Quality control and interpretive criteria for the azlocillin disk diffusion susceptibility test

Abstract

The standardized disk diffusion test was performed with 75-,ug azlocillin disks to determine individual test, accuracy, and precision control values with Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Staphylococcus aureus ATCC 25923. In addition, regression lines for correlating inhibitory zone diameters with the 75-,ug azlocillin disk and azlocillin minimal inhibitory concentrations were calculated for gram-negative clinical isolates (including Enterobacteriaceae, P. aeruginosa, other nonfermenters, and Aeromonas hydrophila). Criteria for distinguishing susceptible isolates from resistant isolates, based on an error-rate bound classification scheme, are proposed.Azlocillin is a new ureidopenicilin which is similar to mezlocillin but is more active against Pseudomonas aeruginosa and less active against Enterobacteriaceae (2, 7). In this study, values were established for monitoring quality control when performing the disk diffusion susceptibility test with 75-,Lg azlocillin disks. In addition, regression lines for correlating inhibitory zone diameters with the 75-,ug azlocillin disk and azlocillin minimal inhibitory concentrations (MICs) were calculated for a variety of gramnegative bacilli. Regression line analyses. A total of 300 clinical isolates were studied. They included 100 strains of Enterobacteriaceae (10 strains each of E. coli, Klebsiella pneumoniae, Enterobacter cloacae, Enterobacter aerogenes, Serratia marcescens, Citrobacter diversus, Citrobacterfreundii, Proteus mirabilis, and Morganella morganii and five strains each of Proteus vulgaris and Providencia stuartil), 100 strains of P. MATERIALS AND METHODSaeruginosa, 90 strains of other nonfermenters (10 strains each of Pseudomonas maltophilia, Pseudomonas cepacia, Pseudomonasfluorescens-putida, Acinetobacter calcoaceticus subsp. anitratus, Acinetobacter calcoaceticus subsp. lwoffii, Achromobacter xylosoxidans, Alcaligenes odorans, Bordetella bronchiseptica, and Flavobacterium sp.), and 10 strains of Aeromonas hydrophila. Individual strains were selected to represent a spectrum of susceptibility to azlocillin.Using the same culture for each of the study strains, inhibitory zone diameters for the 75-,ug azlocillin disk were determined as described above, and azlocillin MICs were determined by a microtube broth dilution test (3). For the latter test, laboratory-standard azlocillin was diluted in cation-supplemented Mueller-Hinton broth (Difco) and then dispensed into microdilution plates with a Dynatech MIC-2000 96-channel dispenser (Dynatech Laboratories, Alexandria, Va.) so that final concentrations consisted of log2 dilutions ranging from 512 to 0.25 ,ug/ml. Antibiotic-containing microdilution plates were inoculated with disposable inoculators (Dynatech) so that the final inoculum was approximately 1 x 105 to 5 x 105 colony-forming units per ml.

Importance of methodology in determining bactericidal and bacteriostatic activities of azlocillin and ticarcillin against Pseudomonas aeruginosa

“…Strains that gave zones of inhibition of 13 mm or less were regarded as resistant. This is consistent with the criteria laid down by Brown and Blowers (1978) for gentamicin, Phaneuf and Neu (1979) for ticarcillin and Fass (1982) for azlocillin. Disks were controlled with a standard sensitive strain, P .…”

Quality surveillance of surface water catchments in selected Obokun rural communities, in South-Western Nigeria

“…The stock culture was transferred into nutrient broth tubes and incubates at 37ºC for 18-24 h. To the gram negative plates, Mueller Hilton Agar (MHA) in plates were inoculated with the 18-24 h old culture isolate, after which the gram negative antibiotics multidisc (Abtek Biologicals, UK) labeled Nitrofurantoin (300 µg), Oflaxacin (30 µg), Gentamicin (10 µg), Ceftazidime (30 µg), Cefixime (5 µg), Ciprofloxacin (5 µg), Cefuroxime (30 µg) and Augmentin (30 µg) was aseptically placed at the centre of the plates using sterile forceps and incubated at 37°C for 18-24 h. To the gram positive plates, the MHA media was inoculated with a swab full of the 18-24 h old broth culture, after which gram positive antibiotics multidisc (Abtek Biologicals, UK) labeled Ampicillin (25 µg), Tetracycline (10 µg), Penicillin (10 µg), Streptomycin (10 µg), Erythromycin (5 µg), Chloramphenicol (30 µg), Cloxacillin (5 µg) and Gentamicin (10 µg) was aseptically placed at the centre of the plates with the aid of sterile forceps and incubated at 37°C for 18-24 h. Plates were examined for sensitivity of the organisms to the various antibiotics based on the size of their zones of inhibition. Isolates with zones of inhibition of 13 mm or less around the antibiotic disc were termed resistant while those above 13 mm were regarded as sensitive (Craven et al, 1977;Brown and Blowers, 1978;Phaneuf and Neu, 1979;Fass, 1982;Brumfitt et al, 1984).…”