Specific spoilage organisms in breweries and laboratory media for their detection

Zymomonas mobilis subspecies identification by amplified ribosomal DNA restriction analysis

AIMS

To identify strains of Zymomonas mobilis at the subspecies level by a fast and reliable technique.

METHODS AND RESULTS

Amplified ribosomal DNA restriction analysis (ARDRA) was used to identify strains of Z. mobilis at the subspecies level using the restriction enzyme StuI. This technique allowed for easy and quick differentiation between Z. mobilis subsp. mobilis and Z. mobilis subsp. pomaceae. By using other enzymes, the presence of two ARDRA profiles within the subspecies Z. mobilis subsp. pomaceae was observed, one profile corresponded to collection strains (British origin) while the other corresponded to wild type strains isolated from 'framboise' ciders in France.

CONCLUSIONS

A rapid method for identification of strains of Z. mobilis at the subspecies level was developed and shown to be more reliable and faster than the conventional method based on physiological tests. Furthermore, consistent differences in the 16S rDNA sequences between collection and wild type strains of Z. mobilis subsp. pomaceae were observed suggesting that the French isolates correspond to a new genomovar within this subspecies.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first description of a molecular method for the identification of Z. mobilis strains at the subspecies level. It will certainly prove to be useful in identifying this beer and cider spoiling micro-organism.

Detection of resistance of lactic acid bacteria to a mixture of the hop analogue compounds tetrahydroiso-alpha-acids by noninvasive measurement of intracellular pH

AIMS

To examine the resistance of beer isolates of lactic acid bacteria (LAB) towards a mixture of tetrahydroiso-alpha-acids (Tetra) by growth experiments as well as by measurement of intracellular pH.

METHODS AND RESULTS

Beer LAB isolates were identified to species level by SDS-PAGE of whole-cell proteins. Beer isolates of Lactobacillus brevis showed better ability for growth in the presence of Tetra than nonbeer isolates of the L. brevis or other species of LAB including beer and nonbeer isolates. The antimicrobial effect of Tetra was also examined by noninvasive measurement of intracellular pH by fluorescence ratio imaging microscopy for selected beer isolates of L. brevis and Pediococcus inopinatus. Strains of L. brevis showing limited decrease of intracellular pH during exposure to Tetra also showed better ability for growth in the presence of these compounds as well as in commercial beer products.

CONCLUSIONS

It was possible to apply a method for noninvasive measurement of intracellular pH to predict the resistance of beer spoilage LAB towards the Tetra hop analogue compounds.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrated the usability of a new rapid method for detecting hop-resistant variants of known beer spoilage LAB species.

Microbial composition of biofilms in a brewery investigated by fatty acid analysis, fluorescence in situ hybridisation and isolation techniques

Biofilms associated with brewery plants can harbour spoiling microorganisms that potentially damage the final product. Most beer-spoiling microorganisms are thought to depend on numerous interactions with the accompanying microbiota. However, there is no information on the microbial community structure of biofilms from bottling plants. The conveyors that transport the bottles to and from the plant are known as potential sources of microbial contamination of beer. Consequently, the material buildup from two conveyors was analysed using a cultivation/isolation approach, and the culture-independent techniques of whole cell fatty acid analysis and fluorescence in situ hybridisation (FISH). Heterogeneous communities were present at both conveyors. Although characteristic fatty acids for Eukarya were present, FISH-signals for Eukarya were extremely low. The Proteobacteria, in particular the Gammaproteobacteria, were abundant at both sample sites. Bacterial isolates were obtained for every dominating group detected by FISH: the Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria, the Xanthomonadaceae, the Actinobacteria, the Bacteroidetes and the Firmicutes.

Beer spoilage bacteria and hop resistance

For brewing industry, beer spoilage bacteria have been problematic for centuries. They include some lactic acid bacteria such as Lactobacillus brevis, Lactobacillus lindneri and Pediococcus damnosus, and some Gram-negative bacteria such as Pectinatus cerevisiiphilus, Pectinatus frisingensis and Megasphaera cerevisiae. They can spoil beer by turbidity, acidity and the production of unfavorable smell such as diacetyl or hydrogen sulfide. For the microbiological control, many advanced biotechnological techniques such as immunoassay and polymerase chain reaction (PCR) have been applied in place of the conventional and time-consuming method of incubation on culture media. Subsequently, a method is needed to determine whether the detected bacterium is capable of growing in beer or not. In lactic acid bacteria, hop resistance is crucial for their ability to grow in beer. Hop compounds, mainly iso-alpha-acids in beer, have antibacterial activity against Gram-positive bacteria. They act as ionophores which dissipate the pH gradient across the cytoplasmic membrane and reduce the proton motive force (pmf). Consequently, the pmf-dependent nutrient uptake is hampered, resulting in cell death. The hop-resistance mechanisms in lactic acid bacteria have been investigated. HorA was found to excrete hop compounds in an ATP-dependent manner from the cell membrane to outer medium. Additionally, increased proton pumping by the membrane bound H(+)-ATPase contributes to hop resistance. To energize such ATP-dependent transporters hop-resistant cells contain larger ATP pools than hop-sensitive cells. Furthermore, a pmf-dependent hop transporter was recently presented. Understanding the hop-resistance mechanisms has enabled the development of rapid methods to discriminate beer spoilage strains from nonspoilers. The horA-PCR method has been applied for bacterial control in breweries. Also, a discrimination method was developed based on ATP pool measurement in lactobacillus cells. However, some potential hop-resistant strains cannot grow in beer unless they have first been exposed to subinhibitory concentration of hop compounds. The beer spoilage ability of Pectinatus spp. and M. cerevisiae has been poorly studied. Since all the strains have been reported to be capable of beer spoiling, species identification is sufficient for the breweries. However, with the current trend of beer flavor (lower alcohol and bitterness), there is the potential risk that not yet reported bacteria will contribute to beer spoilage. Investigation of the beer spoilage ability of especially Gram-negative bacteria may be useful to reduce this risk.

Food spoilage--interactions between food spoilage bacteria

Food spoilage is a complex process and excessive amounts of foods are lost due to microbial spoilage even with modern day preservation techniques. Despite the heterogeneity in raw materials and processing conditions, the microflora that develops during storage and in spoiling foods can be predicted based on knowledge of the origin of the food, the substrate base and a few central preservation parameters such as temperature, atmosphere, a(w) and pH. Based on such knowledge, more detailed sensory, chemical and microbiological analysis can be carried out on the individual products to determine the actual specific spoilage organism. Whilst the chemical and physical parameters are the main determining factors for selection of spoilage microorganisms, a level of refinement may be found in some products in which the interactive behavior of microorganisms may contribute to their growth and/or spoilage activity. This review gives three such examples. We describe the competitive advantage of Pseudomonas spp. due to the production of iron-chelating siderophores, the generation of substrates for spoilage reactions by one organism from another microorganism (so-called metabiosis) and the up-regulation of phenotypes potentially involved in spoilage through cell-to-cell communication. In particular, we report for the first time the widespread occurrence of N-acyl homoserine lactones (AHL) in stored and spoiling fresh foods and we discuss the potential implications for spoilage and food preservation.

Phenotypic and genetic diversity of Saccharomyces contaminants isolated from lager breweries and their phylogenetic relationship with brewing yeasts

A taxonomic study was carried out for isolates of Saccharomyces spp. identified as contaminants ("wild yeast") in 24 different lager breweries. With reference to the current taxonomy all isolates were found to belong to the Saccharomyces sensu stricto complex and 58% of the isolates were further identified as S. cerevisiae, 26% as S. pastorianus and 3% as S. bayanus. The remaining isolates (13%) could not be identified to the species level based on their phenotypic characteristics. However, some of these isolates were identified as S. cerevisiae by HaeIII restriction digest of PCR-amplified intergenic transcribed spacer (ITS) regions. Chromosome length polymorphism (CLP) was evident among the Saccharomyces brewing contaminants with chromosome profiles typical of Saccharomyces sensu stricto. Based upon cluster analysis of their chromosome profiles the majority of the brewing contaminants could be grouped as either S. cerevisiae or S. pastorianus/S. bayanus. Further, the technique was able to differentiate between almost all brewing contaminants and to separate them from any specific lager brewing yeast. The diversity of the Saccharomyces brewing contaminants clearly demonstrated by their CLP was further reflected by MAL genotyping. For the majority of the isolates more than two MAL loci were found with MAL1, MAL2 MAL3, MAL4 and MAL11, MAL31, MAL41 as the dominant genotypes. For all isolates MAL11 and MAL31 were found whereas MAL61 only was found for one isolate. The high number of MAL loci found in the SaccharomYces brewing contaminants indicate their adaptation to a maltose-enriched environment.

Effect of rapid cooling and acidic pH on cellular homeostasis of Pectinatus frisingensis, a strictly anaerobic beer-spoilage bacterium

Pectinatus frisingensis is a strictly anaerobic mesophilic bacterium involved in bottled beer spoilage. Cellular volume, adenylate energy charge, intracellular pH and intracellular potassium concentration measurements were performed in late exponential-phase cell suspensions placed in different physiological conditions, to evaluate the capability of this bacterium to maintain cellular homeostasis. The intracellular pH was calculated from the intracellular accumulation of a [carboxyl-14C]benzoic acid. Optimum physiological conditions were the presence of a carbon source and pH of 6.2, hostile conditions were a pH 4.5, absence of a carbon source, and rapid cooling treatment. The cell was able to maintain a higher intracellular pH than the external pH under all conditions. Intracellular volume was lower at pH 4.5 than at pH 6.2. A low net potassium efflux rate was routinely measured in starving cells, while glucose addition promoted immediate net potassium uptake from the medium. Cooling treatment resulted in sudden net potassium efflux from the cell, a decrease of the intracellular pH, and low modifications of the adenylate energy charge in metabolizing-glucose cell suspensions. Thus, cold treatment perturbs the P. frisingensis homeostasis but the bacteria were able to restore their homeostasis in the presence of a carbon source, and under warm conditions.