Flow cytometry as a tool to discriminate respiratory-competent and respiratory-deficient yeast cells

Metabolic remodeling and de novo mutations transcend cryptic variation as drivers of adaptation in yeast

Many organisms live in predictable environments with periodic variations in growth conditions. Adaptation to these conditions can lead to loss of nonessential functions, which could be maladaptive in new environments. Alternatively, living in a predictable environment can allow populations to accumulate cryptic genetic variation that may have no fitness benefit in that condition, but can facilitate adaptation to new environments. However, how these processes together shape the fitness of populations growing in predictable environments remains unclear. Through laboratory evolution experiments in yeast, we show that populations grown in a nutrient-rich environment for 1,000 generations generally have reduced fitness and lower adaptability to novel stressful environments. These populations showed metabolic remodeling and increased lipid accumulation in rich medium which seemed to provide osmotic protection in salt stress. Subsequent adaptation to stressors was primarily driven by de novo mutations, with very little contribution from the mutations accumulated prior to the exposure. Thus, our work suggests that without exposure to new environments, populations might lose their ability to respond effectively to these environments. Furthermore, our findings highlight a major role of exaptation and de novo mutations in adaptation to new environments but do not reveal a significant contribution of cryptic variation in this process.

Unmasking the Antifungal Activity of Anacardium occidentale Leaf Extract against Candida albicans

Invasive fungal disease causes high morbidity and mortality among immunocompromised patients. Resistance to conventional antifungal drugs and the toxicity associated with high doses highlight the need for effective antifungal therapies. In this study, the antifungal potential of the ethanolic extract of Anacardium occidentale (Cashew Leaf) leaves were evaluated against Candida albicans and C. auris. The antifungal activity was tested by the broth microdilution method and growth kinetic test. To further explore its antifungal action mode, spectrofluorophotometry, confocal microscopy and scanning and transmission electron microscopy were performed. Additionally, heterozygous knockout strains associated with resistance to oxidative stress were included in the study. We found that A. occidentale could inhibit the proliferation and growth of C. albicans at concentrations of 62.5 and 125 μg/mL. The doubling time was also drastically affected, going from 2.8 h to 22.5 h, which was also observed in C. auris. The extract induced the accumulation of intracellular reactive oxygen species (ROS), resulting in endoplasmic reticulum stress and mitochondrial dysfunction, while it did not show cytotoxicity or hemolytic activity at the concentrations evaluated. Our work preliminarily elucidated the potential mechanisms of A. occidentale against C. albicans on a cellular level, and might provide a promising option for the design of a new treatment for invasive candidiasis.

Metabolic Engineering of Probiotic Saccharomyces boulardii

Saccharomyces boulardiiis a probiotic yeast that has been used for promoting gut health as well as preventing diarrheal diseases. This yeast not only exhibits beneficial phenotypes for gut health but also can stay longer in the gut than Saccharomyces cerevisiae Therefore, S. boulardiiis an attractive host for metabolic engineering to produce biomolecules of interest in the gut. However, the lack of auxotrophic strains with defined genetic backgrounds has hampered the use of this strain for metabolic engineering. Here, we report the development of well-defined auxotrophic mutants (leu2,ura3,his3, and trp1) through clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9-based genome editing. The resulting auxotrophic mutants can be used as a host for introducing various genetic perturbations, such as overexpression or deletion of a target gene, using existing genetic tools forS. cerevisiae We demonstrated the overexpression of a heterologous gene (lacZ), the correct localization of a target protein (red fluorescent protein) into mitochondria by using a protein localization signal, and the introduction of a heterologous metabolic pathway (xylose-assimilating pathway) in the genome ofS. boulardii We further demonstrated that human lysozyme, which is beneficial for human gut health, could be secreted by S. boulardii Our results suggest that more sophisticated genetic perturbations to improveS. boulardii can be performed without using a drug resistance marker, which is a prerequisite for in vivo applications using engineeredS. boulardii.

Live-Cell Imaging of Mitochondria and the Actin Cytoskeleton in Budding Yeast

Maintenance and regulation of proper mitochondrial dynamics and functions are necessary for cellular homeostasis. Numerous diseases, including neurodegeneration and muscle myopathies, and overall cellular aging are marked by declining mitochondrial function and subsequent loss of multiple other cellular functions. For these reasons, optimized protocols are needed for visualization and quantification of mitochondria and their function and fitness. In budding yeast, mitochondria are intimately associated with the actin cytoskeleton and utilize actin for their movement and inheritance. This chapter describes optimal approaches for labeling mitochondria and the actin cytoskeleton in living budding yeast cells, for imaging the labeled cells, and for analyzing the resulting images.

Mechanisms of azole resistance in 52 clinical isolates of Candida tropicalis in China

OBJECTIVES

To explore the mechanisms underlying azole resistance in clinical isolates of Candida tropicalis collected in China by focusing on their efflux pumps, respiratory status and azole antifungal target enzyme.

METHODS

Fifty-two clinical isolates of C. tropicalis were collected from five hospitals in four provinces of China and antifungal susceptibility tests were performed. Rhodamine 6G and rhodamine 123 were used to investigate the efflux pumps and respiratory status, respectively. Transporter-related genes CDR1 and MDR1, mitochondrial gene CYTb, as well as ERG11, were quantified by real-time RT-PCR. Meanwhile, ergosterol content was analysed using liquid chromatography-mass spectrometry/mass spectrometry. An ERG11-deficient (erg11Δ) Saccharomyces cerevisiae strain was generated to study the function of mutations in ERG11.

RESULTS

MICs showed that 31 isolates were resistant to at least one type of azole antifungal. Flow cytometry using rhodamine 123 revealed increased respiration for the azole-resistant isolates, but CYTb was not overexpressed. No significant difference in the efflux of rhodamine 6G was found, which was consistent with the comparable expression levels of CDR1 and MDR1. In contrast, the azole-resistant isolates overexpressed ERG11 and showed increased ergosterol content. Moreover, the isolates resistant to three azole antifungals expressed higher levels of ERG11 mRNA than those resistant to only fluconazole or itraconazole. Two ERG11 mutations, Y132F and S154F, were found in azole-resistant isolates and could be shown to mediate azole resistance by expression in S. cerevisiae.

CONCLUSIONS

The up-regulation and mutations of ERG11 mediate azole resistance of C. tropicalis.

Relationship between respiration deficiency and azole resistance in clinical Candida glabrata

Candida glabrata has become a leading cause of invasive infections around the world and is exhibiting growing resistance to azole antifungals. To study the mechanism of its azole resistance, we analyzed the efflux pumps and found well known increased efflux expression and low metabolic state in all azole-resistant strains. The latter finding led us to further investigate the relationship between respiration status and azole antifungal susceptibility in clinical C. glabrata by growing them on glycerol-containing agar, measuring the cellular ATP, reactive oxygen species (ROS) levels, oxygen consumption and transmission electron microscopy. All azole-resistant isolates were respiratory-deficient, with reduced generation of ATP and ROS and decreased oxygen consumption; two isolates grew as small colonies and exhibited mitochondrial deficiency. Spot assays and agarose disc diffusion tests were performed to evaluate the effects of respiratory chain inhibitors, sodium azide and salicylhydroxamic acid, on antifungal susceptibility. The results of antifungal susceptibility showed that inhibition of alternative respiration with salicylhydroxamic acid enhanced azole susceptibility of C. glabrata. In conclusion, clinical azole-resistant C. glabrata isolates harbor respiratory deficiency exhibiting petite mutant or normal phenotype. The alternative respiratory pathway plays an important role in the decreased susceptibility to azole antifungals.

Compartmentalization and regulation of mitochondrial function by methionine sulfoxide reductases in yeast

Elevated levels of reactive oxygen species can damage proteins. Sulfur-containing amino acid residues, cysteine and methionine, are particularly susceptible to such damage. Various enzymes evolved to protect proteins or repair oxidized residues, including methionine sulfoxide reductases MsrA and MsrB, which reduce methionine (S)-sulfoxide (Met-SO) and methionine (R)-sulfoxide (Met-RO) residues, respectively, back to methionine. Here, we show that MsrA and MsrB are involved in the regulation of mitochondrial function. Saccharomyces cerevisiae mutant cells lacking MsrA, MsrB, or both proteins had normal levels of mitochondria but lower levels of cytochrome c and fewer respiration-competent mitochondria. The growth of single MsrA or MsrB mutants on respiratory carbon sources was inhibited, and that of the double mutant was severely compromised, indicating impairment of mitochondrial function. Although MsrA and MsrB are thought to have similar roles in oxidative protein repair each targeting a diastereomer of methionine sulfoxide, their deletion resulted in different phenotypes. GFP fusions of MsrA and MsrB showed different localization patterns and primarily localized to cytoplasm and mitochondria, respectively. This finding agreed with compartment-specific enrichment of MsrA and MsrB activities. These results show that oxidative stress contributes to mitochondrial dysfunction through oxidation of methionine residues in proteins located in different cellular compartments.

Development of a robust flow cytometric assay for determining numbers of viable bacteria

Several fluorescent probes were evaluated as indicators of bacterial viability by flow cytometry. The probes monitor a number of biological factors that are altered during loss of viability. The factors include alterations in membrane permeability, monitored by using fluorogenic substrates and fluorescent intercalating dyes such as propidium iodide, and changes in membrane potential, monitored by using fluorescent cationic and anionic potential-sensitive probes. Of the fluorescent reagents examined, the fluorescent anionic membrane potential probe bis-(1,3-dibutylbarbituric acid)trimethine oxonol [DiBAC(inf4)(3)] proved the best candidate for use as a general robust viability marker and is a promising choice for use in high-throughput assays. With this probe, live and dead cells within a population can be identified and counted 10 min after sampling. There was a close correlation between viable counts determined by flow cytometry and by standard CFU assays for samples of untreated cells. The results indicate that flow cytometry is a sensitive analytical technique that can rapidly monitor physiological changes of individual microorganisms as a result of external perturbations. The membrane potential probe DiBAC(inf4)(3) provided a robust flow cytometric indicator for bacterial cell viability.

Hypersusceptibility to azole antifungals in a clinical isolate of Candida glabrata with reduced aerobic growth

Petite mutations have been described in Saccharomyces cerevisiae and pathogenic yeasts. However, previous studies of the phenotypic traits of these petite mutants reported that they express azole resistance. We describe a clinical isolate of Candida glabrata with a striking association between increased susceptibility to azoles and respiratory deficiency. This isolate was obtained from a urine sample together with a respiration-competent C. glabrata isolate which exhibited azole resistance. The respiratory status of the two isolates was confirmed by cultivation on glycerol-containing agar and oxygraphy. Flow cytometry revealed the normal incorporation of rhodamine 123, and mitochondrial sections with typical cristae were seen by transmission electron microscopy for both isolates. Together, these results suggested a nuclear origin for the reduced respiratory capacity of the hypersusceptible isolate. The sterol contents of these isolates were similar to the sterol content of a reference strain. Sequencing of the ERG11 and PDR1 genes revealed that the sequences were identical in the two isolates, demonstrating their close relatedness. In addition to silent mutations, they carried a nonsense mutation in PDR1 that led to the truncation of transcription factor Pdr1p. They also overexpressed both PDR1 and one of its targets, CDR1, providing a possible explanation for the azole resistance of the respiration-competent isolate. In conclusion, in addition to azole resistance, which is a common feature of C. glabrata mitochondrial petite mutants, the mutation of a nuclear gene affecting aerobic growth may lead to azole hypersusceptibility; however, the mechanisms underlying this phenotype remain to be determined.

Live-cell imaging of the cytoskeleton and mitochondrial-cytoskeletal interactions in budding yeast

This chapter describes labeling methods and optical approaches for live-cell imaging of the cytoskeleton and of a specific organelle-cytoskeleton interaction in budding yeast.

Oxylipin and mitochondrion probes to track yeast sexual cells

When oxylipin and mitochondrion probes, i.e., fluorescing antibodies specific for 3-hydroxy fatty acids (3-OH oxylipins) and rhodamine 123 (Rh123), were added to yeast cells, these probes accumulated mainly in the sexual cells (i.e., both associated with ascospores) and not in the vegetative cells. This suggests increased mitochondrial activity in asci, since 3-OH oxylipins are mitochondrially produced and it is known that Rh123 accumulates selectively in functional mitochondria that maintain a high transmembrane potential (ΔΨm). This increased activity may be necessary for the production and effective release of the many spores found in single-celled asci. These results may be useful in the rapid identification of asci and in yeast sexual spore mechanics, which may find application in yeast systematics as well as hydro-, aero-, and nano-technologies.

Fluorescence imaging of mitochondria in yeast

The budding yeast Saccharomyces cerevisiae has many advantages as a model system, but until recently high-resolution microscopy was not often attempted in this organism. Its small size, rounded shape, and rigid cell wall were obstacles to exploring the cell biology of this model eukaryote. However, it is now feasible for laboratories to acquire and analyze high-resolution, multidimensional images of yeast cell biology, including the mitochondria. As a result, imaging of yeast has emerged as an important tool in eukaryotic cell biology. This chapter describes labeling methods and optical approaches for visualizing yeast mitochondria using fluorescence microscopy.

Relationships between respiration and susceptibility to azole antifungals in Candida glabrata

Over the past two decades, the incidence of infections due to Candida glabrata, a yeast with intrinsic low susceptibility to azole antifungals, has increased markedly. Respiratory deficiency due to mutations in mitochondrial DNA (mtDNA) associated with resistance to azoles frequently occurs in vitro in this species. In order to specify the relationships between respiration and azole susceptibility, the effects of respiratory chain inhibitors on a wild-type isolate of C. glabrata were evaluated. Respiration of blastoconidia was immediately blocked after extemporaneous addition of potassium cyanide, whereas a 4-h preincubation was required for sodium azide. Antifungal susceptibility determined by a disk diffusion method on Casitone agar containing sodium azide showed a significant decrease in the susceptibility to azoles. Biweekly subculturing on Casitone agar supplemented with sodium azide was therefore performed. This resulted after 40 passages in the isolation of a respiration-deficient mutant, as suggested by its lack of growth on glycerol-containing agar. This respiratory deficiency was confirmed by flow cytometric analysis of blastoconidia stained with rhodamine 123 and by oxygraphy. Moreover, transmission electron microscopy and restriction endonuclease analysis of the mtDNA of mutant cells demonstrated the mitochondrial origin of the respiratory deficiency. Finally, this mutant exhibited cross-resistance to all the azoles tested. In conclusion, blockage of respiration in C. glabrata induces decreased susceptibility to azoles, culminating in azole resistance due to the deletion of mtDNA. This mechanism could explain the induction of petite mutations by azole antifungals which have been demonstrated to act directly on the mitochondrial respiratory chain.

Assessment of mitochondrial membrane potential in yeast cell populations by flow cytometry

In yeast the use of rhodamine 123 (Rh123) has been restricted to the evaluation of mitochondrial respiratory function including the discrimination between respiratory-competent and -deficient cells. This study describes the optimization and validation of a low-concentration Rh123 staining protocol for the flow-cytometric assessment of mitochondrial membrane potential (Delta Psi m) changes in whole yeast cells. The optimized protocol was validated by the use of compounds that specifically affect mitochondrial energetics. Epifluorescence microscopy was used to monitor Rh123 distribution within the cell. Incubation of yeast cell suspensions with Rh123 (50 nM, 10 min) gave minimal non-specific binding and cytotoxicity of the dye. The ratio (R) between the green fluorescence and forward scatter (both measured as log values) was used to measure Delta Psi m with only little dependence on cell 'volume' and mitochondrial concentration. Cells treated with mitochondrial membrane de- or hyper-polarizing agents displayed a decrease and an increase of R values respectively, indicating that changes of the Rh123 distribution in cells indicate variations in the Delta Psi m. Live and dead cells also displayed significantly different R values. The method described here allows assessment of Delta Psi m changes in whole yeast cells in response to a given drug. Moreover, the relationship between drug effects and disorders of mitochondrial energetics might be addressed.

A mutation in the yeast mitochondrial ribosomal protein Rml2p is associated with a defect in catalase gene expression

Yeast strains containing a new temperature-sensitive allele of the RML2 gene, encoding a component of the large subunit of the mitochondrial ribosome, display normal growth on acetate, slowed growth on glycerol and an inability to grow on oleic acid. These cells, denoted rml2(fat21), have an apparent inability to induce peroxisomal function, as evidenced by a deficiency in oleic acid induction of beta-oxidation. However, the oleic acid regulation of genes encoding core enzymes of peroxisomal beta-oxidation is normal. In contrast, up-regulation of CTA1 (catalase) mRNA expression and enzyme activity is interrupted. Upon comparison of the induction requirements of catalase and the genes of beta-oxidation, we hypothesized that the rml2(fat21) mutation alters the activity of the transcription factor Adr1p. In support of this hypothesis, over-expression of ADR1 in rml2(fat21) cells restores CTA1 induction. Several assays of mitochondria from rml2(fat21) strains suggest normal mitochondrial function. Thus, the modulation of Adr1p-associated gene regulation is not due to overt mitochondrial dysfunction.

The beauty of the yeast: live cell microscopy at the limits of optical resolution

The yeast Saccharomyces cerevisiae is a very powerful system for cell biological research. Recent advances in electronic light microscopy together with the application of green fluorescent protein and other in vivo staining techniques have allowed novel and exciting insights into structural organization and dynamics of cells as small as yeast. Methods for staining yeast for microscopic inspection and for introducing tags for localization studies of proteins in living or fixed cells are summarized. Electronic light microscopy, video/deconvolution methods, and confocal laser scanning microscopy as novel tools for structural analyses, and their practical applications in yeast, are discussed.

Relating growth dynamics and glucoamylase excretion of individual Saccharomyces cerevisiae cells

We have developed a novel flow cytometric procedure that allows determinations of properties of protein excretion in the growth medium on a cell-by-cell basis in Saccharomyces cerevisiae. The procedure is based on labelling of a periplasmically secreted protein with antibodies conjugated to a fluorescent marker such as fluorescein isothiocyanate (FITC). The staining conditions did not perturb cell growth after resuspension of stained cells in growth medium. Decrease in fluorescence was found to correlate with excretion of glucoamylase into the growth medium. The analysis of the staining pattern over time provides information on the behaviour of individual cells belonging to different cell-cycle phases and can be used to calculate the specific excretion rate of the overall population.

Overexpression of catalase in cytosolic or mitochondrial compartment protects HepG2 cells against oxidative injury

HepG2 cells were transfected with vectors containing human catalase cDNA and catalase cDNA with a mitochondrial leader sequence to allow comparison of the effectiveness of catalase overexpressed in the cytosolic or mitochondrial compartments to protect against oxidant-induced injury. Overexpression of catalase in cytosol and in mitochondria was confirmed by Western blot, and activity measurement and stable cell lines were established. The intracellular level of H(2)O(2) induced by exogenously added H(2)O(2) or antimycin A was lower in C33 cell lines overexpressing catalase in the cytosol and mC5 cell lines overexpressing catalase in the mitochondria as compared with Hp cell lines transfected with empty vector. Cell death caused by H(2)O(2), antimycin A, and menadione was considerably suppressed in both the mC5 and C33 cell lines. C33 and mC5 cells were also more resistant to apoptosis induced by H(2)O(2) and to the loss of mitochondrial membrane potential induced by H(2)O(2) and antimycin A. In view of the comparable protection by catalase overexpressed in the cytosol versus the mitochondria, catalase produced in both cellular compartments might act as a sink to decompose H(2)O(2) and move diffusable H(2)O(2) down its concentration gradient. The present study suggests that catalase in cytosol and catalase in mitochondria are capable of protecting HepG2 cells against cytotoxicity or apoptosis induced by oxidative stress.

Ectopic potassium uptake in trk1 trk2 mutants of Saccharomyces cerevisiae correlates with a highly hyperpolarized membrane potential

Null trk1 trk2 mutants of Saccharomyces cerevisiae exhibit a low-affinity uptake of K+ and Rb+. We show that this low-affinity Rb+ uptake is mediated by several independent transporters, and that trk1Delta cells and especially trk1Delta trk2Delta cells are highly hyperpolarized. Differences in the membrane potentials were assessed for sensitivity to hygromycin B and by flow cytometric analyses of cellular DiOC6(3) fluorescence. On the basis of the latter analyses, it is proposed that Trk1p and Trk2p are involved in the control of the membrane potential, preventing excessive hyperpolarizations. K+ starvation and nitrogen starvation hyperpolarize both TRK1 TRK2 and trk1Delta trk2Delta cells, thus suggesting that other proteins, in addition to Trk1p and Trk2p, participate in the control of the membrane potential. The HAK1 K+ transporter from Schwanniomyces occidentalis suppresses the K+-defective transport of trk1Delta trk2Delta cells but not the high hyperpolarization, and the HKT1 K+ transporter from wheat suppresses both defects, in the presence of Na+. We discuss the mechanism involved in the control of the membrane potential by Trk1p and Trk2p and the causal relationship between the high membrane potential (negative inside) of trk1Delta trk2Delta cells and its ectopic transport of alkali cations.

The ability of membrane potential dyes and calcafluor white to distinguish between viable and non-viable bacteria

Various dyes were assessed for their ability to discriminate between viable and non-viable bacteria. Two methods of killing were employed: by heat treatment or by gramicidin treatment. Staining was carried out in two ways; by staining directly in the medium or by washing cells prior to staining in buffer. Carbocyanine and rhodamine 123 dyes only exhibited small changes in fluorescence between viable and non-viable populations of bacteria. Both oxonol dye (bis 1,3-dibutylbarbituric acid trimethine oxonol) and calcafluor white proved much more useful.

Microbial carotenoids

Carotenoids occur universally in photosynthetic organisms but sporadically in nonphotosynthetic bacteria and eukaryotes. The primordial carotenogenic organisms were cyanobacteria and eubacteria that carried out anoxygenic photosynthesis. The phylogeny of carotenogenic organisms is evaluated to describe groups of organisms which could serve as sources of carotenoids. Terrestrial plants, green algae, and red algae acquired stable endosymbionts (probably cyanobacteria) and have a predictable complement of carotenoids compared to prokaryotes, other algae, and higher fungi which have a more diverse array of pigments. Although carotenoids are not synthesized by animals, they are becoming known for their important role in protecting against damage by singlet oxygen and preventing chronic diseases in humans. The growth of aquaculture during the past decade as well as the biological roles of carotenoids in human disease will increase the demand for carotenoids. Microbial synthesis offers a promising method for production of carotenoids.

Mitochondrial dysfunction in yeast expressing the cytoplasmic male sterility T-urf13 gene from maize: analysis at the population and individual cell level

The urf13TW gene, which is derived from the mitochondrial T-urf13 gene responsible for Texas cytoplasmic male sterility in maize, was expressed in Saccharomyces cerevisiae by targeting its translation product into mitochondria. Analysis by oxygraphy at the population level revealed that in the presence of methomyl the oxygen uptake of intact yeast cells carrying the targeted protein is strongly stimulated only with ethanol as respiratory substrate and not with glycerol, lactate, pyruvate, or acetate. When malate is the substrate oxidized by isolated mitochondria, interaction between the targeted protein and methomyl results in significant inhibition of oxygen uptake. This inhibition is eliminated and oxygen uptake is stimulated by subsequent addition of NAD+. Using 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)] as probe, interactive laser scanning and flow cytometry, which permit analysis at the individual cell level, demonstrated that specific staining of the mitochondrial compartment is obtained and that DiOC6(3) fluorescence serves as a measure of the membrane potential. Finally, it was shown that, as in T cytoplasm maize mitochondria, HmT toxin and methomyl dissipate the membrane potential of yeast mitochondria that carry the foreign protein. Furthermore, the results suggest that the HmT toxin and methomyl response is related to the plasmid copy number per cell and that the deleterious effect induced by HmT toxin is stronger than that of methomyl.

Rapid assessment of bacterial viability by flow cytometry

The ability of a flow cytometer to rapidly assess microbial viability was investigated using three vital stains: rhodamine 123 (Rh123); 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)] and fluorescein diacetate (FDA). Rh123 was found to clearly differentiate viable from non-viable bacteria. The methodology for staining bacteria with this dye was optimised. Rh123 was shown to stain and discriminate several different species of viable bacteria although this was not universal. Viable cells of Bacillus subtilis were found to stain better with FDA than with Rh123. The results demonstrate the ability of flow cytometry to rapidly detect and estimate the viability of bacterial populations.

A fluorescence-microscopic and flow-cytometric study of HeLa cells with an experimentally induced respiratory deficiency

HeLa cells, cultured over a long period in a medium containing low doses of ethidiumbromide, were used as a model system for flow-cytometric detection of human cells with impaired mitochondrial respiratory function. Based on laserscan and flowcytometric analysis after rhodamine 123 staining, the mitochondrial membrane potential of respiratory deficient cells seems unchanged as compared to control cells. Maintenance of this membrane potential in respiration-impaired cells requires glycolytic ATP generation, as transient inhibition of glycolysis by sodium fluoride affects rhodamine 123 accumulation in ethidiumbromide-treated cells, but not in control cells. We present a protocol which allows the detection and separation of respiratory deficient cells by flow cytometry.