Validation of Etest for seven antimicrobial agents using regulatory criteria for the assessment of antimicrobial susceptibility devices

Evaluation of direct Etest for antimicrobial susceptibility testing of bacteria isolated from synovial fluid of horses using enrichment bottles

This study evaluated the Etest for direct antimicrobial susceptibility testing (AST) of bacteria from equine synovial specimens, incubated in BACTEC enrichment bottles. Ninety-four culture-positive broths were inoculated onto agar to directly determine the minimum inhibitory concentrations (MICs) of 13 antimicrobials, using the Etest (direct Etest). Results were compared with those obtained with the agar dilution reference method, the standard Etest, and the disc diffusion method, after subculture and standardisation of the inoculum. For categorical comparison of AST results, MICs were translated into susceptibility categories, using clinical breakpoints. The direct Etest predicted categorical susceptibility/resistance of bacteria from equine synovial fluid with acceptable accuracy (overall categorical agreement, 91%) and was more reliable than the disc diffusion test. The direct Etest was less accurate than the standard Etest for generating MICs ± 1 log dilution relative to the reference method (overall essential agreement, 69% vs. 89%). As the Etest generated a high percentage of inaccuracies with trimethoprim and sulfadiazine, these were less suitable antimicrobial agents for inclusion. In conclusion, the direct Etest reliably predicted the susceptibility of isolates from equine synovial fluid for the tested antimicrobials, except for trimethoprim and sulfadiazine. Since it did not require subculture and preparation of a standardised inoculum, direct Etest results were available at least 24 h earlier than with other methods, which could facilitate the diagnosis of synovial infections. However, when accuracy is prioritised over speed for MIC determination, the standard Etest is preferred over the direct Etest.

Partial Characterization of Bacteriocins Produced by Two New Enterococcus faecium Isolated from Human Intestine

This study aimed at characterizing two novel bacteriocin-producing enterococcal strains isolated from human intestine. A total of 200 lactic acid bacteria were isolated from a woman stool sample. Two of them were selected for characterization due to their high antimicrobial activity against five strains of Listeria monocytogenes. The selected bacteria were identified as two different strains of Enterococcus faecium and designated MT 104 and MT 162. The bacteriocins produced by MT 104 and MT 162 were stable at different pH ranging from 2 to 11 and were active after different treatments such as heat, enzymes, detergents, and γ-irradiation. The two isolated strains exhibited some probiotic properties such as survival in simulated gastric fluid and intestinal fluid, lack of expression of bile salt hydrolase or hemolytic activity, adhesion to Caco-2 cells efficiently, and sensitivity to clinical antimicrobial agents. Thus, the two isolated strains of E. faecium could become new probiotic bacteria and their bacteriocins could be used for controlling L. monocytogenes in combination with irradiation for food preservation.

Agar and broth dilution methods to determine the minimal inhibitory concentration (MIC) of antimicrobial substances

The aim of broth and agar dilution methods is to determine the lowest concentration of the assayed antimicrobial agent (minimal inhibitory concentration, MIC) that, under defined test conditions, inhibits the visible growth of the bacterium being investigated. MIC values are used to determine susceptibilities of bacteria to drugs and also to evaluate the activity of new antimicrobial agents. Agar dilution involves the incorporation of different concentrations of the antimicrobial substance into a nutrient agar medium followed by the application of a standardized number of cells to the surface of the agar plate. For broth dilution, often determined in 96-well microtiter plate format, bacteria are inoculated into a liquid growth medium in the presence of different concentrations of an antimicrobial agent. Growth is assessed after incubation for a defined period of time (16-20 h) and the MIC value is read. This protocol applies only to aerobic bacteria and can be completed in 3 d.

Breakpoints for predicting Pseudomonas aeruginosa susceptibility to inhaled tobramycin in cystic fibrosis patients: use of high-range Etest strips

Inhaled administration of tobramycin assures high concentrations in cystic fibrotic lungs, improving the therapeutic ratio over that of parenteral tobramycin levels, particularly against Pseudomonas aeruginosa. Conventional Clinical and Laboratory Standards Institute (CLSI; formerly National Committee for Clinical Laboratory Standards) breakpoints only consider parenteral levels and do not take into account these high antimicrobial concentrations. The Spanish Antibiogram Committee (The MENSURA Group) has tentatively defined specific breakpoint values for inhaled tobramycin when testing P. aeruginosa isolates from cystic fibrosis (CF) patients (susceptible, < or =64 microg/ml; resistant, > or =128 microg/ml). The antimicrobial susceptibilities of 206 prospectively collected CF P. aeruginosa isolates were determined by the reference agar dilution method. For tobramycin, the performance of high range tobramycin Etest strips (AB Biodisk, Solna, Sweden) and conventional tobramycin disks were assessed with the same collection. Applying MENSURA proposed breakpoints, 95.1% of the strains were categorized as susceptible to tobramycin, either using agar dilution or Etest high-range strips (99% categorical agreement between both methods). With CLSI breakpoints, susceptibility rates decreased to 79.1 and 81.1% for agar dilution and Etest strips, respectively (83.5% categorical agreement). Minor, major, and very major errors for Etest strips (CLSI criteria) were 13.6, 1.2, and 14.8%, respectively. Upon applying the new proposed criteria for inhaled tobramycin, only one major and one very major error were observed with Etest strips. Whenever inhaled tobramycin is considered for therapy, we suggest that P. aeruginosa strains from CF patients categorized as intermediate or resistant to tobramycin according to the CLSI criteria should be retested with high-range Etest strips and recategorized using MENSURA interpretive criteria. CLSI breakpoints should still be followed when intravenous tobramycin is used in CF patients, particularly during the course of exacerbations.

Case management of a multidrug-resistant Shigella dysenteriae serotype 1 outbreak in a crisis context in Sierra Leone, 1999-2000

From December 1999 to the end of February 2000, 4218 cases of dysentery were reported in Kenema district, southeastern Sierra Leone, by a Médecins Sans Frontières team operating in this region. Shigella dysenteriae serotype 1 was isolated from the early cases. The overall attack rate was 7.5% but higher among children under 5 years (11.2%) compared to the rest of the population (6.8%) (RR = 1.6; 95% CI 1.5-1.8). The case fatality ratio was 3.1%, and higher for children under 5 years (6.1% vs. 2.1%) (RR = 2.9; 95% CI 2.1-4.1). A case management strategy based on stratification of affected cases was chosen in this resource-poor setting. Patients considered at higher risk of death were treated with a 5 day ciprofloxacin regimen in isolation centres. Five hundred and eighty-three cases were treated with a case fatality ratio of 0.9%. Patients who did not have signs of severity when seen by health workers were given hygiene advice and oral rehydration salts. This strategy was effective in this complex emergency.

Mutations in gyrA, gyrB, parC, and parE in quinolone-resistant strains of Neisseria gonorrhoeae

Mutations in the genes for the subunits GyrA and ParC of the target enzymes DNA gyrase and topoisomerase IV are important mechanisms of resistance in quinolone-resistant bacteria, including Neisseria gonorrhoeae. The target enzymes also consist of the subunits GyrB and ParE, respectively, though their role in quinolone-resistance has not been fully investigated. We sequenced the quinolone-resistance-determining regions (QRDR) of gyrA, gyrB, parC, and parE in 25 ciprofloxacin-resistant strains from Bangladesh (MIC 4-->32 mg/l) and 5 susceptible strains of N. gonorrhoeae. All the resistant strains had three or four mutations. Two of these were at positions 91 and 95 of gyrA. Fourteen strains had an additional mutation in parC at position 91, and 17 strains had an additional mutation in parE in position 439. No alterations were found in gyrB. The five susceptible strains had identical DNA sequences. Data indicate that the mutations detected in the QRDR of gyrA and parC may be important in the development of quinolone resistance. According to transformation experiments we assume that the alteration in parE is not related to a high degree of quinolone resistance. There was no correlation between ciprofloxacin MICs and pattern or number of mutations in the target genes.

Cefdinir: an advanced-generation, broad-spectrum oral cephalosporin

BACKGROUND

Cefdinir is an advanced-generation, broad-spectrum cephalosporin antimicrobial agent that has been approved for the treatment of community-acquired pneumonia, acute bacterial exacerbations of chronic bronchitis, acute maxillary sinusitis, pharyngitis/tonsillitis, acute bacterial otitis media, and uncomplicated skin and skin-structure infections in adult and pediatric patients.

OBJECTIVE

The purpose of this article was to review the in vitro antimicrobial activity, pharmacokinetics, clinical efficacy, safety, and potential role of cefdinir.

METHODS

Studies were identified by a MEDLINE search (January 1983-September 2001) of the English-language medical literature, a review of identified articles and their bibliographies, and a review of data on file with the manufacturer. Clinical efficacy data were selected from all published trials mentioning cefdinir. Information concerning in vitro susceptibility, safety, chemistry, and the pharmacokinetic profile of cefdinir also was reviewed.

RESULTS

Cefdinir has a broad spectrum of activity against many gram-negative and gram-positive aerobic organisms, including Streptococcus pneumoniae, Staphylococcus aureus, Streptococcus pyogenes, Haemophilus influenzae, and Moraxella catarrhalis. Cefdinir is stable to hydrolysis by 13 of the common beta-lactamases. It is rapidly absorbed from the gastrointestinal tract (mean time to peak plasma concentration, 3 hours) and is almost entirely eliminated via renal clearance of unchanged drug. The terminal disposition half-life of cefdinir is approximately 1.5 hours. Efficacy has been demonstrated in 19 clinical trials in adults and children with upper and lower respiratory tract infections (eg, pharyngitis, sinusitis, acute otitis media, acute bronchitis, acute bacterial exacerbation of chronic bronchitis, community-acquired pneumonia), and skin and skin-structure infections. The adverse-event profile is similar to that of comparator agents, although in 4 adult and adolescent studies and 1 adult study, diarrhea occurred significantly more frequently in cefdinir recipients than in recipients of penicillin V, cephalexin, cefaclor, and cefprozil.

CONCLUSIONS

Cefdinir is an alternative to other antimicrobial agents and can be dosed once or twice daily for the treatment of upper and lower respiratory tract infections and skin and skin-structure infections. Similar to other oral expanded-spectrum cephalosporins, cefdinir has activity against common pathogens of the respiratory tract and skin and is stable in the presence of selected beta-lactamases. The clinical choice of an oral expanded-spectrum cephalosporin will be based on patient acceptance, frequency of administration, and cost.

Cefdinir: an expanded-spectrum oral cephalosporin

OBJECTIVE

To review the antimicrobial activity, pharmacokinetics, clinical efficacy, and tolerability of cefdinir, an expanded-spectrum oral cephalosporin.

DATA SOURCES

Literature was identified by a MEDLINE search (January 1983-November 1999) of the medical literature, review of English-language literature and bibliographies of these articles, and product information.

STUDY SELECTION

Clinical efficacy data were selected from all published trials mentioning cefdinir. Additional information concerning in vitro susceptibility, safety, chemistry, and pharmacokinetic profile of cefdinir was also reviewed.

DATA SYNTHESIS

Cefdinir, an oral expanded-spectrum cephalosporin, has a broad spectrum of activity against many gram-negative and -positive aerobic organisms, including Streptococcus pneumoniae, Staphylococcus aureus, Streptococcus pyogenes, Haemophilus influenzae, and Moraxella catarrhalis. Cefdinir is stable to hydrolysis by many common beta-lactamases. Cefdinir is rapidly absorbed from the gastrointestinal tract and is primarily eliminated via renal clearance of unchanged drug. The terminal disposition half-life of cefdinir is approximately 1.5 hours. Efficacy has been demonstrated in a number of clinical trials in adults and children with upper and lower respiratory tract infections (e.g., pharyngitis, sinusitis, acute otitis media, acute bronchitis, acute exacerbation of chronic bronchitis, community-acquired pneumonia) and skin and skin-structure infections. The adverse event profile is similar to that of comparator agents.

CONCLUSIONS

Cefdinir is a second-line alternative to first-line antimicrobial agents, with convenient once- or twice-daily dosing in the treatment of upper and lower respiratory tract infections and skin and skin-structure infections. Similar to other oral expanded-spectrum cephalosporins, cefdinir has activity against common pathogens of the respiratory tract and skin and is stable in the presence of many beta-lactamases. The clinical choice of an oral expanded-spectrum cephalosporin will be based on patient acceptance, frequency of administration, and cost.

Mechanisms of quinolone resistance in clinical strains of Pseudomonas aeruginosa

Principal mechanisms of bacterial resistance to quinolones are modification of target enzymes, DNA gyrase (gyrA) and topoisomerase IV (parC), or reduction of intracellular concentration due to mutations in the regulatory genes for efflux systems, such as mexR and nfxB. We have examined gyrA, parC, mexR, and nfxB genes from 16 quinolone-resistant clinical isolates of Pseudomonas aeruginosa to determine the relation between mutations in DNA replicating enzymes or regulatory genes for efflux systems and to correlate the mutations with minimal inhibitory concentrations (MICs). The quinolone resistance-determining regions (QRDR) of these genes were amplified by PCR and sequenced by capillary electrophoresis. Fourteen of 16 isolates had mutations in gyrA, and 13/14 strains with MIC to norfloxacin > or = 8 mg/L had threonine at position 83 changed to isoleucine. Seven of 8 strains with MIC > or = 32 mg/L had mutations in parC. One of these strains showed a parC mutation at position 74 without any mutation in gyrA. Four strains had mexR and two strains nfxB mutations. The data indicate that gyrA mutation is the most important component of quinolone resistance, and simultaneous presence of parC mutations is associated with high-level resistance. parC mutation alone may contribute to resistance, and gyrA mutation may not be a prerequisite for parC mutation to express resistance. mexR and nfxB mutations were found mostly in strains with high-level resistance.

Medium compositions and culture conditions for the assay of fosfomycin susceptibility by Etest

The Etest (AB Biodisk, Solna, Sweden), a susceptibility testing method, was used for fosfomycin and was evaluated by comparison with the agar dilution method for 73 clinical isolates of Escherichia coli, including two resistant strains, and an optimal Etest method for fosfomycin was determined. Media and culture conditions greatly affected the minimum inhibitory concentrations (MICs) determined by the Etest for fosfomycin, as shown for the agar dilution methods. Our results showed that the most favorable conditions for the Etest for fosfomycin were with Mueller-Hinton agar (Becton-Dickinson Japan, Tokyo, Japan) under aerobic conditions. However, the MICs for the resistant strains were much higher than those determined by agar dilution methods, using Nutrient agar (Becton-Dickinson Japan) under anaerobic conditions. The addition of glucose-6-phosphate did not significantly affect the Etest results.

Evaluation of the in vitro antimicrobial activity of cefepime compared to other broad-spectrum beta-lactams tested against recent clinical isolates from 10 Chinese hospitals. Chinese Antimicrobial Resistance Study Group

A surveillance study was initiated in China in 1998 in which 10 medical centers participated. The susceptibility profiles of 996 commonly occurring pathogens belonging to 10 different species groups were tested by the Etest (AB BIODISK, Solna, Sweden) against six broad-spectrum beta-lactam antimicrobial agents (cefepime, ceftazidime, ceftriaxone, imipenem, cefoperazone/sulbactam and piperacillin or oxacillin). Quality control was closely monitored and cefepime- and/or imipenem-resistant Enterobacteriaceae were referred to the reference laboratory (University of Iowa College of Medicine, Iowa City, IA) for confirmation. The isolates of Citrobacter spp. and Enterobacter spp. were generally inhibited by imipenem (100% susceptible) and cefepime (89-94%), but were more resistant to the other drugs tested (< or = 74% susceptible). The indole-positive Proteus spp. and Serratia spp. isolates were > 94% susceptible to all tested beta-lactams except piperacillin. Organisms capable of producing extended spectrum beta-lactamases (ESBLs), which included Klebsiella spp. and Escherichia coli, were most susceptible to imipenem (100%) and cefepime (> 90%). Among the non-enteric Gram-negative bacilli, all drugs were marginally active against Pseudomonas aeruginosa (MIC90s, 32-> 256 ug/mL) and the Acinetobacter spp. were rather resistant to all the compounds, except imipenem (96% susceptible). All strains of Staphylococcus spp. were susceptible to the tested antimicrobials except for ceftazidime, which had a low potency (MIC90, 12-16 micrograms/mL) against Chinese isolates with MICs that fell into the intermediate category. Cefepime, the fourth-generation cephalosporin, showed a very broad spectrum of activity against Gram-negative pathogens as well as oxacillin-susceptible Staphylococcus spp. that was comparable with imipenem (widest spectrum) and superior to the other tested beta-lactams overall. Continued monitoring of clinical strains in China seems necessary to guide chemotherapy.