A note on the microbiology of retail packs of prepared salad vegetables

Retail packs of mixed, prepared salad vegetables from two different manufacturers were stored at 7 degrees C until the end of storage-life (sell-by date plus 1 d), when the microbial flora was examined. The quality of the salads was acceptable at the end of storage life. The oxygen concentrations in packs were lower, and the carbon dioxide concentrations were higher, than those in air. High numbers of bacteria were present, with Pseudomonas spp. and Enterobacter agglomerans predominating in packs of both salads, together with lactic acid bacteria in one of the salads. Significant numbers of pectolytic bacteria including Pseudomonas spp. and Erwinia carotovora were detected. Despite the presence of high numbers of coliform bacteria, Escherichia coli was not detected in batches of one salad, and was detected in relatively low numbers in batches of the other. Yersinia spp., predominantly environmental strains of Yersinia enterocolitica, were isolated by enrichment from all samples tested; Staphylococcus aureus and enterococci were not detected.     

Prepared salads and public health

In recent years the importance of prepared salads as potential vehicles of gastrointestinal infection has been highlighted by several large outbreaks both nationally and across international boundaries. Between 1992 and 2006, 2274 foodborne general outbreaks of infectious intestinal disease were reported in England and Wales, of which 4% were associated with the consumption of prepared salads. In total, 3434 people were affected, with 66 hospitalizations and one death reported. The attribution of prepared salad types and pathogens among prepared salad associated outbreaks are presented and discussed. Findings from UK studies on salad vegetables, fruit and mixed salads from 1995 to 2007 (21 247 samples) indicate that most bacteria of concern with regard to human health are relatively rare in these products (98.6% of satisfactory quality); however, outbreaks of salmonellosis were uncovered associated with bagged salad leaves and fresh herbs during two such studies. Although it is known that fresh salad vegetables, herbs or fruit may become contaminated from environmental sources, only in recent years has the association of foods of nonanimal origin, such as salad vegetables, with foodborne illness become evident and recurrent, demonstrating that major health problems can arise from consumption of contaminated prepared salads if hygiene practices breakdown.     

Denaturing gradient gel electrophoresis for nonlethal detection of Aeromonas salmonicida in salmonid mucus and its potential for other bacterial fish pathogens

Denaturing gradient gel electrophoresis (DGGE) of 16S rDNA was used to nonlethally detect Aeromonas salmonicida and other bacteria in salmonid skin mucus. Mucus samples from wild spawning coho salmon (Oncorhynchus kisutch) with endemic A. salmonicida and from cultured lake trout (Salvelinus namaycush) were tested by PCR-DGGE and were compared with mucus culture on Coomassie brilliant blue agar and internal organ culture. PCR-DGGE gave a highly reproducible 4-band pattern for 9 strains of typical A. salmonicida, which was different from other Aeromonas spp. Aeromonas salmonicida presence in mucus was evident as a band that comigrated with the bottom band of the A. salmonicida 4-band pattern and was verified by sequencing. PCR-DGGE found 36 of 52 coho salmon positive for A. salmonicida, compared with 31 positive by mucus culture and 16 by organ culture. Numerous other bacteria were detected in salmonid mucus, including Pseudomonas spp., Shewanella putrefaciens, Aeromonas hydrophila and other aeromonads. However, Yersinia ruckeri was not detected in mucus from 27 lake trout, but 1 fish had a sorbitol-positive Y. ruckeri isolated from organ culture. Yersinia ruckeri seeded into a mucus sample suggested that PCR-DGGE detection of this bacterium from mucus was possible. PCR-DGGE allows nonlethal detection of A. salmonicida in mucus and differentiation of some Aeromonas spp. and has the potential to allow simultaneous detection of other pathogens present in fish mucus.     

Effect of dyes on the quantitative recovery of Yersinia enterocolitica.

A total of 69 dyes were incorporated separately at different concentrations into an agar medium for evaluation of their effects on the quantitative recovery of five serotypes of Yersinia enterocolitica. One strain of Pseudomonas aeruginosa and one strain of Bacillus cereus were included for comparative purposes. Certain dyes were evaluated further for their selective properties with five additional serotypes of Y. enterocolitica, three strains of P. aeruginosa, and two of Engerobacter spp. Metanil yellow was the only dye which was tolerated bettr by Y. enterocolitica than by P. aeruginosa. Dye sensitivity was variable amond strains of the same serotype of Y. enterocolitica. In general, Y. enterocolitica showed a tolerance to dyes greater than that of gram-positive bacteria and similar to that of other gram-negative bacteria.     

Effect of dyes on the quantitative recovery of Yersinia enterocolitica.

A total of 69 dyes were incorporated separately at different concentrations into an agar medium for evaluation of their effects on the quantitative recovery of five serotypes of Yersinia enterocolitica. One strain of Pseudomonas aeruginosa and one strain of Bacillus cereus were included for comparative purposes. Certain dyes were evaluated further for their selective properties with five additional serotypes of Y. enterocolitica, three strains of P. aeruginosa, and two of Engerobacter spp. Metanil yellow was the only dye which was tolerated bettr by Y. enterocolitica than by P. aeruginosa. Dye sensitivity was variable amond strains of the same serotype of Y. enterocolitica. In general, Y. enterocolitica showed a tolerance to dyes greater than that of gram-positive bacteria and similar to that of other gram-negative bacteria.     

Effect of Storage Temperature on the Microbial Composition of Ready-to-Use Vegetables

Four different salad preparations were investigated from microbiological point of view: two were packaged in air and two under Modified Atmosphere. The samples were stored at 4 and 10 °C, and analysed at established times. Total bacterial count (TBC) was taken as the most relevant index to define their hygiene and quality at both temperatures. Lactic acid bacteria, yeasts and moulds were found only occasionally. In general, the most important factor was the packaging technique: TBC was lower when the product is packed under modified conditions. The packaging technique also influences the microbial population: Gram-negative aerobic rods are dominant in air-packaged products, whilst the presence of Enterobacteriaceae becomes important in salads packaged under Modified Atmosphere. Pseudomonas fluorescens, with all its biovars, was the most frequently found species amongst the aerobic isolates, whilst for the Enterobacteriaceae strains, there was no dominant species.     

Microbiological profile in Serra ewes' cheese during ripening

The microflora of Serra cheese was monitored during a 35 d ripening period at three different periods within the ewe's lactation season. After 7 d ripening, the numbers of micro-organisms reached their maximum, and lactic acid bacteria (LAB) and coliforms were the predominant groups. Pseudomonads were not detected after 1 week of ripening. At all stages of ripening, cheeses manufactured in spring exhibited the lowest numbers of LAB and yeasts, whereas cheeses manufactured in winter showed the lowest numbers of coliforms and staphylococci.
Leuconostoc lactis was the most abundant LAB found in Serra cheese whereas Enterococcus faecium and Lactococcus lactis spp. lactis exhibited the highest decrease in percentage composition. Numbers of both Leuc. mesenteroides and Lactobacillus paracasei tended to increase throughout ripening. The most abundant coliform was Hafnia alvei. Klebsiella oxytoca was found in curd but declined in number during ripening. Staphylococcal flora of curd was mainly composed of Staphylococcus xylosus, Staph. aureus and Staph. epidermidis. Staphylococcus xylosus was the major species found at the end of ripening. Pseudomonas fluorescens , was the only Pseudomonas species isolated from the curd. Although a broad spectrum of yeasts were found in Serra cheese, Sporobolomyces roseus was the most abundant yeast isolated.     

A survey of the prevalence of selected bacteria in wild birds

We determined the prevalence of six genera of bacteria from a sample of 387 cloacal swabs from 364 passerines and woodpeckers. The prevalence of bacteria were as follows: Escherichia coli (1%), Pseudomonas spp. (22%), Salmonella spp. (0%), Staphylococcus spp. (15%), Streptococcus spp. (18%), and Yersinia spp. (1%). The prevalence of Streptococcus spp. was higher in omnivorous species than in granivorous species (20% versus 8%). Individuals captured at feeders had a lower prevalence of both Streptococcus spp. (15% versus 33%) and Escherichia coli (0.5% versus 4%) than birds that did not have access to feeders. These differences are probably not due to the feeder per se, but instead to other site related differences. The prevalence of bacteria did not differ between male and female black-capped chickadees, Parus atricapillus. For 279 color marked black-capped chickadees, we calculated the cumulative mortality rate during 12 wk following swabbing. Although the cumulative mortality rates of infected birds were consistently higher than the rates of non-infected birds, none of these differences were significant. Infections may cause slight reductions in survival rates, but we were not able to confirm this with our data.     

Application of antimicrobial-producing lactic acid bacteria to control pathogens in ready-to-use vegetables

Five psychrotrophic strains of lactic acid bacteria (Lactobacillus casei, Lact. plantarum and Pediococcus spp.) were isolated from 22 samples of commercial salads. These strains were shown to inhibit Aeromonas hydrophila, Listeria monocytogenes, Salmonella typhimurium and Staphylococcus aureus on MRS agar, in salads and in juice prepared from vegetable salads. Lactobacillus casei IMPCLC34 was most effective in reducing total mesophilic bacteria and the coliform group; Aer. hydrophila, Salm. typhimurium and Staph. aureus disappeared after 6 d of storage, while the counts for L. monocytogenes remained constant. The potential application of antimicrobial-producing lactic acid bacteria as biopreservatives of ready-to-use vegetables is suggested.     

Residual effect of storage in an elevated carbon dioxide atmosphere on the microbial flora of rock cod (Sebastes spp.).

A residual inhibitory effect on microbial growth due to modified-atmosphere (MA) storage (MA, 80% CO2-20% air) was demonstrated for rock cod fillets stored in MA and transferred to air at 4 degrees C. Results of measurements of CO2 concentrations of the fillets suggested that the residual effect after transfer from MA to air was not due to retention of CO2 at the surface of the fillets but was probably due to the microbial ecology of the system. Lactobacillus spp. and tan Alteromonas spp. (TAN) predominated after 7 and 14 days of storage in MA. During storage in MA, Pseudomonas spp. were inhibited or killed. Following transfer from MA to air, the percentage of the total flora represented by Lactobacillus spp. and TAN bacteria decreased, and 6 days after transfer Pseudomonas spp. were again dominant.     

Residual effect of storage in an elevated carbon dioxide atmosphere on the microbial flora of rock cod (Sebastes spp.)

A residual inhibitory effect on microbial growth due to modified-atmosphere (MA) storage (MA, 80% CO2-20% air) was demonstrated for rock cod fillets stored in MA and transferred to air at 4 degrees C. Results of measurements of CO2 concentrations of the fillets suggested that the residual effect after transfer from MA to air was not due to retention of CO2 at the surface of the fillets but was probably due to the microbial ecology of the system. Lactobacillus spp. and tan Alteromonas spp. (TAN) predominated after 7 and 14 days of storage in MA. During storage in MA, Pseudomonas spp. were inhibited or killed. Following transfer from MA to air, the percentage of the total flora represented by Lactobacillus spp. and TAN bacteria decreased, and 6 days after transfer Pseudomonas spp. were again dominant.     

Isolation and characterization of Legionella spp. and Pseudomonas spp. from greenhouse misting systems

AIMS: Greenhouse misting systems used for watering plants produce fine aerosols. They are a possible cause for bacterial infections. This study investigates the colonization of greenhouse misting systems with Legionella spp. and Pseudomonas spp. and evaluates a possible health hazard. METHODS AND RESULTS: Between June and September 2003, a total of 80 water samples were collected in 20 different greenhouse systems in Germany, each tested on two different occasions. Each time, water was drawn at a central tap and at the outlet of spray nozzles. Sampled greenhouses were used to cultivate various plants and trees for commercial, recreational or scientific reasons, some of them in tropical conditions. Legionella spp. were detected in 10% of the systems (two systems), but only in low numbers. On the contrary, Pseudomonas spp. were recovered from 70% of the greenhouse watering systems (14 systems), occasionally at counts greater than 10,000 CFU per 100 ml. A random amplified polymorphic DNA polymerase chain reaction typing method was used to demonstrate that each colonized greenhouse had one or several individual strains of Legionella and Pseudomonas that could not be detected in any other system. CONCLUSIONS: This study demonstrates that aerosolizing greenhouse watering systems may be contaminated with Legionella or Pseudomonas which under certain circumstances could become a potential source of infection for workers and visitors. SIGNIFICANCE AND IMPACT OF THE STUDY: The study results indicate that greenhouse misting systems should be included in Legionella and Pseudomonas monitoring and control programs.     

A comparison of media and methods for the recovery of Yersinia enterocolitica and Yersinia enterocolitica-like bacteria from milk containing simulated raw milk microfloras

A number of plating and enrichment media proposed for the isolation of Yersinia enterocolitica from foodstuffs were examined for their ability to recover the type strains of Y. enterocolitica sensu stricto, Y. intermedia, Y. frederiksenii and Y. kristensenii. Nine selective plating media were evaluated for the quantitative recovery of the type strains in pure culture, and their inhibition of other organisms typical of both milk and enteric microfloras. Cefsulodin-irgasan-novobiocin (CIN) agar, incubated for 48 h at 25°C, allowed a high recovery of all the Yersinia spp. and was the most selective medium. The same four type strains were added to UHT milk that had been previously inoculated with bacteria to simulate either freshly drawn or cold stored milk microfloras. Twenty-six enrichment procedures (including cold enrichment, selective enrichment at higher temperatures, two-step procedures and a post-enrichment alkali treatment) were assessed for the efficiency of recovery of the Yersinia spp. Pre-enrichment in trypticase-soy broth (TSB) for 24 h at 22°C followed by selective enrichment in bile-oxalate-sorbose (BOS) medium for 5 d at 22°C and plating on CIN agar (48 h at 25°C) allowed the greatest increase in the numbers of Yersinia spp. and maximum inhibition of the competing microflora.     

A comparison of media and methods for the recovery of Yersinia enterocolitica and Yersinia enterocolitica-like bacteria from milk containing simulated raw milk microfloras

A number of plating and enrichment media proposed for the isolation of Yersinia enterocolitica from foodstuffs were examined for their ability to recover the type strains of Y. enterocolitica sensu stricto, Y. intermedia, Y. frederiksenii and Y. kristensenii. Nine selective plating media were evaluated for the quantitative recovery of the type strains in pure culture, and their inhibition of other organisms typical of both milk and enteric microfloras. Cefsulodin-irgasan-novobiocin (CIN) agar, incubated for 48 h at 25 degrees C, allowed a high recovery of all the Yersinia spp. and was the most selective medium. The same four type strains were added to UHT milk that had been previously inoculated with bacteria to simulate either freshly drawn or cold stored milk microfloras. Twenty-six enrichment procedures (including cold enrichment, selective enrichment at higher temperatures, two-step procedures and a post-enrichment alkali treatment) were assessed for the efficiency of recovery of the Yersinia spp. Pre-enrichment in trypticase-soy broth (TSB) for 24 h at 22 degrees C followed by selective enrichment in bile-oxalate-sorbose (BOS) medium for 5 d at 22 degrees C and plating on CIN agar (48 h at 25 degrees C) allowed the greatest increase in the numbers of Yersinia spp. and maximum inhibition of the competing microflora.     

Yersinia species in the dunnock (Prunella modularis) in sub-alpine habitats of the Western Carpathians

The study presents the prevalence of Yersinia species in dunnok Prunella modularis from the sub-alpine zone of the Western Carpathians. Bacteria were detected from cloacal and pharyngeal swabs from 97 specimens using PCR assay. Yersinia enterocolitica showed the highest prevalence (47.4%) from among the determined Yersinia species. Yersinia species (except Y frederiksenii) were detected more frequently in pharyngeal than cloacal samples. The highest prevalence of yersiniosis was detected in April (Yersinia spp. - 80%, Y. enterocolitica - 70%). No statistically differences were observed in the prevalence of Yersinia spp. between males and females and between juveniles and adult birds. Bacterial contamination did not affect body weight or tarsus length.     

Microflora of the honeybee gastrointestinal tract

Microorganisms in the midgut and rectum of the honeybee were enumerated and characterized. Counts of aerobic microorganisms were distinctly lower than counts of anaerobes (10(5)-10(6) viable cells per g of intestinal content vs. 10(8)-10(9) per g). Total numbers of anaerobic microorganisms were almost identical with the count of anaerobic Gram-positive acid resistant rods. A higher number of coliform bacteria and Bacillus spp. was detected in the rectum (10(5) per g). Anaerobic and aerobic microorganisms, coliforms, enterococci, Bacillus spp., Pseudomonas spp. and yeasts were found in all bees; lactobacilli, staphylococci and moulds were not found.     

Salmonella spp., Vibrio spp., Clostridium perfringens, and Plesiomonas shigelloides in Marine and Freshwater Invertebrates from Coastal California Ecosystems

The coastal ecosystems of California are highly utilized by humans and animals, but the ecology of fecal bacteria at the land–sea interface is not well understood. This study evaluated the distribution of potentially pathogenic bacteria in invertebrates from linked marine, estuarine, and freshwater ecosystems in central California. A variety of filter-feeding clams, mussels, worms, and crab tissues were selectively cultured for Salmonella spp., Campylobacter spp., Escherichia coli-O157, Clostridium perfringens, Plesiomonas shigelloides, and Vibrio spp. A longitudinal study assessed environmental risk factors for detecting these bacterial species in sentinel mussel batches. Putative risk factors included mussel collection near higher risk areas for livestock or human sewage exposure, adjacent human population density, season, recent precipitation, water temperature, water type, bivalve type, and freshwater outflow exposure. Bacteria detected in invertebrates included Salmonella spp., C. perfringens, P. shigelloides, Vibrio cholerae, Vibrio parahaemolyticus, and Vibrio alginolyticus. Overall, 80% of mussel batches were culture positive for at least one of the bacterial species, although the pathogens Campylobacter, E. coli-O157, and Salmonella were not detected. Many of the same bacterial species were also cultured from upstream estuarine and riverine invertebrates. Exposure to human sewage sources, recent precipitation, and water temperature were significant risk factors for bacterial detection in sentinel mussel batches. These findings are consistent with the hypothesis that filter-feeding invertebrates along the coast concentrate fecal bacteria flowing from land to sea and show that the relationships between anthropogenic effects on coastal ecosystems and the environmental niches of fecal bacteria are complex and dynamic.     

Role of dissolution rate and solubility in biodegradation of aromatic compounds

Strains of Moraxella sp., Pseudomonas sp., and Flavobacterium sp. able to grow on biphenyl were isolated from sewage. The bacteria produced 2.3 to 4.5 g of protein per mol of biphenyl carbon, and similar protein yields were obtained when the isolates were grown on succinate. Mineralization of biphenyl was exponential during the phase of exponential growth of Moraxella sp. and Pseudomonas sp. In biphenyl-supplemented media, Flavobacterium sp. had one exponential phase of growth apparently at the expense of contaminating dissolved carbon in the solution and a second exponential phase during which it mineralized the hydrocarbon. Phase-contrast microscopy did not show significant numbers of cells of these three species on the surface of the solid substrate as it underwent decomposition. Pseudomonas sp. did not form products that affected the solubility of biphenyl, although its excretions did increase the dissolution rate. It was calculated that Pseudomonas sp. consumed 29 nmol of biphenyl per ml in the 1 h after the end of the exponential phase of growth, but 32 nmol of substrate per ml went into solution in that period when the growth rate had declined. In a medium with anthracene as the sole added carbon source, Flavobacterium sp. converted 90% of the substrate to water-soluble products, and a slow mineralization was detected when the cell numbers were not increasing. Flavobacterium sp. and Beijerinckia sp. initially grew exponentially and then arithmetically in media with phenanthrene as the sole carbon source. Calculations based on the growth rates of these bacteria and the rates of dissolution of phenanthrene suggest that the dissolution rate of the hydrocarbon may limit the rate of its biodegradation.