The Molecular Weaponry Produced by the Bacterium Hafnia alvei in Foods

Hafnia alvei is receiving increasing attention from both a medical and veterinary point of view, but the diversity of molecules it produces has made the interest in this bacterium extend to the field of probiotics, the microbiota, and above all, to its presence and action on consumer foods. The production of Acyl Homoserine Lactones (AHLs), a type of quorum-sensing (QS) signaling molecule, is the most often-studied chemical signaling molecule in Gram-negative bacteria. H. alvei can use this communication mechanism to promote the expression of certain enzymatic activities in fermented foods, where this bacterium is frequently present. H. alvei also produces a series of molecules involved in the modification of the organoleptic properties of different products, especially cheeses, where it shares space with other microorganisms. Although some strains of this species are implicated in infections in humans, many produce antibacterial compounds, such as bacteriocins, that inhibit the growth of true pathogens, so the characterization of these molecules could be very interesting from the point of view of clinical medicine and the food industry. Lastly, in some cases, H. alvei is responsible for the production of biogenic amines or other compounds of special interest in food health. In this article, we will review the most interesting molecules that produce the H. alvei strains and will discuss some of their properties, both from the point of view of their biological activity on other microorganisms and the properties of different food matrices in which this bacterium usually thrives.

Group II intron-mediated deletion of lactate dehydrogenase gene in an isolated 1,3-propanediol producer Hafnia alvei AD27

Our previous studies showed that glycerol fermentation by Hafnia alvei AD27 strain was accompanied by formation of high quantities of lactate. The ultimate aim of this work was the elimination of excessive lactate production in the 1,3-propanediol producer cultures. Group II intron-mediated deletion of ldh (lactate dehydrogenase) gene in an environmental isolate of H. alvei AD27 strain was conducted. The effect of the Δldh genotype in H. alvei AD27 strain varied depending on the culture medium applied. Under lower initial glycerol concentration (20 gL^-1), lactate and 1,3-propanediol production was fully abolished, and the main carbon flux was directed to ethanol synthesis. On the other hand, at higher initial glycerol concentrations (40 gL^-1), 1,3-propanediol and lactate production was recovered in the recombinant strain. The final titers of 1,3-propanediol and ethanol were similar for the recombinant and the WT strains, while the Δldh genotype displayed significantly decreased lactate titer. The by-products profile was altered upon ldh gene deletion, while glycerol utilization and biomass accumulation remained unaltered. As indicated by flow-cytometry analyses, the internal pH was not different for the WT and the recombinant Δldh strains over the culture duration, however, the WT strain was characterized by higher redox potential.

Fractionation and analysis of lipopolysaccharide-derived oligosaccharides by zwitterionic-type hydrophilic interaction liquid chromatography coupled with electrospray ionisation mass spectrometry

Lipopolysaccharide (LPS, endotoxin) is a main surface antigen and virulence factor of Gram-negative bacteria. Regardless of the source of LPS, this molecule, isolated from the smooth forms of bacteria, is characterised by a general structural layout encompassing three regions: (i) an O-specific polysaccharide (O-PS) - a polymer of repeating oligosaccharide units, (ii) core oligosaccharide (OS), and (iii) the lipid A anchoring LPS in the outer membrane of the cell envelope of Gram-negative bacteria. Structural analysis usually requires degradation of LPS and further efficient separation of various poly- and oligosaccharide glycoforms. The hydrophilic interaction liquid chromatography (HILIC) was shown as an efficient technique for separation of labelled or native neutral and acidic glycans, glycopeptides, sialylated glycans, glycosylated and nonglycosylated peptides. Herein we adopted ZIC(®) (zwitterionic stationary phase covalently attached to porous silica)-HILIC technology in combination with electrospray ionisation mass spectrometry to separate different LPS-derived oligosaccharides. As a result three effective procedures have been developed: (i) to separate different core oligosaccharides of Escherichia coli R1 LOS, (ii) to separate RU-[Hep]-Kdo oligosaccharides from core OS glycoforms of Hafnia alvei PCM 1200 LPS, and (iii) to separate Hep and Kdo-containing mono, di-, tri- and tetrasaccharides of H. alvei PCM 1200 LPS. Moreover, some of developed analytical procedures were scaled to semi-preparative protocols and used to obtain highly-purified fractions of the interest in larger quantities required for future evaluation, analysis, and biological applications.

Degradation of phytate by the 6-phytase from Hafnia alvei: a combined structural and solution study

Phytases hydrolyse phytate (myo-inositol hexakisphosphate), the principal form of phosphate stored in plant seeds to produce phosphate and lower phosphorylated myo-inositols. They are used extensively in the feed industry, and have been characterised biochemically and structurally with a number of structures in the PDB. They are divided into four distinct families: histidine acid phosphatases (HAP), β-propeller phytases, cysteine phosphatases and purple acid phosphatases and also split into three enzyme classes, the 3-, 5- and 6-phytases, depending on the position of the first phosphate in the inositol ring to be removed. We report identification, cloning, purification and 3D structures of 6-phytases from two bacteria, Hafnia alvei and Yersinia kristensenii, together with their pH optima, thermal stability, and degradation profiles for phytate. An important result is the structure of the H. alvei enzyme in complex with the substrate analogue myo-inositol hexakissulphate. In contrast to the only previous structure of a ligand-bound 6-phytase, where the 3-phosphate was unexpectedly in the catalytic site, in the H. alvei complex the expected scissile 6-phosphate (sulphate in the inhibitor) is placed in the catalytic site.

Biofilm formation and acyl homoserine lactone production in Hafnia alvei isolated from raw milk

The objective of this study was to detect the presence of acyl homoserine lactones (AHLs), signal molecules of the quorum sensing system in biofilm formed by Hafnia alvei strains. It also evaluated the effect of synthetic quorum sensing inhibitors in biofilm formation. AHLs were assayed using well diffusion techniques, thin layer chromatography (TLC) and detection directly in biofilm with biomonitors. The extracts obtained from planktonic and sessile cell of H. alvei induced at least two of three monitor strains evaluated. The presence of AHLs with up to six carbon atoms was confirmed by TLC. Biofilm formation by H. alvei was inhibited by furanone, as demonstrated by 96-well assay of crystal violet in microtitre plates and by scanning electron microscopy. The H. alvei 071 hall mutant was deficient in biofilm formation. All these results showed that the quorum sensing system is probably involved in the regulation of biofilm formation by H. alvei.

Immunochemical studies of the lipopolysaccharides of Hafnia alvei PCM 1219 and other strains with the O-antigens containing D-glucose 1-phosphate and 2-deoxy-2-[(R)-3-hydroxybutyramido]-D-glucose

Introduction:

Hafnia alveiis the only species of the genus Hafnia, which belongs to the family of Enterobacteriaceae. These Gram-negative bacteria are commonly distributed in the natural environment and are often the cause of human opportunistic infections. Their lipopolysaccharides (LPSs) are important surface antigens which are responsible for the serological specificity and numerous cross-reactions with other enterobacterial genera. So far, 29 different O-polysaccharide (OPS, O-antigen) structures in Hafnias LPSs have been established and for some of them the molecular basis of the serological activity has been elucidated.

Materials and Methods:

OPS from H. alvei strain PCM 1219 was obtained by mild acid hydrolysis of the LPS followed by gel permeation chromatography of carbohydrate material on Sephadex G-50 column. The polysaccharide structure was determined using chemical methods as well as ¹³C NMR and ¹H NMR spectroscopy. For serological studies, SDS-PAGE, immunoblotting, and passive hemagglutination tests were used.

Results:

The serological studies revealed a cross-reactivity of the LPSs of H. alvei PCM 1219 and a group of H. alvei strains with an O-antigen containing D-glucose 1-phosphate and [(R)-3-hydroxybutyramido]-D-glucose. The following structure of the OPS was established: where Acyl stands for (R)-3-hydroxybutyryl and the degree of O-acetylation is ~70%. The structure of the core oligosaccharide was found to be typical of the genus Hafnia.

Conclusions:

Based on the OPS structure and serological results it was concluded that H. alvei strain PCM 1219 should be classified in the same serogroup as the H. alvei type strain ATCC 13337 and five other strains containing D-glucose 1-phosphate and 2-deoxy-2-[(R)-3-hydroxybutyramido]-D-glucose in their O-antigens.

Presence of acylated homoserine lactones (AHLs) and AHL-producing bacteria in meat and potential role of AHL in spoilage of meat

Quorum-sensing (QS) signals (N-acyl homoserine lactones [AHLs]) were extracted and detected from five commercially produced vacuum-packed meat samples. Ninety-six AHL-producing bacteria were isolated, and 92 were identified as Enterobacteriaceae. Hafnia alvei was the most commonly identified AHL-producing bacterium. Thin-layer chromatographic profiles of supernatants from six H. alvei isolates and of extracts from spoiling meat revealed that the major AHL species had an R(f) value and shape similar to N-3-oxo-hexanoyl homoserine lactone (OHHL). Liquid chromatography-mass spectrometry (MS) (high-resolution MS) analysis confirmed the presence of OHHL in pure cultures of H. alvei. Vacuum-packed meat spoiled at the same rate when inoculated with the H. alvei wild type compared to a corresponding AHL-lacking mutant. Addition of specific QS inhibitors to the AHL-producing H. alvei inoculated in meat or to naturally contaminated meat did not influence the spoilage of vacuum-packed meat. An extracellular protein of approximately 20 kDa produced by the H. alvei wild-type was not produced by the AHL-negative mutant but was restored in the mutant when complemented by OHHL, thus indicating that AHLs do have a regulatory role in H. alvei. Coinoculation of H. alvei wild-type with an AHL-deficient Serratia proteamaculans B5a, in which protease secretion is QS regulated, caused spoilage of liquid milk. By contrast, coinoculation of AHL-negative strains of H. alvei and S. proteamaculans B5a did not cause spoilage. In conclusion, AHL and AHL-producing bacteria are present in vacuum-packed meat during storage and spoilage, but AHL does not appear to influence the spoilage of this particular type of conserved meat. Our data indicate that AHL-producing H. alvei may induce food quality-relevant phenotypes in other bacterial species in the same environment. H. alvei may thus influence spoilage of food products in which Enterobacteriaceae participate in the spoilage process.

Genetic and ecological structure of Hafnia alvei in Australia

Multilocus enzyme electrophoresis of 161 Hafnia alvei isolates from 158 hosts and 3 water column samples collected in Australia revealed that this species consists of two genetically distinct groups. The two groups of H. alvei differed significantly in their genetic structure and host distribution. The taxonomic class of the host but not geographic locality explained a significant proportion of the observed genetic and biochemical variation among strains within each genetic group.

Gas formation in ground beef chubs due to Hafnia alvei is reduced by multiple applications of antimicrobial interventions to artificially inoculated beef trim stock

Gas-forming microorganisms were isolated from gas-swollen ground beef chubs obtained from a commercial source and were phenotypically identified as Hafnia alvei. In in situ experiments, the isolated H. alvei strains produced gas in inoculated irradiation-sterilized ground beef chubs. A five-strain cocktail of H. alvei isolates was inoculated on beef trim. The inoculated beef trim samples were treated with either a water wash (W) at 65 psi for five passes (a pass refers to the application of successive multiple antimicrobial treatments to inoculated beef trim on a moving processing conveyor belt at a speed of 1 cm/s under heat ducts or oscillating spray nozzles), W plus a 2% (vol/vol) lactic acid wash (L) at room temperature at 30 psi for three passes (W/L), or a combination treatment (COMB) consisting of W plus 82 degrees C water for three passes plus 510 degrees C hot air for six passes plus L, or were not treated (control). After treatment, the beef trim was ground and vacuum packaged. The numbers of H. alvei were reduced with water alone and with the aforementioned antimicrobial intervention treatments. For the untreated and inoculated control samples, the numbers of H. alvei increased from 7.03 to 8.40 log CFU/g after 7 days of incubation at 4 degrees C. However, the numbers of H. alvei treated by successive antimicrobial interventions (COMB) were initially reduced to 5.25 log CFU/g and increased to just 6.9 log CFU/g after 7 days of incubation at 4 degrees C. Gas was produced in untreated control samples after 3 days at 15 degrees C (15 of 15 inoculated chubs). However, in meat treated with W, W/L, and COMB, gas was produced after 4 to 5, 7 to 8, and 9 to 10 days of storage at 15 degrees C, respectively. These results demonstrate the effectiveness of multiple antimicrobial interventions in reducing H. alvei numbers on beef trim and subsequently delaying gas formation in the resulting ground beef chubs.

Modification of Niven's medium for the enumeration of histamine-forming bacteria and discussion of the parameters associated with its use

The objective of this study was to improve Niven's medium (NM) for the optimized enumeration of histamine-forming bacteria (HFB). The parameters modified related to solidification of the agar at low pH values (pH 5.3 to 5.8), incubation time (24, 48, and 72 h) and temperature (30 and 37 degrees C), number of colonies developed on the plate to allow enumeration of HFB, and color differentiation. Strains of HFB, Morganella morganii, Klebsiella pneumoniae, and Hafnia alvei were examined for their ability to change color on NM. The three microorganisms produced different colors on the medium, which can be used for preliminary identification of HFB. Quantitative analysis of HFB proved to be achievable, with the prerequisite that only 1 to 80 colonies developed on the medium allow effective enumeration. A larger number of colonies results in color development throughout the medium, making the distinction between HFB and other bacteria unachievable. Growth of prolific HFB was noticeably better at pH values from 5.3 to 5.5, compared to 6.3, on NM. Growth at 5.3 and 5.5 on NM also presented a significant advantage in comparison to growth on plate count agar (PCA; pH 7) at the same incubation temperature. The increased agar concentration of 3% was found to give better solidification at pH 5.3 to 6.0, compared to 2%. This agar concentration also allows autoclaving for 12 min at 121 degrees C, overcoming the hydrolysis problems that appear at the lower concentration of 2%. The construction of a color chart for the recognition of the pH change due to histidine decarboxylase activity was also achieved.

Characterisation of Hafnia alvei isolates from human clinical extra-intestinal specimens: haemagglutinins, serum resistance and siderophore synthesis

Extra-intestinal Hafnia alvei isolates are rarely considered to be pathogenic. To investigate whether such strains are able to produce virulence factors, a total of 70 clinical H. alvei isolates was compared with clinical extra-intestinal isolates of other members of the enterobacterial tribe Klebsiellae (Kiebsiella pneumoniae, Enterobacter cloacae, Serratia marcescens). Whereas mannose-sensitive haemagglutination (MSHA) was less common in H. alvei (59%) than in K. pneumoniae (86%) and E. cloacae (89%) isolates, the incidences of mannose-resistant haemagglutination indicative of type 3 pili (MR/K-HA) and of serum resistance properties were not lower. All H. alvei strains secreted siderophores but, unlike the other enterobacterial species examined, the siderophore type was neither enterobactin nor aerobactin. Although the low pathogenicity of H. alvei isolates could not be attributed to any of the factors investigated, the mean number of factors expressed by each H. alvei isolate was significantly lower than that expressed by K. pneumoniae and E. cloacae isolates but did not differ significantly from that of S. marcescens. Based on these findings, the low pathogenicity of H. alvei appears to be due to its low frequency of expression of virulence factors as compared with clinically significant species such as K. pneumoniae and E. cloacae.

Heterogeneity of AmpC cephalosporinases of Hafnia alvei clinical isolates expressing inducible or constitutive ceftazidime resistance phenotypes

Ten unrelated Hafnia alvei clinical isolates were grouped according to either their low-level and inducible cephalosporinase production or their high-level and constitutive cephalosporinase production phenotype. Their AmpC sequences shared 85 to 100% amino acid identity. The immediate genetic environment of ampC genes was conserved in H. alvei isolates but was different from that found in other ampC-possessing enterobacterial species.

Changes in protein composition and hydrolytic enzyme activity of Escherichia coli and Hafnia alvei grown in human fluids

Growing of Escherichia coli and Hafnia alvei cells in several cell-free human fluids, such as normal serum, serum from diabetic patients, pleural, ascitic and spinal fluid, revealed that various biochemical changes occurred. Protein profile on SDS-PAGE as well as acid and alkaline phosphohydrolytic enzymes on native gels of cell extracts were affected after culturing of bacteria in the above fluids. Gelatinolytic and hyaluronolytic activity was of interest because both of them are histolytic enzymes. Although there was a potential appearance of gelatinolytic bands on gelatin-SDS-PAGE in cells starved in seawater, none of these activities were expressed in cells grown in human fluids. A hyaluronolytic activity of approximately 45 KDa was present in cells cultured in Mueller Hinton broth. This enzyme was decreased either in cells starved in seawater or in cells grown in human fluids to an almost invisible band on hyaluronan-SDS-PAGE.