A differential medium for the isolation of Kluyveromyces marxianus and Kluyveromyces lactis from dairy products

The Effects of NaCl and Temperature on Growth and Survival of Yeast Strains Isolated from Danish Cheese Brines

Yeasts play an important role in cheese making, by contributing to microbial community establishment and improving flavor. This study aimed at investigating the impact of NaCl and temperature on growth and survival of 20 strains belonging to the yeast species Candida intermedia (2 strains), Debaryomyces hansenii (11), Kluyveromyces lactis (1), Papiliotrema flavescens (1), Rhodotorula glutinis (1), Sterigmatomyces halophilus (2) and Yamadazyma triangularis (2) isolated from Danish cheese brines. All yeasts could grow in Malt Yeast Glucose Peptone (MYGP) medium with low NaCl (≤ 4%, w/v) concentrations at 25 °C and 16 °C. Further, none of the strains, except for one strain of D. hansenii (KU-9), were able to grow under a condition mimicking cheese brine (MYGP with 23% (w/v) NaCl and 6.3 g/L lactate) at 25 °C, while all yeasts could grow at 16 °C, except for the two strains of C. intermedia. In the survival experiment, D. hansenii, S. halophilus and Y. triangularis survived in MYGP with 23% (w/v) NaCl throughout 13.5 days at 25 °C, with Y. triangularis and S. halophilus being the most NaCl tolerant, while the remaining yeasts survived for less than 7 days. These results enable the selection of relevant yeasts from cheese brines for potential use in the cheese industry.

Investigation of physicochemical composition and microbial communities in traditionally fermented vrum from Inner Mongolia

Mongolian traditionally fermented vrum is known for its functional characteristics, and indigenous microbial flora plays a critical role in its natural fermentation. However, studies of traditionally fermented vrum are still rare. In this study, we investigated the artisanal production of traditionally fermented vrum from Inner Mongolia. In general, its physicochemical composition was characterized by 34.5 ± 8% moisture, 44.9 ± 12.1% fat, 10.6 ± 3.2% protein, and 210 ± 102°T. The total lactic acid bacteria and yeast counts ranged from 50 to 2.8 × 10⁸ cfu/g and from 0 to 1.1 × 10⁶ cfu/g, respectively. We studied bacterial and fungal community structures in 9 fermented vrum; we identified 5 bacterial phyla represented by 11 genera (an average relative abundance >1%) and 8 species (>1%), and 3 fungal phyla represented by 8 genera (>1%) and 8 species (>1%). Relative abundance values showed that Lactococcus and Lactobacillus were the most common bacterial genera, and Dipodascus was the predominant fungal genus. This scientific investigation of the nutritional components, microbial counts, and community profiles in Mongolian traditionally fermented vrum could help to develop future functional biomaterials and probiotics.

Case Report of a Tubo-ovarian Abscess Caused by Candida kefyr

BACKGROUND

Candida species are harmless commensals of hosts, including humans, but they can cause infection when the immune system is compromised. Infections with non-albicans species can occur, ranging from urinary tract infections to sepsis, especially among patients in intensive care units.

CASE

The patient, a 37-year-old woman, presented with severe abdominal pain, fever, and vomiting. The patient's symptoms and fever continued in spite of treatment with antibiotics, and she underwent exploratory laparotomy. Cyst content culture results showed that Candida kefyr was present in the cyst.

CONCLUSION

To the best of our knowledge, this is the first case report of a tubo-ovarian abscess caused by C. kefyr. Rare pathogens can be found in patients with a tubo-ovarian abscess, so culture of the abscess material is important for determining subsequent treatment, particularly in women who require an operation for tubo-ovarian abscess.

Genetically Engineered Yeast Expressing a Lytic Peptide from Bee Venom (Melittin) Kills Symbiotic Protozoa in the Gut of Formosan Subterranean Termites

The Formosan subterranean termite, Coptotermes formosanus Shiraki, is a costly invasive urban pest in warm and humid regions around the world. Feeding workers of the Formosan subterranean termite genetically engineered yeast strains that express synthetic protozoacidal lytic peptides has been shown to kill the cellulose digesting termite gut protozoa, which results in death of the termite colony. In this study, we tested if Melittin, a natural lytic peptide from bee venom, could be delivered into the termite gut via genetically engineered yeast and if the expressed Melittin killed termites via lysis of symbiotic protozoa in the gut of termite workers and/or destruction of the gut tissue itself. Melittin expressing yeast did kill protozoa in the termite gut within 56 days of exposure. The expressed Melittin weakened the gut but did not add a synergistic effect to the protozoacidal action by gut necrosis. While Melittin could be applied for termite control via killing the cellulose-digesting protozoa in the termite gut, it is unlikely to be useful as a standalone product to control insects that do not rely on symbiotic protozoa for survival.

Protozoacidal Trojan-Horse: use of a ligand-lytic peptide for selective destruction of symbiotic protozoa within termite guts

For novel biotechnology-based termite control, we developed a cellulose bait containing freeze-dried genetically engineered yeast which expresses a protozoacidal lytic peptide attached to a protozoa-recognizing ligand. The yeast acts as a ‘Trojan-Horse’ that kills the cellulose-digesting protozoa in the termite gut, which leads to the death of termites, presumably due to inefficient cellulose digestion. The ligand targets the lytic peptide specifically to protozoa, thereby increasing its protozoacidal efficiency while protecting non-target organisms. After ingestion of the bait, the yeast propagates in the termite's gut and is spread throughout the termite colony via social interactions. This novel paratransgenesis-based strategy could be a good supplement for current termite control using fortified biological control agents in addition to chemical insecticides. Moreover, this ligand-lytic peptide system could be used for drug development to selectively target disease-causing protozoa in humans or other vertebrates.

Characterization and stability of lactobacilli and yeast microbiota in kefir grains

Characterization and stability of lactobacilli and yeasts from kefir grains using culture-dependent and culture-independent methods were investigated in this study. Culture-dependent analysis, followed by sequencing of 16S ribosomal DNA for bacteria and 26S rRNA gene for yeasts, revealed 3 different species of lactobacilli and yeasts, respectively. The most frequently isolated bacterial species were Lactobacillus kefiranofaciens ssp. kefirgranum, Lb. parakefiri, and Lb. kefiri, whereas yeasts belonged to Kluyveromyces marxianus, Kazachstania exigua, and Rhodosporidium kratochvilovae. This study is the first to report on the presence of R. kratochvilovae in kefir grains. On the other hand, PCR-denaturing gradient gel electrophoresis in the culture-independent method showed that the dominant microorganisms were Lb. kefiranofaciens ssp. kefirgranum, Kl. marxianus and Ka. exigua, but did not reveal bands corresponding to Lb. parakefiri, Lb. kefiri, or R. kratochvilovae. Our results support the necessity of combining more techniques for detailed and reliable study of microbial communities in kefir grains. Another interesting finding confirmed that the detected dominant microbiota of kefir grains is very stable and did not change over experimental time. This finding is important to ensure consistent product quality.

Candidiasis caused by Candida kefyr in a neonate: case report

BACKGROUND

Systemic Candidia infections are of major concern in neonates, especially in those with risk factors such as longer use of broad spectrum antibiotics. Recent studies showed that also term babies with underlying gastrointestinal or urinary tract abnormalities are much more prone to systemic Candida infection. We report a very rare case of candidiasis caused by Candida kefyr in a term neonate.

CASE PRESENTATION

Renal agenesis on the left side was diagnosed antenatally and anal atresia postnatally. Moreover, a vesico-ureteral-reflux (VUR) grade V was detected by cystography. The first surgical procedure, creating a protective colostoma, was uneventful. Afterwards our patient developed urosepsis caused by Enterococcus faecalis and was treated with piperacillin. The child improved initially, but deteriorated again. A further urine analysis revealed Candida kefyr in a significant number. As antibiotic resistance data about this non-albicans Candida species are limited, we started liposomal amphotericin B (AMB), but later changed to fluconazole after receiving the antibiogram. Candiduria persisted and abdominal imaging showed a Candida pyelonephritis. Since high grade reflux was prevalent we instilled AMB into the child's bladder as a therapeutic approach. While undergoing surgery (creating a neo-rectum) a recto-vesical fistula could be shown and subsequently was resected. The child recovered completely under systemic fluconazole therapy over 3 months.

CONCLUSIONS

Candidiasis is still of major concern in neonates with accompanying risk factors. As clinicians are confronted with an increasing number of non-albicans Candida species, knowledge about these pathogens and their sensitivities is of major importance.

A beta-glucuronidase-based agar medium for the differential detection of the yeast Debaryomyces hansenii from foods

A selective and differential solid medium, Debaryomyces differential medium (DDM), was used for the isolation of Debaryomyces hansenii. This medium is formulated to allow detection of the beta-glucuronidase enzyme using the chromogenic substrate magenta-glucuro.CHA (5Br-6Cl-3indolyl-beta-D-glucuronide, cyclohexylammonium salt). Of the more than 120 microorganisms tested, including yeasts, bacteria, and a filamentous fungus, only D. hansenii produced violet colonies, thus permitting its easy discrimination from other organisms. When quality assessment tests were performed, optimal productivity and selectivity were obtained for D. hansenii. The medium was also satisfactory when used to test naturally contaminated food products.

Yeast populations in Sardinian feta cheese

In this study, the yeast populations in feta cheese from two different Sardinian dairies were examined. Samples of good quality feta (32) and samples of feta with a slimy surface defect (10) were examined from Dairy A. Similar, samples of good quality feta (23), feta with slimy surface defects (14) and samples with swelling defects (6) were examined from Dairy B. Kluyveromyces lactis was the dominating species in feta from Dairy A (95.2% of samples) followed by Debaryomyces hansenii (76.2%), Dekkera anomala (28.6%) and Dek. bruxellensis (19%). D. hansenii was dominant in samples from Dairy B (93%), followed by K. lactis (23.3%), Geotrichum candidum (23.3%) and Dek. anomala (18.6%). No significant difference was observed between the occurrence of yeast species in feta of good quality and in feta with slimy surface defects, thus confirming that slimy production is not associated with yeast contaminations. The swelling of samples observed in Dairy B seems to be caused by Dek. anomala. In fact, this strong fermenting species was present in all swelled samples in numbers exceeding 10(6) CFU g(-1), while it was isolated in very low concentration in only 5.4% of good samples.

Rapid differentiation of the closely related Kluyveromyces lactis var. lactis and K. marxianus strains isolated from dairy products using selective media and PCR/RFLP of the rDNA non transcribed spacer 2

PCR/RFLP of the NTS2 (IGS2) of rDNA was applied to differentiate two closely related yeast species, Kluyveromyces lactis var. lactis (referred to as K. lactis) and K. marxianus. Using specific primers, the NTS2 region was amplified from DNA of both K. lactis and K. marxianus type and collection strains. AluI restriction of amplified fragments generated patterns characteristic for each species. The NTS2 region from K. lactis var. drosophilarum and related species K. aestuarii, K. africanus, K. dobzhanskii, and K. wickerhamii could also be amplified with the same primers, but AluI patterns generated were clearly different. PCR/RFLP of the NTS2 appears thus to be a convenient method for rapid identification of K. lactis and K. marxianus, frequently found in dairy products. This test was validated therefore on K. lactis and K. marxianus from natural habitats. We showed that all yeast strains collected from whey samples and scoring blue on X-gal glucose plates were either K. lactis or K. marxianus. For application purposes, we propose here an approach for quickly screening for K. lactis/marxianus and Saccharomyces cerevisiae in dairy products using X-gal coloured and lysine growth media.

Genetics and molecular physiology of the yeast Kluyveromyces lactis

With the recent development of powerful molecular genetic tools, Kluyveromyces lactis has become an excellent alternative yeast model organism for studying the relationships between genetics and physiology. In particular, comparative yeast research has been providing insights into the strikingly different physiological strategies that are reflected by dominance of respiration over fermentation in K. lactis versus Saccharomyces cerevisiae. Other than S. cerevisiae, whose physiology is exceptionally affected by the so-called glucose effect, K. lactis is adapted to aerobiosis and its respiratory system does not underlie glucose repression. As a consequence, K. lactis has been successfully established in biomass-directed industrial applications and large-scale expression of biotechnically relevant gene products. In addition, K. lactis maintains species-specific phenomena such as the "DNA-killer system, " analyses of which are promising to extend our knowledge about microbial competition and the fundamentals of plasmid biology.

A chromogenic medium for the detection of yeasts with beta-galactosidase and beta-glucosidase activities from intermediate moisture foods

A selective and differential solid medium for the specific detection of some common yeasts frequently causing spoilage in intermediate moisture foods is described. The principle of the method is based on the detection of two enzymes, beta-glucosidase and beta-galactosidase, using the chromogenic substrates salmon-Gluc and X-Gal. Over 140 yeasts and bacteria were tested, and Debaryomyces hansenii and Kluyveromyces marxianus strains produced salmon and dark blue colonies, respectively, thus permitting their clear discrimination from other yeasts common in intermediate moisture foods. The medium was very satisfactory when intermediate moisture foods were tested.