An automated antimicrobial susceptibility system versus the Kirby-Bauer disk diffusion test for antimicrobial susceptibility.
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Methods↗
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Antimicrobial susceptibility of flavobacteria.
Antimicrobial susceptibility patterns of 28 clinical isolates of Flavobacterium sp. were determined by standard disk diffusion technique and by antimicrobial dilution in agar. Rifampin, clindamycin, trimethoprim-sulfamethoxazole, cefoxitin, and vancomycin are among the antimicrobial agents which may be clinically useful to treat infections caused by flavobacteria. All 28 isolates were resistant to erythromycin with minimal inhibitory concentrations of 32 mug/ml or more. Currently recommended interpretive zones of inhibition by disk diffusion did not reliably predict antimicrobial susceptibility of the 28 flavobacteria isolates when compared with the agar dilution technique, and, therefore, a more direct measurement of minimal inhibitory or bactericidal concentration is recommended.
Anti-Bacterial Agents↗
Antimicrobial susceptibility testing of Mycobacterium fortuitum complex.
A total of 24 strains of the Mycobacterium fortuitum complex were tested for susceptibility to antimicrobial agents by the disk diffusion and agar dilution techniques. By comparing zones of inhibition obtained with the disk diffusion technique with results of minimal inhibitory concentration determinations, it was shown that disk diffusion results could predict in vitro susceptibility to selected antimicrobial agents. All of 17 strains of M. fortuitum were susceptible to </=1 mug of amikacin per ml. The corresponding average zone of inhibition around a 10-mug amikacin disk was 37 mm. Seven M. chelonei strains were more resistant to amikacin, with minimal inhibitory concentrations ranging from 1 to 32 mug/ml, and the corresponding average zone size was 21 mm. Susceptibility of both M. fortuitum and M. chelonei to tetracycline was variable and none of the M. chelonei strains was inhibited by polymyxin B, whereas M. fortuitum strains consistently had zones of inhibition around the polymyxin disk. It appears that identification to species of the M. fortuitum complex may be of importance with regard to antibiotic susceptibility. Separation of M. fortuitum and M. chelonei was readily accomplished in the present study by the nitrate reduction and 3-day arylsulfatase tests.
Aminoglycosides↗
Yersinia enterocolitica: in vitro antimicrobial susceptibility.
Using 21 antimicrobial agents, agar dilution susceptibility tests were carried out against 190 strains of Yersinia enterocolitica. Isolates from human, environmental, and animal sources had essentially equal susceptibility patterns.
Anti-Bacterial Agents↗
Antimicrobial susceptibility testing of pneumococci: determination of Kirby-Bauer breakpoints for penicillin G, erythromycin, clindamycin, tetracycline, chloramphenicol, and rifampin.
Antimicrobial susceptibility testing of pneumococci is now essential to monitor for the presence of resistance to agents such as the penicillins, macrolides, lincomycins, chloramphenicol, and tetracycline. In this study, clinical isolates of a selection of resistant South African strains were tested for antimicrobial susceptibility by minimal inhibitory concentration (MIC) determination and by a modified Kirby-Bauer disk diffusion technique, using Mueller-Hinton medium supplemented with 5% horse blood. Disk diffusion breakpoints were determined for penicillin G, erythromycin, clindamycin, tetracycline, chloramphenicol, and rifampin. Reliable results were obtained on disk diffusion for all these agents except for penicillin G. With 6-mug penicillin G disks, zones of strains with intermediate penicillin susceptibility overlapped those of sensitive and resistant strains. With 5-mug methicillin disks, clearer separation of strains based on susceptibility to penicillin G occurred. Strains with zones of <35 mm around penicillin G disks and <25 mm around methicillin disks should have penicillin G MICs determined to confirm their resistance to penicillin G. In view of the potential for pneumococci to be resistant to the agents used in this study, antimicrobial susceptibility of all clinically significant isolates should be determined.
Chloramphenicol↗
Antimicrobial susceptibility and selection of resistance among Staphylococcus epidermidis isolates recovered from patients with infections of indwelling foreign devices.
Twenty-seven isolates of Staphylococcus epidermidis from patients with prosthetic valve endocarditis or infected cerebrospinal fluid shunts were examined for susceptibility to antimicrobial agents. Subpopulations resistant to 20 and 100 mug of methicillin per ml were present in 63% of the isolates (methicillin-resistant isolates). Subpopulations resistant to 20 mug of nafcillin and cephalothin per ml were found in every methicillin-resistant isolate but with frequencies (10(-5.0 +/- 0.5) and 10(-6.4 +/- 0.9), respectively) which were not always detectable by susceptibility testing. Resistance to >/=1.6 mug of penicillin per ml was found in 80% of isolates. Cephalothin, cefazolin, and cefamandole were more active than cefoxitin or cephradine, and gentamicin was more active than tobramycin or amikacin; rifampin was the single most active agent against all isolates. There was no difference in susceptibility between prosthetic valve endocarditis and cerebrospinal fluid shunt infection isolates. Among methicillin-resistant isolates, the phenotypic expression of resistance to methicillin or nafcillin but not to cephalothin could be enhanced by 48 h of incubation with each drug. Isolates containing no methicillin-resistant subpopulations were killed by incubation with methicillin, nafcillin, or cephalothin. High-level resistance to rifampin emerged in both methicillin-resistant and methicillin-sensitive isolates after 8 to 24 h of incubation with this drug. The presence or absence of antibiotic-resistant subpopulations among S. epidermidis isolates and their selection during treatment should be considered when therapy is devised.
Anti-Bacterial Agents↗
Antimicrobial susceptibility patterns of Streptococcus pneumoniae.
Fifty clinical isolates of Streptococcus pneumoniae received by the Streptococcus Laboratory of the Center for Disease Control from August 1976 through March 1977 and 50 pneumococcal strains retrieved from 13- to 16-year storage (originally isolated October 1961 through December 1964) were tested for susceptibility to 10 antimicrobial agents by disk-agar diffusion and agar dilution tests. No appreciable differences in susceptibility patterns were apparent between the two groups, and, except for one isolate, all were highly susceptible to every drug tested except gentamicin. This single isolate required higher drug concentrations to inhibit macroscopic growth and had corresponding decrements in zones of disk inhibition with penicillin, ampicillin, and cephalothin. An additional 43 pneumococci recently received from various areas of the United States and Canada were screened by a disk agar diffusion method for penicillin resistance. Four of these isolates had penicillin zone diameters <30 mm, and subsequent agar dilution test results showed that the penicillin minimum inhibitory concentrations were elevated with these organisms. Antimicrobial susceptibility patterns of pneumococci to antimicrobials other than penicillin and its analogs have not changed substantially in over a decade. However, due to the emergence of strains with decreased susceptibility to penicillin, the screening test for penicillin resistance in pneumococci, especially in isolates from spinal fluid and blood, could be clinically useful as an aid in selecting optimal therapy.
Cephalosporinase↗
Moraxella species isolated from blood cultures in Europe (MORAXEu): a multicentre study of epidemiology and antimicrobial susceptibility with complementary phylogenomic analysis of publicly available genomes.
INTRODUCTION: Moraxella species are fastidious Gram-negative bacteria capable of causing opportunistic infections, including bloodstream infections, especially in immunocompromised patients. Data on their epidemiology, antimicrobial susceptibility, and phylogenomics in Europe remains limited. METHODS: We conducted a multicentre, retrospective, observational study across 56 European hospital centres between January 1st 2020 and December 31st 2024. All Moraxella species isolated from blood cultures (BCs) were included. Species distribution and antimicrobial susceptibility profiles were analysed. We also performed a phylogenomic analysis of Moraxella genomes deposited in GenBank. RESULTS: A total of 709 Moraxella isolates were included. Moraxella osloensis (61.1%; n = 433/709) and Moraxella catarrhalis (20.4%; n = 145/709) were the most frequently identified species, followed by Moraxella nonliquefaciens (6.5%; n = 46/709) and Moraxella atlantae (4.8%; n = 34/709). Species distribution differed by age. M. catarrhalis was predominant in paediatric patients, whereas M. atlantae was more common in adults. Most isolates showed > 90% susceptibility to amoxicillin/clavulanate, cefotaxime, fluoroquinolones, and trimethoprim/sulfamethoxazole. Cefotaxime resistance in M. osloensis was more frequent in adults than in children (49% vs. 8%, p = 0.009). Phylogenomic analysis demonstrated the distinction of a core Moraxella group from the divergent Faucicola lineage, with M. osloensis and M. atlantae clustering within the latter. CONCLUSIONS: The epidemiology of Moraxella species from BCs in Europe showed age-group-specific differences in species distribution and generally favourable antimicrobial susceptibility patterns. Phylogenomic data corroborated recent taxonomic revisions, highlighting the need for improved diagnostics, harmonized nomenclature and sustained surveillance to inform management and stewardship of Moraxella bacteraemia.
Faucicola↗
Detection and antimicrobial susceptibility patterns of Salmonella enterica subsp. arizonae and Proteus spp. associated with gastrointestinal disease in rescued hedgehogs (Erinaceus europaeus).
Western European hedgehogs (Erinaceus europaeus) are frequently admitted to wildlife rehabilitation centres, where infectious diseases may affect recovery and raise One Health concerns. This study aimed to identify bacterial isolates recovered from hedgehog samples submitted for suspected gastrointestinal infection and to characterise their antimicrobial susceptibility profiles. Five bacterial isolates were analysed using the MicroScan WalkAway Plus® system with the Neg-Urine-Combo 98 panel, and the results were interpreted in accordance with EUCAST guidelines. The identified bacteria included one isolate of Salmonella enterica subsp. arizonae, three isolates of Proteus mirabilis and one isolate of Proteus penneri. The Salmonella enterica subsp. arizonae isolate was susceptible to all antimicrobials for which a valid result was obtained. Proteus spp. isolates were susceptible to cefotaxime, nalidixic acid, ciprofloxacin, levofloxacin, norfloxacin, amikacin, gentamicin, tobramycin, aztreonam, cefoxitin, ceftazidime and fosfomycin. However, resistance was observed to amoxicillin-clavulanic acid, ampicillin, ertapenem, meropenem, trimethoprim-sulfamethoxazole, cefuroxime, piperacillin-tazobactam, colistin and nitrofurantoin, with the latter two showing resistance in all Proteus spp. The Proteus penneri isolate displayed the broadest resistance profile, including resistance to several β-lactams, carbapenems. As expected, all Proteus spp. showed intrinsic non-susceptibility to colistin and nitrofurantoin. Although the Salmonella enterica subsp. arizonae isolate was susceptible to the tested agents, Proteus spp. from hedgehog samples may display relevant antimicrobial resistance (AMR) patterns. Therefore, continuous bacteriological monitoring and antimicrobial susceptibility testing are important in wildlife rehabilitation settings to guide treatment decisions and support One Health surveillance.
Antimicrobial resistance↗
Rapid antimicrobial susceptibility test using tetrazolium reduction.
A rapid method of antimicrobial susceptibility testing has been developed, which uses a modified microdilution procedure and an inoculum of 10(7) bacteria per ml. Results are determined within 4 h with an indicator consisting of 2(p-iodophenyl)-3(p-nitrophenyl)-5-phenyltetrazolium chloride. The precipitation of a red formazan by bacteria uninhibited by antimicrobials is accelerated by the addition of phenazine methosulfate. Isolates are classified as resistant, indeterminate, or susceptible, based on growth in up to two antimicrobial concentrations which conform closely to concentrations which correlate with the millimeter breakpoints used in the Bauer-Kirby method. Results of testing 10 antimicrobial agents against 1,126 isolates were compared with results obtained when the Bauer-Kirby method and the agar dilution procedure were used as reference methods. Enterococci were excluded because of false resistance. Discrepancies were classified as very major (false susceptibility), major (false resistance), and minor (combinations of susceptibility or resistance with indeterminate results). The rapid method versus the agar dilution method yielded 2.3% very major, 0.7% major, and 2.9% minor discrepancies, for a total of 6.0%. Of 58 organism-antimicrobial agent combinations tested, 23 displayed 1% very major discrepancies between the rapid method and the agar dilution method. Six were not therapeutically important. The remainder involved Staphylococcus aureus, Staphylococcus epidermidis, Acinetobacter sp., and most organisms tested with chloramphenicol. It is suggested that adjustments in antibiotic concentrations and/or inoculum size may eliminate these discrepancies. The rapid method appeared economical when compared with Autobac 1 and the Bauer-Kirby procedure.
Evaluation Studies as Topic↗
Evaluation of Oxford nanopore sequencing for antimicrobial resistance surveillance in Salmonella: comparison with phenotypic antimicrobial susceptibility in a large-scale study.
UNLABELLED: Salmonella is a major zoonotic foodborne pathogen, and antimicrobial resistance (AMR) in Salmonella presents a significant public health challenge. Compared with conventional antimicrobial susceptibility testing (AST), whole-genome sequencing (WGS) provides a more rapid and comprehensive approach to AMR characterization, thereby informing antimicrobial selection and supporting public health surveillance. In this study, Oxford Nanopore Technology (ONT)-based WGS was performed on 1,490 Salmonella isolates collected through nationwide surveillance in Taiwan in 2025. Genotypic resistance inferred from WGS data was compared with phenotypic AST results to assess the performance of ONT-WGS. Overall, WGS-inferred resistance showed high concordance with phenotypic resistance for most antimicrobials. However, major genotype-phenotype discordance was observed, attributed to four categories: (i) breakpoint-dependent classification, (ii) reduced or absent phenotypic expression of resistance genes, (iii) minimum inhibitory concentration (MIC) modulation by ramAp, and (iv) absence of known AMR determinants. Notable discrepancies included tigecycline resistance without known genetic determinants, nalidixic acid resistance linked to ramAp-mediated MIC elevation, and a high prevalence of colistin resistance (35.7%) in S. Enteritidis, with most resistant isolates lacking identifiable AMR determinants. Additionally, a significant proportion of ESBL- and AmpC-producing isolates were classified as susceptible or intermediate to cefotaxime and ceftazidime under CLSI criteria, highlighting the potential for misclassification and treatment failure. These findings demonstrate that ONT-WGS enables accurate and comprehensive AMR characterization by directly identifying resistance determinants and avoiding potential misclassification associated with breakpoint-based AST interpretations. When interpreted appropriately, WGS can support better antimicrobial selection and serve as a valuable alternative to conventional susceptibility testing. IMPORTANCE: Accurate prediction of antimicrobial resistance is essential for appropriate therapy and effective surveillance of Salmonella. However, discordance between genotype-based predictions and phenotypic antimicrobial susceptibility testing (AST) can complicate clinical interpretation. In this nationwide study of 1,490 Salmonella isolates, we show that Oxford Nanopore Technology-based whole-genome sequencing (ONT-WGS) provides rapid and comprehensive detection of antimicrobial resistance determinants with high concordance to phenotypic AST. We further identify four major mechanisms underlying genotype-phenotype discordance, including breakpoint-dependent classification, reduced or absent phenotypic expression of resistance genes, minimum inhibitory concentration (MIC) modulation by ramAp, and the absence of known AMR determinants. These findings demonstrate how WGS can complement conventional AST, improve interpretation of challenging susceptibility results, and strengthen genomic surveillance of emerging antimicrobial-resistant Salmonella.
Microbial Sensitivity Tests↗
Evaluation of microdilution trays (sensititre) for antimicrobial susceptibility testing of Bacteroides fragilis.
Microdilution trays (Sensititre) for antimicrobial susceptibility testing of Bacteroides fragilis has been compared for accuracy with the standard agar dilution method. The microtrays were found to be reliable for all antimicrobials tested (benzylpenicillin, clindamycin, doxycycline, chloramphenicol, fusidic acid, cefoxitin, and cephalotin) but not for metronidazole.
Anti-Bacterial Agents↗
In vitro antimicrobial susceptibility of Streptococcus pneumoniae in New Zealand.
Ninety-seven isolates of Streptococcus pneumoniae referred to the National Health Institute between January 1976 and March 1978 were tested for their susceptibility to 10 antimicrobials. All were susceptible to penicillin, erythromycin, chloramphenicol, vancomycin, clindamycin, cephalothin and rifampicin. Resistance to tetracycline was found in 9.3 percent of the isolates, and 9.7 percent were resistant to a combination of sulphamethoxazole and trimethoprim.
Anti-Bacterial Agents↗
Antimicrobial susceptibility of rhamnose-positive Yersinia enterocolitica.
Agar dilution antimicrobial susceptibility tests were carried out against rhamnose-positive Y. enterocolitica strains isolated from various origins. All the strains were susceptible to aminoglycosides, chloramphenicol, tetracycline, minocycline, rifampicin, trimethprim and sulfamethoxazole. All but two strains were susceptible to polymyxin. Resistnace to ampicillin, carbenicillin and cephalothin was noted with the majority of the strains.
Anti-Bacterial Agents↗
Genomic population structure, antimicrobial susceptibility, and clinical features of Mycobacterium xenopi isolates, Frankfurt, Germany, 1995-2020.
Mycobacterium xenopi causes non-tuberculous mycobacterial pulmonary disease (NTM-PD) that is difficult to treat. However, data on the genomic population structure, antimicrobial susceptibility, and the clinical significance of this pathogen remain scarce. We analyzed 76 clinical M. xenopi isolates from 70 patients collected between 1995 and 2020 in Frankfurt am Main, Germany. All isolates underwent phenotypic drug susceptibility testing and whole-genome sequencing. Cluster analysis, including isolates from this study and all hitherto available high-quality M. xenopi genome data sets in the Sequence Read Archive (n = 11), was performed by core genome multilocus sequence typing. In our cohort, only 26.5% of patients met criteria for clinically relevant NTM-PD. Phylogenetic analysis identified three large hospital-associated clusters (≤10 allelic difference), each involving between 7 and 20 patients and persisting for over 18 years, suggesting prolonged transmission chains or a common environmental source. We also defined three major clades (≤50 allelic difference), two of which contained isolates from the United Kingdom. Clofazimine and guideline-recommended antimycobacterial agents showed good in vitro efficacy, except rifampicin, with 23.6% resistance. This study represents a major expansion of M. xenopi genomic resources and provides insights into the genomic population structure, phenotypic susceptibility, and clinical characteristics of M. xenopi. Guideline-recommended antimycobacterials show good in vitro activity, while clofazimine may be a valuable addition to M. xenopi therapy. The identified clusters underscore the need for further investigation into transmission dynamics and globally successful clones.IMPORTANCEMycobacterium xenopi is an increasingly recognized opportunistic lung pathogen that is difficult to treat. Infections often occur in patients with pre-existing health conditions and can present substantial diagnostic and therapeutic challenges. A deeper understanding of its genetic diversity and resistance mechanisms is essential for optimal patient management and for clarifying potential transmission routes. By analyzing 76 whole-genome sequences together with detailed clinical information and phenotypic drug-susceptibility data, this study substantially expands the available genomic repertoire for M. xenopi. While clinical relevance was limited in our cohort, most guideline-recommended antimicrobial agents showed good efficacy in vitro. The detection of closely related strains might point toward a common environmental source of infection. These findings highlight the need for continued surveillance and provide a comprehensive foundation that supports more accurate monitoring, improved understanding of disease behavior, and future investigations into M. xenopi pathogenicity.
Humans↗
Comparison of rapid methods of antimicrobial susceptibility in Haemophilus influenzae.
A method is described for the rapid, simultaneous determination of the MIC's of chloramphenicol and ampicillin to Haemophilus influenzae. Excellent agreement was observed between the Autobac method and the agar dilution method for antimicrobial susceptibility. All ampicillin resistant Haemophilus isolates produced beta lactamase and none of the suceptible strains produced this enzyme.
Ampicillin↗
Antimicrobial susceptibility of Escherichia coli isolated at a children's hospital.
97 strains of Escherichia coli (49 from urine and 48 from non-urine sources) were isolated from patients at Children's Memorial Hospital, Oklahoma City, and studied for antimicrobial susceptibility. The proportion of highly resistant strains (growth at a concentration of 50 microgram/ml or more) was 27% with ampicillin, 25% with tetracycline, 23% with cephalothin, 6% with kanamycin and neomycin, 4% with chloramphenicol, and zero with colistin, gentamicin, and tobramycin. 23 strains (24%) were found resistant to two or more antibiotics simultaneously. The incidence of strains resistant to ampicillin, cephalothin, kanamycin, neomycin, and tetracycline was higher among urine isolates than among non-urine isolates.
Anti-Bacterial Agents↗
Antimicrobial susceptibility patterns of commensal fecal bacteria isolated from pigs with an intentional genomic alteration that included the selectable marker gene nptII.
INTRODUCTION: Animals with intentional genomic alterations (IGAs) hold promise for meeting increasing worldwide demand for animal-source proteins. As part of regulatory risk assessment for introducing animals with IGAs into the food chain, monitoring commensal bacterial microbiota is recommended due to concern that antimicrobial resistance genes used during IGA selection could be transferred, via horizontal gene transfer, to gastrointestinal or environmental bacterial populations, potentially contributing to antimicrobial resistance. The objective of this study was to assess the antimicrobial susceptibility patterns in commensal bacteria isolated from fecal samples of GalSafe™ pigs that have an IGA that includes the aminoglycoside resistance gene nptII. METHODS: Antimicrobial resistance rates observed in Escherichia coli, Salmonella, Campylobacter and Enterococcus isolated from GalSafe™ pigs were compared to resistance rates observed in conventional pigs at slaughter. Bacterial isolates were tested for antimicrobial resistance genes by PCR and one isolate underwent whole genome sequencing. RESULTS: In total, 137 bacterial isolates recovered from 55 fecal samples collected from 47 individual adult GalSafe™ pigs were evaluated. Prevalence of antimicrobial resistance in GalSafe™ pigs was generally similar to, or lower than, resistance prevalence reported from conventional pigs at slaughter, based on National Antimicrobial Resistance Monitoring System (NARMS) data. Higher resistance rates in GalSafe™ pigs were observed only for quinolones in Campylobacter coli (ciprofloxacin and nalidixic acid) and nitrofurantoin in Enterococcus spp. One isolate (E. coli) was positive for nptII neomycin resistance gene, the same gene used for IGA selection in GalSafe™ pigs, and the remaining 136 isolates were negative for nptII. However, the positive isolate did not appear to contain nptII derived from the GalSafe™ pig genome as the sequences flanking the gene did not match the IGA. DISCUSSION: We did not detect evidence of nptII gene transformation into bacterial species of potential human health importance in this population of GalSafe™ pigs.
NARMS↗