Susceptibility testing with the sensititer breakpoint broth microdilution system

A survey of methods used for antimicrobial susceptibility testing in veterinary diagnostic laboratories in the United States

Antimicrobial resistance is a serious threat to animal and human health worldwide, requiring a collaborative, holistic approach. The U.S. Government has developed a national strategy to address antimicrobial resistance, with one component being to monitor antimicrobial resistance in agricultural settings. We developed a survey to collect information about antimicrobial susceptibility testing (AST) from the veterinary diagnostic laboratory community in the United States, assessing current practices and technologies and determining how AST information is shared. Of the 132 surveys administered, 52 (39%) were returned. Overall, responding laboratories conducted susceptibility tests on 98,788 bacterial isolates in 2014, with Escherichia coli being the most common pathogen tested across all animal species. The 2 most common AST methods employed were the disk diffusion method (71%) and the Sensititre platform broth microdilution system (59%). Laboratories primarily used the Clinical Laboratory Standards Institute (CLSI) VET-01 standard (69%) and the automatically calculated interpretations provided by the commercial AST systems (61%) for interpreting their AST data. Only 22% of laboratories published AST data on a periodic basis, usually via annual reports published on the laboratory's website or through peer-reviewed journals for specific pathogens. Our results confirm that disk diffusion and broth microdilution remain the standard AST methods employed by U.S. veterinary diagnostic laboratories, and that CLSI standards are commonly used for interpreting AST results. This information will help determine the most efficient standardized methodology for future surveillance. Furthermore, the current infrastructure within laboratories, once harmonized, will help provide a mechanism for conducting national surveillance programs.

Diagnostic accuracy assessment of Sensititre and agar disk diffusion for determining antimicrobial resistance profiles of bovine clinical mastitis pathogens

Determining the accuracy and precision of a measuring instrument is pertinent in antimicrobial susceptibility testing. This study was conducted to predict the diagnostic accuracy of the Sensititre MIC mastitis panel (Sensititre) and agar disk diffusion (ADD) method with reference to the manual broth microdilution test method for antimicrobial resistance profiling of Escherichia coli (n = 156), Staphylococcus aureus (n = 154), streptococcal (n = 116), and enterococcal (n = 31) bovine clinical mastitis isolates. The activities of ampicillin, ceftiofur, cephalothin, erythromycin, oxacillin, penicillin, the penicillin-novobiocin combination, pirlimycin, and tetracycline were tested against the isolates. Diagnostic accuracy was determined by estimating the area under the receiver operating characteristic curve; intertest essential and categorical agreements were determined as well. Sensititre and the ADD method demonstrated moderate to highly accurate (71 to 99%) and moderate to perfect (71 to 100%) predictive accuracies for 74 and 76% of the isolate-antimicrobial MIC combinations, respectively. However, the diagnostic accuracy was low for S. aureus-ceftiofur/oxacillin combinations and other streptococcus-ampicillin combinations by either testing method. Essential agreement between Sensititre automatic MIC readings and MIC readings obtained by the broth microdilution test method was 87%. Essential agreement between Sensititre automatic and manual MIC reading methods was 97%. Furthermore, the ADD test method and Sensititre MIC method exhibited 92 and 91% categorical agreement (sensitive, intermediate, resistant) of results, respectively, compared with the reference method. However, both methods demonstrated lower agreement for E. coli-ampicillin/cephalothin combinations than for Gram-positive isolates. In conclusion, the Sensititre and ADD methods had moderate to high diagnostic accuracy and very good essential and categorical agreement for most udder pathogen-antimicrobial combinations and can be readily employed in veterinary diagnostic laboratories.

Current antimicrobial susceptibility of cutaneous bacteria to first line antibiotics

Antimicrobial susceptibility of common bacterial species occurring on human skin appears to be falling. Data for the antimicrobial susceptibility of major groups of bacteria isolated from human skin during routine cultures were complied and analysed over a period of 9 months. Routine diagnostics of specimens from skin lesions and normal human skin were analysed for the presence of specified groups of bacteria. The species were identified using standard methods. Anti-microbial susceptibility was determined using a broth microdilution system giving breakpoints, the Sensititre system. Of the 333 Staphylococcus aureus, 129 Streptococcaceae, 180 Enterobacteriaceae and 120 Pseudomonadaceae strains investigated more than 5% of Staphylococcus aureus strains were resistant to flucloxacillin and thus methicillin (MRSA). More than 25% of Staphylococcus aureus strains were resistant to tetracycline and erythromycin. Many MRSA strains were found multi-resistant. Gentamicin was active against a large majority of Enterobacteriaceae strains but many Pseudomonadaceae strains were resistant. Compared with previous corresponding surveys methicillin-resistant Staphylococcus aureus strains are clearly on the increase. To prevent a further increase of resistant strains a defined strategy for antibiotic use is needed in dermatology.

Comparison of a veterinary breakpoint minimal inhibitory concentration system and a standardized disk agar diffusion procedure for antimicrobic susceptibility testing

The correlation between the antimicrobic standard disk agar diffusion procedure and a veterinary antimicrobic breakpoint minimal inhibitory concentration system was evaluated. Bacterial isolates representing 5 different genera were tested against 15 antimicrobics. There were 3,795 tests performed; 77.3% of the test results were in agreement, 22.6% were in disagreement. Thirty-one percent of the conflicting results were due to the organism being intermediate on the standard disk agar diffusion procedure. Results suggest that the breakpoint system needs more than 1 antimicrobic dilution per antimicrobic, or a change in some of the single dilutions in the breakpoint system would eliminate some of these discrepancies. Another possibility would be to utilize more than 1 panel design, i.e., 1 for gram-positive organisms, 1 for gram-negative organisms, and 1 for fastidious organisms.

Rapid and overnight microdilution antibiotic susceptibility testing with the Sensititre Breakpoint Autoreader system

The Sensititre Breakpoint Autoreader system (SBAS) is a broth microdilution method with one to three concentrations of each antibiotic and innovative fluorescence technology to define inhibitory endpoints. We tested 248 gram-negative bacilli and 80 gram-positive cocci using both the rapid (5 h) and overnight (18 h) SBAS procedures. Inhibitory endpoints were also determined by visual inspection of the microdilution trays after 18 h of incubation. SBAS results were compared with those obtained by a standardized disk diffusion (SDD) procedure. Agreement between the rapid SBAS and SDD results for all antibiotic-organism combinations was found in 3,730 of 4,571 (81.6%) tests, with 3.9% very major, 6.5% major, and 7.9% minor discrepancies noted. Data analysis by organism group revealed 86.8, 57.3, 71.4, and 62.3% agreement for members of the family Enterobacteriaceae, Pseudomonas spp., staphylococci, and enterococci, respectively. The results of the overnight SBAS and SDD agreed in 4,154 of 4,654 (89.2%) tests, with 2.3% very major, 1.3% major, and 7.1% minor discrepancies recorded. Concordance was noted in 90.4, 78.1, 90.6, and 83.3% of the comparisons for the members of the Enterobacteriaceae, Pseudomonas spp., staphylococci, and enterococci, respectively. The inhibitory endpoints determined with the Autoreader were as reliable as those determined by visual inspection after 18 h of incubation.

Collaborative investigation of the accuracy and reproducibility of Sceptor Breakpoint susceptibility panels

A combination Sceptor Breakpoint/ID panel (Johnston Laboratories, Inc., Towson, Md.), which determines interpretive susceptibility results (susceptible, moderately susceptible, and resistant) using two to three selected concentrations of antimicrobial agents, was tested in comparison with full-range Sceptor microdilution MIC panels. The inter- and intralaboratory interpretive reproducibilities for 24 control strains tested in three laboratories on three consecutive days were 97.0 and 95.7%, respectively. The equivalency of breakpoint results to category results obtained by the microdilution MIC procedure for 10,368 control organism-antimicrobial agent comparisons was 94.1%. The level of interpretive agreement between breakpoint and MIC category results using 101 fresh clinical isolates was 97.0% for 51 gram-negative and 50 gram-positive bacteria. Among the total 4,872 clinical organism-antimicrobial agent comparisons, major and very major discrepancies were seen in 0.2% of gram-negative bacteria and very major discrepancies were seen in 0.9% of gram-positive bacteria. All very major discrepancies with gram-positive organisms were associated with trailing endpoints using trimethoprim or sulfisoxazole and staphylococci. The breakpoint concept of testing selective antimicrobial agent concentrations was highly reproducible and accurate and allows for placement of more antimicrobial agents into a panel than is possible with full-dilution MIC testing.

Sensititre autoreader for same-day breakpoint broth microdilution susceptibility testing of members of the family Enterobacteriaceae

The Sensititre Autoreader system is an instrument-assisted broth microdilution susceptibility test procedure based on the detection of fluorogenic growth substrate metabolism by test bacteria with different concentrations of antimicrobial agents. In the current investigation, this system was assessed as a means for predicting the in vitro activity of 17 antimicrobial agents versus numerous species of the family Enterobacteriaceae and Pseudomonas aeruginosa by using a breakpoint broth microdilution test format. Same-day and overnight determinations of susceptibility were made with the Sensititre Autoreader system, and in both cases, the results were compared with those obtained with a manual overnight breakpoint broth microdilution susceptibility test. Among a total of 6,086 organism-antimicrobial agent comparisons with Enterobacteriaceae, concordance was noted between the results of the same-day Autoreader system and the manual overnight test in 94.4% of cases. The same-day Autoreader results with members of the Enterobacteriaceae other than Proteus spp. were determined after 4 h of incubation; with Proteus spp. the same-day Autoreader results were determined after 5 h of incubation. When the Enterobacteriaceae Autoreader results were determined after 18 h of incubation, concordance was noted in 97.2% of comparisons. Among a total of 1,377 organism-antimicrobial agent comparisons with P. aeruginosa after 18 h of incubation, agreement of results from the manual overnight test and the Autoreader system was achieved in 92.2% of cases.