J. Andrew Hudson

Growth of Listeria monocytogenes, Aeromonas hydrophila, and Yersinia enterocolitica on Vacuum and Saturated Carbon Dioxide Controlled Atmosphere-Packaged Sliced Roast Beef

The growth of Listeria monocytogenes , Aeromonas hydrophila and Yersinia enterocolitica on sliced roast beef packaged under vacuum or saturated CO2 controlled atmosphere conditions was measured. At -1.5°C, the pathogens declined in numbers in the controlled atmosphere packs but were able to grow under vacuum packaging. At 3°C, growth occurred, with maximum numbers being reached at the end of the products storage life. The increased shelf life of sliced roast beef incubated under a carbon dioxide controlled atmosphere compared to vacuum-packaged beef did not, therefore, bring with it a concomitant risk associated with the increased growth of these three pathogens over the extended storage period.

Incidence and coincidence of Listeria spp., motile aeromonads and Yersinia enterocolitica on ready-to-eat fleshfoods

A survey for the presence of Listeria spp., Yersinia enterocolitica and motile aeromonads in 203 samples of ready-to-eat fleshfoods purchased from retail outlets was conducted. Overall, 39.4%, 3.4% and 23.2% of samples were positive for the presence of Listeria spp., Y. enterocolitica and motile aeromonads respectively. Two factors have been identified as contributing to contamination of fleshfoods by these cold-tolerant bacteria. These are (i) the method of sale; delicatessen-bought foods were notably more contaminated than similar products bought pre-packaged, and (ii) the method of preservation. For motile aeromonads fermented foods were the least contaminated, whereas smoked and cooked products had similar incidence rates. For L. monocytogenes, significantly more (41.9%) smoked products were contaminated than fleshfoods preserved by other methods. For Y. enterocolitica, only cooked products were contaminated. In the case of cooked fleshfoods it must be assumed that most contamination occurs post-cooking and that contamination rates are increased by poor food handling procedures. Of the three possible pairwise combinations of these organisms, the coincidence of Y. enterocolitica and motile aeromonads was the only one that differed significantly from a random distribution (P less than 0.001), indicating that fleshfoods contaminated with Y. enterocolitica are probably also contaminated by motile aeromonads.

A Numerical Classification of Some Thermus Isolates

Summary: A numerical classification was performed on a collection of 45 Thermus isolates recovered from New Zealand hot pools and on six type strains including T. aquaticus and “T. thermophilus”. Unweighted average linking (UPGMA) and single linkage clustering methods were applied to similarity matrices derived from simple matching (SSM ) and Jaccard similarity (SJ ) coefficients. Differences were observed between phenograms derived from SSM and SJ coefficients, indicating that some of the clusters formed were derived from a significant component of negative matches. Test error was estimated at 2.9%. In the UPGMA/SSM phenogram, seven clusters were formed. A majority of the New Zealand isolates did not cluster with non-New Zealand isolates. Analysis of variance showed that there was a relationship between the composition of the clusters and the temperature and pH of the source of the isolate. Chi-squared testing showed that, within New Zealand, the geographical source of the isolate had no bearing on the clusters formed.

Numerical Classification of Thermus Isolates from Globally Distributed Hot Springs

Summary A numerical classification was carried out on a collection of 131 Thermus strains isolated predominantly from geothermal sources in New Zealand (North and South Islands), Iceland and Yellowstone National Park (USA) with some strains from New Mexico (USA), Japan, Fiji, USSR and the UK. The reference strains comprised T. aquaticus YT-1 T , T. ruber T , T. filiformis Wai33 A1 T , “T. thermophilus” HB8, “T. flavus” AT-62, “T. lacteus” and “T. rubens” . Results from unweighted average linkage (UPGMA) clustering applied to a similarity matrix derived from the simple matching (S SM ) coefficient showed the formation of 14 clusters at the 73% S SM level. These clusters showed a clear relationship to the geographical sources of the isolates. Analysis of variance also showed a relationship between the cluster composition and the pH and temperature values of the isolate sources. The recognised species were separated at 65% S SM and if this value is taken to define the species level then there appear to be 8 species among the strains studied.

Presence and significance of Bacillus cereus in dehydrated potato products.

Dehydrated potato contains Bacillus cereus at a prevalences of 10 to 40% and at numbers usually less than 10(3) CFU g(-1). B. cereus in dehydrated potato is likely to be present as spores that are able to survive drying of the raw vegetable and may represent a significant inoculum in the reconstituted (rehydrated) product where conditions favor germination of, and outgrowth from, spores. Holding rehydrated mashed potato alone, or as part of another product (e.g., potato-topped pie), at temperatures above 10 degrees C and below 60 degrees C may allow growth of vegetative B. cereus. Levels exceeding 10(4) CFU g(-1) are considered hazardous to human health and may be reached within a few hours if stored inappropriately between these temperatures. Foods incorporating mashed potato prepared from dehydrated potato flakes have been implicated in B. cereus foodborne illness. This review is a summary of the information available concerning the prevalence and numbers of B. cereus in dehydrated potato flakes and the rate at which growth might occur in the rehydrated product.

Thermonema lapsum gen. nov., sp. nov., a thermophilic gliding bacterium

We previously reported the isolation of five strains of a thermophilic gliding organism. These strains are described here as a new genus and species, Thermonema lapsum. The isolates can be readily distinguished from other thermophilic gliding bacteria as they are apparently unicellular aerobic filaments that grow optimally at 60°C. Their cell walls are similar in ultrastructure to those of gram-negative cells, but they are susceptible to penicillin G. Our isolates can be grown on a fully defined medium containing amino acids. Oxidation-versus-fermentation tests indicate that deamination takes place. The type strain, strain 23/9, has been deposited with the American Type Culture Collection as strain ATCC 43542.

Isolation and lytic activity of the Listeria bacteriophage endolysin LysZ5 against Listeria monocytogenes in soya milk.

The endolysin gene (lysZ5) from the genome of the Listeria monocytogenes phage FWLLm3 was cloned in Escherichia coli and characterized. Comparative sequence analysis revealed that lysZ5 resembled the murein hydrolase ply511 encoded by L. monocytogenes phage A511. The encoded protein LysZ5 had a predicted molecular mass of 35.8 kDa and was expressed in E. coli as an N-terminal fusion protein of 41.5 kDa. Addition of purified fusion protein to lawns of indicator bacteria showed that LysZ5 could lyse L. monocytogenes, Listeria innocua and Listeria welshimeri, but not Staphylococcus aureus or Enterococcus faecalis. The purified protein was able to kill L. monocytogenes growing in soya milk, with the pathogen concentration reduced by more than 4 log₁₀ CFU ml⁻¹ after 3 h incubation at 4 °C. As far as we know, this is the first report of a Listeria phage endolysin to control pathogens in soya milk and to demonstrate endolysin activity in foods at refrigeration temperatures. Moreover, LysZ5 may also be useful for biocontrol in other ready-to-eat foods.

Numerical Classification of some Thermus Isolates from Icelandic Hot Springs

Summary A numerical classification was performed on a collection of 51 Thermus isolates from Icelandic hot pools, six reference strains including T. aquaticus YT-1, “ T. thermophilus ” HB8 and T. ruber , and on seven strains respresenting clusters produced by a previous numerical classification of Thermus isolates from New Zealand hot pools. Unweighted average linking (UPGMA) and single linkage clustering methods were applied to similarity matrices derived from simple matching (S SM ) and Jaccard similarity (S J ) coefficients. Differences were observed between the phenograms derived from the SSM and S J coefficients indicating that some of the clusters formed were derived from a significant component of negative matches. Test error was estimated at 6.4%. In the UPGMA/S SM phenogram four clusters were formed. A majority of the Icelandic isolates did not cluster with non-Icelandic isolates. Analysis of variance showed there to be a relationship between the composition of the clusters and the pH of the isolate source, but not to the isolate source temperature. Cluster composition did not appear to reflect the geographical sources within Iceland.

Presence of Listeria monocytogenes, motile aeromonads and Yersinia enterocolitica in environmental samples taken from a supermarket delicatessen.

Environmental swab samples collected on three occasions from a supermarket delicatessen were examined for the presence of Listeria monocytogenes, motile aeromonads and Yersinia enterocolitica. Strains of Y. enterocolitica belonging to a pathogenic biotype were not recovered, but isolates of L. monocytogenes and motile aeromonads were found. Contaminated surfaces and items that came into direct contact with ready-to-eat food included a slicing machine, a knife and parsley that was to be used as a garnish. Most sites sampled near a display of processed meats requiring cooking, such as bacon and sausages, yielded positive results and could therefore act as a source of cross contamination for ready-to-eat meats.

Presence and Growth of Bacillus cereus in Dehydrated Potato Flakes and Hot-Held, Ready-to-Eat Potato Products Purchased in New Zealand

Potato products prepared from dehydrated potato flakes have been implicated in foodborne illness incidents involving Bacillus cereus intoxications. B. cereus can survive as spores in potato flakes and can germinate and multiply in the rehydrated product. This study assessed the frequency and concentration of B. cereus in dehydrated potato flakes and hot-held, ready-to-eat mashed potato products. Of 50 packets of potato flakes tested, eight contained greater than 100 CFU/g B. cereus (maximum 370 CFU/g). The temperature of the potato portion of 44 hot-held food products was measured immediately after purchase, and 86% were below the safe hot-holding temperature of 60 degrees C. The potato portions were subsequently tested for B. cereus. Only two of the potato portions contained B. cereus at greater than 100 CFU/g, a potato-topped pastry (1000 CFU/g) and a container of potato and gravy (120 CFU/g). To assess multiplication of B. cereus in this food, we held rehydrated potato flakes with naturally occurring B. cereus at 37, 42, and 50 degrees C and tested them over 6 h. By 6 h, the number of B. cereus in potato stored at 37 degrees C had exceeded 10(3) CFU/g, was greater than 10(4) CFU/g at 50 degrees C, and was close to 10(6) CFU/g at 42 degrees C. Growth data were compared to predictions from the U.S. Department of Agriculture Pathogen Modeling Program (PMP 7.0). The PMP predictions were found to simulate the measured growth better at 42 degrees C than at 37 degrees C. Hot-held potato products should be safe for consumption if held at 60 degrees C or above or discarded within 2 h.