Xylose Lysine Desoxycholate Research Articles

THERMAL RESISTANCE OF ANTIBIOTIC‐RESISTANT AND ANTIBIOTIC‐SENSITIVE SALMONELLA SPP. ON CHICKEN MEAT

ABSTRACT The aim of this research was to investigate the potential relationship, if any, between the acquisition/possession of antibiotic resistance genes in strains of Salmonella and its resistance to heat stress. Chicken pieces were inoculated with antibiotic sensitive (AS) strains of S. Enteritidis and S. Typhimurium, its laboratory‐acquired antibiotic‐resistant (AR) mutant strains (nalidixic acid and streptomycin), or a multiresistant strain of S. Typhimurium DT104. Half of these samples were heat‐shocked (48C for 30 min) and all were heat‐challenged at 55C for up to 30 min. Samples were then plated on xylose lysine desoxycholate (XLD) and tryptone soya agar (TSA) overpoured with XLD. Heat‐shocked cultures of S. Typhimurium DT104 had significantly higher D‐values (the time required for a 1 log reduction in the number of bacteria) than their non‐heat‐shocked counterparts (P < 0.05). No significant differences were observed between AR and their AS. However, the D‐values for S. Typhimurium DT104 were significantly higher than the D‐values for S. Typhimurium (AS) and S. Enteritidis (AS) (P < 0.05). This study concluded that laboratory‐acquired antibiotic‐resistant mutation did not affect heat resistance of the Salmonella strains studied and suggested a potential link between multiantibiotic resistance and heat resistance in S. Typhimurium DT104.

Evaluation of three enrichment broths and five plating media for Salmonella detection in poultry

We evaluated the effectiveness of Selenite Cystine (SC), Tetrathionate Brilliant Green (TBG) and Rappaport Vassiliadis (RV) broths for Salmonella isolation. We also tested three classic plating media, Salmonella-Shigella Agar (SS), Brilliant Green Agar (BGA), Xylose Lysine Desoxycholate Agar (XLD) and two chromogenic agars, Rambach (RA) and CHROMagar Salmonella (CAS). Among 100 poultry carcasses, 29 were positive for Salmonella using all plating media combined. RV broth (69%) and TT broth (58.6%) were more effective than SC broth (24.1%). The chromogenic media gave better results than the classic ones with less false-positive colonies. The most effective isolation medium was CHROMagar, where Salmonella was identified in 23 (79.3%) of the 29 positive samples, followed by Rambach (48%). Positivity for Salmonella using classic media was 13.8% for BGA, 27.6% for SS and 34.5% for XLD.

Thermal Resistance of Heat-, Cold-, and Starvation-Injured Salmonella in Irradiated Comminuted Turkey

To investigate the effects of sublethal stress on Salmonella thermal inactivation kinetics, an eight-strain Salmonella cocktail was subjected to heat shock (30 min at 54°C), cold shock (2 h at 4°C), and starvation stress (10 days in phosphate buffer at 4°C), harvested by centrifugation, and inoculated into irradiated comminuted turkey. Immediately after stressing, the Salmonella cocktails contained 89.1% heat-injured, 44.7% cold-injured, and 67.7% starvation-injured cells, as determined by plating on selective and nonselective media. D60°C-values for the heat-shocked cocktail (0.64 min on Trypticase soy agar containing 0.6% yeast extract [TSAYE], 0.35 min on xylose lysine desoxycholate [XLD] agar) were higher (P < 0.05) than those for the unshocked control (0.41 min on TSAYE, 0.17 min on XLD), whereas D60°C-values for the cold-shocked cocktail (0.38 min on TSAYE, 0.17 min on XLD) were not significantly different from those for the control. Starved cells had the same D60°C-value on TSAYE as did the unshocked cocktail, but the D60°C-value on XLD was significantly lower (0.14 min). Although starvation and cold shock were not thermally protective, heat shock increased thermal resistance, indicating that product history and the physiological state of the Salmonella cells should be considered when developing and validating thermal processes. D60°C-values observed on selective media were significantly lower than those observed on nonselective media for all stress treatments and for the control. Therefore, nonselective culture media should be used to assess the response of microorganisms to a thermal challenge when developing performance standards for lethality.

Factors affecting survival, growth, and retrieval of Salmonella Poona on intact and wounded cantaloupe rind and in stem scar tissue

Studies were done to determine the survival and recovery of Salmonella enterica serotype Poona from cantaloupe rind as affected by environmental conditions between the time of contamination and analysis. Detection and enumeration of the pathogen as influenced by analytical methods were also investigated. Combinations of preenrichment broth (lactose broth or universal preenrichment broth), enrichment broth (Rappaport–Vassiliadis broth or tetrathionate broth), and selective agar medium (bismuth sulfite agar or xylose lysine desoxycholate agar) for detecting S. Poona on inoculated cantaloupes stored at 4°C for 7 days or 21°C for 3 days were equivalent in performance. The use of nalidixic acid resistance as a marker in S. Poona and nalidixic acid in media used to enhance detection or enumeration of the pathogen by inhibiting background micro-flora in sanitizer efficacy studies, for example, would not adversely affect its survival on or recovery from cantaloupes. Overall, the composition of the carrier (water or 5% horse serum, a high organic matrix) used to prepare inocula did not influence the number of S. Poona recovered from the intact rind surface, wounds in the surface, or the stem scar tissue. Regardless of inoculation site or composition of the carrier, populations on spot inoculated melons stored at 4°C remained constant between 2 and 24 h after inoculation. The pathogen grew within 24 h in wounds of spot- and dip-inoculated cantaloupes stored at 21°C and 37°C. The addition of up to 1.0% Tween 80 to 0.1% peptone used to remove S. Poona from the rind surface did not adversely affect viability and may have enhanced detachment. Consideration of these observations is recommended when developing a method to test the efficacy of sanitizers in killing salmonellae on the rind surface of inoculated cantaloupes and to detect or enumerate salmonellae that may be natural contaminants.

Evaluation of VIDAS® Immuno-Concentration Salmonella (ICS) Plus Selective Plate Method (Hektoen Enteric, Bismuth Sulfite, Xylose Lysine Desoxycholate) for Detection of Salmonella in Selected Foods (Method Modification 2001.08): Collaborative Study

A new method for detection of Salmonella in foods in 48 h has been granted AOAC First Action approval in selected foods (Official Method 2001.08) using both the VIDAS Immuno-Concentration Salmonella (ICS) method and a combination of 3 selective plates: Hektoen enteric (HE), bismuth sulfite (BS), and xylose lysine desoxycholate (XLD).

Inactivation of Salmonella Typhimurium and Escherichia coli O157:H7 in Apple Juice by a Combination of Nisin and Cinnamon

Pasteurized apple juice with nisin (0, 25, 50, 100, and 200 ppm, wt/vol) and cinnamon (0 and 0.3%, wt/vol) was inoculated with Salmonella Typhimurium and Escherichia coli O157:H7 at 104 CFU/ml and stored at 5 and 20°C. Counts on tryptic soy agar (TSA), selective medium (xylose lysine desoxycholate agar for Salmonella Typhimurium, and MacConkey sorbitol agar for E. coli O157:H7), and thin agar layer (TAL) were determined at 1 h and 1, 3, 7, and 14 days. The TAL method (selective medium overlaid with TSA) was used for recovery of sublethally injured cells. The pathogens were gradually inactivated by the acidic pH of apple juice. Nisin and cinnamon greatly contributed to the inactivation. The killing effect was more marked at 20°C, with counts in all treated samples being undetectable by direct plating in 3 days for Salmonella Typhimurium and 7 days for E. coli O157:H7. Thus, several factors influenced the decrease in counts: low pH, addition of nisin and cinnamon, and storage temperature. The TAL method was as effective as TSA in recovering injured cells of the pathogens. The combination of nisin and cinnamon accelerates death of Salmonella Typhimurium and E. coli O157:H7 in apple juice and so enhances the safety of the product.

Preenrichment Versus Direct Selective Agar Plating for the Detection of Salmonella Enteritidis in Shell Eggs

The relative effectiveness of two methods for the recovery of Salmonella Enteritidis (SE) from jumbo and medium shell eggs was compared. The first method used in the comparison consisted of a preenrichment of the sample, and the second method was developed by the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS). Three bulk lots of blended, pooled eggs, each containing 220 liquid whole eggs that were thoroughly mixed manually were artificially inoculated with different levels of SE cells between approximately 100 and 103 CFU/ml. Twenty samples containing the contents of approximately 10 eggs each (by weight) were withdrawn from each of the inoculated bulk lots and incubated for 4 days at room temperature (ca. 23°C). For the APHIS method, each sample was cultured by direct plating onto brilliant green (BG), brilliant green with novobiocin (BGN), xylose lysine desoxycholate (XLD), and xylose lysine agar Tergitol 4 (XLT4) agars. For the preenrichment method, 25-g portions from each pool were enriched in modified tryptic soy broth with 30 mg/liter of FeSO4. After 24 h of incubation, the preenrichments were subcultured to tetrathionate and Rappaport-Vassiliadis broths, and streaked to BG, BGN, bismuth sulfite, XLD, and XLT4 agar plates. SE isolates were confirmed biochemically and serologically. In all of the experiments, the preenrichment method recovered significantly more SE isolates (P < 0.05) of all the phage types and inoculum levels than did the APHIS method. From a total of 539 jumbo egg test portions analyzed, 381 (71%) were SE-positive by the preenrichment method and 232 (43%) were positive by the APHIS method. From a total of 360 medium egg test portions analyzed, 223 (62%) were SE-positive by the preenrichment method and 174 (48%) were positive by the APHIS method. The preenrichment method provided greater sensitivity for the isolation of SE in contaminated egg slurries than did the APHIS method.

Evaluation of Several Modifications of an Ecometric Technique for Assessment of Media Performance

Recovery of Listeria monocytogenes 101M, Jonesia denitrificans, salmonellae, and Pediococcus sp. NRRL B-2354 across nine media was evaluated with three modified versions of an ecometric method. Two approaches involved the use of broth cultures (108 to 109 CFU/ml) of individual strains and either large (10-μl) or small (1-μl) presterilized plastic loops. The third approach involved precultured slants and the inoculation of media with presterilized plastic inoculating needles (104 CFU per needle). Absolute growth indices (AGIs) were compared. No significant differences (P < 0.05) between methods were found when tryptic soy agar supplemented with 0.6% yeast extract (TSAYE) was used for the recovery of L. monocytogenes, J. denitrificans, Pediococcus sp. NRRL B-2354, and Salmonella spp. However, the small loop–broth technique recovered significantly fewer Salmonella enterica Typhimurium DT104 and Salmonella Senftenberg 775W cells than the other two techniques did. The performance of each individual bacterial strain on each of nine media was assayed. The recovery of L. monocytogenes was excellent (AGI > 4.8) with TSAYE, PALCAM, modified Oxford medium (MOX), and Baird-Parker agar and slight with modified PRAB (AGI = 0.4) and deMan Rogosa Sharpe (MRS) agar (<0.1), and the organism was not recovered with the remaining media (modified lysine iron agar [MLIA], xylose lysine desoxycholate [XLD] agar, and xylose lysine tergitol 4 [XLT4] agar). The recovery of J. denitrificans with TSAYE and MOX was excellent, significantly better than that achieved with PALCAM (AGI = 3.0), but the organism was not recovered with Baird-Parker agar or with the other media tested. The recovery of Pediococcus sp. NRRL B-2354 was excellent with TSAYE and modified PRAB medium > Baird-Parker agar > acidified MRS agar, but the organism was not recovered with any of the other media tested. The best recovery of S. enterica Typhimurium DT104 was achieved with TSAYE > MLIA ≥ XLD agar ≥ XLT4 agar > Baird-Parker > PALCAM, MOX, acidified MRS agar, modified PRAB, and MRS agar. The best recovery of Salmonella Senftenberg 775W was achieved with TSAYE, MLIA, and XLD agar > XLT4 agar, but the organism was not recovered with the other media evaluated.

Molecular Characterization of Salmonella spp. Isolated from Bulk Tank Milk and Cull Dairy Cow Fecal Samples

The consumption of meat from cull dairy cows and of raw milk has been associated with foodborne salmonellosis. This survey was conducted to establish the prevalence of Salmonella in cull dairy cow fecal samples and bulk tank milk and to determine the proportion of Salmonella-positive dairy farms (n = 30) in east Tennessee. Food and Drug Administration bacteriological analytical protocols were generally used for Salmonella isolation. Primary enrichment was performed with lactose broth, and secondary enrichment was conducted with tetrathionate broth. Eosin methylene blue, hektoen enteric, xylose lysine desoxycholate, bismuth sulfite, and brilliant green (BG) were used as isolation agars. BG agars supplemented with individual antibiotics and/or sulfur compounds were also evaluated. Six of 268 (2.24%) bulk tank milk samples and 9 of 415 (2.17%) fecal samples from 7 of 30 (25.3%) dairy farms were Salmonella-positive. Most isolates (11 of 15) were obtained between September and December. Salmonella isolates were further characterized using polyvalent somatic O Salmonella antiserum, o-nitrophenyl-β-d-galactopyranoside (ONPG), and Analytical Profile Index (API) 20E strips for Enterobacteriaceae. Serological evaluation of presumptive positive Salmonella isolates resulted in substantial numbers of false positives (41.2%). ONPG and API 20E tests enabled further biochemical distinction of the majority of Salmonella spp. from Salmonella Arizonae and closely related members of Enterobacteriaceae like Citrobacter youngae. Pulsed-field gel electrophoresis of SpeI-digested Salmonella DNA was used to subtype isolates. The isolates grouped into four clusters. The baseline information generated in this survey is being used to develop preharvest pathogen reduction programs on selected farms.

Salmonella in Selected Foods by VIDAS® Immuno-Concentration Salmonella Plus Selective Plate Method (Hektoen Enteric, Xylose Lysine Desoxycholate, Bismuth Sulfite): Collaborative Study

The VIDAS Immuno-concentration Salmonella (ICS) plus selective plate method (Hektoen enteric, xylose lysine desoxycholate, bismuth sulfite) method for the detection of Salmonella was compared to the Bacteriological Analytical Manual (BAM)/AOAC culture method in a collaborative study. Thirty-two laboratories participated in the evaluation. Each laboratory tested one or more of the 6 test products: milk chocolate, nonfat dry milk, dried whole egg, soy flour, ground black pepper, and ground raw turkey. The 2 methods were in agreement for 1,297 of the 1,455 samples. Of the 158 samples not in agreement, 82 were VIDAS ICS plus selective plate-positive and BAM/AOAC-negative, and 76 were VIDAS ICS plus selective plate-negative and BAM/AOAC-positive.

Development of membrane filter holder (MFH) method for recovery of heat-injured Escherichia coli O157:H7 and Salmonella typhimurium.

A method of recovering sublethally heat-injured bacteria was developed with specific apparatus (membrane filter holder; MFH) which was originally used for Iso-Grid Hydrophobic membrane. filter holder. The procedure used a non-selective agar underlayed with a selective medium with a MFH. A non-selective agar was poured on upper part (compartment A) of MFH, and then injured foodborne pathogens were inoculated on the non-selective medium. After 3-h repair incubation period, selective agar was added to the bottom of the chamber (compartment B) of the MFH and further incubated. By diffusing through the non-selective top agar, selective agents from the underlay medium impart selectivity to the system. Using the MFH method, recovery of heat-injured foodborne pathogens (Escherichia coli O157:H7 and Salmonella typhimurium) were not different (P > 0.05) from recoveries with non-selective media (TSA). However, the recoveries of foodborne pathogens on MFH were significantly higher (P < 0.05) than those of direct plating on selective medium such as SMAC (MacConkey Sorbitol Agar) or XLD (Xylose Lysine Desoxycholate). In conclusion, the MFH method is a simple and convenient method for recovery of heat-injured foodborne pathogens.

Evaluation of culture media for enrichment and isolation of Shigella sonnei and S. flexneri

The performance of Gram-negative (GN) broth with (10 μg/ml) and without novobiocin, Shigella broth (SB) with 0.5 and 3.0 μg/ml novobiocin, all incubated at 37 °C (SB with 3.0 μg/ml novobiocin also at 42 °C) and buffered brilliant green bile glucose (EE) broth with 1.0 μg/ml novobiocin incubated at 37 and 42 °C were evaluated for resuscitation and growth of Shigella sonnei and S. flexneri (eight strains; unstressed, chill-stressed and acid-stressed) and non-shigellae (11 strains). GN broth with or without novobiocin supported significantly less growth of Shigella sp. No significant differences in growth of shigellae were obtained between the other culture media. Performance depended more on the Shigella strain used. None of the tested media were significantly superior for suppressing the competitive flora. Electivity and selectivity of MacConkey agar (MAC), tergitol-7 agar (T7), desoxycholate citrate agar (DCA), xylose lysine desoxycholate agar (XLD), Salmonella Shigella agar and Hektoen enteric agar (HEA) were determined by ecometric testing. HEA confirmed to be a high selective medium for both non-shigellae and stressed Shigella sp. Klebsiella sp., Enterobacter sp., Citrobacter sp., Salmonella sp. and the Escherichia strains can mask the presence of shigellae. In vitro competition experiments and experiments with artificially contaminated foods showed higher resistance of S. sonnei than S. flexneri towards the stress imposed by the food matrix and its indigenous flora. Reliable detection, however, of shigellae in foods with the current enrichment and isolation media was not achieved.

SURVIVAL OF THREE SALMONELLA SEROTYPES ON BEEF TRIMMINGS DURING SIMULATED COMMERCIAL FREEZING AND FROZEN STORAGE

ABSTRACT This study investigated the survival of three Salmonella serotypes (S. Brandenberg, S. Dublin and S. Typhimurium) on beef trimmings during simulated commercial freezing, frozen storage for 9 months and subsequent abusive slow thawing and refreezing conditions. This was achieved by plating samples monthly and after thawing and refreezing on nonselective Tryptic Soy Agar (TSA) and selective Xylose Lysine Desoxycholate Agar (XLD) and incubating both at 37C for 24 h to determine Salmonella counts, aerobic counts and the presence, if any, of sublethal injury of this pathogen. Two freezing temperatures (−18C or −35C) to simulate slow or rapid freezing respectively, and two inoculation levels (103 cfu g−1 or 105 cfu g−1) were used. Aerobic counts and counts of all the Salmonella serotypes did not change significantly (p &gt; 0.05) during frozen storage or for any of the other treatments applied in this study. This finding was attributed to the insulating nature of the subcutaneous fat layer in this manufacturing cut. These results are important with respect to food safety associated with ground beef processing.

Survival of Escherichia coli O157:H7 and Salmonella on chill-stored vacuum or carbon dioxide packaged primal beef cuts

The ability of two Escherichia coli O157:H7 strains (E27, a cattle isolate, and B6-914 gfp-91, a fluorescent marker strain) and two Salmonella serotypes ( S. typhimurium and S. brandenberg) to survive on chilled preservatively packaged primal beef cuts was examined. Each of the strains was inoculated separately at two dilution levels (10 3 and 10 5 cfu g −1) onto 500 g beef steaks, packaged under vacuum or 100% carbon dioxide, and stored, with uninoculated controls, for 6 weeks at −1.5°C, then for 2 weeks at 4°C. Bacterial numbers were determined by dilution and incubation at 37°C for 24 h on either Sorbitol McConkey Agar or Xylose Lysine Desoxycholate Agar for E. coli O157:H7 and Salmonella samples, respectively. Counts were corrected for background growth and their accuracy checked using immunological tests. Fluorescent E. coli O157:H7 B6-914 gfp-91 was also counted under ultra-violet light. No significant changes in numbers of the E. coli O157:H7 or Salmonella strains occurred during storage at either −1.5 or 4°C packaged under either vacuum or carbon dioxide. The ability of these pathogens to survive standard preservative packaging conditions is different from that reported from their generic counterparts and therefore a cause for public health concern.

Improved Detection of Nontyphoid and Typhoid Salmonellae with Balanced Agar Formulations

A strategically balanced medium was developed for the improved detection of nontyphoid and typhoid salmonellae. Its balanced sugar (cellobiose, lactose, mannitol, and trehalose) and protein (beef extract and polypeptone peptone) formulation provided Salmonella with a selective growth advantage over non-Salmonella enteric organisms. The formulations promoted the production and detection of H2S production levels that otherwise might be missed with traditional agar formulations. In combination, these advantages contributed to increased sensitivity without the loss of specificity. In comparative studies using 86 samples of meat products (beef, pork, and chicken), the new media, Miller-Mallinson (MM) agar and xylose lysine tergitol (Niaproof) 4 agar, possessed significantly higher sensitivity (P < 0.001) and an improved specificity over bismuth sulfite, hektoen enteric, and xylose lysine desoxycholate agars. However, these samples did not contain nontyphoid salmonellae with weak to ultraweak H2S production characteristics. Modified formulations of MM agar were generally similar to bismuth sulfite and hektoen enteric agars in the identification of four of seven globally diverse strains of Salmonella serotype Typhi. Two of these seven strains were found to produce more readily identifiable black (H2S-positive) colonies on MM agar, whereas one of the seven was not readily detected by any of the media. The improved detection of nontyphoid and typhoid salmonellae attests to the sensitivity of MM agar and to its potentially broad utility in both clinical and food quality laboratories.

Update on media for isolation of Enterobacteriaceae from foods

Testing for `total' Enterobacteriaceae, coliforms and Escherichia coli as marker organisms in foods and detection of specific pathogens of the family Enterobacteriaceae, including pathogenic E. coli, Salmonella, Shigella and Yersinia spp. is widely applied in many food control laboratories. This review describes some recent developments in culture media for these organisms. Methods for enumeration of E. coli include the standard MPN technique, a membrane-filter method and the use of media containing chromogenic and fluorogenic indicators for β- d-glucuronidase (GUD) activity. Shiga toxin-producing E. coli O157 strains usually do not ferment sorbitol and are GUD-negative. These characteristics are used in selective media for these organisms, such as cefixime tellurite sorbitol MacConkey agar. For the detection of salmonellae, motility enrichment in Modified Semisolid Rappaport–Vassiliadis (MSRV) medium shows equal or better results than the use of standard Rappaport–Vassiliadis broth. Addition of nitrofurantoin to diagnostic semisolid salmonella agar and to xylose lysine desoxycholate agar favours the isolation of S. enteritidis. Recently developed salmonella media use different selective and diagnostic properties, such as acid formation from propylene glycol, glucuronate fermentation, fermentation of glycerol and addition of Tergitol 4 as selective agent. The isolation of Shigella spp. from foods is rather difficult and further evaluation of suggested isolation systems and the development of more effective methods for the isolation of this pathogen are needed. Yersinia enterocolitica includes both pathogenic and nonpathogenic biotypes and serogroups. As no single procedure will recover all pathogenic strains of Y. enterocolitica, the use of two isolation procedures in parallel is recommended.

Rambach agar as a new plate differential medium for the identification of some enteric pathogens in meat products

An investigation of the efficacy of both Rambach agar and xylose-lysine-desoxycholate (XLD) agar as media for isolation of some enteric pathogenic bacteria from raw meat and ready-to-eat meat products was conducted. For this purpose, 50 samples representing 20 and 30 samples of raw and ready-to-eat meat products respectively were examined and the growth characteristics of various Gram-negative bacteria on XLD and Rambach agars were compared. In the case of raw meat products 33.3% of suspected salmonella colonies showing growth on XLD agar and not on Rambach agar gave false results in biochemical and serological examinations. In contrast, all suspected salmonella colonies on Rambach agar were positive biochemically and serologically. Furthermore, suspected salmonella colonies isolated from ready-to-eat meat products on XLD agar showed 83.3% false positive cases while in case of Rambach agar no false positive cases were detected. Therefore, Rambach agar excelled XLD agar in the isolation of salmonella from both raw and ready-to-eat products.

Comparison between a cultural procedure using Rappaport-Vassiliadis broth and motility enrichments on modified semisolid Rappaport-Vassiliadis medium for Salmonella detection from food and feed

Motility enrichment on modified semisolid Rappaport-Vassiliadis (MSRV) medium was evaluated for a rapid Salmonella detection in naturally contaminated food- and feed samples. The results of this procedure were compared with a conventional cultural procedure using Rappaport-Vassiliadis (RV) broth as selective enrichment and modified brilliant green-, xylose lysine desoxycholate- and mannitol lysine crystal violet brilliant green agar for selective plating. The productivity of motility enrichment was 87% compared to a productivity of 99% for the cultural procedure. Statistical analysis showed that there was a significant difference at the 5% level. The present study shows that a combination of direct motility enrichment with the cultural procedure could be used as a very rapid and highly cost-efficient method for Salmonella detection.

Relative Effectiveness of Selective Plating Agars for Recovery of Salmonella Species from Selected High-Moisture Foods

The relative effectiveness of 6 selective plating media were compared for effectiveness in recovery of Salmonella spp. from selected high-moisture foods. Three new plating agars (EF-18, Rambach, and xylose lysine Tergitol-4) and 3 selective plating agars (bismuth sulfite, Hektoen enteric, and xylose lysine desoxycholate) recommended by AOAC INTERNATIONAL and the Bacteriological Analytical Manual (BAM) were compared. The agars were streaked from cultures selectively enriched in selenite cystine broth, tetrathionate broth, and Rappaport-Vassiliadis medium. The high-moisture foods studied were naturally contaminated pork sausage, chicken parts, turkey parts, and frog legs and artificially contaminated shrimp, oysters, egg yolks, and lettuce. The relative effectiveness of each selective plating agar was determined by recovery of Salmonella spp. and enumeration of false-positive and false-negative reactions. Although the new selective plating agars compared favorably with the AOAC/BAM-recommended agars, they offered no advantage. Incubation of selective enrichment broths at elevated temperatures decreased the numbers of false-positive and false-negative reactions for all 6 selective plating agars.

Microbiological Safety of Blue and Cheddar Cheeses Containing Naturally Modified Milk Fat

Milk containing naturally modified fat was obtained by feeding lactating dairy cows a Control diet and two experimental diets containing either extruded soybeans or sunflower seeds. Milk from cows fed the experimental diets contained higher levels of both long chain (C18-C18:2) and unsaturated fatty acids than the milk from cows fed the Control diet. Each milk was pasteurized, standardized to 3.6% milk fat, and inoculated with Listeria monocytogenes (strains Scott A and V7), Salmonella typhimurium and Salmonella senftenberg, before manufacturing into Blue or stirred-curd Cheddar cheeses. Populations of L. monocytogenes and Salmonella spp. were monitored during manufacture and aging using Oxford and Xylose Lysine Desoxycholate agars, respectively. During the manufacture of Blue and Cheddar cheese, and during the aging of Blue cheese, behavior of Salmonella spp. and L. monocytogenes in the experimental cheese was similar to the Control cheese. During aging of Cheddar cheese, the rate and extent of decline of Salmonella spp. and L. monocytogenes varied among the cheeses. Declines correlated with the accumulation of specific fatty acids, namely C12, C14, C18:1 and C18:2. These fatty acids were also found to be inhibitory to S. typhimurium and L. monocytogenes when incorporated into tryptic soy agar plates at 37°C. Therefore, the natural fat modification of Blue and Cheddar cheeses enhanced the safety of these cheeses.