Survival of Listeria monocytogenes during storage on dried apples, strawberries, and raisins at 4 °C and 23 °C.

The survival of Listeria monocytogenes was assessed during long-term storage on three dried fruits: dried apples, raisins and dried strawberries. Using sand as a carrier, the dried fruits were dry-inoculated with a four-strain cocktail of L. monocytogenes to achieve numbers of 4.0 to 4.6 log CFU/g. The inoculated foods were stored at 4 °C, 25-81% relative humidity (RH) and 23 °C, 30-35% RH for 336 days. Colonies of L. monocytogenes could not be recovered from the dried apples after inoculation, i.e., day 0. Concentrations of L. monocytogenes decreased rapidly on the raisins and dried strawberries during storage at 23 °C, with enhanced survival observed at 4 °C. Linear rates of decline for populations of L. monocytogenes during storage at 4 °C on the raisins and dried strawberries were 0.1 and 0.2 log CFU/g/month, respectively. The relative distribution of the four L. monocytogenes strains making up the cocktail was determined by multiplex PCR at the beginning of storage and after 336 days on the dried fruits. At day 0, L. monocytogenes populations were predominantly composed of the serotype 1/2a and 3a strains on both the raisins and dried strawberries. After long-term storage at 4 °C, a relative decrease in serotype 1/2a was observed on both fruits, coupled with relative increases in the serotype 3a strain during storage on both fruits, in addition to the serotype 1/2b strain on the raisins. These results demonstrate that L. monocytogenes is rapidly inactivated during storage on raisins and dried strawberries at 23 °C, but it is capable of long-term survival at 4 °C. Improved knowledge on the survival of L. monocytogenes on these commodities is important for predictive modeling and can be used to better inform microbial health risk assessments.

Survival and Virulence of Listeria monocytogenes during Storage on Chocolate Liquor, Corn Flakes, and Dry-Roasted Shelled Pistachios at 4 and 23°C.

ABSTRACT: The survival and virulence of Listeria monocytogenes was assessed during storage on three low-moisture foods (LMFs): chocolate liquor, corn flakes, and shelled, dry-roasted pistachios (water activity [aw] of 0.18, 0.27, and 0.20, respectively). The LMFs were inoculated with a four-strain cocktail of L. monocytogenes at 8 log CFU/g, dried, held until the aw stabilized, and then stored at 4°C and 25 to 81% relative humidity (RH) and at 23°C and 30 to 35% RH for at least 336 days. At 4°C, L. monocytogenes remained stable on the LMFs for at least 336 days. At 23°C, L. monocytogenes levels declined on the chocolate liquor, corn flakes, and pistachios at initial rates of 0.84, 0.88, and 0.32 log CFU/g/month, respectively. After 8 months at 23°C, L. monocytogenes levels on the chocolate liquor and corn flakes decreased to below the limit of detection (i.e., 0.48 log CFU/g). Relative populations of each strain were assessed before storage (i.e., day 0) and after 6 and 12 months of storage at 23 and 4°C, respectively. Generally, a decline in the relative level of the serotype 1/2a strain was observed during storage, coupled with the relative increase in other strains, depending on the LMF and storage temperature. The total viable populations of L. monocytogenes determined by the PMAxx quantitative PCR method after >12 months of storage at 4°C were significantly (1.8- to 3.7-log) higher than those obtained by plating on tryptic soy agar with yeast extract. Decreases in the culturable population of L. monocytogenes during storage on the LMFs were the result of both cellular inactivation and transition to a viable-but-nonculturable state. The surviving cells, specifically after long-term storage at 4°C on the chocolate liquor and pistachios, remained infectious and capable of intracellular replication in Caco-2 enterocytes. These results are relevant for predictive modeling used in microbial health risk assessments and support the addition of LMFs to food safety questionnaires conducted during listeriosis outbreaks.

Methods for the Control of Foodborne Pathogens in Low-Moisture Foods.

Low-moisture foods (LMFs) have been defined as those food products with a water activity (aw) less than 0.85 and are generally considered less susceptible to microbial spoilage and the growth of foodborne pathogens. However, in recent years, outbreaks linked to LMFs have increased, with Salmonella spp., Bacillus cereus, Cronobacter sakazakii, Clostridium spp., Escherichia coli O157:H7, non-O157 E. coli, and Staphylococcus aureus being the principal pathogens involved. Because of the new concerns raised as a result of recent outbreaks, new approaches need to be developed to control foodborne pathogens in LMFs. This review summarizes the recent research on novel inactivation methods suitable for use on LMFs. Among the methods discussed are the nonthermal inactivation methods as well as other novel methods such as radio-frequency and microwave heating. Additional research is needed to evaluate older technologies and develop new technologies, either alone or in combination, to understand the mechanisms of inactivation.

Prevalence and antimicrobial resistance of Salmonella isolated from two pork processing plants in Alberta, Canada.

This study investigated the frequency of Salmonella serovars on pig carcasses at various processing steps in two commercial pork processing plants in Alberta, Canada and characterized phenotypic and genotypic antimicrobial resistance (AMR) and PFGE patterns of the Salmonella isolates. Over a one year period, 1000 swab samples were collected from randomly selected pigs at two slaughter plants. Sampling points were: carcass swabs after bleeding (CSAB), carcass swabs after de-hairing (CSAD, plant A) or skinning (CSASk, plant B), carcass swabs after evisceration (CSAE), carcass swabs after pasteurization (CSAP, plant A) or washing (CSAW, plants B) and retail pork (RP). For plant A, 87% of CSAB and 8% of CSAE were positive for Salmonella while at plant B, Salmonella was recovered from 94% of CSAB and 10% of CSAE. Salmonella was not recovered from the RP samples at either plant, indicating that the plants used effective control measures. Salmonella enterica serovar Derby was the most common serotype (23%, 29/127) recovered in plant A and plant B (61%, 76/124). For plant A, 35% (45/127) of isolates were resistant to at least one antimicrobial. Five isolates (3.9%), 4 serovar Ohio strains and one serovar I:Rough-O:I,v:-, strain were simultaneously resistant to antimicrobials of very high (Category I), high (Category II), and medium (Category III) importance to human medicine. The 4 S. Ohio isolates were recovered from 3 different steps of pork processing on the same sampling day and displayed resistance to 5-7 antimicrobials, with all of them displaying resistance to ceftiofur and ceftriaxone (Category I). An I:Rough-O:l,v:- isolate, recovered on a different sampling day, was resistant to 7 antimicrobials that included resistance to ampicillin/clavulanic acid, ceftiofur and ceftriaxone (Category I). Salmonella strains isolated from plant A harbored 12 different AMR genes. The most prevalent genes were sul1, sul2, tet(A), tet(B), aadA, strA/strB, aac(3)IV and aphA1. For Salmonella isolates from plant B, 7 resistance genes were identified alone or in combination where tet(B) was found in 77 (62.3%) of the isolates. For plant A, 19 different PFGE subtypes of Salmonella isolates that displayed phenotypic and/or genotypic resistance were observed while 13 different PFGE subtypes were observed for plant B. The lack of detection of Salmonella on the surfaces of RP suggests that current pork processing practices can dramatically reduce Salmonella. Salmonella isolates from pig carcasses at various steps displayed multidrug resistance, including to those of very high importance in human medicine, which represent a public health concern.

Glutamine, glutamate, and arginine-based acid resistance in Lactobacillus reuteri.

This study aimed to determine whether glutamine deamidation improves acid resistance of Lactobacillus reuteri, and to assess whether arginine, glutamine, and glutamate-mediated acid resistance are redundant or complementary mechanisms of acid resistance. Three putative glutaminase genes, gls1, gls2, and gls3, were identified in L. reuteri 100-23. All three genes were expressed during growth in mMRS and wheat sourdough. L. reuteri consistently over-expressed gls3 and the glutamate decarboxylase gadB. L. reuteri 100-23ΔgadB over-expressed gls3 and the arginine deiminase gene adi. Analysis of the survival of L. reuteri in acidic conditions revealed that arginine conversion is effective at pH of 3.5 while glutamine or glutamate conversion were effective at pH of 2.5. Arginine conversion increased the pHin but not ΔΨ; glutamate decarboxylation had only a minor effect on the pHin but increased the ΔΨ. This study demonstrates that glutamine deamidation increases the acid resistance of L. reuteri independent of glutamate decarboxylase activity. Arginine and glutamine/glutamate conversions confer resistance to lactate at pH of 3.5 and phosphate at pH of 2.5, respectively. Knowledge of L. reuteri's acid resistance improves the understanding of the adaptation of L. reuteri to intestinal ecosystems, and facilitates the selection of probiotic and starter cultures.