In vitro susceptibilities of clinical yeast isolates to three antifungal agents determined by the microdilution method

Fatty acid synthase impacts the pathobiology of Candida parapsilosis in vitro and during mammalian infection

Cytosolic fungal fatty acid synthase is composed of two subunits α and β, which are encoded by Fas1 and Fas2 genes. In this study, the Fas2 genes of the human pathogen Candida parapsilosis were deleted using a modified SAT1 flipper technique. CpFas2 was essential in media lacking exogenous fatty acids and the growth of Fas2 disruptants (Fas2 KO) was regulated by the supplementation of different long chain fatty acids, such as myristic acid (14∶0), palmitic acid (16∶0), and Tween 80, in a dose-specific manner. Lipidomic analysis revealed that Fas2 KO cells were severely restricted in production of unsaturated fatty acids. The Fas2 KO strains were unable to form normal biofilms and were more efficiently killed by murine-like macrophages, J774.16, than the wild type, heterozygous and reconstituted strains. Furthermore, Fas2 KO yeast were significantly less virulent in a systemic murine infection model. The Fas2 KO cells were also hypersensitive to human serum, and inhibition of CpFas2 in WT C. parapsilosis by cerulenin significantly decreased fungal growth in human serum. This study demonstrates that CpFas2 is essential for C. parapsilosis growth in the absence of exogenous fatty acids, is involved in unsaturated fatty acid production, influences fungal virulence, and represents a promising antifungal drug target.

Perfil de sensibilidad antifúngica de especies de Candida aisladas de hemocultivos en un Hospital Universitario, Maracaibo, Venezuela

The aim of this study was to determine the in vitro susceptibility of amphotericin B, fluconazole and itraconazole, to several Candida spp recovered from blood cultures on hospitalized patients at the University Hospital of Maracaibo, Venezuela. The determination of the antifungal susceptibility was carried out according to the microdilution method in broth developed by The European Committee for Antimicrobial Susceptibility Testing (EUCAST). The profile of susceptibility of the 74 isolates showed that all the studied species were susceptible to amphotericin B, and 97.2% and 89.2% to fluconazole and itraconazole, respectively.

In vitro antifungal susceptibility testing

With the rising frequency of fungal infections, as well as increasing reports of resistance to antifungal agents, it is imperative that clinically applicable antifungal susceptibility testing be available. In 1997 the National Committee for Clinical Laboratory Standards published standard guidelines for antifungal susceptibility testing of Candida sp and Cryptococcus neoformans with amphotericin B, flucytosine, fluconazole, itraconazole, and ketoconazole. Although the methods are standard, they are time consuming, can be difficult to interpret, and are approved only for testing limited organisms and drugs. Modifications to the methods and alternative approaches have been proposed to make these tests more convenient and efficient, applicable to a greater number of species, and appropriate for performing in the clinical laboratory.

Typing of Candida glabrata in clinical isolates by comparative sequence analysis of the cytochrome c oxidase subunit 2 gene distinguishes two clusters of strains associated with geographical sequence polymorphisms

We tested whether comparative sequence analysis of the mitochondrion-encoded cytochrome c oxidase subunit 2 gene (COX2) could be used to distinguish intraspecific variants of Candida glabrata. Mitochondrial genes are suitable for investigation of close phylogenetic relationships because they evolve much faster than nuclear genes, which in general exhibit very limited intraspecific variation. For this survey we used 11 clinical isolates of C. glabrata from three different geographical locations in Brazil, 10 isolates from one location in the United States, 1 American Type Culture Collection strain as an internal control, and the published sequence of strain CBS 138. The complete coding region of COX2 was amplified from total cellular DNA, and both strands were sequenced twice for each strain. These sequences were aligned with published sequences from other fungi, and the numbers of substitutions and phylogenetic relationships were determined. Typing of these strains was done by using 17 substitutions, with 8 being nonsynonymous and 9 being synonymous. Also, cDNAs made from purified mitochondrial polyadenylated RNA were sequenced to confirm that our sequences correspond to the expressed copies and not nuclear pseudogenes and that a frameshift mutation exists in the 3' end of the coding region (position 673) relative to the Saccharomyces cerevisiae sequence and the previously published C. glabrata sequence. We estimated the average evolutionary rate of COX2 to be 11.4% sequence divergence/10(8) years and that phylogenetic relationships of yeasts based on these sequences are consistent with rRNA sequence data. Our analysis of COX2 sequences enables typing of C. glabrata strains based on 13 haplotypes and suggests that positions 51 and 519 indicate a geographical polymorphism that discriminates strains isolated in the United States and strains isolated in Brazil. This provides for the first time a means of typing of Candida strains that cause infections by use of direct sequence comparisons and the associated divergence estimates.

Novel microtiter plate format for testing germ tube formation and proposal of a cost-effective scheme for yeast identification in a clinical laboratory

The germ tube test is most widely used for presumptive identification of Candida albicans. Conventional testing is relatively time-consuming due to the hands-on time involved in preparing and viewing each isolate. In order to reduce work-load and costs we have developed a novel microtiter plate test format that offers several advantages: (i) use of removable strips of microtiter wells placed in lockwell frames, (ii) only one pipetting step for each isolate, (iii) direct micromorphological evaluation using an inverse microscope, (iv) use of a novel synthetic germ tube test medium, (v) reduction of the inoculum, permitting testing of minute colonies, (vi) thus, testing of different colonies in potentially mixed primary cultures of clinical specimens is encouraged and facilitated. Implementing this microtiter based germ tube test with simultaneous trehalase test for presumptive identification of Torulopsis (Candida) glabrata, we propose an identification scheme including this test format. This has been implemented in our routine laboratory permitting cost-effective presumptive identification of almost all clinically relevant yeast species.

The epidemiology of candidemia in two United States cities: results of a population-based active surveillance

We conducted prospective, active population-based surveillance for candidemia (defined as any Candida species isolated from blood) in Atlanta and San Francisco (total population, 5.34 million) during 1992-1993. The average annual incidence of candidemia at both sites was 8 per 100,000 population. The highest incidence (75 per 100,000) occurred among infants </=1 year old. In 19% of patients, candidemia developed prior to or on the day of admission. Underlying medical conditions included cancer (26%), abdominal surgery (14%), diabetes mellitus (13%), and human immunodeficiency virus infection (10%). In 47% of cases, species of Candida other than Candida albicans were isolated, most commonly Candida parapsilosis, Candida glabrata, and Candida tropicalis. Antifungal susceptibility testing of 394 isolates revealed minimal levels of azole resistance among C. albicans, C. tropicalis, and C. parapsilosis. These data document the substantial burden of candidemia and its changing epidemiology. Continued surveillance will be important to monitor the epidemiology of candidemia and to detect emergence of resistance to azoles.

Fluconazole disk diffusion susceptibility testing of Candida species

We describe a simple procedure for detecting fluconazole-resistant yeasts by a disk diffusion method. Forty clinical Candida sp. isolates were tested on RPMI-glucose agar with either 25- or 50-microgram fluconazole disks. With 25-microgram disks, zones of inhibition of >/=20 mm at 24 h accurately identified 29 of 29 isolates for which MICs were /=27 mm identified 28 of 29 such isolates. All 11 isolates for which MICs were >8 microgram/ml were identified by using either disk. Disk diffusion may be a useful screening method for clinical microbiology laboratories.

International surveillance of bloodstream infections due to Candida species: frequency of occurrence and antifungal susceptibilities of isolates collected in 1997 in the United States, Canada, and South America for the SENTRY Program. The SENTRY Participant Group

An international program of surveillance of bloodstream infections (BSIs) in the United States, Canada, and South America between January and December 1997 detected 306 episodes of candidemia in 34 medical centers (22 in the United States, 6 in Canada, and 6 in South America). Eighty percent of the BSIs were nosocomial and 50% occurred in patients hospitalized in an intensive care unit. Overall, 53.3% of the BSIs were due to Candida albicans, 15.7% were due to C. parapsilosis, 15.0% were due to C. glabrata, 7.8% were due to C. tropicalis, 2.0% were due to C. krusei, 0.7% were due to C. guilliermondii, and 5.8% were due to Candida spp. However, the distribution of species varied markedly by country. In the United States, 43.8% of BSIs were due to non-C. albicans species. C. glabrata was the most common non-C. albicans species in the United States. The proportion of non-C. albicans BSIs was slightly higher in Canada (47.5%), where C. parapsilosis, not C. glabrata, was the most common non-C. albicans species. C. albicans accounted for 40.5% of all BSIs in South America, followed by C. parapsilosis (38.1%) and C. tropicalis (11.9%). Only one BSI due to C. glabrata was observed in South American hospitals. Among the different species of Candida, resistance to fluconazole (MIC, > or = 64 microg/ml) and itraconazole (MIC, > or = 1.0 microg/ml) was observed with C. glabrata and C. krusei and was observed more rarely among other species. Isolates of C. albicans, C. parapsilosis, C. tropicalis, and C. guilliermondii were all highly susceptible to both fluconazole (99.4 to 100% susceptibility) and itraconazole (95.8 to 100% susceptibility). In contrast, 8.7% of C. glabrata isolates (MIC at which 90% of isolates are inhibited [MIC90], 32 microg/ml) and 100% of C. krusei isolates were resistant to fluconazole, and 36.9% of C. glabrata isolates (MIC90, 2.0 microg/ml) and 66.6% of C. krusei isolates were resistant to itraconazole. Within each species there were no geographic differences in susceptibility to fluconazole or itraconazole.

National surveillance of nosocomial blood stream infection due to Candida albicans: frequency of occurrence and antifungal susceptibility in the SCOPE Program

Surveillance of nosocomial blood stream infections (BSI) in the USA between April 1995 and June 1996 revealed that Candida was the fourth leading cause of nosocomial BSI, accounting for 8% of all infections. Fifty-two percent of 379 episodes of candidemia were due to Candida albicans. In vitro susceptibility studies using the 1997 National Committee for Clinical Laboratory Standards reference method demonstrated that 92% of C. albicans isolates were susceptible to 5-fluorocytosine and 90% were susceptible to fluconazole and itraconazole. Geographic variation in susceptibility of fluconazole and itraconazole was observed. Isolates from the Northwest and Southeast regions were more frequently resistant to fluconazole (13.3-15.5%) and to itraconazole (17.2-20.0%) than those from the Northeast and Southwest regions (2.9-5.5% resistant to fluconazole and itraconazole). Continued surveillance for infections caused by C. albicans and other species of Candida among hospitalized patients is recommended.

National surveillance of nosocomial blood stream infection due to species of Candida other than Candida albicans: frequency of occurrence and antifungal susceptibility in the SCOPE Program. SCOPE Participant Group. Surveillance and Control of Pathogens of Epidemiologic

A national surveillance program of nosocomial blood stream infections (BSI) in the USA between April 1995 and June 1996 revealed that Candida was the fourth leading cause of nosocomial BSI, accounting for 8% of all infections. Forty-eight percent of 379 episodes of candidemia were due to species other than Candida albicans. The rank order of non-C. albicans species was C. glabrata (20%) > C. tropicalis (11%) > C. parapsilosis (8%) > C. krusei (5%) > other Candida spp. (4%). The species distribution varied according to geographic region, with non-C. albicans species predominating in the Northeast (54%) and Southeast (53%) regions, and C. albicans predominating in the Northwest (60%) and Southwest (70%) regions. In vitro susceptibility studies demonstrated that 95% of non-C. albicans isolates were susceptible to 5-fluorocytosine, and 84% and 75% were susceptible to fluconazole and itraconazole, respectively. Geographic variation in susceptibility to itraconazole, but not other agents, was observed. Isolates from the Northwest and Southeast regions were more frequently resistant to itraconazole (29-30%) than those from the Northeast and Southwest regions (17-18%). Molecular epidemiologic studies revealed possible nosocomial transmission (five medical centers). Continued surveillance for the presence of non-C. albicans species among hospitalized patients is recommended.

Pharmacokinetics of fluconazole in children requiring peritoneal dialysis

After major open heart surgery, a significant number of infants and children are dependent on peritoneal dialysis as a result of renal impairment. They often require broad-spectrum antibiotics for the treatment of ongoing infections or as prophylactic therapy and have an increased risk of fungal infection. Fluconazole is a new thiazole antifungal agent that has been widely used in adults, but its use in children with acute renal impairment requiring peritoneal dialysis has not been documented. The purpose of this investigation was to study the pharmacokinetics of fluconazole in infants and children who developed various degrees of renal impairment, with or without the need for peritoneal dialysis, after major open heart surgery. Between January 1992 and June 1995, 17 children ranging in age from 2 weeks to 3 years (mean, 6 months) who received fluconazole therapy intravenously (3 mg/kg per day for 2 to 3 weeks) after major open heart surgery were enrolled in a prospective study. They were divided into two groups--those who required peritoneal dialysis (PD group; n = 8) and those who did not (non-PD group; n = 9). Blood, urine, and peritoneal dialysate samples were collected for 4 days to determine the pharmacokinetics of the drug, and data were compared between the two groups. The two groups of children had similar demographic characteristics, and their plasma concentrations of fluconazole showed no statistically significant differences. For children dependent on peritoneal dialysis, fluconazole was excreted almost solely through dialysis. Despite a significantly longer terminal elimination half-life for this group of infants, they tended to have a marginally larger volume of distribution of the drug. This, coupled with the continuous hourly exchange of dialysate, provided a large sink volume to effectively remove flu- conazole from the circulation. Thus the plasma clearance and the accumulation ratio were comparable for the two groups of children. Continuous cycling peritoneal dialysis effectively removed fluconazole from the circulation and was the main mode of excretion of the drug in children dependent on dialysis.

In vitro studies of activities of the antifungal triazoles SCH56592 and itraconazole against Candida albicans, Cryptococcus neoformans, and other pathogenic yeasts

We investigated the effects of various assay conditions on the activities of two antifungal drugs, SCH56592 and itraconazole, against seven species of fungi by the broth macrodilution testing procedure proposed by the National Committee for Clinical Laboratory Standards (NCCLS). For both drugs, which are insoluble in water, the concentration and type of solubilizing agent produced differences in drug activity. Starting inoculum size differences from 10(2) to 10(5) yeast cells per ml resulted in approximately a fourfold effect on the MIC of both drugs, but other significant differences were not observed with variations in synthetic medium composition, pH, buffering reagent, or incubation temperature. Under standardized conditions of reference method M27-T with 1% polyethylene glycol as the solubilizing agent, median MICs of SCH56592 and itraconazole of 60 and 125 mg/ml, respectively, were demonstrated for 110 strains (12 to 23 strains for each of seven species). Broth microdilution results were typically severalfold higher than broth macrodilution results. We conclude that the NCCLS standard reference method can be applied without modification to the testing of SCH56592 and itraconazole, but particular attention to solubilizing the agents is critical to obtaining consistent results.

Fluconazole disk diffusion procedure for determining susceptibility of Candida species

A simple disk diffusion test was defined for quick determination of the susceptibility of Candida species to fluconazole. The standard macrotube dilution reference method, the fluconazole E test, and a 25-microgram fluconazole disk test were all performed with each of 250 Candida species selected to provide a broad range of fluconazole MICs. All three methods were in excellent agreement. On RPMI 1640-glucose agar, isolates with inhibition zone diameters of > or = 19 mm were all susceptible (MIC, < or = 8.0 micrograms/ml) by the E test and 94% were susceptible by the macrotube method. Strains with smaller zones were either resistant, intermediate (dose-dependent susceptibility), or susceptible by the reference methods. The disk test did not adequately separate fully resistant strains from those with dose-dependent susceptibility: additional quantitative tests are needed for the few strains that are not unequivocally susceptible by the disk method.

Strain variation among and antifungal susceptibilities of isolates of Candida krusei

Candida krusei is an emerging pathogen that is well known for its propensity to develop resistance to fluconazole and other azoles. Despite the potential clinical significance of C. krusei, little is known of its epidemiology and genetic diversity as defined by the newer DNA-based typing methods. We investigated the genotypic diversity and antifungal susceptibility of 67 clinical isolates from 44 patients and 5 health care workers from six different medical centers. Strain delineation was performed by restriction endonuclease analysis of genomic DNA (REAG) with the restriction enzyme HinfI followed by conventional electrophoresis. The susceptibility of the isolates to the antifungal agents amphotericin B, flucytosine, fluconazole, and itraconazole was determined by methods recommended by the National Committee for Clinical Laboratory Standards. The MICs at which 90% of the isolates were inhibited ranged from 1.0 microgram/ml for itraconazole to 64 micrograms/ml for fluconazole. In general, isolates from a given patient, or epidemiologically related isolates from a single institution, were identical by molecular typing methods. Epidemiologically unrelated isolates were distinctly different by the REAG typing method employed. These data document the genetic diversity and antifungal susceptibility of clinical isolates of C. krusei.

DNA macrorestriction profiles and antifungal susceptibility of Candida (Torulopsis) glabrata

Candida (Torulopsis) glabrata is an emerging nosocomial pathogen that may be relatively resistant to fluconazole. A series of 75 isolates (blood, urine, tissue, and other sites) from 16 patients (1 to 12 isolates per patient) at a single university medical center were analyzed by pulsed field gel electrophoresis (PFGE) of restriction endonuclease digests of chromosomal DNA. The MICs of the isolates for amphotericin B, flucytosine, fluconazole, and itraconazole, were determined by a microdilution broth method. A preliminary study of seven restriction enzymes, three producing small fragments (Hinf I, Hind III, Eco RI) and four producing large fragments (Eag I, BssH II, Sfi I, Not I) identified Not I as giving interpretable banding patterns. Isolates were considered of different types if they differed by two or more bands. Sixteen distinct DNA types (A to P) were identified. Karyotyping was used an an additional technique to compare strains with a common PFGE type. Most patients were colonized or infected with a single type at multiple body sites and over time. One PFGE type was shared by four patients housed in different areas of the institution at different times. For two of these four patients, the karyotype was also indistintuishable. Five patients were each colonized with two distinct types. The MIC of the strains studied were amphotericin B 0.5-1.0 microgram/ml (MIC90 = 1.0 microgram/ ml), 5-fluorocytosine 0.25-->256 micrograms/ml (MIC90 = 2 micrograms/ml), fluconazole 0.25-->128 micrograms/ml (MIC90 = 32 micrograms/ml), and itraconazole 0.06-8.0 micrograms/ml). Molecular typing by PFGE using Not I digestion is a useful technique for epidemiological investigation as epidemiologically related isolates are generally identical and epidemiologically unrelated isolates are different by this method.

Clinical evaluation of a dried commercially-prepared microdilution panel for antifungal susceptibility testing

A commercially-prepared dried broth microdilution panel (Sensititre) was compared with a reference microdilution method for antifungal susceptibility testing of two reference yeast strains and 98 clinical isolates of Candida spp. The antifungal agents tested include 5-fluorocytosine (5FC), fluconazole, itraconazole, and D0870. Microdilution testing was performed according to National Committee for Clinical Laboratory Standards (NCCLS) recommendations. Minimum inhibitory concentration (MIC) endpoints were read visually after 48 hours of incubation and were assessed independently for each microdilution panel. The MICs for the reference strains were within published control limits for both reference and Sensititre microdilution panels. Discrepancies among MIC endpoints of no more than two dilutions (two wells) were used to calculate the percent agreement. An acceptable level of agreement between Sensititre and reference panels was observed for all antifungal agents when tested against the 98 clinical isolates. Agreement ranged from 83% for itraconazole to 93% for 5FC. The Sensititre dried microdilution panel appears to be a viable alternative to inhouse prepared microdilution panels and to the NCCLS microdilution reference method.