4 CFU inoculum, the sensitivities were 84 and 99% at 24 and 48 h, respectively. The 10 5 CFU inoculum increased the sensitivities at 24 and 48 h to 91 and 100%, respectively. The specificity was 100% for the 10 4 CFU inoculum at 24 h and 97% for the other combinations. The sensitivity and specificity for the Vitek 2 card were 93 and 100%, respectively. The performance of both the agar dilution method and the Vitek 2 card was good, but these methods were not as sensitive as the D-zone test at 24 h.

Disk diffusion was a reliable, easy, and inexpensive method for testing the susceptibility of Campylobacter jejuni to erythromycin (215 susceptible and 45 resistant isolates) and to ciprofloxacin (154 susceptible, two intermediate, and 124 resistant isolates) using, respectively, an erythromycin disk and ciprofloxacin and nalidixic acid disks.Campylobacter jejuni subsp. jejuni (C. jejuni) is a major human pathogen responsible of 85 to 95% of enterocolitis caused by Campylobacter spp. These infections need to be treated with an antimicrobial agent in less than 50% of cases (1). Macrolides and fluoroquinolones are the first-and second-choice antimicrobial agents for that purpose (1,4,8,9,11,13). With the development of resistance of C. jejuni strains to both erythromycin and ciprofloxacin, routine susceptibility testing has become a very important tool for appropriate antimicrobial treatment when needed (4, 6-9, 11, 13). The main erythromycin resistance mechanism of C. jejuni strains (mutation of 23S ribosomal rRNA and proteins) confers a high-level resistance with MICs of Ն128 g/ml (9). The main quinolone resistance mechanism (mutation[s] of DNA gyrase A) can confer a low, intermediate, or high resistance level to ciprofloxacin and confers a high resistance level to nalidixic acid (4,14).Antimicrobial susceptibility testing of Campylobacter spp. with an agar dilution method has been standardized by the CLSI (2), and the erythromycin, ciprofloxacin, tetracycline, and doxycycline MIC interpretive criteria for C. jejuni and Campylobacter coli have been reported previously (3). The agar dilution method, however, is not convenient for testing a few isolates at a time, and the disk diffusion method has not been standardized yet.The aim of this study was to compare the results obtained by the disk diffusion method with those obtained with the reference agar dilution method to test the susceptibility of C. jejuni isolates to erythromycin and ciprofloxacin.

Direct plating of simulated stool specimens on MacConkey agar (MCA) with 10-g ertapenem, meropenem, and imipenem disks allowed the establishment of optimal zone diameters for the screening of carbapenem-resistant Gram-negative rods (CRGNR) of <24 mm (ertapenem), <34 mm (meropenem), and <32 mm (imipenem). Screening of stool specimens is recommended by the Centers for Disease Control and Prevention as well as the Institut national de santé publique du Québec to identify carriers of carbapenem-resistant Gram-negative rods (CRGNR) and initiate appropriate infection control measures (1, 2). Lolans and colleagues reported that an ertapenem zone diameter of Յ27 mm on MacConkey agar (MCA) was highly sensitive for the detection of Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae in rectal swab specimens (3). However, the zone diameter interpretive criteria for imipenem and meropenem placed directly on MCA have not yet been established. This study compares the performances of the screening method using MCA and 10-g carbapenem disks (ertapenem, meropenem, and imipenem) and defines the optimal inhibition zone diameters for detecting CRGNR using simulated stool specimens.Thirty-nine clinical isolates have been well characterized, phenotypically and genotypically, as described in Table 1. Twenty carbapenemase-producing isolates (17 Enterobacteriaceae and 3 nonfermenters) were selected based on the presence of genes coding for different carbapenemases. Nineteen non-carbapenemase-producing Enterobacteriaceae (18 extended-spectrum ␤-lactamase [ESBL]-or plasmid-mediated AmpC [pAmpC] lactamase-producing isolates) and a susceptible wild-type Escherichia coli strain were also selected as negative controls. The MICs of ceftazidime, cefotaxime, ertapenem, meropenem, and imipenem were determined by the microdilution method according to the Clinical and Laboratory Standards Institute (4).A stool specimen obtained from a normal volunteer was used to prepare all the simulated clinical specimens. To ensure that the specimen did not harbor any ␤-lactam-resistant bacteria, screening tests were performed using ChromID ESBL, CHROMagar KPC, and MCA with ertapenem, meropenem, and imipenem disks. Each plate was inoculated with 100 l of liquefied stool and incubated 24 h aerobically at 35°C. There was no growth on the two selective chromogenic agar plates. For the MCA, inhibition diameters around the antibiotic disks were 29 mm, 39 mm, and 35 mm for ertapenem, meropenem, and imipenem, respectively.Dilutions of the 39 isolates were mixed with aliquots of stool. The simulated fecal material was inoculated onto the screening MCA medium to obtain final challenge concentrations of 10 4 to 10 1 CFU/ml for each strain. The fecal inoculum was spread on MCA by rotation using a rake spreader, and disks of the carbapenems were individually placed onto MCA. After incubation, the diameters of the inhibition zones around each carbapenem disk were measured.The results of all inhibition diameters obtained with the 4 dilutions tested for each...

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