Development of detection medium for hard-to-culture beer-spoilage lactic acid bacteria

Induction and Recovery of the Viable but Nonculturable State of Hop-Resistance Lactobacillus brevis

Lactobacillus brevis is a major hop-resistance bacterium which poses significant challenge for the brewing industry, mainly due to the difficulty or incapability in detection by routine culturing methodology and its beer spoilage ability.This study aimed at investigating the VBNC state of a hop-resistance strain, L. brevis BM-LB13908. The culturable, total and viable numbers of L. brevis cells were calculated by MRS agar plate counting, acridine orange direct count (AODC) method and Live/Dead BacLight bacterial viability kit with fluorescence microscope. VBNC formation was induced by 189 ± 5.7 days under low-temperature storage or 27 ± 1.2 subcultures by continuous passage in beer, and VBNC cells induced by both strategies were recovered by adding catalase. In addition, insignificant difference in beer-spoilage ability was found in 3 states of L. brevis, including logarithmic growing, VBNC and recovered cells. This is the first study on the formation of VBNC state for L. brevis and beer-spoilage ability of both VBNC and recovered cells, which indicate L. brevis strain could cause beer spoilage without being detected by routine methodologies. The results derived from this study may support further study on L. brevis and other hop-resistance bacteria, and guidance on beer spoilage prevention and control, such as improvement for brewers on the microbiological quality control by using the improved culture method with catalase supplementation.

Discovery and control of culturable and viable but non-culturable cells of a distinctive Lactobacillus harbinensis strain from spoiled beer

Occasional beer spoilage incidents caused by false-negative isolation of lactic acid bacteria (LAB) in the viable but non-culturable (VBNC) state, result in significant profit loss and pose a major concern in the brewing industry. In this study, both culturable and VBNC cells of an individual Lactobacillus harbinensis strain BM-LH14723 were identified in one spoiled beer sample by genome sequencing, with the induction and resuscitation of VBNC state for this strain further investigated. Formation of the VBNC state was triggered by low-temperature storage in beer (175 ± 1.4 days) and beer subculturing (25 ± 0.8 subcultures), respectively, and identified by both traditional staining method and PMA-PCR. Resuscitated cells from the VBNC state were obtained by addition of catalase rather than temperature upshift, changing medium concentration, and adding other chemicals, and both VBNC and resuscitated cells retained similar beer-spoilage capability as exponentially growing cells. In addition to the first identification of both culturable and VBNC cells of an individual L. harbinensis strain from spoiled beer, this study also for the first time reported the VBNC induction and resuscitation, as well as verification of beer-spoilage capability of VBNC and resuscitated cells for the L. harbinensis strain. Genes in association with VBNC state were also identified by the first genome sequencing of beer spoilage L. harbinensis. The results derived from this study suggested the contamination and spoilage of beer products by VBNC and resuscitated L. harbinensis strain BM-LH14723.

Study on spoilage capability and VBNC state formation and recovery of Lactobacillus plantarum

OBJECTIVE

The present study aimed at investigating the capability of L. plantarum strain BM-LP14723 to enter into and recover from the viable but nonculturable (VBNC) state and to cause beer spoilage.

METHODS

VBNC state was induced by incubating in beer with subculturing or low temperature treatment. Culturable, total, and viable cells numbers were assessed by MRS agar plate counting, acridine orange direct counting, and Live/Dead BacLight bacterial viability kit, respectively. Organic acids concentrations were measured by reversed-phase high performance liquid chromatography.

RESULTS

VBNC L. plantarum cells were detected after 189 ± 1.9 days low temperature treatment or 29 ± 0.7 subcultures in beer. The VBNC L. plantarum retained spoilage capability. Addition of catalase is an effective method for the recovery of the VBNC L. plantarum cells.

CONCLUSION

L. plantarum strain BM-LP14723 is capable of entering into and recovery from the VBNC state and maintained spoilage capability. The current study presented that beer-spoilage L. plantarum can hide both in breweries and during transporting and marketing process and thus lead to beer-spoilage incidents.

First study on the formation and resuscitation of viable but nonculturable state and beer spoilage capability of Lactobacillus lindneri

OBJECTIVES

This study aimed to investigate the spoilage capability of Lactobacillus lindneri during the induction and resuscitation of viable but nonculturable (VBNC) state.

METHODS

L. lindneri strain was identified by sequencing the PCR product (amplifying 16S rRNA gene) using ABI Prism 377 DNA Sequencer. During the VBNC state induction by low temperature storage and beer adaption, total, culturable, and viable cells were assessed by acridine orange direct counting, plate counting, and Live/Dead BacLight bacterial viability kit, respectively. Organic acids and diacetyl concentration were measured by reversed-phase high performance liquid chromatography and head dpace gas chromatography, respectively.

RESULTS

VBNC state of L. lindneri was successfully induced by both beer adaption and low temperature storage, and glycerol frozen stock was the optimal way to maintain the VBNC state. Addition of catalase was found to be an effective method for the resuscitation of VBNC L. lindneri cells. Furthermore, spoilage capability remained similar during the induction and resuscitation of VBNC L. lindneri.

CONCLUSIONS

This is the first report of induction by low temperature storage and resuscitation of VBNC L. lindneri strain, as well as the first identification of spoilage capability of VBNC and resuscitated L. lindneri cells. This study indicated that the potential colonization of L. lindneri strain in brewery environment, formation and resuscitation of VBNC state, as well as maintenance in beer spoilage capability, may be an important risk factor for brewery environment.

Heterogeneity between and within Strains of Lactobacillus brevis Exposed to Beer Compounds

This study attempted to investigate the physiological response of six Lactobacillus brevis strains to hop stress, with and without the addition of Mn²⁺ or ethanol. Based on the use of different fluorescent probes, cell viability and intracellular pH (pHi) were assessed by fluorescence microscopy combined with flow cytometry, at the single cell level. The combined approach was faster than the traditional colony based method, but also provided additional information about population heterogeneity with regard to membrane damage and cell size reduction, when exposed to hop compounds. Different physiological subpopulations were detected under hop stress in both hop tolerant and sensitive strains. A large proportion of cells were killed in all the tested strains, but a small subpopulation from the hop tolerant strains eventually recovered as revealed by pHi measurements. Furthermore, a short term protection against hop compounds was obtained for both hop tolerant and sensitive strains, by addition of high concentration of Mn²⁺. Addition of ethanol in combination with hop compounds caused an additional short term increase in damaged subpopulation, but the subsequent growth suggested that the presence of ethanol provides a slight cross resistance toward hop compounds.

In situ examination of Lactobacillus brevis after exposure to an oxidizing disinfectant

Beer is a hostile environment for most microorganisms, but some lactic acid bacteria can grow in this environment. This is primarily because these organisms have developed the ability to grow in the presence of hops. It has been speculated that hop resistance is inversely correlated to resistance against oxidation, and this would have great impact on the use of various disinfectants in the brewing industry. In this study, we cultivated bacteria under aerobic and anaerobic conditions, and then investigated the in situ outgrowth of individual cells into microcolonies on de Man Rogosa Sharpe (MRS) agar after exposure to the oxidizing agent peracetic acid (PAA). An automated microscope stage allowed us to analyse a much larger number of cells over extended periods of incubation. After PAA treatment, the lag time increased markedly, and extensive variation in morphology, μmax as well as stress resistance was observed between and within the tested Lactobacillus brevis strains. The results suggest that aerobic cultivation increased the oxidative stress tolerance in Lactobacillus brevis. The results also show that dead cells are randomly distributed in a microcolony and the majority of non-growing individual cells do not stain with a membrane impermanent dye (Propidium iodide), which indicates that PAA may not destroy the plasma membrane. In conclusion, the developed microscopic analysis of individual cells on MRS agar can provides faster results and more details of cell physiology compared to the traditional CFU method.

Identification of beer-spoilage bacteria using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Applicability of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for identification of beer-spoilage bacteria was examined. To achieve this, an extensive identification database was constructed comprising more than 4200 mass spectra, including biological and technical replicates derived from 273 acetic acid bacteria (AAB) and lactic acid bacteria (LAB), covering a total of 52 species, grown on at least three growth media. Sequence analysis of protein coding genes was used to verify aberrant MALDI-TOF MS identification results and confirmed the earlier misidentification of 34 AAB and LAB strains. In total, 348 isolates were collected from culture media inoculated with 14 spoiled beer and brewery samples. Peak-based numerical analysis of MALDI-TOF MS spectra allowed a straightforward species identification of 327 (94.0%) isolates. The remaining isolates clustered separately and were assigned through sequence analysis of protein coding genes either to species not known as beer-spoilage bacteria, and thus not present in the database, or to novel AAB species. An alternative, classifier-based approach for the identification of spoilage bacteria was evaluated by combining the identification results obtained through peak-based cluster analysis and sequence analysis of protein coding genes as a standard. In total, 263 out of 348 isolates (75.6%) were correctly identified at species level and 24 isolates (6.9%) were misidentified. In addition, the identification results of 50 isolates (14.4%) were considered unreliable, and 11 isolates (3.2%) could not be identified. The present study demonstrated that MALDI-TOF MS is well-suited for the rapid, high-throughput and accurate identification of bacteria isolated from spoiled beer and brewery samples, which makes the technique appropriate for routine microbial quality control in the brewing industry.

Rapid detection and identification of beer-spoilage lactic acid bacteria by microcolony method

We evaluated a microcolony method for the detection and identification of beer-spoilage lactic acid bacteria (LAB). In this approach, bacterial cells were trapped on a polycarbonate membrane filter and cultured on ABD medium, a medium that allows highly specific detection of beer-spoilage LAB strains. After short-time incubation, viable cells forming microcolonies were stained with carboxyfluorescein diacetate (CFDA) and counted with muFinder Inspection System. In our study, we first investigated the growth behavior of various beer-spoilage LAB by traditional culture method, and Lactobacillus lindneri and several L. paracollinoides strains were selected as slow growers on ABD medium. Then the detection speeds were evaluated by microcolony method, using these slowly growing strains. As a result, all of the slowly growing beer-spoilage LAB strains were detected within 3 days of incubation. The specificity of this method was found to be exceptionally high and even discriminated intra-species differences in beer-spoilage ability of LAB strains upon detection. These results indicate that our microcolony approach allows rapid and specific detection of beer-spoilage LAB strains with inexpensive CFDA staining. For further confirmation of species status of detected strains, subsequent treatment with species-specific fluorescence in situ hybridization (FISH) probes was shown as effective for identifying the CFDA-detected microcolonies to the species level. In addition, no false-positive results arising from noise signals were recognized for CFDA staining and FISH methods. Taken together, the developed microcolony method was demonstrated as a rapid and highly specific countermeasure against beer-spoilage LAB, and compared favorably with the conventional culture methods.