Thermal resistance of Salmonella serovars isolated from raw, frozen chicken nuggets/strips, nugget meat and pelleted broiler feed.

Raw, frozen chicken nuggets/strips available at retail and prepared at home before consumption have been identified as a significant risk factor in contracting food-borne salmonellosis. Cases of salmonellosis from consumption of these products may be due, in part, to Salmonella strains originating in broiler feed. In this study the thermal resistances of Salmonella strains isolated from chicken nuggets and strips, chicken nugget/strip meat and broiler feed were determined to assess whether they exhibited enhanced thermal resistance. Thermal resistances (D- and z- values) of 7 cocktails (25 isolates, 4 serovars) were determined in commercially prepared irradiation-treated chicken nugget/strip meat blend, and heated in a constant temperature waterbath. The thermal resistances found were lower than those reported for similar strains in the literature. D-values ranged from 6.93 to 0.12 min at 55 and 62 degrees C respectively, with z-values from 4.10 to 5.17 degrees C. Two strains of S. Enteritidis separately isolated from pelleted feed and chicken nugget meat blend, with indistinguishable geno- and phenotypes, had lower (and probably identical) thermal resistances than the other isolates. Results indicated that the strains of Salmonella isolated from raw, frozen chicken nuggets/strips and pelleted broiler feed did not exhibit unusually high thermal resistance, and that normal heating (71 degrees C) prior to consumption should eliminate these organisms from chicken nuggets/strips.

Evaluation of the BAX gel and fluorometric systems for the detection of foodborne Salmonella.

The present study compared the sensitivity of the BAX automated fluorometric and the recently discontinued BAX gel electrophoresis systems with a standard culture method to detect Salmonella in 333 high-moisture and 171 low-moisture foods. A total of 95 naturally contaminated foods, including 63 high-moisture and 32 low-moisture foods, were detected by the standard culture method. No contaminated samples were identified exclusively by the BAX systems. By means of the analytical protocol stipulated by the manufacturer, the BAX fluorometric system detected 36 (57.1%) and 29 (90.6%) of the contaminated high- and low-moisture foods, respectively. Similar results were obtained with the BAX gel electrophoresis system, which identified 40 (63.5%) and 26 (81.3%) of the contaminated high- and low-moisture foods. The rate of false-positive reactions with the BAX systems was low. Our results indicate that the low sensitivity of the BAX systems with high-moisture foods, notably raw meats and poultry products, was serovar-independent. The high levels of background microflora that commonly occur in raw meat and on fresh fruit and vegetable products, and the high successive dilutions of test materials for PCR analysis, suggestively undermined the sensitivity of the gel and the fluorometric BAX assays. The potential benefits of immunomagnetic separation of Salmonella in preenrichment cultures, of selective broth enrichment following preenrichment to markedly reduce levels of background microflora in PCR test materials, and the use of larger portions of test materials in PCR analyses should be investigated.

Occurrence and characterization of Salmonella from chicken nuggets, strips, and pelleted broiler feed.

Raw, frozen chicken nuggets and strips have been identified as a significant risk factor in contracting foodborne salmonellosis. Cases of salmonellosis as a result of consuming partly cooked chicken nuggets may be due in part to Salmonella strains originating in broiler feed. This study was undertaken to determine the occurrence and characterize the strains of Salmonella contaminating chicken nuggets, strips, and pelleted feeds, in an attempt to demonstrate whether the same Salmonella strains present in broiler feed could be isolated from raw, frozen chicken nuggets and strips available for human consumption. Salmonellae were recovered using the Health Canada MFHPB-20 method for the isolation and identification of Salmonella from foods. Strains were characterized by serotyping, phage typing, antimicrobial resistance typing (R-typing), and by pulsed-field gel electrophoresis (PFGE). Salmonellae were isolated from 25-g samples in 27% (n=92) of nugget and strip samples, 95% (n=20) of chicken nugget meat samples, and from 9% (n=111) of pelleted feed samples. Salmonella Heidelberg, Salmonella Enteritidis, and Salmonella Orion were the most commonly isolated serovars from chicken nuggets and strips, nugget and strip meat, and pelleted broiler feeds, respectively. Salmonella Enteritidis phage type (PT) 13a with PFGE pattern SENXAI.0006 and R-type sensitive as well as Salmonella Enteritidis PT13a with PFGE pattern SENXAI.0068 and R-type sensitive were isolated from pelleted feed, and chicken nugget and strip meat in two separate instances. Data showed that Salmonella strains isolated from broiler feed were indistinguishable from strains isolated from packaged raw, frozen chicken nuggets and strips. However, results did not rule out the possibility that breeding stock or contamination during processing may have contributed to chicken meat contamination by Salmonella.

Salmonella and the international food trade.

Non-typhoid Salmonella spp. continue to figure prominently in many national epidemiological registries as the leading cause of bacterial foodborne disease. Although Salmonella enterocolitis is generally a self-limiting illness that may require fluid and electrolyte replacement, the disease can spread systemically and degenerate into a chronic condition such as reactive arthritis, osteomyelitis, cardiac inflammation or neural disorders. Ampicillin, chloramphenicol and trimethoprim-sulfamethoxazole have provided the mainstay of therapy for the clinical management of bacteremic salmonellosis. However, the increasing occurrence of strains that are resistant to one or more of these traditional antibacterial drugs has resulted in the wider use of quinolones for the treatment of Salmonella septicaemia. Successful clinical results with these newer drugs are already being overshadowed by the emergence of salmonellae that are resistant to these therapeutic agents. A rapidly growing international trade in agricultural, aquacultural and manufactured food products has greatly facilitated the introduction of new Salmonella serovars within the geographical boundaries of importing countries. This paper reviews the prevalence of Salmonella in selected food types that are subject to the import-export market and attendant epidemiological overtones. More specifically, the importance of fresh fruits and vegetables, spices, cheese, and aquacultural products as vehicles of human infection will be underlined. The potential impact of the widespread use of antibiotics of importance in human medicine in the aquaculture industry will also be discussed. The ubiquitous distribution of Salmonella in the natural environment and its prevalence in the global food chain, the physiological adaptability and virulence of this important human bacterial pathogen, and its potentially serious economic impact on the food industry predicate the need for continued vigilance and stringent controls at all levels of food production.

Psychrotrophy and foodborne Salmonella.

Recent reports on the behaviour of Salmonella at chill temperatures (less than 10 degrees C) raise concerns on the purported safety of refrigerated foods. The propensity for growth of salmonellae within 10-28 days in complex broth (5.9 degrees C) and agar (4.0 degrees C) media is overshadowed by more recent evidence on their capability to proliferate in fresh meats (2.0 degrees C) and shell eggs (4.0 degrees C) within 6 and 10 days, respectively. Such findings, together with the inability of many domestic refrigerators to maintain uniformly cold temperatures, are disquieting. Gaseous mixtures of CO2, N2 and O2 are widely used to extend the shelf life of chilled foods, notably fresh meats. The high levels of CO2 used in modified atmosphere packaging or generated by endogenous microflora in vacuum-packaged foods effectively inhibit the growth of psychrotrophic spoilage bacteria. Current evidence suggests that this industrial practice also arrests the growth of Salmonella but exerts little or no effect on their survival. Enhancement of the bacteriostatic potentials of pH and NaCl as temperature deviates from the optimum for growth to lower values could further contribute to the safety of chilled foods.

Pathogenicity of foodborne Salmonella.

Salmonella remains a leading etiological agent in bacterial foodborne diseases. Although human salmonellosis generally presents as a self-limiting episode of enterocolitis, the disease can degenerate into chronic and debilitating conditions. Antibiotic treatment of uncomplicated salmonellosis is contra-indicated because it tends to prolong the carrier state. Clinical management of systemic infections with newer drugs such as third-generation cephalosporins and quinolones is most promising, particularly in light of the increasing resistance of Salmonella to the traditional ampicillin, chloramphenicol and trimethoprim sulfamethoxazole therapeutic agents. Research into the development of effective vaccines from avirulent auxotrophic or from virulence plasmid-cured strains may ultimately facilitate the control of salmonellosis in human populations and in various agricultural sectors. Human salmonellosis reflects the outcome of a confrontation between humoral and cellular immune responses of the host, and virulence determinants of the invasive pathogen. Following an adhesion-dependent attachment of salmonellae to lumenal epithelial cells, the invasive pathogen is internalized within an epithelial cell by a receptor-mediated endocytotic process. Cytotoxin localized in the bacterial cell wall suggestively may facilitate Salmonella entry into the epithelial layer. Cytoplasmic translocation of the infected endosome to the basal epithelial membrane culminates in the release of salmonellae in the lamina propria. During this invasive process, Salmonella secretes a heat-labile enterotoxin that precipitates a net efflux of water and electrolytes into the intestinal lumen. Although non-typhoid salmonellae generally precipitate a localized inflammatory response in deeper tissues via lymphatics and capillaries, and elicit a major immune response. Current research efforts have focused on the molecular characterization and role of virulence plasmids and chromosomal genes in Salmonella pathogenicity.

Efficacy of prolonged (48 h) selective enrichment for the detection of foodborne Salmonella.

The effect of prolonged (48 h) incubation on the productivity of five enrichment-temperature conditions (tetrathionate brilliant green, 35 and 43 degrees C; Muller-Kauffman tetrathionate brilliant green, 43 degrees C; Rappaport-Vassiliadis, 43 degrees C; selenite cystine, 35 degrees C) was compared to homologous results obtained under standard (24 h) conditions of selective enrichment. Of 797 high moisture and 166 low moisture foods tested, 171 (21.5%) and 80 (48.2%), respectively, were found to contain salmonellae by one or more analytical condition. Combined results of the five enrichment conditions after 24 and 48 h of incubation identified 247 (98.4%) and 250 (99.6%) of the 251 contaminated samples identified in this study. Our results are at variance with earlier reports on the greater method sensitivity with extended (greater than or equal to 48 h) periods of selective enrichment. The productivities of individual enrichment conditions after each period of incubation varied markedly where recovery rates with TBG43 and MKTBG43 exceeded that obtained with SC35 and TBG35. Our findings also underline the determinant role of enrichment at an elevated temperature (43 degrees C), and use of multiple enrichment and plating media for the optimal recovery of foodborne Salmonella.

A comparison of standard cultural methods for the detection of foodborne Salmonella.

The sensitivity of the standard cultural method of the International Organization for Standardization (ISO 6579 and ISO 3565 combined) was compared to that of the Health Protection Branch (HPB) procedure for the detection of foodborne Salmonella. Of 195 foods tested, 84 (43.1%) were found to contain salmonellae by one or more cultural conditions. Of these, 75 (89.3%) and 68 (81.0%) were identified by the ISO and HPB methods, respectively. The apparent lack of agreement between both methods likely stemmed from the low indigenous numbers of salmonellae in several food homogenates, and unequal transfer of the target microorganism into homologous ISO and HPB pre-enrichment broths. The sensitivities of the commercially available Muller-Kauffmann tetrathionate broth (MKTBG43, Oxoid CM343), and a closely-related medium prepared with Oxoid CM29 tetrathionate base varied from 86.9 to 89.3%, and were deemed equivalent to that obtained with the ISO formulation of MKTBG43 (89.3%). Comparatively fewer contaminated samples were identified from selenite cystine (SC35) and selenite brilliant green (SBG35) enrichment cultures (82.1-83.3%). The high selectivity and saccharide-independent response of the bismuth sulfite agar medium warrants its consideration as a mandatory plating medium in ISO methodologies for the effective detection of typical and atypical biotypes of foodborne Salmonella spp.

Thermal resistance of Listeria monocytogenes in inoculated and naturally contaminated raw milk.

Raw whole milk inoculated with 10(5) CFU/ml of Listeria monocytogenes was thermally processed at 60-72 degrees C for a minimum holding time of 16.2 s with survival being observed at temperatures up to 67.5 degrees C. In addition, milk naturally contaminated with L. monocytogenes serotype 1 (around 10(4) CFU/ml) was pooled for 2 to 2.5 days and then run through an HTST pasteurizer at temperatures ranging from 60-78 degrees C. Viable L. monocytogenes were detected in the temperature range of 60-66 degrees C. No viable Listeria were detected after treatment at temperatures of 69 degrees C and above in any of five trials. Efficacy of pasteurization and widespread use of processing conditions well above the minimum HTST guidelines ensure the absence of Listeria in pasteurized milk products. However, survival of Listeria at sub-pasteurization temperatures (60-67.5 degrees C) is of concern with regard to heat-treated or raw-milk cheeses.

Survival of Listeria spp. on raw whole chickens cooked in microwave ovens.

The prevalence of microwave ovens in North American homes has increased dramatically within the last decade. Although microwave ovens are primarily used for reheating of foods, they are now more commonly being applied to the cooking of raw foods. Although cooking of raw foods, according to manufacturers' instructions targets an organoleptically acceptable end product, the process does not address the microbiological safety of the cooked food. Seventeen microwave ovens from various commercial suppliers were used to cook naturally contaminated whole raw broilers (< or = 1.8 kg) and roasters (> 1.8 kg) according to manufacturers' instructions. Temperature probes (six per chicken) were used to measure the temperature of chickens immediately after cooking and during the holding period. Of 81 Listeria-positive raw broilers and 93 raw roasters, 1 (1.2%) and 9 (9.7%), respectively, yielded viable Listeria spp. after microwave cooking. Of these, two were undercooked (visual inspection), one was over the maximum weight stipulated by the oven manufacturer and another one was over the maximum weight and undercooked. A significantly greater proportion of contaminated cooked birds was observed with roasters than with broilers, where for one of these contaminated roasters, the temperature at all six measured sites was > or = 87 degrees C. Most of the postcook Listeria-positive birds were associated with 2 of the 17 microwave ovens. Factors such as wattage, cavity size, and the presence or absence of a turntable seemingly did not play a significant role in the survival of Listeria spp. in microwave-cooked chicken. However, the general inability of microwave ovens to uniformly heat chicken carcasses was noted. In order to promote greater safety of microwave-cooked foods, general recommendations for consumers are provided.

Detection of Salmonella in dry foods using refrigerated pre-enrichment and enrichment broth cultures: interlaboratory study.

An interlaboratory study was performed in 11 laboratories to validate the use of pre-enrichment and tetrathionate brilliant green (TBG35) and selenite cystine (SC35) enrichment cultures refrigerated 72 h at 2-5 degrees C for greater analytical flexibility in the detection of Salmonella in dry foods. Productivities of refrigerated pre-enrichment and enrichment cultures were compared with that of the AOAC/Bacteriological Analytical Manual (BAM) procedure using 4 food types: whole egg powder, milk chocolate, animal feed, and instantized skim milk powder. Uninoculated and inoculated samples were included in each food group. There was complete agreement between the results obtained by the standard AOAC/BAM procedure and the 2 refrigeration procedures. Of 660 samples tested, the AOAC/BAM procedure identified 393 contaminated samples that were readily detected from the corresponding refrigerated pre-enrichment cultures and from the combined productivity of homologous refrigerated TBG35 and SC35 cultures. Refrigeration (72 h) of pre-enrichment or enrichment cultures for greater analytical flexibility and laboratory productivity in the examination of dry foods is under review for adoption by AOAC International.

Detection of Salmonella in dry foods using refrigerated pre-enrichment and enrichment broth cultures: summary of collaborative study.

A collaborative study was conducted to compare the productivity of refrigerated pre-enrichment and enrichment broth cultures with the U.S. Food and Drug Administration culture methods for detection of Salmonella. The refrigerated pre-enrichment and selective enrichment broth culture methods for detection of Salmonella in dry foods have been adopted first action by AOAC INTERNATIONAL.

Recovery of Salmonella spp. from refrigerated preenrichment cultures of dry food composites.

Refrigerated preenrichment 72 h and selective enrichment cultures arising from 25 g analytical units of dry foods can be used to increase the analytical flexibility and productivity of laboratories for the detection of foodborne Salmonella spp. by AOAC method 994.04. Results of this intralaboratory study using artificially contaminated dry foods validate the extended application of the refrigerated preenrichment approach to dry food composites (375 g). All samples found to be contaminated by AOAC/Bacteriological Analytical Manual methods were identified readily from the homologous, refrigerated preenrichment broth cultures. This extended application of the refrigeration approach was recently adopted First Action by AOAC and was included as a modification to method 994.04. In addition, ancillary work on the diagnostic value of prolonged (48 h) incubation of lysine iron (LI) agar as described in the AOAC Official Method 967.26 led to a recommendation that the 48 h period of incubation be revoked in favor of a 24 h incubation of inoculated LI medium.

Limitations of lysine-iron-cystine-neutral red broth in the presumptive identification of Salmonella.

Lysine-iron-cystine-neutral red broth performed satisfactorily in the presumptive identification of Salmonella in preenriched food and animal feed samples enriched in tetrathionate-brilliant green broth. Homologous results from selenite-cystine enrichment broths yielded unacceptably high numbers of false negative reactions.

Salmonella and the Chocolate Industry. A Review.

This paper reviews the microbiology of milk and bitter chocolate and of ingredients used in their manufacture with emphasis on the incidence and survival of Salmonella in these products. The increased thermal resistance of salmonellae associated with certain physicochemical properties of chocolate and chocolate ingredients underlines limitations of dry roasting of cocoa beans, heating of cocoa liquor, and conching of chocolate as effective bactericidal treatments. The value of quality control programs, irradiation, fumigation, and bacteriological standards as reliable control measures are also discussed.

Manufacture of Dairy Products From Unpasteurized Milk: A Safety Assessment.

The prevalence of Salmonella , Campylobacter , and Yersinia spp. in the food chain, and the more recent emergence of Listeria monocytogenes and hemorrhagic Escherichia coli as foodborne pathogens, are of public health concern. The ability of some of these bacterial agents to grow in milk and dairy products, to survive prolonged periods of refrigerated storage, and to withstand thermal treatments of raw milk at subpasteurizing temperatures, place new emphasis on the need for stringent control of milk processing operations and plant environment. Mandatory use of pasteurized milk may provide the only viable option for production of pathogen-free milk products.

Effect of storage conditions of the performance of bismuth sulfite agar.

Refrigerated storage of bismuth sulfite agar plates for up to 4 days did not adversely affect growth and colonial characteristics of selected Salmonella strains. Incubation of inoculated plates for 48 h favored the development of more salmonellae with typical morphology. Inoculated plates of freshly poured medium incubated for 48 h gave recoveries similar to those on refrigerated plates and showed a high selectivity against Citrobacter freundii and Proteus vulgaris, organisms which mimic the colonial characteristics of Salmonella on this medium. The use of bismuth sulfite plates stored at room temperature for more than 2 days should be avoided.

Recovery of sublethally heat-injured Salmonella typhimurium on supplemented plating media.

The efficacy of 32 additives to Levine eosin-methylene blue-salts agar medium (EMBS) for the recovery of sublethally heat-injured Salmonella typhimurium was evaluated. In order of decreasing effectiveness, lactate, mannitol, and alpha-glycerophosphate mediated 90% or more recovery of injured cells; similar levels of recovery were obtained on EMBS supplemented with 1% (wt/vol) tryptic soy broth, protease peptone, or plate count agar. Other additives showed little or no capacity for repair or strongly inhibited heated and nonheated cell suspensions. Conditions of growth and storage before heat treatment were also found to markedly affect susceptibility to heat injury.