EFFECTS OF TRISODIUM PHOSPHATE ON POPULATIONS OF SALMONELLA TYPHIMURIUM AND SALMONELLA ENTERITIDIS IN VITRO

This study determined the bactericidal activity of different concentrations of trisodium phosphate (TSP) against Salmonella typhimurium and Salmonella enteritidis in vitro. One milliliter of each TSP solution (0.5, 1, 1.5 and 2% w/v) was mixed with 1 mL of each bacterial culture and left to stand at room temperature (18  ±  1C) for 0, 15, 30, 45, 60 or 90 min before enumeration of the surviving pathogens.
The effects of 1.5 and 2% TSP were apparent on both bacterial species from 0 min and bacterial counts reached undetectable ( < 1.0  ×  10¹ cfu/mL) levels immediately. The 1% TSP solution effected close to 4.20–5.72 log reduction after 15 min. However, the reduction effect of 0.5% TSP was a function of treatment time, and some culturable bacteria remained even after 90 min of treatment. These results suggest that the bactericidal effect of TSP solution on these pathogens depends on its concentration and on the duration of treatment.

PRACTICAL APPLICATIONS

Poultry has been identified as one of the most important reservoirs of Salmonella typhimurium and Salmonella enteritidis in food chain. Efforts to eliminate or reduce Salmonella spp. and other pathogens attached to poultry carcasses during processing have been made for many years. In 1992, the U.S. Department of Agriculture approved the use of trisodium phosphate (TSP) as a processing aid to reduce Salmonella spp. on raw poultry carcasses. The aim of the present study was to determine the bactericidal effects of different concentrations of TSP on pure cultures of these pathogens in vitro .     

DETECTION OF SALMONELLA TYPHIMURIUM IN OYSTERS BY PCR AND MOLECULAR HYBRIDIZATION

Because shellfish (oysters, clams and mussels) are filter feeders, i.e., able to concentrate pathogens from the surrounding waters within their tissues, they have been widely associated with outbreaks illness. The incidence of salmonellosis caused by the consumption of raw or undercooked shellfish, is a primary concern of public health agencies. Then, in recent years, more rapid and specific methods based on the DNA sequence of salmonella genes have been developed to detect low levels of pathogens in environmental and food samples. In this study, we developed a sensitive method to detect low levels of Salmonella typhimurium in oyster tissues (0.1 cfu/g). This methodology consisted of dissection of the gastrointestinal oyster tract, pre‐enrichment of the samples in nonselective medium, DNA extraction and polymerase chain reaction followed by molecular hybridization using a digoxygenin‐labeled amplicon‐derived probe. These results can benefit the public health agencies and shellfish producers concerning microbiological and quality aspects of the commercial oyster production.     

CAFFEINE IS NON-MUTAGENIC TO SALMONELLA TYPHIMURIUM AND HUMAN CELLS IN CULTURE

Caffeine (1,3,7-trimethylxanthine) was inactive as a mutagen at the hgprt locus in two human lymphoblast lines at concentrations up to 20 mM. In addition, caffeine did not induce 8-azaguanine-resistant mutants in Salmonella typhimurium at similar levels.     

EFFECT OF ELECTRICAL STIMULATION ON KILLING SALMONELLA TYPHIMURIUM IN VARIOUS SALT SOLUTIONS

Electrical stimulation was evaluated as a method to kill Salmonella typhimurium in various salt solutions at different concentration . Salmonella typhimurium at 2 × 10⁵ CFU/ml was treated at 22–24C for 60 min in each salt solution using electricity at 10 mA/cm² current, 1 kHz frequency, and 50% duty cycle. Samples taken at various times were serially diluted, plated on tryptic soy agar and xylose lysine desoxycholate agar, and incubated at 37C for 18–24 h. To detect injured cells, samples were also pre-enriched in buffered peptone water at 37C for 4–5h before being plated. Results indicated all salmonellae were electrically killed at 5 min in NaCl, at 30 min in NaNO₃, and at 45 min in NaC₂H₃O₂ at 0.15 and 0.015 M concentrations. Salmonellae were also killed at 45 min in Na₃PO₄ and at 60 min in Na₂CO₃ at 0.0015 M concentration by electricity in combination with high pH .     

EFFECT OF MICROWAVE HEATING ON SURVIVAL OF SALMONELLA TYPHIMURIUM IN ARTIFICIALLY CONTAMINATED READY-TO-EAT FOODS

This research evaluated the destruction of Salmonella typhimurium during reheating of foods in two different types of microwave ovens: a conventional 750W oven and 700W oven with preset controls. The heating times in the conventional microwave oven were established as 50 s for baby food and 75 s for mashed potatoes as well as for the beef stroganoff samples, while for the preset oven those periods of time were determined by a built-in temperature sensor. The percentage of food samples positive for S. typhimurium in conventional microwave was 47.8% while in microwave with preset controls it was 93.3%. Our results suggest that reheating contaminated foods in microwave ovens may, therefore, not be sufficient to destroy S. thyphimurium in order to assure food safety .     

LACTIC ACID INACTIVATION OF SALMONELLA TYPHIMURIUM ATTACHED TO CATFISH SKIN

This study was conducted to test sanitizer efficacy of lactic acid on Salmonella typhimurium populations that were firmly attached or loosely attached on catfish skin with or without mucus. For both cell types, counts of S. typhimurium were reduced from 5.9 log¹⁰ CFU/skin to below the limit of detection of 2.0 log¹⁰ CFU/skin by a 5 min exposure to 2% lactic acid. Exposure for 1 min reduced a 10⁴ CFU/skin inoculum to below the limit of detection. Catfish skin mucus slightly decreased the antimicrobial effect of lactic acid against firmly attached S. typhimurium.     

D VALUES OF COMPOSITE SALMONELLA ENTERICA SEROTYPES TYPHIMURIUM AND ENTERITIDIS IN PHILIPPINE NATIVE ORANGE JUICE

ABSTRACT

This study established the decimal reduction times (D values) of a composite inoculum of Salmonella enterica serotypes Typhimurium (American Type Culture Collection 13311 and E 1366) and Enteritidis (BIOTECH 1963) in Philippine native orange juice (pH 3.0, 0.84 % citric acid, 8.4°Brix). Acid‐adapted cells (10 log₁₀ cfu/g) were heated to 50–70C for 0–1.5 min and recovered by surface plating onto a selective (bismuth sulfite agar) and a nonselective medium (nutrient agar [NA]). Results showed that in both enumerating media and in all heating temperatures, survivor populations significantly decreased (P < 0.05) with increasing treatment time. The greater populations enumerated on NA in all treatment times were attributed to the sublethal injuries imparted by the acidity of the suspending medium and the heat treatments. The D values calculated from the survivor populations on both media were not significantly different (P > 0.05). The D values ranged from 0.16 to 0.40 min.

PRACTICAL APPLICATIONS

This study established the thermal inactivation rates of a composite Salmonella spp. inoculum in a Philippine orange juice medium. Salmonella is a target organism pertinent in the assuring citrus juices. Hence, appropriate thermal process schedules for citrus juices with similar physicochemical properties as the test medium, against the test organisms may be calculated using the results obtained from this study.

     

IN VITRO COMPARISON OF ANAEROBIC AND AEROBIC GROWTH RESPONSE OF SALMONELLA TYPHIMURIUM TO ZINC ADDITION

Zinc supplemented diets have been used to provide zinc as a nutrient and higher concentrations have been used to induce molt in laying hens. It is not known if the zinc in these diets would inhibit Salmonella spp growth. This study examines the effects of zinc compounds on the growth of S. typhimurium poultry isolate under aerobic and anaerobic conditions. The aerobic growth response of S. typhimurium was determined either in tryptic soy broth (TSB) or minimal (M9) broth containing five different concentrations (0.67, 2.01, 3.35, 4.69, and 6.03% [wt/vol]) of either Zn acetate [Zn(C₂H₂O₂)₂2H₂O] or Zn sulfate [ZnSO₄7H₂O] while anaerobic growth response was determined in M9 broth with or without reductants (L-cysteine hydrochloride [C₃H₇NO₂SHCl], and sodium sulfide [Na₂S 9H₂O]). Aerobic growth rates inhibited (P < 0.05) by Zn acetate than by Zn sulfate in TSB medium. The Zn source and concentration decreased (P < 0.05) aerobic growth response of S. typhimurium poultry isolate in M9 medium. The growth rates of S. typhimurium under anaerobic growth conditions were less responsive to Zn salts but were generally lower (P < 0.05) in the presence of reductant than in the absence of reductants at each concentration of Zn compound. The results in this study provide evidence that Zn may inhibit S. typhimurium under in vitro aerobic or anaerobic atmospheric conditions and S. typhimurium grows less optimally under anaerobic growth conditions.     

INHIBITION OF SALMONELLA TYPHIMURIUM ON POULTRY BY THE LACTOPEROXIDASE SYSTEM1

The antibacterial activity of the lactoperoxidase (LP) system on the growth and survival of a naladixic acid resistant strain of Salmonella typhimurium on poultry was determined. The LP system treatment, which consisted of lactoperoxidase (1 μg/ml), potassium thiocyanate (5.9 mM), and hydrogen peroxide (2.5 mM) (final concentrations) in water, reduced the level of Salmonella on inoculated chicken legs. The magnitude of reduction was dependent on temperature and time. For a water temperature of 25C and a time of 30 min, a 13.2% reduction was seen, as compared to a water temperature of 60C and a time of 15 min which showed a 80.6% reduction. In a 48 h shelf-life study, the LP system controlled growth of psychrotrophic bacteria on chicken legs. Hunterlab color values (L, a, b) for chicken thigh skin and oxidative deterioration as measured by TBA values for chicken thigh meat did not significantly differ (p < 0.01 and p < 0.05, respectively) between control and treated thighs.     

EFFECT OF GAMMA RADIATION ON TOXICITY AND IMMUNOGENICITY OF SALMONELLA TYPHIMURIUM

Salmonella typhimurium H391 neurotoxin was sensitive to gamma radiation under in vivo conditions but resistant under in vitro conditions. Cells were detoxified after exposure to a radiation dose of 500 krad while isolated neurotoxin retained toxicity up to a dose of 3,000 krad. Detoxification of S. typhimurium H391 cells after exposure to 500 krad was accompanied by loss of antigenic properties which may be attributed to changes in the physico-chemical behavior of the neurotoxin moiety after in vivo radiation.     

SURVIVAL OF SALMONELLA TYPHIMURIUM IN ROASTING CHICKENS COOKED IN A MICROWAVE, CONVECTION MICROWAVE, AND A CONVENTIONAL ELECTRIC OVEN

Fresh whole roasting chickens were inoculated with Salmonella typhimurium and cooked either in a microwave, convection microwave, or conventional electric oven to an internal temperature of 74°C, 77°C, 79°C, or 85°C. Cooked chickens were tested for the presence of viable Salmonella organisms using an Enzyme Linked Immunosorbent Assay (ELISA). No chicken (0/12) cooked to ± 79°C in the convection microwave or conventional oven tested positive for Salmonella. Two out of three chickens cooked to 79°C and all three chickens cooked to 85°C in the microwave tested Salmonella positive. Tetrathionate broth dilutions from all 11 microwave cooked chickens produced isolated colonies on Brilliant Green Agar. No growth was observed on Brilliant Green Agar plates inoculated with tetrathionate broth dilutions from chickens cooked to an internal temperature of 79°C or 85°C in the conventional oven or to 85°C in the convection microwave.     

DEVELOPMENT OF STRESS AND SURVIVAL OF SALMONELLA TYPHIMURIUM ATCC 7136 IN A TAPIOCA SOYSTARCH PRODUCT CONTAINING POTASSIUM SORBATE

The effects of reduced water activity and 0.1% sorbate on Salmonella typhimurium ATCC 7136 in a soy-starch product were examined. The formulated product (final pH 6.7), was inoculated with cells to give a final concentration of approximately 2 × 10⁶ cells/g. Ten gram samples were placed in a series of desiccators over saturated salt solutions and stored at either 22°C or 40°C. Water activities ranged from 0.79 to 0.97. Samples were withdrawn at prescribed intervals and appropriate dilutions made. A dual plating procedure was used to monitor growth (tryptic soy agar) and development of injury (Levine's Eosin Methylene Blue Agar supplemented with 2% NaCl and 0.05% sodium desoxycholate). Cells stored at 22°C remained viable longer at an a_w of 0.79 than at a_w values of 0.96 to 0.97. As storage time increased, cells were stressed at a faster rate at higher a_w values. This effect was accelerated at 40°C and as storage time increased.