Effects of preparation method, age, and plating technique of thin agar layer media on recovery of Escherichia coli O157:H7 injured by sodium chloride

The thin agar layer (TAL) method was experimentally tested to determine its ability to recover Escherichia coli O157:H7 injured by sodium chloride (NaCl). Cells grown in Brain Heart Infusion broth with 0%, 5%, or 7.5% (w/v) NaCl were spread and spiral plated onto Tryptic Soy agar (TSA), MacConkey Sorbitol agar (MSA), and TSA/MSA TAL combinations. Generally, TSA recovered more injured cells than TAL (p < or =0.05), and TAL recovered more cells than MSA (p < or =0.05). Preparation mode (two vs. three layers) and age (0, 1, or 7 days) of TAL had negligible effect on resuscitation of injured cells (p > 0.05). TAL, which is conventionally used to recover heat, cold, and acid-injured foodborne pathogens, may be used to recover NaCl-injured E. coli O157:H7.

Detection of extracellular bound proteinase in EPS-producing lactic acid bacteria cultures on skim milk agar

AIMS

Skim milk agar was developed to investigate extracellular cell-bound proteinase in yogurt cultures, Streptococcus thermophilus and Lactobacillus bulgaricus.

METHODS AND RESULTS

The Lact. bulgaricus cultures produced more extracellular cell-bound proteinase than did Strep. thermophilus cultures. Strong positive correlations between the size of the exopolysaccharide (EPS) layer and extracellular cell-bound proteinase were found for both Streptococcus and Lactobacillus cultures.

CONCLUSION

Strong positive linear relationships existed between the EPS size and colony size and the diameter of clear zone and colony size for Streptococcus cultures, whereas weak positive linear relationships were observed for Lactobacillus cultures.

SIGNIFICANCE AND IMPACT OF THE STUDY

These data are useful to validate the relationship between extracellular proteinase and the EPS size of LAB. Also, a convenient medium to detect the presence of extracellular cell-bound proteinase of LAB is valuable for dairy industries.

Evaluation of thin agar layer method for recovery of acid-injured foodborne pathogens

The thin agar layer (TAL) method of Kang and Fung was used to enumerate acid-injured foodborne pathogens. This method involves overlaying 14 ml of nonselective medium (tryptic soy agar [TSA]) onto a prepoured and solidified pathogen-specific, selective medium in a petri dish. After surface plating, injured cells resuscitated and grew on TSA during the first few hours of incubation; then, the selective agents from the selective medium diffused to the top layer, interacted with the recovered microorganisms, and started to produce typical reactions. Foodborne pathogens were exposed to 2% acetic acid for 1, 2, or 4 min, and the recovery rate with the TAL method was compared with the rate of TSA and pathogen-specific, selective media. No significant difference occurred between TSA and TAL (P > 0.05) for enumeration of acid-injured Escherichia coli O157:H7, Salmonella Typhimurium, Staphylococcus aureus, and Yersinia enterocolitica, and both recovered significantly higher numbers than the selective medium for each respective pathogen (P < 0.05). For recovery of acid-injured Listeria monocytogenes, no difference (P > 0.05) occurred among TSA, TAL, and selective media. However, fewer cells were recovered in the selective media. The TAL method is a one-step, convenient procedure for recovery of acid-injured cells.

Enrichment media for isolation of Campylobacter jejuni from inoculated ground beef and chicken skin under normal atmosphere

The efficiency of Hunt broth containing Oxyrase was compared with the gas replacement method for detection of Campylobacter jejuni in inoculated ground beef and chicken skin. Five strains of C. jejuni were inoculated individually into samples and cultured with various media under conditions generated by either flushing with a mixture of gases or supplementing with Oxyrase. Oxyrase media added with 7% lysed blood, 2.5% charcoal, or 6% ground cooked meat were compared with examinations from chicken skin samples. Campylobacter counts from enrichments were performed at 6, 12, 20, and 28 h of incubation. From inoculated ground beef, counts at 20 h increased by 4 to 7 log CFU/ml depending on strains and initial concentration of inocula. The efficiencies of Hunt medium using gassing and those with Oxyrase added were similar (P > 0.05). Broth containing 0.15 U/ml of Oxyrase without blood effectively supported the growth of all strains (P > 0.05). From inoculated chicken skin, 20-h incubation counts increased by 3.0 to 7.5 log CFU/ml for the gassing method and by 2.7 to 7.3 log CFU/ml for supplementation with 0.6 U/ml of Oxyrase and blood. The addition of 7% lysed sheep blood provided better Campylobacter growth than supplementing with 2.5% charcoal or 6% ground cooked meat. Enrichment media incorporating with Oxyrase is a simple, convenient, and time-saving method to replace flushing with mixed gas for isolation of Campylobacter jejuni.

Evaluation of a 5'-nuclease (TaqMan) assay for the detection of virulent strains of Yersinia enterocolitica in raw meat and tofu samples

Culture methods for detecting virulent Yersinia enterocolitica require selective enrichment and a series of confirmatory tests that are time-consuming, costly, and laborious. The objective of this study was to evaluate a fluorogenic 5'-nuclease assay for detecting the enterotoxin yst gene of virulent Y. enterocolitica in pure cultures, inoculated ground pork samples, and naturally contaminated food samples. These results were then compared with "gold standard" methods recommended by the U.S. Food and Drug Administration in the Bacteriological Analytical Manual for detecting pathogenic Y. enterocolitica. The 5'-nuclease assay was able to identify the organism in 100% of the repetitions when 10(2) CFU/ml or more organisms were present in pure cultures and 10(3) CFU/g or more organisms were present in ground pork. Similar recovery efficiency on cefsulodin-irgasan-novobiocin (CIN) agar plates was only evident when 10(5) CFU/ml or more organisms were present in pure culture and 10(6) CFU/g or more organisms were present in inoculated ground pork. The 5'-nuclease assay indicated a contamination rate of 35.5% (94/265) in various meats and tofu, whereas the CIN plating method indicated a contamination rate of 28.3% (75/265). This resulted in 100% sensitivity and 64.5% specificity for the 5'-nuclease assay when compared with the standard culture recovery method. Only 75% (60/80) of the Yersinia spp. isolated on CIN was identified as containing a virulence plasmid by autoagglutination and crystal violet binding tests. These results indicate that the true rate of contamination of virulent Y. enterocolitica in pork and other processed meats and foods is being underestimated using current detection methods. This study demonstrates the potential of the 5'-nuclease assay for rapidly and specifically detecting virulent Y. enterocolitica in processed foods with the added advantage of being an automated detection system with high-throughput capability.

Enrichment procedures and plating media for isolation of Yersinia enterocolitica

A shortened enrichment procedure (25 degrees C for 24 h) was compared with cold enrichment procedures (4 degrees C for 1 to 3 weeks) and direct plating for isolation of Yersinia enterocolitica from commercial ground meat samples. The combined data of all recovery procedures showed that this organism was isolated from 34% of the ground beef samples. The highest isolation rate was 32% for the 4 degrees C/3-week enrichment, followed by 28% for the 4 degrees C/2-week enrichment, 26% for the 25 degrees C/24-h enrichment, 22% for the 4 degrees C/1-week enrichment, and 10% for direct plating. No significant differences (P > 0.05) in isolation rate occurred between the 4 degrees C/3-week, 4 degrees C/2-week, 25 degrees C/24-h, and 4 degrees C/1-week enrichments. The combined data of all recovery procedures showed that Y. enterocolitica was isolated from 64% of ground pork samples. The highest isolation rate was 48% for the 4 degrees C/3-week enrichment, followed by 40% for the 25 degrees C/24-h enrichment, 34% for the 4 degrees C/2-week enrichment, 24% for the 4 degrees C/1-week enrichment, and 24% for direct plating. No significant differences (P > 0.05) in isolation rate occurred between the 4 degrees C/3-week, 25 degrees C/24-h, and 4 degrees C/2-week enrichments. During the plating phase of the experiment, the efficiency of a dye-containing, Yersinia-selective medium (KV202) was compared with that of a commercially available cefsulodin-irgasan-novobiocin medium. Recovery rates were similar for both media. However, KV202 agar differentiated Y. enterocolitica from such contaminating bacteria as Enterobacter, Serratia, and Salmonella by colony morphologic characteristics and color.

Stimulation of starter culture for further reduction of foodborne pathogens during salami fermentation

This study was conducted to determine if stimulated meat starter culture (MSC; Pediococcus acidilactici) would further control Escherichia coli O157:H7, Listeria monocytogenes, and Staphylococcus aureus during salami fermentation. Manganese ion (0.005% of MnSO4) was used as a stimulator for the growth and acid production of MSC. After 24-h salami fermentation, nonstimulated MSC and stimulated MSC reduced E. coli O157:H7 levels by 1.3 and 2.3 log10 units, respectively. Nonstimulated MSC reduced L. monocytogenes levels by 1.2 log10 units, whereas the stimulated MSC achieved a 2.2-log10 reduction after 24-h fermentation. In the case of S. aureus, nonstimulated MSC and stimulated MSC reduced S. aureus levels by 1.3 and 2.3 log10 units after 24-h fermentation, respectively. Stimulated MSC by MnSO4 reduced those foodborne pathogens more effectively compared with nonstimulated MSC (P < 0.05).

Control of foodborne pathogens during sufu fermentation and aging

Control of the foodborne pathogens Escherichia coli O157:H7, Salmonella typhimurium, Staphylococcus aureus, and Listeria monocytogenes during sufu fermentation was evaluated. Before fermentation, pathogens were inoculated onto tofu (substrate for sufu) at 5 log cfu/g or 3 log cfu/g, and starter culture (Actinomucor elegans) was inoculated at 3 log cfu/g. After 2 days of fermentation at 30 degrees C, the four pathogens reached 7 to 9 log cfu/g, and the mold count reached 6 to 7 log cfu/g. After fermentation, sufu samples were aged in a solution of 10% alcohol + 12% NaCl. After 1 month of aging, the total bacterial count was 6 to 7 log cfu/g, but all foodborne pathogens and mold were reduced to nondetectable levels. The total bacterial count decreased after aging for 2 months and 3 months, but the differences were not significant (P > 0.05) compared with the count after 1 month. Microorganism in experimental sufu from different aging periods and in commercial sufu were compared. A total of 270 isolates were purified and identified by the BBL Crystal Identification System. From the experimental sufu samples, 49 Bacillus spp. (20.4%), 167 Enterococcus spp. (69.6%), 6 Shewanella putrefaciens (2.4%), and 18 miscellaneous Gram-negative bacilli (7.5%) were identified. From commercial sufu samples, 17 Bacillus spp. (56.7%), 2 Enterococcus durans (6.7%), 5 miscellaneous Gram-negative bacilli (16.7%), 5 Corynbacterium aquaticum (16.7%), and 1 Shewanella putrefaciens (3.3%) were obtained. Although the longer aging period did not significantly decrease the total bacterial count, it may help in the development of sufu flavor. This study showed that sufu fermentation and aging can control common foodborne pathogens, so sufu is a safe product even though its preparation does not include pasteurization.

Rapid 5' nuclease (TaqMan) assay for detection of virulent strains of Yersinia enterocolitica

We have developed a rapid procedure for the detection of virulent Yersinia enterocolitica in ground pork by combining a previously described PCR with fluorescent dye technologies. The detection method, known as the fluorogenic 5' nuclease assay (TaqMan), produces results by measuring the fluorescence produced during PCR amplification, requiring no post-PCR processing. The specificity of the chromosomal yst gene-based assay was tested with 28 bacterial isolates that included 7 pathogenic and 7 nonpathogenic serotypes of Y. enterocolitica, other species of Yersinia (Y. aldovae, Y. pseudotuberculosis, Y. mollaretti, Y. intermedia, Y. bercovieri, Y. ruckeri, Y. frederiksenii, and Y. kristensenii), and other enteric bacteria (Escherichia, Salmonella, Citrobacter, and Flavobacterium). The assay was 100% specific in identifying the pathogenic strains of Y. enterocolitica. The sensitivity of the assay was found to be >/=10(2) CFU/ml in pure cultures and >/=10(3) CFU/g in spiked ground pork samples. Results of the assay with food enrichments prespiked with Y. enterocolitica serotypes O:3 and O:9 were comparable to standard culture results. Of the 100 field samples (ground pork) tested, 35 were positive for virulent Y. enterocolitica with both 5' nuclease assay and conventional virulence tests. After overnight enrichment the entire assay, including DNA extraction, amplification, and detection, could be completed within 5 h.

Application of thin agar layer method for recovery of injured Salmonella typhimurium

Xylose lysine decarboxylase (XLD) medium, a selective plating medium, can inhibit heat-injured Salmonella typhimurium from growing, whereas tryptic soy agar (TSA), a nonselective medium, does not. To facilitate recovery of heat-injured S. typhimurium cells while providing selectivity of isolation of S. typhimurium from other bacteria in the sample, a thin agar layer (TAL) procedure was developed by overlaying 14 ml of nonselective medium (TSA) onto prepoured and solidified XLD medium in a 8.5 cm diameter Petri dish. During the first few hours of incubating the plate, the injured S. typhimurium repaired and started to grow in the TSA. During the resuscitation of injured cells, the selective agents from XLD were diffused to the TSA top layer part. Once the selective agents diffused to the top part of the TAL, the resuscitated S. typhimurium started to produce a typical reaction (black color) and other microorganisms were inhibited by the selective agents. The recovery rate for heat-injured (55 degrees C for 15 min) S. typhimurium with the TAL method was compared with TSA, XLD, and the traditional overlay method (OV; pouring selective agar on top of resuscitated cells on TSA agar 3-4 h after incubation). No significant difference occurred among TSA, OV, and TAL (P > 0.05) for enumeration of heat-injured S. typhimurium, but they recovered significantly higher numbers than from XLD agar (P < 0.05).

Control of foodborne pathogens during sufu fermentation and aging

Control of the foodborne pathogens Escherichia coli O157:H7, Salmonella typhimurium, Staphylococcus aureus, and Listeria monocytogenes during sufu fermentation was evaluated. Before fermentation, pathogens were inoculated onto tofu (substrate for sufu) at 5 log cfu/g or 3 log cfu/g, and starter culture (Actinomucor elegans) was inoculated at 3 log cfu/g. After 2 days of fermentation at 30 degrees C, the four pathogens reached 7 to 9 log cfu/g, and the mold count reached 6 to 7 log cfu/g. After fermentation, sufu samples were aged in a solution of 10% alcohol + 12% NaCl. After 1 month of aging, the total bacterial count was 6 to 7 log cfu/g, but all foodborne pathogens and mold were reduced to nondetectable levels. The total bacterial count decreased after aging for 2 months and 3 months, but the differences were not significant (P > 0.05) compared with the count after 1 month. Microorganism in experimental sufu from different aging periods and in commercial sufu were compared. A total of 270 isolates were purified and identified by the BBL Crystal Identification System. From the experimental sufu samples, 49 Bacillus spp. (20.4%), 167 Enterococcus spp. (69.6%), 6 Shewanella putrefaciens (2.4%), and 18 miscellaneous gram-negative bacilli (7.5%) were identified. From commercial sufu samples, 17 Bacillus spp. (56.7%), 2 Enterococcus durans (6.7%), 5 miscellaneous gram-negative bacilli (16.7%), 5 Corynbacterium aquaticum (16.7%), and 1 Shewanella putrefaciens (3.3%) were obtained. Although the longer aging period did not significantly decrease the total bacterial count, it may help in the development of sufu flavor. This study showed that sufu fermentation and aging can control common foodborne pathogens, so sufu is a safe product even though its preparation does not include pasteurization.

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.

Reduction of Escherichia coli O157:H7 by stimulated Pediococcus acidilactici

This study was conducted to determine if stimulating the growth of meat starter culture (Pediococcus acidilactici) in a laboratory medium (Brain Heart Infusion broth +2.3% NaCl + 1.5% sucrose; LBHI) and during meat fermentation would control Escherichia coli O157:H7. In LBHI medium without P. acidilactici, the numbers of E. coli O157:H7 increased from 4.00 to 8.34 log10 cfu ml-1, whereas in the presence of P. acidilactici (approximately 6.0 log10 cfu ml-1) in LBHI, LBHIM (LBHI + 0.005% MnSO4), LBHIO (LBHI + 0.3 unit ml-1 Oxyrase), and LBHIMO (LBHI + M + O), the numbers of E. coli O157:H7 increased from 4.00 to 8.05, 7.50, 7.99, and 6.50 log10 cfu ml-1, respectively, after incubation at 40 degrees C for 15 h. During salami fermentation, the numbers of E. coli O157:H7 changed from 7.00 to 6.40 and 5.10 log10 cfu g-1 without and with P. acidilactici (approximately 7.0 log10 cfu g-1), respectively. Stimulated P. acidilactici by M, O, and MO further reduced the number of E. coli O157:H7 from 7.00 to 4.00, 4.80, and 3.65 log10 cfu g-1, respectively. The combination of MO was a better growth stimulator for P. acidilactici, which controlled E. coli O157:H7 in both systems (P < 0.05).

Effect of diacetyl on controlling Escherichia coli O157:H7 and Salmonella Typhimurium in the presence of starter culture in a laboratory medium and during meat fermentation

Diacetyl is a flavor compound that possesses antimicrobial activity and is found in several dairy products. The effect of diacetyl on controlling the growth of two foodborne pathogens, Escherichia coli O157:H7 and Salmonella Typhimurium, when grown with Pediococcus acidilactici as a meat starter culture was evaluated in a laboratory medium and during salami fermentation. Diacetyl (50 ppm) added to each mixed culture system strongly inhibited the growth of E. coli O157:H7 and Salmonella Typhimurium in the laboratory medium (brain heart infusion, 2.3% of NaCl, 0.75% of dextrose) (P < 0.05). During meat fermentation, the growth of E. coli O157:H7 and Salmonella Typhimurium was inhibited significantly by addition of diacetyl (300 ppm) (P < 0.05) after 24 h fermentation. However, the acid production and growth of P. acidilactici were not affected by the addition of diacetyl (P > 0.05). After 24 h meat fermentation, about a 1.0-log CFU/g difference occurred in numbers of each foodborne pathogen mixed with P. acidilactici (P < 0.05) with and without 300 ppm diacetyl. Diacetyl and the acid produced by the meat starter culture reduced the growth of the two foodborne pathogens during salami fermentation. These results suggest that diacetyl can be used as a food ingredient during meat fermentation to control E. coli O157:H7 and Salmonella Typhimurium without harmful effects on the growth and acid production of P. acidilactici.

Comparison of five anaerobic incubation methods for enumeration of Clostridium perfringens from foods

This study compared the cost, speed, convenience, and sensitivity of five anaerobic systems. Fung's double-tube (FDT) method, the minitube method (MT), the sandwiched microtiter plate (SMP) method, and the Mitsubishi AnaeroPack System were compared with the Brewer anaerobic jar for total anaerobic bacterial counts in foods. Incubation was at 37 degrees C for 4, 8, 12, and 24 h. The results indicated that FDT, MT, SMP, and the Mitsubishi AnaeroPack System were as sensitive as the Brewer anaerobic jar for the detection and enumeration of Clostridium perfringens from food products. The FDT, MT, and SMP methods recovered higher numbers of C. perfringens compared to the Brewer anaerobic jar (P < 0.05) after 12 and 24 h incubation. The Brewer anaerobic jar was the most expensive method.

Development and evaluation of a 24 well microtitre plate method for isolation of Listeria spp. or Listeria monocytogenes from foods

A rapid and simple method (24M) using 24 well microtitre plates was developed to determine the presence of Listeria monocytogenes or Listeria spp. in food samples. The 24M was composed of two 24 well microtitre plates connected with a yellow tip. The 24M was evaluated with pathogen cocktails and ground beef samples and compared with the conventional method for presumptive identification of Listeria spp. Only food-borne pathogen cocktails and ground beef samples containing L. monocytogenes or Listeria spp. showed a positive reaction in 24M after 24 h incubation at 35 degrees C. Test results were the same with the conventional method and the 24M method and showed high efficiency for recovery of Listeria spp. from foods. This new, convenient and economical method can isolate Listeria spp. simultaneously from 24 different food samples.

Development of a medium for differentiation between Escherichia coli and Escherichia coli O157:H7

A new medium (Escherichia coli O157:H7 medium: EOH) was developed for differentiation between E. coli and E. coli O157:H7. The EOH medium was compared with sorbitol MacConkey agar (SMAC), which is the most popular medium to enumerate E. coli O157:H7. Several combinations of 35 dyes were evaluated to develop the new medium. Indigo carmine (0.03) g/liter) and phenol red (0.036 g/liter) were found as the best combination for differentiation between E. coli O157:H7 and E. coli and added to the basal agar medium (SMAC medium excluding neutral red and crystal violet) for EOH medium. On the dark blue EOH medium, E. coli produced a yellow color with clear zone, whereas E. coli O157:H7 produced a red color without clear zone. For differentiation between E. coli and E. coli O157:H7, EOH has much better potential than SMAC. Furthermore. the red color produced by normal E. coli in SMAC may mask the light gray color produced by E. coli O157: H7, whereas the yellow color with clear zone did not mask the red color without clear zone in the EOH medium. The recovery numbers of E. coli O157:H7 from inoculated ground beef, pork, and turkey were not significantly different between SMAC and EOH media (P > 0.05). The recovery rates of heat- and cold-injured E. coli O157:H7 also were not significantly different (P > 0.05).

Alkaline-fermented foods: a review with emphasis on pidan fermentation

Alkaline-fermented foods constitute a group of less-known food products that are widely consumed in Southeast Asia and African countries. They can be made from different raw ingredients. For instance, Japanese natto, Thai thua-nao, and kinema are made from cooked soybeans, dawadawa from African locust beans, ogiri from melon seeds, ugba from African oil beans, kawal from fresh legale leaves, owoh from cotton seeds, and pidan from fresh poultry eggs. In alkaline-fermented foods, the protein of the raw materials is broken down into amino acids and peptides; ammonia is released during the fermentation, raising the pH of the final products and giving the food a strong ammoniacal smell. Most alkaline fermentations are achieved spontaneously by mixed bacteria cultures, principally dominated by Bacillus subtilis. In other cases, pure cultures can be used. For example, Japanese natto is inoculated with a pure culture of B. subtilis var natto. Pidan is a special example of alkaline fermentation. Instead of using microorganisms, pidan is made using an alkali-treated fermentation. Sodium hydroxide (NaOH) is produced from the reaction of sodium carbonate (Na2CO3), water (H2O), and calcium oxide (CaO) of pickle or coating mud. NaOH penetrates into the eggs, causing the physicochemical changes, color changes, and gelation. The appearance of pidan differs from fresh eggs in that the white becomes a semitransparent tea-brown color, and the yolk is solid or semisolid with a dark-green color. The nutritional value of pidan is slightly decreased compared with fresh eggs, but pidan has an extremely long shelf life and a pleasant, fragrant taste that is preferred by most people in Southeast Asian countries. In a small-scale laboratory study conducted by the authors, B. subtilis was not found in pidan. Four Staphylococcus spp. (S. cohnii, S. epidermidis, S. haemolyticus, and S. warneri) and two strains of Bacillus spp. (B. cereus and B. macerans) were isolated from pidan. Staphylococcus spp. did not contribute to the fermentation and were considered contaminants.

Salmonellae and food safety

Salmonella is one of the most important foodborne pathogens around the world. The knowledge that very low numbers of Salmonella cells can be infectious emphasizes the need for stringent food safety measures Traditional methods for isolating and identifying Salmonella in food rely on preenrichment, selective enrichment in selective and differential media, biochemical tests, and serological confirmation. Recent advances in diagnostic technology have considerably altered testing methods for foodborne Salmonella. Many commercial assay systems and kits that use newer technologies are available to facilitate the identification of Salmonella in foods. These systems include miniaturized biochemical tests, new media formulations, automated instrumentation, DNA/RNA probes, antibody-dependent assays, and polymerase chain reaction. The technologies used for these systems are described, and the various kit formats are compared. Among the limitations of detection methods in terms of food safety are timeliness, limits of detection, and differentiation of virulent and nonvirulent isolates. Current efforts of prevention measures and strategies at different links of the food chain such as consumer education and hazard analysis and critical control point (HACCP) programs are reviewed, Global approaches to food safety are needed..

Microbiology of fresh and restructured lamb meat: a review

Microbiology of meats has been a subject of great concern in food science and public health in recent years. Although many articles have been devoted to the microbiology of beef, pork, and poultry meats, much less has been written about microbiology of lamb meat and even less on restructured lamb meat. This article presents data on microbiology and shelf-life of fresh lamb meat; restructured meat products, restructured lamb meat products, bacteriology of restructured meat products, and important foodborne pathogens such as Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes in meats and lamb meats. Also, the potential use of sodium and potassium lactates to control foodborne pathogens in meats and restructured lamb meat is reviewed This article should be of interest to all meat scientists, food scientists, and public health microbiologists who are concerned with the safety of meats in general and lamb meat in particular.

Five-tube most-probable-number method using the Fung-Yu tube for enumeration of Listeria monocytogenes in restructured meat products during refrigerated storage

The survival of Listeria monocytogenes in cooked, chopped hams stored at 5 degrees C for 5 weeks was studied. Slices of chopped ham (25 g) were inoculated with a three-strain mixture of L. monocytogenes (LM 101M, LM 103M, and Scott A) at levels of < or = 350 cfu/25 g and packaged in Stomacher bags. Prior to inoculation, L. monocytogenes cells were subjected to either heat-injury (56 degrees C, 30 min) or freeze-injury (-18 degrees C, 14 days). The organisms were detected by a five-tube most-probable-number (MPN) technique using the motility enrichment Fung-Yu tube, and the direct plating method. After storage at 5 degrees C for 1 week, there was a one log10 reduction in counts. Thereafter, Listeria recovered and heat- and freeze-injured cells grew to 10(7) and 10(8) within 5 weeks, respectively. Similar results were obtained from both methods. However, direct plating could not recover L. monocytogenes at low levels (< or = 100/25 g), whereas MPN counts were obtained at these low levels. The pH (6.22 +/- 0.10) of the chopped hams remained constant throughout the study. These results indicated that low numbers of L. monocytogenes surviving sublethal heat- or freeze-injury could initiate growth after recovery in chopped hams. The five-tube MPN method using the Fung-Yu tube was effective in enumerating both low and high levels of L. monocytogenes in food.

Tempeh: a mold-modified indigenous fermented food made from soybeans and/or cereal grains

A variety of indigenous fermented foods exist today; however, tempeh has been one of the most widely accepted and researched mold-modified fermented products. Tempeh is a traditional fermented food made from soaked and cooked soybeans inoculated with a mold, usually of the genus Rhizopus. After fermentation has occurred, the soybeans are bound together into a compact cake by dense cottony mycelium. An important function of the mold in the fermentation process is the synthesis of enzymes, which hydrolyze soybean constituents and contribute to the development of a desirable texture, flavor, and aroma of the product. Enzymatic hydrolysis also may decrease or eliminate antinutritional constituents; consequently, the nutritional quality of the fermented product may be improved. Current technology and new scientific advancements have enabled researchers to examine specific strains of Rhizopus and new substrates such as cereal grains. Because Kansas produces numerous cereal grains, production of a fermented tempeh-like product using wheat, sorghum (milo), oats, rye, barley, corn, and triticale is a definite possibility for generating a Kansas Value-Added Product. In this study, several different tempeh-like products were produced using various cereal grains inoculated with Rhizopus oligosporus NRRL 2549 or R. oligosporus NRRL 2710. Grains used included hard red winter wheat, triticale, yellow sorghum (milo), and red sorghum (milo). The grain source as well as the strain of R. oligosporus used influenced the product's appearance, flavor, and patty integrity. Results showed that R. oligosporus NRRL 2549 produced more mycelium at a more rapid rate than did the R. oligosporus NRRL 2710 strain. The combination of red sorghum and R. oligosporus NRRL 2549 yielded a product with good patty texture, aroma, and appearance. Furthermore, the red sorghum fermented product was well suited for slicing. On the other hand, yellow sorghum inoculated with either R. oligosporus NRRL 2549 or R. oligosporus NRRL 2710 failed to produce an organoleptically suitable product. Triticale also was found to be an unacceptable substrate for the production of a tempeh-like product. Although the fermented wheat product had a desirable aroma and flavor, it lacked patty integrity and crumbled when sliced. Further research is needed to evaluate the economic significance and industrial applications of these tempeh-like products.

New aniline blue dye medium for rapid identification and isolation of Candida albicans

Organic dyes have long been used in diagnostic microbiology to differentiate species by color reactions. We studied the ability of a new noninhibitory medium, YM agar containing 0.01% aniline blue WS dye, Colour Index 42780 (YMAB), to identify Candida albicans among 1,554 yeast specimens obtained from seven clinical laboratories. Appropriate American Type Culture Collection and other characterized strains served as controls. A total of 487 of the clinical strains were identified as C. albicans. The remainder were other Candida species and non-Candida yeasts. Clinical isolates and controls were grown on Sabouraud agar for 18 h at 30 degrees C and then transferred to YMAB. Plates were incubated for 12 to 18 h at 30 degrees C, and colonies were observed for yellow-green fluorescence under long-wave UV light (A365). All control strains of C. albicans and Candida stellatoidea fluoresced, as did 480 of the 490 isolates designated as C. albicans (which included 3 strains of C. stellatoidea). Cells of C. albicans grown on YMAB produced germ tubes in serum. Only five of the other 1,062 non-C. albicans yeasts fluoresced. The sensitivity and specificity were 98.0 and 99.5%, respectively, with a predictive value of 99.1%. A fluorescent metabolite was found in cell wall particulate fractions of C. albicans sonic extracts grown on YMAB but not in non-C. albicans yeasts. This metabolite showed the same spectral curve as those of metabolites from whole cells in a recording spectrofluorometer when it was excited at 400 nm and scanned from 420 to 550 nm. Thus, growth on YMAB generates the production of a fluorescent moiety that can be used to specifically identify C. albicans within 12 to 18 h.

Aeromicrobiology--a review

This article reviews the presence of microorganisms in air and their sources, the relation of airborne dust and endotoxin, the sources of atmospheric microbial contamination in food-processing plants, the mechanisms of airborne particle deposition, the importance of airborne microbes, the survival of microorganisms in air, methods of air sampling, airborne microbial populations in food-processing plants, control of airborne microorganisms in food-processing plants, and the general issue of microorganisms in air and their impact on food safety. The purpose was to bring together scattered information about airborne microorganisms and review their importance in food protection and sanitation.

Conventional and rapid methods for yeast identification

Identification of yeasts depends on data obtained from morphological features and physiological characteristics. This article reviews the commonly used conventional methods for the identification of yeast to set the stage for a critical review of rapid systems for yeast identification. Comparative analyses of percentage agreement of API 20C, Uni-Yeast Tek, Minitek, Mycotube, Micro-Drop, Auxodisk, Iatron, Autobac 1, AMS, Abbott MS-2, and Abbott Quantum 11 against the conventional method and, in some cases, against each other are tabulated. Gas liquid chromatography method, enzyme method, MUG, and the Fung's mini-system are also mentioned as possible rapid methods for yeast identification. The purpose of this review is to update the rapidly expanding information concerning diagnostic kits and systems for yeast identification and analyze some of the advantages, disadvantages and potential uses of rapid systems of yeast identification.

Critical review of water activities and microbiology of drying of meats

The drying of meat is one of the least expensive and most effective ways to preserve meat. This article reviews the influence of intrinsic and extrinsic parameters of foods in relation to the drying of meat and the role of microbial activities in the drying and storage of meat. The critical role of water activity and its relationship with other physical and chemical parameters and microbial survival in the drying of meat is also reviewed in detail. This article ends with a description of the various methods to measure water activity in foods. The conclusion is that the drying of meat is a highly complex phenomenon which is influenced by the composition of meat, its water holding capacity, the processing of meat, and various microorganisms found on meat before and after the drying process. There is no doubt that the drying of meat is an effective preservation process, but attention should be placed on the possible survival of microorganisms in the process in order to prevent spoilage and food-borne diseases.

Off-flavors in milk

This report is a review of off-flavors encountered in fluid milk. It includes sections on transmitted, microbial, lipolyzed, heated, light-activated, oxidized, and miscellaneous flavors. Finally, the flavor of ultrahigh temperature (UHT) milk is reviewed. Most of the literature cited for different off-flavors covers the period since a comprehensive review was done of milk flavors by Strobel et al. (1953). In addition to causative agents and volatile materials associated with off-flavors, methods for control of each of the off-flavors are presented. This report is designed to serve the dairy production and processing industry, students and faculty of food science curricula, dairy product evaluation students, regulatory people, and fieldmen.

Effect of phenolic antioxidants on microbial growth

Antioxidants belong to a class of compounds used to retard oxidation of chemicals in foods. These compounds, such as BHA, BHT, TBHQ, PG, etc. are approved to be used in foods by government agencies. In the past 10 years considerable interest has been directed to the antimicrobial properties of these compounds due to the observations by various scientists that many of these compounds can suppress the growth of viruses, protozoa, bacteria, yeast, and molds and their subsequent production of toxic materials in foods. Thus, the dual purpose usage of these compounds (i.e., antioxidation and antimicrobial) has been the subject of many research papers. This review is designed to summarize major publications on this subject as well as present some detailed studies on the effect of major antioxidants on bacteria and mold generated in the laboratory of the author in recent years.

The effect of microwaves on nutrient value of foods

Microwave cooking has gained considerable importance as an energy-saving, convenient, and time-saving cooking method. This article reviews the state of the art of microwave cooking and the existing publishing data on the effects of microwave cooking on nutritive values of moisture, protein, carbohydrate, lipid, minerals, and vitamins. Most reports indicated that microwave cooking resulted in higher moisture losses compared with conventional methods. Overall, the nutritional effects of microwaves on protein, lipid, and minerals appear minimal. There is no report on the effects of microwaves on carbohydrate fraction in foods. A large amount of data is available on the effects of microwaves on vitamins. It is concluded that there are only slight differences between microwave and conventional cooking on vitamin retention in foods. In conclusion, no significant nutritional differences exist between foods prepared by conventional and microwave methods. Any differences reported in the literature are minimal.

Acriflavine violet red bile agar for the isolation of Klebsiella

A medium for the isolation and detection of Klebsiella is described. It contains 0.06% Acriflavine in Violet Red Bile agar (Difco). Klebsiella appeared as 5 to 7 mm mucoid (24 h at 37 degrees C) golden-yellow colonies. Pseudomonas aeruginosa and Enterobacter spp. appeared as small, brown to dark brown colonies. Escherichia coli as well as many other gram negative organisms and gram positive organisms did not grow on this medium.

The excretion of enterotoxin B from nongrowing cells of Staphylococcus aureus 4916

The physiological conditions for the excretion of staphylococcal enterotoxin B (SEB) by nongrowing cells have been determined. In general, SEB excretion appears to be best when amino acids serve as sources of carbon, nitrogen, and energy. Little toxin was produced in a nitrogen-free medium with glucose as the sole carbon source. Inhibitor studies have shown that most of the toxin excreted by nongrowing cells was the result of de novo protein synthesis. Optimum toxin excretion was also obtained under conditions of aeration, and at or near optimum growth temperature, those conditions at which the cells are metabolically and biosynthetically most active. Levels of cell-associated SEB decreased during toxin excretion. However, these levels of toxin could not account for the high levels of toxin released during excretion. Glucose and several other rapidly fermentable carbohydrates inhibited toxin excretion by as much as 80% under conditions of uncontrolled decrease in pH: however, when the pH was kept at either 6.2 or 7.5 a stimulation of toxin excretion was observed. Protoplast studies have suggested that SEB production does not take place in the absence of an intact cell wall.

The occurrence of cell-associated enterotoxin B in Staphylococcus aureus

Cell-associated enterotoxin B was detected in lysates of cells of Staphylococcus aureus S-6 and 4916 disrupted by sonication or lysostaphin treatment. As much as 67% of this total cell-associated toxin was surface-bound, located outside the cytoplasmic membrane, and was released during protoplasting of this organism by lysostaphin treatment in hypertonic medium. The remainder of the cell-associated toxin was termed cytoplasmic and was released during osmotic lysis of the protoplasts. Levels of cell-associated toxin as a function of the age of the cells showed a rapid increase in both surface-bound, cytoplasmic, and total cell-associated toxin levels during the period of active toxin synthesis (late exponential phase of growth). These cell-associated toxin levels then reached a peak as the culture entered stationary phase, at a time corresponding to a decrease in the rate of toxin synthesis, and decreased slowly thereafter.

Thermal inactivation of staphylococcal enterotoxins B and C

Thermal inactivation profiles of staphylococcal enterotoxins B (SEB) and C (SEC) at 80, 100, and 121 C showed that SEC is more resistant than SEB to heat. After 24 h of incubation at 25 C, some reactivation (recovery of serological reactivity) occurred in toxins that had been inactivated by heat. If the toxin was stirred during heating, reactivation did not occur. An examination of the reactivation kinetics of heat-treated SEC showed that reactivation was temperature dependent. At 25 C, the incubation temperature of heat-treated crude SEC (80 C for 10 min), 100% reactivation occurred after 24 h, whereas at 4 C only slight reactivation was observed. We and others observed that heat-treated toxins initially lost more serological activity when heated at a low temperature (80 C) than at a higher temperature (100 C); in the present study we demonstrate that this is a reversible phenomenon.

Polyvalent antiserum agar system for the detection of staphylococcal enterotoxins A, B, C, and E

A polyvalent antiserum agar system in capillary tubes was developed and evaluated for the detection of enterotoxins A, B, C, and/or E present in culture supernatant fluids.

Capillary tube catalase test

A capillary tube procedure for detection of catalase activities of isolated colonies of pure and mixed cultures on nutrient and blood agar plates is proposed.

Effect of dyes on bacterial growth

A rapid screening procedure was used to test the effect of 42 dyes on growth of 30 bacteria on solid media. The results indicated that many readily available dyes might have potential application for selective isolation of specific bacterial groups as well as value in differentiating between closely related bacterial taxa. Separation of Enterobacter from Escherichia, Salmonella from Shigella, and Staphylococcus from Micrococcus by selected dyes was also evaluated.

Amino acid requirements for the production of enterotoxin B by Staphylococcus aureus S-6 in a chemically defined medium

A minimal chemically defined medium has been developed for growth (approximately 25 Klett units) and production of detectable enterotoxin B (approximately 5-6 mug/ml) by Staphylococcus aureus S-6. This medium contains monosodium glutamate as a source of carbon, nitrogen, and energy, three additional amino acids (arginine, cystine, and phenylalanine), six inorganic salts, and four vitamins. Increasing the concentrations of several amino acids in a series of defined media gave no increase in enterotoxin production. Apparently the limiting factor for growth and enterotoxin production in these media is the biosynthesis of one or more missing amino acids, rather than the concentration of the amino acids present in the media. An additional requirement for proline and valine was observed when glucose was added as the primary source of energy. When compared to complex media, our results indicated that the inhibitory effect of glucose on enterotoxin synthesis in defined media was less evident or totally absent.

Optimum temperature for enterotoxin production by Staphylococcus aureus S-6 and 137 in liquid medium

The temperature for maximum toxin yield by Staphylococcus aureus S-6 and 137 in liquid medium cultured under aerobic conditions was determined to be 40 C.

Rapid characterization of bacteria, with emphasis on Staphylococcus aureus

Several miniaturized microbiological techniques were devised. These techniques were combined with multiple-inoculation methods for rapid characterization of Staphylococcus aureus strains isolated from turkey products. These procedures are applicable to a variety of other ecological investigations; they result in substantial savings of space, material, labor, and time.

Chemically defined medium for growth and sporulation of Clostridium perfringens

A chemically defined medium was developed that could support sporulation and growth of Clostridium perfringens strains ATCC 12916 and H9. This medium consisted of a modification of the basal medium of Boyd et al. plus 0.1% sodium thioglycolate and 0.5% monosodium glutamate. Five other strains grew, but did not sporulate, in this medium. With the addition of more vatamins into the medium, two more strains grew but did not sporulate. The effects of glucose, monosodium glutamate, ammonium glutamate, and sodium thioglycolate on growth and sporulation of C. perfringens ATCC 12916 in the defined medium was investigated.

Capillary tube assay for staphylococcal enterotoxins A, B, and C

A miniaturized single-gel diffusion procedure for detection of staphylococcal enterotoxin is proposed. The technique effects substantial savings of reagents and is easy to perform.

Rapid procedure for the detection of acid and gas production by bacterial cultures

A rapid miniaturized procedure, with a Microtiter plate and the Amojell-overlay technique, is described for the detection of acid and gas production by bacterial cultures.

Dye-diffusion test for semiquantitative staphylocoagulase assay

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Microtiter method for the evaluation of viable cells in bacterial cultures

A microtiter technique was investigated as a means of evaluating viable cells in bacterial cultures. Parallel experiments were performed employing the conventional agar plate method along with the microtiter procedure. Statistical analyses showed that the correlation between the two methods was highly significant. With this new method, many samples were analyzed simultaneously, and readable results were obtained in 12 to 15 hr. Other advantages of this method were substantial savings of time, space, and materials. Also, the applicability of this method to estimates of mixed bacterial populations was demonstrated by studying the associative growth of two bacterial cultures.

Nutritional requirements of Staphylococcus aureus S-6

A synthetic medium was devised for growth of Staphylococcus aureus strain S-6. The growth yield in synthetic medium was compared to that in complex medium containing an equivalent amount of protein hydrolysate. Enterotoxin B formation in the two media was also compared. The defined medium was composed of inorganic salts, 11 amino acids (glycine, valine, leucine, threonine, phenylalanine, tyrosine, cysteine, methionine, proline, arginine, and histidine), and three vitamins (thiamine, nicotinic acid, and biotin). Biotin was a growth factor requirement of S-6 when glutamic acid but not glucose was used as a carbon source. The quantity of enterotoxin B produced in the defined medium was about one-seventh of that produced in complex medium, even though the growth yields were similar.