Formation of guaiacol in chocolate milk by the psychrotrophic bacterium Rahnella aquatilis

Phenotypic and genomic characterizations of Klebsiella pneumoniae ssp. pneumoniae and Rahnella inusitata strains reveal no clear association between genetic content and ropy phenotype

Ropy defect of pasteurized fluid milk is a type of spoilage which manifests itself by an increased viscosity, slimy body, and string-like flow during pouring. This defect has, among other causes, been attributed to the growth, proliferation and exopolysaccharide production by coliform bacteria, which are most commonly introduced in milk as post-pasteurization contaminants. As we identified both Klebsiella pneumoniae ssp. pneumoniae and Rahnella inusitata that were linked to a ropy defect, the goal of this study was to characterize 3 K. pneumoniae ssp. pneumoniae strains and 2 R. inusitata for (1) their ability to grow and cause ropy defect in milk at 6°C and 21°C and to (2) probe the genetic basis for observed ropy phenotype. Although all K. pneumoniae ssp. pneumoniae and R. inusitata strains showed net growth of >4 log10 over 48 h in UHT milk at 21°C, only R. inusitata strains displayed growth during 28-d incubation period at 6°C (>6 log10). Two out of 3 K. pneumoniae ssp. pneumoniae strains were capable of causing the ropy defect in milk at 21°C, as supported by an increase in the viscosity of milk and string-like flow during pouring; these 2 strains were originally isolated from raw milk. Only one R. inusitata strains was able to cause the ropy defect in milk; this strain was able to cause the defect at both 6°C and 21°C, and was originally isolated from a pasteurized milk. These findings suggest that the potential of K. pneumoniae ssp. pneumoniae and R. inusitata to cause ropy defect in milk is a strain-dependent characteristic. Comparative genomics provided no definitive answer on genetic basis for the ropy phenotype. However, for K. pneumoniae ssp. pneumoniae, genes rffG, rffH, rfbD, and rfbC involved in biosynthesis and secretion of enterobacterial common antigen (ECA) could only be found in the 2 strains that produced ropy defect, and for R. inusitata a set of 2 glycosyltransferase- and flippase genes involved in nucleotide sugar biosynthesis and export could only be identified in the ropy strain. Although these results provide some initial information for potential markers for strains that can cause ropy milk, the relationship between genetic content and ropiness in milk remains poorly understood and merits further investigation.

Rapid headspace solid-phase microextraction sheets with direct analysis in real time mass spectrometry (SPMESH-DART-MS) of derivatized volatile phenols in grape juices and wines

Volatile phenols possess "smoky, spicy" aromas and are routinely measured in grapes, wines and other foodstuffs for quality control. Routine analyses of volatile phenols rely on gas chromatography - mass spectrometry (GC-MS), but slow throughput of GC-MS can cause challenges during times of surge demand, i.e. following 'smoke taint' events involving forest fires near vineyards. Parallel extraction of headspace volatiles onto sorbent sheets (HS-SPMESH) followed by direct analysis in real time mass spectrometry (DART-MS) is a rapid alternative to conventional GC-MS approaches. However, HS-SPMESH extraction is poorly suited for lower volatility odorants, including volatile phenols. This work reports development and validation of an HS-SPMESH-DART-MS approach for five volatile phenols (4-ethylphenol, 4-ethylguiacol, guaiacol, 4-methylguaiacol, and cresols). Prior to HS-SPMESH extraction, volatile phenols were acetylated to facilitate their extraction. A unique feature of this work was the use of d₆-Ac₂O as a derivatizing agent to overcome issues with isobaric interferences inherent to chromatography-free MS techniques. The use of alkaline conditions during derivatization resulted in cumulative measurement of both free and bound forms of volatile phenols. The validated HS-SPMESH-DART-MS method achieved a throughput of 24 samples in ∼60 min (including derivatization and extraction time) with low limits of detection (<1 μg L^-1) and good repeatability (3-6% RSD) in grape and wine matrices. Validation experiments with smoke-tainted grape samples indicated good correlation between total (free + bound) volatile phenols measured by HS-SPMESH-DART-MS and a gold standard GC-MS method.

High-Throughput 16S rRNA Sequencing to Assess Potentially Active Bacteria and Foodborne Pathogens: A Case Example in Ready-to-Eat Food

Technologies to detect the entire bacterial diversity spectra and foodborne pathogens in food represent a fundamental advantage in the control of foodborne illness. Here, we applied high-throughput 16S rRNA sequencing of amplicons obtained by PCR and RT-PCR from extracted DNA and RNA targeting the entire bacterial community and the active bacterial fraction present in some of the most consumed and distributed ready-to-eat (RTE) salad brands in Europe. Customer demands for RTE food are increasing worldwide along with the number of associated foodborne illness and outbreaks. The total aerobic bacterial count in the analyzed samples was in the range of 2-4 × 10⁶ CFU/g (SD ± 1.54 × 10⁶). Culture validated methods did not detect Salmonella spp., Escherichia coli, and other fecal coliforms. 16S rRNA gene Illumina next-generation sequencing (NGS) data were congruent with these culture-based results and confirmed that these and other well-known foodborne bacterial pathogens, such as Listeria, were not detected. However, the fine-resolution of the NGS method unveiled the presence of the opportunistic pathogens Aeromonas hydrophyla and Rahnella aquatilis (relative frequency of 1.33-7.33%) that were metabolically active in addition to non-pathogenic, active members of Yersinia spp. (relative frequency of 0.0015-0.003%). The common ail and foxA marker genes of Yersinia enterocolitica were not detected by qPCR. Finally, our NGS data identified to non-pathogenic Pseudomonas spp. as the most abundant and metabolically active bacteria in the analyzed RTE salads (53-75% of bacterial abundance). Our data demonstrate the power of sequencing, in parallel, both 16S rRNA and rDNA to identify and discriminate those potentially and metabolically active bacteria and pathogens to provide a more complete view that facilitates the control of foodborne diseases, although further work should be conducted to determine the sensitivity of this method for targeting bacteria.

Rahnella spp. are commonly isolated from onion (Allium cepa) bulbs and are weakly pathogenic

AIMS

Bacterial decays of onion bulbs have serious economic consequences for growers, but the aetiologies of these diseases are often unclear. We aimed to determine the role of Rahnella, which we commonly isolated from bulbs in the United States and Norway, in onion disease.

METHODS AND RESULTS

Isolated bacteria were identified by sequencing of housekeeping genes and/or fatty acid methyl ester analysis. A subset of Rahnella spp. strains was also assessed by multilocus sequence analysis (MLSA); most onion strains belonged to two clades that appear closely related to R. aquatilis. All tested strains from both countries caused mild symptoms in onion bulbs but not leaves. Polymerase chain reaction primers were designed and tested against strains from known species of Rahnella. Amplicons were produced from strains of R. aquatilis, R. victoriana, R. variigena, R. inusitata and R. bruchi, and from one of the two strains of R. woolbedingensis.

CONCLUSIONS

Based on binational testing, strains of Rahnella are commonly associated with onions, and they are capable of causing mild symptoms in bulbs.

SIGNIFICANCE AND IMPACT OF THE STUDY

While Rahnella strains are commonly found within field-grown onions and they are able to cause mild symptoms, the economic impact of Rahnella-associated symptoms remains unclear.

Simultaneous analysis of guaiacol and vanillin in a vanilla extract by using high-performance liquid chromatography with electrochemical detection

We developed and validated a new high-performance liquid chromatographic analysis for electrochemically detecting guaiacol and vanillin as important components in vanilla extract. Separation was achieved with Capcell Pak C-18 MG, the potential of the working electrode being set at +1000 mV. The respective calibration curves for guaiacol and vanillin were linear in the range of 1.60-460 µg/L and 5.90-1180 µg/L. The respective limits for the quantities of guaiacol and vanillin were 1.60 µg/L and 2.36 µg/L. The related standard deviations of the intra- and inter-day precision of the retention time and peak area were all less than 4%. The recovery of guaiacol and vanillin was both more than 97%, all of the validation data being within an acceptable range. This analysis method is well suited for the simultaneous and convenient analysis of guaiacol and vanillin in a vanilla extract to evaluate the quality of the vanilla extract.

Infection caused by Rahnella aquatilis

The authors report a case of a 27-year-old African American woman with sickle cell disease who developed septic shock caused by a Rahnella aquatilis infection associated with a peripherally inserted central venous catheter. The infection was treated successfully with ciprofloxacin. R aquatilis is a Gram-negative rod, first isolated from freshwater in 1976, which has been linked to human disease in rare instances, most commonly in immunosuppressed individuals. In addition to this case report, the authors also review the literature.

Alicyclobacillusspp. in the Fruit Juice Industry: History, Characteristics, and Current Isolation/Detection Procedures

The first Alicyclobacillus spp. was isolated in 1982, and was originally thought to be strictly limited to thermophilic and acidic environments. Two years later, another Alicyclobacillus sp., A. acidoterrestris, was identified as the causative agent in spoilage of commercially pasteurized apple juice. Subsequent studies soon found that Alicyclobacillus spp. are soilborne bacteria, and do not strictly require thermophilic and acidic environments. Alicyclobacillus spp. posess several distinct characteristics; the major one is their ability to survive commercial pasteurization processes and produce off-flavors in fruit juices. The fruit juice industry has acknowledged Alicyclobacillus spp. as a major quality control target microorganism. Guaiacol and halophenols were identified as the offensive smelling agent in many Alicyclobacillus spp. related spoilage. Though the exact formation pathway of these off-flavors by Alicyclobacillus spp. are not yet identified, studies report that the presence of Alicyclobacillus spp. in the medium may be a major contributor to the formation of these off-flavors. Many identification methods and isolation media were developed in the last two decades. However, most of these methods were developed specifically for A. acidoterrestris, which was the first identified off-flavor producing Alicyclobacillus. However, recent studies indicate that other species of Alicyclobacillus may also produce guaiacol or the halophenols. In this respect, all Alicyclobacillus spp. should be monitored as potential spoilage bacteria in fruit juices. This article includes an overall review of the history of Alicyclobacillus spp., characteristics, suggested off-flavor production pathways, and commonly used identification methods for the currently identified Alicyclobacillus spp.

Demonstrating Pathogenicity of Enterobacter cloacae on Macadamia and Identifying Associated Volatiles of Gray Kernel of Macadamia in Hawaii

Gray kernel is an important disease of macadamia (Macadamia integrifolia) that affects the quality of kernels, causing gray discoloration and a permeating, foul odor. Gray kernel symptoms were produced in raw, in-shell kernels of three cultivars of macadamia that were inoculated with strains of Enterobacter cloacae. Koch's postulates were fulfilled for three strains, demonstrating that E. cloacae is a causal agent of gray kernel. An inoculation protocol was developed to consistently reproduce gray kernel symptoms. Among the E. cloacae strains studied, macadamia strain LK 0802-3 and ginger strain B193-3 produced the highest incidences of disease (65 and 40%, respectively). The other macadamia strain, KN 04-2, produced gray kernel in 21.7% of inoculated nuts. Control treatments had 1.7% gray kernel symptoms. Some abiotic and biotic factors that affected incidence of gray kernel in inoculated kernels were identified. Volatiles of gray and nongray kernel samples also were analyzed. Ethanol and acetic acid were present in nongray and gray kernel samples, whereas volatiles from gray kernel samples included the additional compounds, 3-hydroxy-2-butanone (acetoin), 2,3-butanediol, phenol, and 2-methoxyphenol (guaiacol). This is believed to be the first report of the identification of volatile compounds associated with gray kernel.

Biological control of postharvest spoilage caused by Penicillium expansum and Botrytis cinerea in apple by using the bacterium Rahnella aquatilis

The epiphytic bacterium Rahnella aquatilis, isolated from fruit and leaves of apples, was tested for antagonistic properties against Penicillium expansum and Botrytis cinerea on Red Delicious apple fruit. In "in vitro" assays, this bacterium inhibited completely the germination of P. expansum and B. cinerea spores, but it needed direct contact with the spores to do it. However the putative mechanism seemed be different for the two pathogens. The bacterium did not produce extracellular antibiotic substances and when the acute toxicity test was performed no mortality, toxicity symptoms or organ alterations of the test animals (Wistar rats) were observed. Assays of biological control of P. expansum and B. cinerea on apple fruit were carried out at different temperatures. At 15 degrees C and 90% RH, the incidence of disease caused by P. expansum on apples stored for 20 days, was reduced by nearly 100% by R. aquatilis (10(6) cells/ml), while in the case of B. cinerea, the reduction of decay severity was nearly 64% but there was no reduction in the incidence of disease. At 4 degrees C and 90% RH the treatment with the bacterium significantly inhibited the development of B. cinerea on apples stored for 40 days and the incidence of disease was reduced by nearly 100%, while the incidence of disease caused by P. expansum at 4 degrees C was 60%. The results obtained show that R. aquatilis would be an interesting microorganism to be used as a biocontrol agent.

Determination of vanillin and related flavor compounds in cocoa drink by capillary electrophoresis

A simple, rapid, and reliable capillary electrophoresis (CE) method using a photodiode array detector determined four flavor components (vanillin, ethylvanillin, 2-methoxyphenol, and 2-ethoxyphenol) in cocoa drink. Simple and rapid sample preparation required only dilution. Separation used 50 mM phosphate buffer and 2 mM cetyltrimethylammonium hydroxide (CTAH) at pH 10 with 10% acetonitrile. Sorbic acid was the internal standard (I.S.). Vanillin and related compounds were determined in 7 min, with the limits of detection at 1.6 microg/ml with S/N > 3 and a quantitation range of 5-500 microg/ml. Recoveries were investigated in cocoa drink samples with four flavor components. Mean recoveries were 96.3-103.8%. Using this method, the four flavor components were determined from cocoa drink, in which vanillin and ethylvanillin were originally contained as a flavoring and into which Bacillus firmus was added.

Isolation and characterization of pHW15, a small cryptic plasmid from Rahnella genomospecies 2

A small cryptic plasmid designated pHW15 was isolated from Rahnella genomospecies 2 WMR15 and its complete nucleotide sequence was determined. The plasmid contained 3002 bp with a G+C content of 47.4%. The origin of replication was identified by deletion analysis as a region of about 600 bp. This region had an identity of 70% to the replication origin of the ColE1 plasmid at the nucleotide level. Sequence analysis revealed the typical elements: RNA I, RNA II and their corresponding promoters, a sequence allowing hybridisation of RNA II to the DNA and favouring processing by RNaseH, a single-strand initiation determinant (ssi) that allows initiation of lagging-strand synthesis, and a terH sequence required for termination of lagging-strand synthesis. The plasmid contained three expressed open reading frames, one of which showed homology to a ColE1 plasmid-encoded protein. Furthermore, a multimer resolution site was identified by sequence analysis. Its deletion resulted in formation of plasmid multimers during growth leading to an increased plasmid loss rate.

Changes in the spoilage-related microbiota of beef during refrigerated storage under different packaging conditions

The microbial spoilage of beef was monitored during storage at 5 degrees C under three different conditions of modified-atmosphere packaging (MAP): (i) air (MAP1), (ii) 60% O2 and 40% CO2 (MAP2), and (iii) 20% O2 and 40% CO2 (MAP3). Pseudomonas, Enterobacteriaceae, Brochothrix thermosphacta, and lactic acid bacteria were monitored by viable counts and PCR-denaturing gradient gel electrophoresis (DGGE) analysis during 14 days of storage. Moreover, headspace gas composition, weight loss, and beef color change were also determined at each sampling time. Overall, MAP2 was shown to have the best protective effect, keeping the microbial loads and color change to acceptable levels in the first 7 days of refrigerated storage. The microbial colonies from the plate counts of each microbial group were identified by PCR-DGGE of the variable V6-V8 region of the 16S rRNA gene. Thirteen different genera and at least 17 different species were identified after sequencing of DGGE fragments that showed a wide diversity of spoilage-related bacteria taking turns during beef storage in the function of the packaging conditions. The countable species for each spoilage-related microbial group were different according to packaging conditions and times of storage. In fact, the DGGE profiles displayed significant changes during time and depending on the initial atmosphere used. The spoilage occurred between 7 and 14 days of storage, and the microbial species found in the spoiled meat varied according to the packaging conditions. Rahnella aquatilis, Rahnella spp., Pseudomonas spp., and Carnobacterium divergens were identified as acting during beef storage in air (MAP1). Pseudomonas spp. and Lactobacillus sakei were found in beef stored under MAP conditions with high oxygen content (MAP2), while Rahnella spp. and L. sakei were the main species found during storage using MAP3. The identification of the spoilage-related microbiota by molecular methods can help in the effective establishment of storage conditions for fresh meat.

Rahnella aquatilis bacteremia from a suspected urinary source

A 76-year-old male with prostatic hyperplasia presented with acute pyelonephritis. Blood cultures yielded Rahnella aquatilis. Treatment with intravenous followed by oral levofloxacin resulted in cure. Important characteristics of this organism include its biochemical similarities to Enterobacter agglomerans, its apparent ability to cause bacteremia from a renal focus, and its response to quinolone therapy.

Degradation of vanillic acid and production of guaiacol by microorganisms isolated from cork samples

The presence of guaiacol in cork stoppers is responsible for some cases of cork taint causing unpleasant alterations to wine. We have performed a characterization of the cork-associated microbiota by isolating 55 different microorganisms: eight yeast, 14 filamentous fungi or molds, 13 actinomycetes and 20 non-filamentous bacteria. A screening for degradation of vanillic acid and guaiacol production showed that none of the filamentous fungi could achieve any of these processes. By contrast, five of the eight yeast strains isolated were able to degrade vanillic acid, although it was not converted to guaiacol. Guaiacol production was only detected in four bacterial strains: one isolate of Bacillus subtilis and three actinomycetes, Streptomyces sp. A3, Streptomyces sp. A5 and Streptomyces sp. A13, were able to accumulate this compound in both liquid media and cultures over cork. These results suggest that guaiacol-mediated cork taint should be attributed to the degradative action of vanillic acid by bacterial strains growing on cork.