James T. Peeler

Thermal resistance of Listeria monocytogenes in milk

The thermal resistance of Listeria monocytogenes associated with a milkborne outbreak of listeriosis was determined in buffer and whole milk. Thermal resistance was stable over a 2-year period and could not be altered by selecting heat-stressed survivors. The rate of inactivation was linear and did not differ significantly between pH 5.5 and 9.0. When portions of whole milk containing 1 × 105 cells of L. monocytogenes /ml were heated at seven temperatures from 52.2 to 74.4°C, the D-values ranged from 1683.7 to 0.7 s, respectively. The zD-value was 6.3°C. The D-value at 71.7°C was 0.9 s. L. monocytogenes would not survive the pasteurization process.

Thermal Resistance of Listeria monocytogenes in Dairy Products

The thermal resistance of Listeria monocytogenes strain Scott A that had been associated with a recent milkborne outbreak of listeriosis was determined in whole and skim milk, heavy cream, and ice cream mix. L. monocytogenes suspended at concentrations of approximately 1 × 105 cells/ml was heated at temperatures ranging from 52.2 to 79.4°C at various contact times. The D71.7°C values computed for milk samples ranged from 0.9 to 2.7 s. The D7.94°C value in ice cream mix was 0.5 s. The zD value for fluid products ranged from 5.8 to 7.1°C; the zF value for ice cream mix was 7.0°C. The L. monocytogenes suspensions would not survive a proper pasteurization process given to raw dairy products.

Hazard assessment of Listeria monocytogenes in the processing of bovine milk

The steps in Grade A production of bovine milk for human consumption were assessed. A cumulative distribution of values, based on published data, was used to evaluate steps for milking, storage, transportation and pasteurization. Conservative estimates of parameters in the distributions were used to compute concentrations and probabilities. Under normal operations, the probability was less than 2 in 100 that one Listeria monocytogenes cell occurs in every 2 gallons of milk processed at exactly 71.7°C for 15 s. The probability was less than 2 in 100 that one cell occurs in 3.8 × 1010 gallons processed at 74.4°C for 20 s.

A Collaborative Study of the Spiral Plate Method for Examining Milk Samples

The spiral plating procedure is a rapid method for determining bacteriological counts. Results from a collaborative study indicate that the procedure should be useful in milk analysis. Typical milk samples (homogenized milk, raw milk, chocolate drink, 2% milk, and 20% cream) were sent to six analysts to be examined by standard plate count (SPC) and spiral plate count (SPLPC). Analysis of duplicate samples shows that the SPC and SPLPC values did not differ at the a = 0.01 level. Components of variance for replicate determinations among laboratories and laboratory-sample interaction were computed. The standard deviation was 0.109 compared to the 0.110 estimate reported for SPC in state laboratories. Results from the SPLPC method compared favorably to the results of conventional (SPC) pour procedure.

Thermal Resistance of Disease-Associated Salmonella typhimurium in Milk

The thermal resistance of Salmonella typhimurium cultures that had been associated with a major milkborne oubreak of salmonellosis was determined in raw whole milk. Thirteen patient stool isolates and 24 implicated pasteurized milk isolates at concentrations of 1 × 105/ml were screened for heat resistance at 51.8°C. A representative milk strain was heated in replicate at four temperatures from 51.8 to 68.3°C. The zD value was calculated to be 5.3°C. Mean D-value estimates at 51.8°C were 24.0 and 22.8 min for patient and milk isolates, respectively. Extrapolated D71.7°C values were 0.24 and 0.22 s, and did not differ significantly (α = 0.05). These isolates would not survive proper pasteurization.

Thermal resistance of Listeria spp. in milk

The thermal resistance of one strain each of Listeria ivanovii , L. seeligeri , and L. welshimeri and three L. monocytogenes strains was determined in raw and sterile milk. Listeria spp. suspended in milk at concentrations of 1 × 105 cells/ml were heated at temperatures ranging from 52.2 to 71.1°C for various contact times. The heat resistance of L. monocytogenes appeared somewhat greater than that of the other Listeria spp. in both milks, but the difference was not statistically significant (α = 0.05). High-temperature, short-time processing is adequate for pasteurization of raw milk.

Persistence of Polioviruses in Shellstock and Shucked Oysters Stored at Refrigeration Temperature

Poliovirus persistence was monitored in shellstock and shucked oysters stored at 5°C. The oysters either bioaccumulated viruses from seawater contaminated with feces-associated or cell-culture-grown viruses or they were innoculated with a needle and syringe. The viruses were detected in the oysters for > 28 d, the longest estimated time period between harvest and consumption of fresh oysters. These results indicate that if oysters are contaminated when harvested, they will probably be contaminated when purchased for processing in the home or food establishment.

Quantitative comparison of two enrichment methods for isolating Listeria monocytogenes from seafoods

Two enrichment methods that had been used as standard procedures by the U.S. Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA) were quantitatively compared for their ability to isolate Listeria monocytogenes from seafoods. Cultures of a clinical sample and a seafood isolate were inoculated into raw and cooked shrimp; cultures heated at 57.8°C for 5 min were added to surimi, cooked crabmeat, and cooked shrimp. With the FDA procedure, which used enrichment intervals of 24 h, 48 h, and 7 d, KOH culture treatment and enrichment for 24 h provided no advantage for Listeria recovery. The FDA procedure isolated heated L. monocytogenes from seafoods at a lower level than the USDA method; however, the two methods isolated unheated cells equally well. The greater selectivity of the USDA procedure may offer an advantage for isolating nonheat-stressed Listeria when the aerobic plate count of the product is high.

Comparison of Five Selective Enrichment Broths and Two Selective Agars For Recovery of Vibrio vulnificus From Oysters

The Bacteriological Analytical Manual (6th edition) specifies use of glucose-salt-teepol (GST) broth for detection of Vibrio vulnificus and other halophilic vibrios in seafood. Since teepol is no longer commercially available, this study compared five enrichment broths for their ability to recover V. vulnificus . Ten samples of seeded oysters were analyzed using a three-tube MPN and enriched in each of five broths in duplicate. Broth cultures were then streaked onto cellobiose-polymyxin B-colistin (CPC) agar and sodium dodecyl sulfate-polymyxin B-sucrose (SDS) agar plates. Average (± standard error) recovery (log MPN/g) from each broth was as follows: Alkaline-peptone-water (APW), 4.16 ± 0.20; Marine (MRN) broth, 3.63 ± 0.16; Hories broth, 2.88 ± 0.17; Monsurs broth, 2.43 ± 0.16; and GST broth, 1.28 ± 0.28. APW and MRN broths yielded significantly (P<0.05) higher recovery than other broths by the Kruskal-Wallis nonparametric rank test. Vibrio vulnificus was isolated with higher frequency from CPC (81%) as compared with SDS (61%) agar plates. Background growth was minimal on CPC agar, facilitating selection of V. vulnificus colonies. Based on these results, APW enrichment broth and CPC isolation agar were more efficient for recovery of V. vulnificus from oysters than other broth and agar combinations.

Aeromonas hydrophila in shellfish growing waters: incidence and media evaluation

Levels of Aeromonas hydrophila determined for the shellfish growing area of Grays Harbor, Washington, ranged from 3 to 4600/100 g in oysters and from 3 to 2400/100 ml in water. Of isolates tested, 80% produced a hemolysin, a trait reported to correlate with enterotoxin production and pathogenicity. Two enrichment broths, Tryptic Soy Broth with ampicillin (TSBA) and Modified Rimler Shotts Broth (MRSB) were compared in combination with three solid agar media: Rimler Shotts (RS), Peptone Beef Extract Glycogen (PBG), and MacConkeys (MCA) agars. TSBA was far superior to MRSB in isolating this species from the environmental samples tested.