes strains were monitored over 4 days. The counts increased for the first 2 days reaching 10(5) cfu/cm2, except for L. monocytogenes 7148 (10(4) cfu/cm2). After 2 days, cell counts remained at 10(5) cfu/cm2 for four strains (tested on days 3 and 4), while L. monocytogenes 7148 continued to grow and reached 10(5) cfu/cm2 on day 4. This difference in biofilm growth was not related to variations in growth rates of planktonic cells suggesting that growth behaviour of Listeria in biofilms may be different from their planktonic growth. CSLM revealed that the biofilms grown under static conditions consisted of two distinct layers with 0.5 log10 higher cell numbers in the bottom layer as compared to the upper layer.     

A comparative study of the 'FDA' and 'USDA' methods for the detection of Listeria monocytogenes in foods

Nineteen laboratories across Canada took part in a comparative study of the 'FDA' and 'USDA' methods for the detection of Listeria monocytogenes in foods and environmental samples. The results show that the enrichment period of the FDA method can be shortened from 7 to 2 days without substantially reducing the number of positive samples. With a limited number of samples, the USDA method proved to be slightly more efficient in isolating L. monocytogenes than the FDA method. Fraser broth, in principle, proved to be useful as a screening tool but is not very selective. Oxford agar and lithium chloride-phenylethanol-moxalactam medium were better than modified McBride's agar in isolating this microorganism.     

Antimicrobial Effect of Cetylpyridinium Chloride on Listeria monocytogenes V7 Growth on the Surface of Raw and Cooked Retail Shrimp

ABSTRACT:  This study examined the concentration of cetylpyridinium chloride (CPC) required to control Listeria monocytogenes on the surfaces of raw and cooked, peeled and shell-on shrimp. Shrimp (5 g) were inoculated by immersion into a 24 h culture of L. monocytogenes (decimally diluted in PBS) for 1 min, followed by air drying for 1 h, to yield between 6.2 log and 7.0 log CFU/g. The raw and cooked shell-on samples had higher L. monocytogenes counts than the peeled shrimp groups after this inoculation process. The shrimp samples were treated by soaking in different concentrations of CPC (0.05, 0.1, 0.2, 0.4, 0.6, 0.8, or 1.0%) solutions for 1 min, with or without a water rinse for 1 min. The samples were bagged, stored at 4 °C for 24 h, and then plated onto Oxford selective media for determination of log CFU/g. All CPC treatments (0.05% to 1.0%) that were followed by a water rinse reduced L. monocytogenes counts on cooked shrimp by about 2.5 log CFU/g. Conversely, treatments not followed by a water rinse reduced L. monocytogenes counts on cooked shrimp by 3 log CFU/g with 0.1, 0.2, or 0.4% CPC, 5 log CFU/g with 0.6% CPC, 6 log CFU/g with 0.8% CPC, and 7.0 log CFU/g with 1.0% CPC. These results indicate that a soaking treatment of CPC has a strong potential to eliminate or reduce L. monocytogenes on the surfaces of shrimp.     

Survival of Listeria monocytogenes on fresh and frozen strawberries

Cut or intact surfaces of fresh strawberries were spot inoculated with a five-strain cocktail of nalidixic-acid resistant Listeria monocytogenes (10(6) (low inoculum) and 10(8) (high inoculum) CFU per three-berry sample). Inoculated strawberries were dried for 1 h at 24 degrees C and were stored in loosely closed containers at 4 or 24 degrees C. An initial population reduction of approximately 0.6 and 1.2 log cycles, high and low inoculum, respectively, was observed on intact but not cut berries after the 1-h drying period. A decrease of 1.4 and 3.3 log cycles per intact sample was observed over 48 h for the high and low inoculum, respectively, when stored at 24 degrees C. When held at 4 degrees C, a reduction of approximately 3 log cycles per intact-berry sample was observed for both inocula over the 7-day storage period. Populations on cut surfaces remained constant at both temperatures and both inoculum densities throughout the storage period. Sliced, inoculated strawberries (6.7 log CFU/25-g sample) with or without 20% sucrose were frozen at -20+/-2 degrees C. After 28 days of frozen storage, populations of L. monocytogenes determined on tryptose phosphate agar supplemented with nalidixic acid (TPAN) had declined by 0 to 1.2 log cycles, with and without 20% sucrose, respectively. Counts on modified Oxford agar supplemented with nalidixic acid were significantly (P< or =0.05) lower (0.5 to 1.8 log CFU/g) than on TPAN indicating that some cell injury had occurred. Results of this study indicate that L. monocytogenes is capable of survival but not growth on the surface of fresh intact or cut strawberries throughout the expected shelf life of the fresh fruit and can survive on frozen strawberries for periods of at least 4 weeks. On whole strawberries held at 24 degrees C, significantly faster declines (P< or =0.05) of L. monocytogenes were observed when lower rather than higher inoculum levels were applied.     

Fate of Escherichia coli O157:H7 and Listeria monocytogenes in strawberry juice and acidified media at different pH values and temperatures

Survival and growth of Escherichia coli O157:H7 and Listeria monocytogenes in strawberry juice and acidified media at different pH levels (pH 3.4 to 6.8) and temperatures were studied. Sterile strawberry juice (pH 3.6) and acidified trypticase soy broth (TSB) media (pH 3.4 to 6.8) were inoculated with approximately 6.7 log CFU/ml E. coli O157:H7 or 7.3 log CFU/ ml L. monocytogenes, incubated for 3 days at 4 and 37 degrees C. Bacterial levels were determined after 2 h, 1 day, and 3 days using surface plating nonselectively on tryptic soy agar and selectively on sorbitol MacConkey agar for E. coli O157:H7 or modified Oxford agar for L. monocytogenes. A spectrophotometer (660 nm) was also used to study growth inhibition of L. monocytogenes in different TSB and strawberry juice media (pH 3.4 to 7.3). E. coli O157:H7 survived well at pH values of 3.4 to 6.8 at 4 degrees C, but the number of injured cells increased as pH decreased and incubation time increased. At 37 degrees C, E. coli O157:H7 was inactivated at pH of < or = 3.6 but could grow at pH 4.7. L. monocytogenes was quickly injured at pH of < or = 4.7 within 2 h of storage at 4 degrees C and then was slightly and gradually inactivated as storage time increased. L. monocytogenes survived well at pH 6.8 at 4 degrees C and grew well at 37 degrees C. Growth of L. monocytogenes at 37 degrees C was inhibited in TSB by 1% citric acid and 0.5% malic acids at pH 3.4 or by 50% strawberry juice at pH 4.7. Bacterial injury and inactivation appeared to be induced by the acids in strawberry juice. The acids, pH value, temperature, and time were important factors for bacterial survival, inactivation, and growth in the media tested.