In vitro susceptibilities of sucrose-negative Candida tropicalis, Candida lusitaniae, and Candida norvegensis to amphotericin B, 5-fluorocytosine, miconazole, and ketoconazole.

The MICs and minimal lethal concentrations of four antimycotics, amphotericin B, 5-fluorocytosine, miconazole nitrate, and ketoconazole, were determined for 25 yeast isolates representing species uncommonly implicated in candidiasis. A microdilution procedure was employed with complex and synthetic media. The isolates, in general, were susceptible to the same antimicrobial agents shown to be effective against Candida albicans, but differences between some of the species in relative susceptibilities to the antifungal agents were noted. Isolates of atypical sucrose-negative Candida tropicalis were similar in their susceptibility patterns to typical isolates of the species. Relative resistance to amphotericin B, miconazole nitrate, and ketoconazole was noted for two Candida lusitaniae isolates, but all strains were susceptible to 5-fluorocytosine. Candida norvegensis isolates were more resistant to miconazole and ketoconazole than C. albicans clinical isolates. The microtiter system was satisfactory for determining minimal inhibitory concentrations, but the system is not recommended for detecting finite differences in drug susceptibilities or for detecting drug synergism.     

In vitro susceptibilities and biotypes of Candida albicans isolates from the oral cavities of patients infected with human immunodeficiency virus.

Candida albicans strains were isolated from the oral cavities of 62 human immunodeficiency virus (HIV)-infected patients at different stages of HIV infection. Only patients with persistent generalized lymphadenopathy-acquired immunodeficiency syndrome (AIDS)-related complex or full-blown AIDS showed typical clinical symptoms for oral candidiasis. In general, the microbiological recovery of Candida strains from the oral cavity increased with more advanced stages of HIV infection. The antifungal activity of ketoconazole, itraconazole, nystatin, amphotericin B, and flucytosine against all 62 strains was evaluated by means of a photometer-read broth microdilution method for determination of the 30% inhibitory concentrations of the drugs. The 95% ranges of 30% inhibitory concentrations were as follows: less than or equal to 0.063 to 32 micrograms/ml for ketoconazole, less than or equal to 0.063 to 8 micrograms/ml for itraconazole, 0.5 to 4 micrograms/ml for nystatin, less than or equal to 0.063 to 4 micrograms/ml for amphotericin B, and less than or equal to 0.063 to 8 micrograms/ml for flucytosine. Two strains were resistant to flucytosine, one was resistant to ketoconazole, and three were resistant to itraconazole. Isolates from patients with full-blown AIDS showed significantly less susceptibility to itraconazole, amphotericin B, and flucytosine. Strains were biotyped by using the API 20C carbohydrate assimilation system. The major biotype accounted for 63.9% of the isolates. At repeated evaluation, a change in biotype pattern was seen in 27.3%.     

Effect of different media on determination of novobiocin resistance for differentiation of coagulase-negative staphylococci

Species identification of coagulase-negative staphylococci often requires the determination of novobiocin susceptibility. Although previous investigators have recommended the use of P agar for this purpose, most clinical laboratories do not routinely utilize this medium. For this reason, studies were performed to compare novobiocin susceptibility results obtained with 11 different species of staphylococci (10 isolates of each species), using P agar, Trypticase soy agar with 5% sheep blood, and Mueller-Hinton agar. Tests performed on 70 susceptible isolates (minimal inhibitory concentration less than 1.6 micrograms/ml) resulted in zones of inhibition around 5-micrograms novobiocin disks ranging from 19.6 to 33.9, 16.2 to 26.6, and 21.3 to 36.4 mm on P agar, Trypticase soy agar with 5% sheep blood, and Mueller-Hinton agar, respectively. Forty resistant isolates (minimal inhibitory concentration greater than or equal to 1.6 micrograms/ml) exhibited zones of inhibition ranging from 6.0 to 11.3 mm on P agar, 6.0 to 11.6 mm on Trypticase soy agar with 5% sheep blood, and 6.0 to 13.5 mm on Mueller-Hinton agar. Using the established cut off of 16 mm to define novobiocin resistance for the identification of coagulase-negative staphylococci, we correctly identified 100% of the strains tested, regardless of the media utilized.     

In Vitro Antifungal Activity of Clotrimazole (Bay b 5097)

The in vitro antifungal activity of clotrimazole (Bay b 5097) was compared with those of amphotericin B, griseofulvin, nystatin, and pyrrolnitrin. The inhibitory activity of clotrimazole against most systemic pathogens was comparable to that of amphotericin B; minimal inhibitory concentrations of the two drugs for Blastomyces dermatitidis, Histoplasma capsulatum, Sporothrix schenckii, Cryptococcus neoformans, and Coccidioides immitis were in the range of 0.20 to 3.13 and 0.10 to 6.25 mug/ml, respectively. One isolate of Allescheria boydii was resistant to 100 mug of amphotericin B per ml but was inhibited by 6.25 mug of clotrimazole per ml. Clotrimazole was less active than amphotericin B against Candida albicans and Aspergillus fumigatus. The activity of clotrimazole against dermatophytes was comparable to that of pyrrolnitrin; 0.78 mug of either compound per ml was fungicidal for most isolates of Trichophyton sp., Microsporum sp. and Epidermophyton floccosum. Both griseofulvin and nystatin were less active than clotrimazole. The size of inoculum was shown to have a significant effect on the results of in vitro susceptibility testing with clotrimazole.     

Distribution and antifungal susceptibility of Candida species causing nosocomial candiduria

The aim of the study was to investigate the distribution of Candida species isolated from urine specimens of hospitalized patients in Akdeniz University Hospital, Antalya, Turkey, as well as their susceptibilities to antifungal agents. A total of 100 patients who had nosocomial candiduria between March 2003 and May 2004 at the facility were included in the study. Organisms were identified by conventional methods and the use of API ID 32C strips. Susceptibilities of the isolates to amphotericin B were determined by Etest, whereas the minimum inhibitory concentration (MIC) values of these same strains to fluconazole, voriconazole and caspofungin were assessed using the broth microdilution method. The most common species recovered was C. albicans 44% of all yeasts, followed by C. tropicalis (20%), C. glabrata (18%), C. krusei (6%), C. famata (5%), C. parapsilosis (4%), C. kefyr (2%) and C. guilliermondii (1%). A total of nine (9%) of the isolates, including five C. krusei and four C. glabrata isolates were susceptible dose-dependent (SDD) to fluconazole. In constrast, only two C. glabrata and one C. krusei isolates were resistant to this antifungal. The voriconazole MICs for all Candida isolates were ≤0.5 μg/ml, except for one C. glabrata isolate with a MIC value of 2 μg/ml. Among all isolates, 94% were susceptible to amphotericin B with MIC values of <1 μg/ml and all isolates were susceptible to caspofungin with MIC values of ≤0.5 μg/ml. Future studies are needed to define better treatment regimens for those patients who have fluconazole-resistant Candida urinary tract infections.     

Improved detection of amphotericin B-resistant isolates of Candida lusitaniae by Etest

Both intrinsic and acquired resistance to amphotericin B have been documented for Candida lusitaniae. Amphotericin B remains the drug of choice for many critical fungal infections, and the detection of resistance is essential to monitor treatment effectively. The limitations of the National Committee for Clinical Laboratory Standards (NCCLS) reference methodology for detection of amphotericin B resistance are well documented, and several alternative methods have been proposed. Etest assays with RPMI and antibiotic medium 3 (AM3) agar were compared to the NCCLS M27-A broth macrodilution method using AM3 for amphotericin B resistance testing with 49 clinical isolates of C. lusitaniae. The panel included nine isolates with known or presumed resistance to amphotericin B on the basis of in vivo and/or in vitro data. The distribution of amphotericin B MICs by Etest with RPMI ranged from 0. 032 to 16 microg/ml and was bimodal. All of the putatively resistant isolates were inhibited by amphotericin B at >/=0.38 microg/ml and could be categorized as resistant using this breakpoint. Etest with AM3 yielded a broader amphotericin B MIC range (0.047 to 32 microg/ml), and there were six putatively resistant isolates for which MICs were >1 microg/ml. The separation of putatively susceptible and resistant isolates was less obvious. Broth macrodilution with AM3 generated a unimodal distribution of MICs (ranging from 0.032 to 2 microg/ml) and failed to discriminate most of the putatively resistant isolates at both 24 and 48 h. Etest using RPMI and, to a lesser extent, using AM3 provided better discrimination between amphotericin B-resistant and -susceptible isolates of C. lusitaniae.     

Identification and susceptibility of clinically isolated yeast-like fungi

We studied the identification and susceptibility of clinically isolated yeast-like fungi at Showa University Hospital from April 1988 to March 1989. Clinically significant of yeast-like fungi were observed in 7.1% of specimens from outpatients, 13.0% of inpatients. In both outpatients and inpatients, yeast-like fungi were isolated mainly from sputum and urine. But, one third of them were considered as non-pathogenic and not identified. The species of isolates were, Candida albicans 57%, Candida tropicalis 14% and Candida glabrata 8% in both inpatients and outpatients, and these species shared most part. The isolation frequency of Candida parapsilosis was higher in blood and cerebrospinal fluid (CSF) specimen than the others. The susceptibility test by agar dilution method indicated most of the isolates were susceptible to Amphotericin B and Miconazole (MIC less than or equal to 25 micrograms/ml). There was no difference in MIC between predominantly isolated fungi and commonly isolated fungi. Notably, isolates from blood and CSF showed a significant high tolerance against Amphotericin B and Miconazole than from the other specimens. The MICs of Fluconazole were shown to be very high (greater than 100 micrograms/ml) in normal Sabouraud agar, were decreased dose-dependently by human sera in the medium. These findings indicated the component(s) of sera enhanced the anti-fungal activity of Fluconazole.     

Emergence of nosocomial candidemia at a teaching hospital in Taiwan from 1981 to 2000: increased susceptibility of Candida species to fluconazole

The incidence of nosocomial Candida fungemia increased 36-fold from 1981 (0.8/10,000 discharges) to 2000 (28.8/10,000 discharges) at the National Taiwan University Hospital, a 2000-bed teaching hospital in northern Taiwan. To understand the current status of resistance to available antifungal agents among Candida species causing invasive infections, the in vitro susceptibilities of 222 isolates (collected from July, 1999-June, 2001) were determined. Among all of the Candida species tested, 6% and 7% were resistant to fluconazole and itraconazole, respectively. The MIC90 values of voriconazole and amphotericin B were 0.5 and 1 microg/ml, respectively, although some isolates of C. krusei (amphotericin B and voriconazole MIC, >64 microg/ml) and C. tropicalis and C. glabrata (voriconazole MICs, >64 microg/ml) were less susceptible to voriconazole or amphotericin B. About one-half of the C. glabrata isolates belonged to susceptible dose-dependent (SDD, 36%) or resistant (12%) categories for fluconazole and 96% belonged to SDD (56%) or resistant (40%) category for itraconazole. When compared with fluconazole susceptibility data of blood Candida isolates recovered from patients treated at the same hospital (NTUH) from two different time periods (January, 1994, to June, 1995, and January, 1997, to June, 1999 described in previous reports), the incidence of increased susceptibility of non-krusei Candida isolates to fluconazole was evident. This trend of increasing susceptibility for fluconazole did not correlate to the increasing use of this agent in the hospital. None of the random amplified polymorphic DNA patterns generated by arbitrarily primed PCR using four random oligonucleotide primers for 14 isolates, which exhibited fluconazole MICs of > or = 16 microg/ml, were identical, indicating an absence of clonal dissemination among these isolates in the hospital.     

Recovery of vancomycin-resistant gram-positive cocci from children.

A cross-sectional survey of vancomycin-resistant gram-positive cocci (VRGPC) in the feces of children was initiated after several bacteremic infections with these organisms occurred at our hospital. A selective medium consisting of colistin-nalidixic acid agar, 5% sheep blood, vancomycin (5 mg/liter), and amphotericin B (8 mg/liter) was developed to isolate VRGPC. A single stool specimen submitted to the clinical microbiology laboratory from each of 48 patients was inoculated onto the medium. Plates were incubated at 35 degrees C with 5% carbon dioxide and examined at 24, 48, and 72 h. Susceptibilities were determined by broth microdilution. A total of 14 isolates from 11 of 48 (22%) children were recovered. The density of growth ranged from a single colony to 2+. The VRGPC were identified as Leuconostoc lactis (n = 2), Lactobacillus confusus (n = 4), Enterococcus species (n = 5), and Lactococcus lactis (n = 3). One strain of Lactobacillus confusus was recovered from both the stool and the blood of one of these patients. The MICs of vancomycin were 4 micrograms/ml for one of the isolates, 8 micrograms/ml for four of the isolates, and more than 16 micrograms/ml for the remaining eight isolates. All isolates were susceptible to both penicillin and ampicillin. Only 1 of the 11 children had received prior treatment with vancomycin. We conclude that low concentrations of VRGPC may be common in the gastrointestinal tracts of children.     

Selective medium with DNase test agar and a modified toluidine blue O technique for primary isolation of Branhamella catarrhalis in sputum.

A selective medium with DNase test agar and incorporating vancomycin (10 micrograms/ml), trimethoprim (8 micrograms/ml), and amphotericin B (2 micrograms/ml) supported the growth of 305 Branhamella catarrhalis isolates. A modified toluidine blue O technique was used after 48 h of incubation in CO2 to overlay suspected B. catarrhalis colonies. A metachromatic color change was observed in 15 min, indicating DNase production. In 200 unselected sputum samples of hospitalized patients, this method was compared with routine microbiologic procedures; 31 B. catarrhalis isolates were recovered with the method, compared with 22 isolated from the clinical laboratory. This medium will be particularly useful for culture of sputum, which shows inflammatory cells and gram-negative diplococci on Gram-stained smears.     

Neisseria gonorrhoeae strains inhibited by vancomycin in selective media and correlation with auxotype

Strains of Neisseria gonorrhoeae that failed to grow on Thayer-Martin (T-M) and Martin-Lewis (M-L) media accounted for 2.0% of isolates at the University of Colorado Hospital and its Venereal Disease Clinic. A total of 31 inhibited and 31 control strains were compared by agar dilution testing for their susceptibilities to 13 antimicrobial agents used for treatment or in selective media. All 62 isolates were resistant to lincomycin, colistin, nystatin, amphotericin B, trimethoprim lactate, polymyxin B, and anisomycin. Vancomycin was the inhibitory antibiotic for N. gonorrhoeae in both T-M and M-L media. The vancomycin-inhibited strains were also significantly more sensitive to penicillin and ampicillin than were the control strains (P less than 0.001). The presence of the other antibiotics in selective media did not affect the minimum inhibitory concentrations of vancomycin for gonococci. All 31 inhibited strains were sensitive to 8.0 micrograms of vancomycin per ml, and 26 of these were sensitive to 2.0 microgram/ml. Decreasing the size of inoculum of gonococci results in greater inhibition by any given concentration of vancomycin. The vancomycin-sensitive strains contained significantly more arginine- hypoxanthine-, and uracil-requiring auxotypes (28 out of 31) than did the control strains (9 out of 31). As with T-M medium, some strains of gonococci will be missed when M-L medium with 4.0 micrograms of vancomycin per ml is the only medium used for the diagnosis of gonorrhea. This may be of particular importance in the confirmation of disseminated infection with Arg- Hyx- Ura- auxotypes of N. gonorrhoeae when cultures of blood, joint fluid, or skin lesions are negative.     

Rapid identification of fluconazole resistance using Chromagar Candida

Although extremely rare 10 years ago, antifungal drug resistance is becoming a major problem in certain populations, especially in those infected with HIV. This study was undertaken to study the resistance of Candida species isolated in our hospital to Fluconazole using Chrom agar Candida. The Candida strains which were routinely isolated from clinical specimens like blood, urine, sputum, pus, fluid and homograft isolates were included in the study. 142 Candida isolates were tested by using Chrom agar Candida incorporated with fluconazole. 16 strains were found to be resistant to Fluconazole and 126 strains sensitive to Fluconazole. Nine were C tropicalis, 3 C krusei, 2 C guillermondii, 1 Geotrichum candidum and one was an unidentified strain of Candida. The MIC of the 16 strains were done using RPMI 1640 medium by macro broth dilution method. MIC of 9 strains was 64 & > 64 micrograms/ml of 6 strains 32 micrograms/ml and 1 strain 16 micrograms/ml.     

Invasive fungal infections in Chile: a multicenter study of fungal prevalence and susceptibility during a 1-year period.

During the first year of an ongoing surveillance program of invasive fungal infections (IFI) a total of 130 patients (56% male) with fungal strains isolated from blood and other sterile sites were reported from 13 hospitals in Chile. Significant yeast isolates were obtained from 118 patients, and molds affected 12 patients. The main patient groups affected were neonates, children less than 1 year old and adults aged 50-79 years. All fungal bloodstream infections (BSI) were due to yeasts; 79 patients (61%) were affected. The main risk factors recorded were antibiotic therapy (76%), stay in the intensive care unit (ICU) (70%) and presence of a central venous catheter (65%). Nosocomial infections were represented in 83.5% of BSI. Overall, Candida albicans (40.8%), C. parapsilosis (13.1%), C. tropicalis (10%) and Cryptococcus neoformans (10%) were the most common species. Aspergillus fumigatus (3.1%) was the most frequent mold. C. albicans (48.1%) and C. parapsilosis (17.7%), were the most frequent agents recovered from blood. Saccharomyces cerevisiae and Trichosporon mucoides, two emerging pathogens, were also isolated. All yeasts tested were susceptible to amphotericin B with minimal inhibitory concentration (MIC) < or = 1 microg/ml. Resistance to itraconazole (MIC > or = 1 microg/ml) and fluconazole (MIC > or = 64 microg/ml) was observed in 4 and 6% of cases, respectively. C. glabrata was the least susceptible species, with 50% of isolates resistant to itraconazole and 33% resistant to fluconazole, with one strain showing combined resistance. Reduction of BSI requires greater adherence to hand-washing and related infection control guidelines.     

Characterisation of yeasts implicated in vulvovaginal candidosis in Irish women

High-vaginal swabs were taken from 98 women attending a genitourinary clinic in Dublin city who presented with symptoms of vaginitis. Twenty-eight (28.6%) proved culture-positive for yeasts. Candida albicans was isolated from 26 of the yeast-positive patients and Candida glabrata and Candida tropicalis were isolated from a further two patients. Two patients were colonized by two yeasts simultaneously (C. albicans and Candida lucitaniae, and C. albicans and Candida krusei). Yeasts were characterised on the basis of their adherence ability, extracellular enzyme production and resistance to antifungal agents. All C. albicans isolates demonstrated high adherence abilities to buccal epithelial cells, but produced relatively low levels of phospholipase and acid proteinase. The non-C. albicans isolates demonstrated low levels of adherence, but no extracellular enzyme production was detected. Isolates were most sensitive to the polyenes amphotericin B and nystatin but a large proportion (50%) of C. albicans isolates were resistant to clotrimazole, which is an important agent in the treatment of vulvovaginal candidosis.     

Antibiotic sensitivity pattern of Citrobacter isolated from various clinical specimens in a tertiary care hospital

A total of 709 isolates of Citrobacter species were recovered from different clinical specimens at Kasturba Medical College, Mangalore. The antimicrobial susceptibility testing of the isolates was performed using Kirby-Bauer's method of disk diffusion. All the isolates were susceptible to Imipenem (100%) and 100% resistance to Ampicillin was recorded. Susceptibility to third generation cephalosporins varied between 29 - 43%. The beta-lactamase inhibitor combination used along with beta-lactam antibiotics increased their sensitivity. The minimum inhibitory concentration (MIC) of cephotaxime to Citrobacter species was determined by agar dilution method (MIC value = or < 8 microg/ml are considered susceptible). Two hundred and fifty three strains were found to be completely susceptible, 71 strains showed intermediate susceptibility and 385 strains were completely resistant to Cephotaxime.     

Catheter-associated fungemia caused by Fusarium chlamydosporum in a patient with lymphocytic lymphoma.

We evaluated the use of mannitol salt agar with oxacillin for use as a primary screening medium for the simultaneous detection and identification of methicillin-resistant Staphylococcus aureus in clinical surveillance specimens. Oxacillin agar dilution susceptibility tests with mannitol salt agar and Mueller-Hinton agar were performed in parallel with disk-agar diffusion testing on 95 oxacillin-susceptible and 105 oxacillin-resistant S. aureus stock isolates. MICs were found to be comparable, showing distinct separation of susceptible and resistant isolates into two groups with MICs of less than or equal to 2 and greater than or equal to 32 micrograms/ml, respectively. In accord with these findings, 4 micrograms of oxacillin per ml was selected for use in the screening medium. For performance evaluation, mannitol salt agar with 4 micrograms of oxacillin per ml was compared with mannitol salt agar without oxacillin by performing parallel screening tests on 153 clinical surveillance specimens. For detection of methicillin-resistant S. aureus, mannitol salt agar with 4 micrograms of oxacillin per ml was as sensitive as mannitol salt agar without oxacillin and required significantly fewer confirmatory tests. For primary identification of methicillin-resistant S. aureus, mannitol salt agar with 4 micrograms of oxacillin per ml was 6.4% false-positive and 1.1% false-negative, with a 93.6% positive predictive value. These findings indicate that mannitol salt agar with 4 micrograms of oxacillin per ml can be used as a reliable and cost-effective screening medium for the simultaneous detection and identification of methicillin-resistant S. aureus in clinical surveillance specimens.     

Lactose-fermenting, multiple drug-resistant Salmonella typhi strains isolated from a patient with postoperative typhoid fever

Two lactose-fermenting Salmonella typhi strains were isolated from bile and blood specimens of a typhoid fever patient who underwent a cholecystectomy due to cholelithiasis. One lactose-fermenting S. typhi strain was also isolated from a pus specimen which was obtained at the tip of the T-shaped tube withdrawn from the operative wound of the common bile duct of the patient. These three lactose-fermenting isolates: GIFU 11924 from bile, GIFU 11926 from pus, and GIFU 11927 from blood, were phenotypically identical to the type strain (GIFU 11801 = ATCC 19430 = NCTC 8385) of S. typhi, except that the three strains fermented lactose and failed to blacken the butt of Kligler iron agar or triple sugar iron agar medium. All three lactose-fermenting strains were resistant to chloramphenicol, ampicillin, sulfomethoxazole, trimethoprim, gentamicin, cephaloridine, and four other antimicrobial agents. The type strain was uniformly susceptible to these 10 drugs. The strain GIFU 11925, a lactose-negative dissociant from strain GIFU 11926, was also susceptible to these drugs, with the sole exception of chloramphenicol (minimal inhibitory concentration, 100 micrograms/ml).     

In vitro activity of nystatin compared with those of liposomal nystatin, amphotericin B, and fluconazole against clinical Candida isolates

We investigated the in vitro activity of nystatin and liposomal nystatin against 103 Candida isolates to determine the effect of both time and medium on MICs. We also compared the nystatin MICs with those of amphotericin B and fluconazole. Testing was performed in accordance with the National Committee for Clinical Laboratory Standards M27-A microdilution methodology with RPMI 1640, RPMI 1640 supplemented with glucose to 2% (RPMI-2), and antibiotic medium 3 supplemented with glucose to 2% (AM3). While nystatin MICs were similar to or slightly lower than liposomal nystatin MICs in RPMI 1640 and RPMI-2, they were markedly higher than liposomal nystatin MICs in AM3. Use of AM3 and determination of the MIC after 24 h of incubation provided a slightly wider range of liposomal nystatin MICs (0.06 to >16 microg/ml). Under these conditions, the MICs at which 90% of isolates were inhibited of nystatin and liposomal nystatin were 2 and 1 microg/ml, respectively. Nystatin and liposomal nystatin in general showed good activity against all Candida spp. tested. Although the MICs of nystatin and liposomal nystatin tended to rise in parallel with the amphotericin B MICs, nystatin and liposomal nystatin MICs of 1 to 2 and 0.5 to 1 microg/ml, respectively, were obtained for seven and six, respectively, of nine isolates for which amphotericin B MICs were >or=0.25 microg/ml. No correlation between fluconazole and nystatin or liposomal nystatin MICs was observed. As amphotericin B MICs of >or=0.25 microg/ml correlate with in vitro resistance, these results suggest that liposomal nystatin might have activity against some amphotericin B-resistant isolates. In vivo testing in animal models is required for clarification of this issue.     

Rapid detection of clindamycin resistance in Bacteroides spp.

High-level resistance to clindamycin can be accurately detected by the Wadsworth disk identification test. Of the 98 isolates of the Bacteroides fragilis group that were tested, 90 were inhibited by the 60-micrograms erythromycin disk and had clindamycin minimal inhibitory concentrations of less than or equal to 3.2 micrograms/ml. Of the remaining eight isolates, all were resistant to the erythromycin disk and had clindamycin minimal inhibitory concentrations of greater than or equal to 100 micrograms/ml.     

Sensitivity of Pseudomonas aeruginosa to beta-lactam antibiotics and effects of beta-lactamases

The activities of carbenicillin, ticarcillin, piperacillin, cefsulodin, ceftriaxone, azlocillin, ceftazidime and apalcillin were evaluated against 128 hospital isolates of Pseudomonas aeruginosa. Against the 85 isolates that were susceptible to carbenicillin, apalcillin had the best minimal inhibitory concentrations, the median MIC being 1, 1 microgram/ml. Against the 43 isolates resistant to carbenicillin, ceftazidime inhibited 90% of organisms at a concentration of 16 micrograms/ml. A constitutive beta-lactamase, was found in 27 of the 43 resistant isolates. All the 128 isolates, but one, were producers of an inducers of an inducible beta-lactamase. Ceftriaxone, piperacillin and azlocillin were highly antagonized by cefoxitin. The extent of antagonism was lower with ticarcillin and carbenicillin.