Methods for detection and identification of beer-spoilage microbes

It is critical that breweries of all sizes routinely monitor the microbiome of their process to limit financial losses due to microbial contamination. Contamination by beer-spoiling microbes (BSMs) at any point during the brewing process may lead to significant losses for breweries if gone undetected and allowed to spread. Testing and detection of BSMs must be routine and rapid, and because even small breweries need the capability of BSM detection and identification, the method also needs to be affordable. Lactic acid bacteria (LAB) are responsible for most spoilage incidents, many of which have been shown to enter the viable but nonculturable (VBNC) state under conditions present in beer such as cold or oxidative stress. These bacteria are invisible to traditional methods of detection using selective media. This article describes several methods of BSM detection and identification that may be useful in the majority of craft breweries. While there are several genomic methods that meet some or many qualifications of being useful in craft breweries, real-time quantitative polymerase chain reaction (qPCR) currently best meets the desired method characteristics and holds the most utility in this industry, specifically SYBR Green qPCR. qPCR is a targeted method of detection and identification of microbes that is affordable, rapid, specific, sensitive, quantitative, and reliable, and when paired with valid DNA extraction techniques can be used to detect BSMs, including those in the VBNC state.

Group-specific PCR-RFLP and real-time PCR methods for detection and tentative discrimination of strictly anaerobic beer-spoilage bacteria of the class Clostridia

The strictly anaerobic brewery contaminants of the genera Pectinatus, Megasphaera, Selenomonas and Zymophilus in the class Clostridia constitute an important group of spoilage bacteria of unpasteurised, packaged beers. The aim of this study was to develop and evaluate group-specific PCR methods to detect and differentiate these bacteria in beer. A group-specific primer pair targeting a 342-bp variable region of the 16S rRNA gene was designed and evaluated in end-point PCR with gel electrophoresis and in real-time PCR with SYBR Green I dye. Significant cross-reactions with DNAs from any of the forty-two brewery-related, non-target microbes or from real brewery samples were not detected in either PCR system. The group-specific end-point and real-time PCR products could be differentiated according to species/genus and spoilage potential using restriction fragment length polymorphism (KpnI, XmnI, BssHII, ScaI) and melting point curve analysis, respectively. In combination with a rapid DNA extraction method, the PCR reactions detected ca 10(0)-10(3) CFU per 25 ml of beer depending on the strain and on the PCR system. The end-point and real-time PCR analysis took 6-7 h and 2-3 h, respectively. Pre-PCR enrichment of beer samples for 1-3 days ensured the detection of even a single cultivable cell. The PCR and cultivation results of real brewery samples were mostly congruent but the PCR methods were occasionally more sensitive. The PCR methods developed allow the detection of all the nine beer-spoilage Pectinatus, Megasphaera, Selenomonas and Zymophilus species in a single reaction and their differentiation below group level and reduce the analysis time for testing of their presence in beer samples by 1-2 days. The methods can be applied for brewery routine quality control and for studying occurrence, diversity and numbers of the strictly anaerobic beer spoilers in the brewing process.

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.