EUCAST expert rules in antimicrobial susceptibility testing

EUCAST expert rules have been developed to assist clinical microbiologists and describe actions to be taken in response to specific antimicrobial susceptibility test results. They include recommendations on reporting, such as inferring susceptibility to other agents from results with one, suppression of results that may be inappropriate, and editing of results from susceptible to intermediate or resistant or from intermediate to resistant on the basis of an inferred resistance mechanism. They are based on current clinical and/or microbiological evidence. EUCAST expert rules also include intrinsic resistance phenotypes and exceptional resistance phenotypes, which have not yet been reported or are very rare. The applicability of EUCAST expert rules depends on the MIC breakpoints used to define the rules. Setting appropriate clinical breakpoints, based on treating patients and not on the detection of resistance mechanisms, may lead to modification of some expert rules in the future.

Phenotypic detection of beta-lactamase-mediated resistance to oxyimino-cephalosporins in Enterobacteriaceae: evaluation of the Mastascan Elite Expert System

OBJECTIVES

The aim of this study was to evaluate phenotypic detection of beta-lactamase-mediated resistance to oxyimino-cephalosporins in Enterobacteriaceae using the Mastascan Elite Expert System challenged with a battery of genotypically characterized organisms.

METHODS

Isolates (n = 120) were identified to species level and antimicrobial susceptibilities were determined using agar incorporation methods and Mastascan Elite. Phenotypes were examined using an Expert System (ES) and putative genotypes were suggested using interpretative reading.

RESULTS

Identification was correct in 119 of 120 isolates. The ES was able to identify the correct beta-lactam phenotype (as deduced from molecular methods) in a single choice in 98 of 120 (81.7%) isolates. In an additional 15 (12.5%) cases, the ES identified the correct beta-lactam phenotype within two or more choices. The detected phenotype was incorrect in seven (5.8%) isolates, but three of these were not inherent to the ES.

CONCLUSIONS

The Mastascan Elite ES is relatively inexpensive and flexible and can identify the mechanism of resistance to oxyimino-cephalosporins in the majority of Enterobacteriaceae without recourse to molecular methods.

Evaluation of the mastascanelite image analysis system for measuring zones of inhibition in disc diffusion susceptibility tests

In this evaluation a mastascanelite image analysis system is compared with manual measurement of disc diffusion inhibition zones. Data for 213 diverse organisms and a total of 1679 organism/antibiotic combinations gave an overall correlation coefficient of 0.988. The mean difference between readings was +0.425 mm, with 95% confidence limits of +/-2.94 mm, and the majority of scanned zones (97.51%) fell within +/-3 mm of the manual measurement. The mastascanelite system forms part of a laboratory suite and was found to be objective, accurate and rapid, reading and interpreting each plate in less than a second. Interfacing to the laboratory computer system facilitated data handling and performance control.

Screening for urinary tract infection with the Mastascan Elite

Mastascan Elite urine screen (MUS) is compared to a conventional method to determine significant growth of urinary pathogens and determine identification and susceptibility. MUS can detect at least 10(3) colony-forming units (cfu)/mL of commonly isolated pathogens and in this study identified 161 true positive bacterial growths (15.2%), 840 true negatives (79.2%), 54 false positives (5.1%) and six false negatives (0.6%). Overall performance in determining susceptibility to nine antibiotics directly from urine was 10.5% true resistance, 82.4% true susceptibility, 5.3% false resistance and 1.8% false susceptibility. Using limited tests, the system correctly identified 80.7% of isolates, 11.8% could not be identified at all and 7.5% were incorrectly identified. In this study, MUS provided 94.7% of results in 24 h, compared to 78.4% produced by the conventional method. Thus, MUS proved an effective and efficient method for processing urines, saving significant time, materials and human resources.

Interpretative reading: recognizing the unusual and inferring resistance mechanisms from resistance phenotypes

If isolates are speciated and if a sufficient range of antibiotics is tested, underlying resistance mechanisms can often be inferred from the antibiogram data. This allows: (i) anomalous combinations of phenotype and organism to be reconsidered; (ii) prediction of further antibiotics that deserve testing; and (iii) the suppression of susceptibilities that are anomalous in the light of the inferred mechanism. This 'interpretative reading' is widely undertaken in France but is largely precluded in the UK by limited speciation and the testing of narrow ranges of antibiotics. Nevertheless, UK laboratories should be aware of: (i) grossly anomalous combinations of species and phenotype, demanding reference laboratory confirmation; (ii) useful indicator drugs, where resistance implies a mechanism conferring other resistances that may be less obvious in direct tests; and (iii) antibiotics that are prone to select resistant mutants of particular species during therapy. Details of these combinations of organism and resistance are presented. Relationships between antibiogram and mechanism are also presented to allow full interpretative reading for those testing wide panels of drugs versus speciated isolates.

A 10 year survey of the antimicrobial susceptibility of urinary tract isolates in the UK: the Microbe Base project

Microbe Base is a national computerized database comprising in excess of 1.7 million patient records down-loaded from the laboratory computer systems of 61 participating UK laboratories over 10 years. This paper highlights the antimicrobial susceptibilities of organisms isolated from the urinary tract which comprise around 50% of all isolates in the database. These data may be used to determine trends in antimicrobial susceptibilities; to formulate local antibiotic policies; to compare local with national data and, overall, to assist clinicians in the rational choice of antibiotic therapy and to prevent misuse, or overuse, of antibiotics.

Applications of pyrolysis mass spectrometry in studies on the mode of action of antimicrobial agents

Changes in overall cell composition during exposure to antimicrobial agents were investigated by pyrolysis mass spectrometry (Py-MS) in parental (O+K+), K antigen deficient (O+K-), and O and K antigen deficient (O-K-) strains of Klebsiella aerogenes NCTC 5055. Changes followed distinct patterns that correlated with mode of action: penicillin binding protein (PBP) 3 blockade (ceftazidime, piperacillin); PBP 2 blockade (imipenem); membrane disruption (colistin); bacteriostatic (chloramphenicol), and bactericidal (gentamicin) protein synthesis inhibition; and DNA synthesis inhibition (ciprofloxacin). Changes were pronounced after exposure for 1 hour at 10 x MIC. In general, the O-K- strain showed the largest compositional shifts, and the parental O+K+ strain the least shifts, correlating with bactericidal kinetic studies. Py-MS may be useful in studies of the mode of action of antimicrobial agents, particularly in early screening of new compounds, and in rapid detection of the effects of antimicrobial agents on micro-organisms.

A pyrolysis mass spectrometry study of temperature-dependent compositional shifts in Xanthomonas (Pseudomonas) maltophilia

Xanthomonas (Pseudomonas) maltophilia strains are frequently susceptible to aminoglycoside antibiotic and polymixin B when incubated at 37 degrees C, but resistant at 30 degrees C. Five strains showing temperature dependent resistance, and five that did not were examined by pyrolysis-mass spectrometry, a characterisation method that gives fingerprint data reflecting cell composition. Cultures grown at 30 degrees C and 37 degrees C in the absence of antibiotic were analysed. Strains showing temperature-dependent resistance exhibited a characteristic compositional difference between cells grown at the two temperatures, whereas strains not showing this type of resistance varied widely in the extent and nature of temperature-dependent differences in composition. The precise nature of the chemical differences cannot be elucidated at present, but could be determined by examining purified cell constituents.

True identity of control Staphylococcus aureus strains and their performance in the tube coagulase test

One hundred laboratories were asked to submit their control Staphylococcus aureus strains to determine the true identity of strains presumed to be S. aureus NCTC 6571, and also to evaluate the performance of those strains being used as controls in the tube coagulase test (TCT). Of the 60 who replied, 55 laboratories sent at least one strain labelled as S. aureus NCTC 6571 (total of 64 strains). Of these, 84% were identified as S. aureus, and were indistinguishable from a fresh type strain by a combination of phenotypic methods including biotyping, antibiotic susceptibility testing and phage typing. Six-to-ten strains (9-16%), depending on the degree of stringency, were not identifiable as S. aureus NCTC 6571. The time since last retrieval from storage ranged from daily to > or = 3 years, but there was no correlation between this duration and the likelihood of differing from S. aureus NCTC 6571. Forty-seven laboratories submitted 51 strains used as controls in the TCT; these included 31 strains labelled as S. aureus NCTC 6571, eight wild strains, three other NCTC strains and nine strains of uncertain origin. Generally, the S. aureus NCTC 6571 strains produced weaker clots than the remainder. None of the S. aureus NCTC 6571 strains was found to be inoculum dependent but four of the other control strains were. The study demonstrates that some laboratories must improve procedures for ensuring that control S. aureus strains retain their true identity, particularly by avoiding repeated subcultures. Laboratories are divided in their use of strong or weak (S. aureus NCTC 6571) positive controls for the TCT. S. aureus NCTC 6571 is a more stringent control for the TCT than other control strains presently being used and is, therefore, to be preferred.

Quality assessment of Microbe Base antimicrobial susceptibility data

AIMS

To assess the quality of centres contributing antimicrobial susceptibility data to a centralised database.

METHODS

Twelve organisms were distributed to 31 regional microbiology laboratories contributing data to a centralised susceptibility database. Participants were asked to determine susceptibilities to certain antibiotics by their routine method and return the data to the Department of Microbiology, Royal Hallamshire Hospital, Sheffield, for analysis.

RESULTS

Results for the overwhelming majority of organism/antibiotic combinations were in agreement with expected results. Reasons for discrepancies included the non-bimodal distribution of susceptibilities, the use of different content discs, and, more importantly, minimum inhibitory concentrations falling close to breakpoint values.

CONCLUSIONS

It is inevitable that any large multicentre database will contain a degree of inaccurate data. This study has highlighted several areas where discrepant results have occurred and has enabled Glaxo Laboratories to approach individual laboratories to address this problem. This study emphasises the value and consistency of Microbe Base as the largest database, of its kind, nationally.

Binding of teicoplanin and vancomycin to polymer surfaces

Both teicoplanin and vancomycin were found to bind to a range of polymer surfaces. The binding of teicoplanin to specimen vessel surfaces was, on average, four times greater than that of vancomycin and was particularly marked with silconized polymers (5.2 micrograms/cm2). Pre-exposure of a polymer surface to human body fluids caused a 60% reduction in teicoplanin binding. Reduction of the negative surface charge on a polymer surface with ferric nitrate resulted in a ten-fold increase in teicoplanin binding. The accumulation of a strain of Staphylococcus epidermidis on silicone rubber catheter segments pre-exposed to glycopeptide antibiotics was examined. In phosphate buffered saline binding of bacteria to vancomycin-treated polymer was greater than to an unexposed control surface. In contrast, in human serum both antibiotics caused reductions in adherent growth. The binding of glycopeptide antibiotics, in particular teicoplanin, to polymer surfaces may interfere with the results of in-vitro assays. However, this phenomenon may be useful in the prevention of bacterial accumulation on the surfaces of medical devices.

Multipoint identification of Enterobacteriaceae: report of the British Society for Microbial Technology collaborative study

AIMS

To evaluate the accuracy and reproducibility of multipoint identification schemes in a multicentre trial.

METHODS

Forty two strains of Enterobacteriaceae were distributed to 22 laboratories for identification by routine multipoint methods. Analysis of results enabled inter- and intralaboratory reproducibility of a variety of tests, and the ability of laboratories to identify individual organisms to be determined.

RESULTS

Interlaboratory reproducibility of most of the biochemical tests was acceptable. The least reproducible tests, both within and between laboratories, were citrate utilisation, production of urease and beta galactosidase, detection of motility, and decarboxylation of lysine and ornithine. Inconsistent results for these tests were often associated with misidentified strains. Most laboratories performed identifications satisfactorily. Most isolates (72.1%) were identified correctly to species level; 9.6% were incorrectly identified, and 6.4% could not be identified at all. The most difficult organisms to identify were Citrobacter freundii, Enterobacter cloacae, Hafnia alvei and Aeromonas hydrophila. Strains of Enterobacter, Serratia sp, and Providencia sp were difficult to speciate. Several laboratories could not identify organisms exhibiting at least one atypical biochemical reaction.

CONCLUSION

This study emphasises the need for quality control of media and reagents for multipoint identification of Gram negative enteric bacilli.

Susceptibility of alpha-haemolytic streptococci causing endocarditis to benzylpenicillin and ten cephalosporins

A clinical case of streptococcal endocarditis in which the isolate proved susceptible to third- but not first-generation cephalosporins prompted us to examine the susceptibility of 44 alpha-haemolytic streptococci from cases of endocarditis to ten cephalosporins and benzylpenicillin. Twenty per cent of strains were resistant to penicillin, and 20% were tolerant. Cefazolin, cefuroxime and cefpirome were the most active first-, second- and third-generation cephalosporins tested. Other first-generation cephalosporins tested compared poorly to cefazolin. Cefotaxime and cefpirome were moderately active against some penicillin-resistant strains. Penicillin tolerance was common in Streptococcus gordonii, but a correlation between tolerance and dextran production could not be confirmed.

A numerical taxonomic study of the "Streptococcus milleri" group based upon conventional phenotypic tests and pyrolysis mass spectrometry

Clinical strains presumptively identified as Streptococcus milleri (60), and blind coded collection strains (21) were characterised in conventional tests and pyrolysis mass spectrometry. Comparison of the clusters found by these two approaches revealed five clearly distinct centres of variation. Three corresponded to the DNA homology groups suggested by Whiley and Hardie (1989) as representing the species S. anginosus, S. intermedius and S. constellatus; a fourth comprised three Lancefield group C beta-haemolytic strains; the fifth may represent a biotype of S. anginosus. The characteristics of the latter group are described.

Streptococcus intermedius, Streptococcus constellatus, and Streptococcus anginosus (the Streptococcus milleri group): association with different body sites and clinical infections

The associations of Streptococcus intermedius, S. constellatus, and S. anginosus (the three species of the S. milleri group) with clinical infections and sites of isolation were investigated by using a simple biochemical scheme to identify a collection of 153 clinical isolates. S. intermedius was associated with abscesses of the brain and liver, while both S. anginosus and S. constellatus were isolated from a wider range of sites and infections. S. anginosus strains predominated in both genitourinary and gastrointestinal sources and exhibited a wider range of phenotypes, particularly in the ability to ferment mannitol and/or raffinose.

Tetracycline resistance in Moraxella (Branhamella) catarrhalis: demonstration of two clonal outbreaks by using pulsed-field gel electrophoresis

Two tetracycline-resistant (Tcr) Moraxella (Branhamella) catarrhalis strains from England were compared with two previously characterized Tcr Texas strains. Both pairs carried the Tet B determinant, which was nontransferable. Pulsed-field gel electrophoresis of their genomic DNA restriction fragments demonstrated that the strains from the same area were identical (clonal); however, the Texas and English strains differed from each other.

Multipoint microbiological assay for detecting beta-lactamase

A multipoint microbiological assay for determining beta-lactamase production by clinical isolates of bacteria was evaluated. With strains of Haemophilus influenzae, Neisseria gonorrhoeae, and Branhamella catarrhalis there was excellent correlation between results obtained using this method and those obtained using the chromogenic cephalosporin reference method. The multipoint method is an inexpensive yet reliable adjunct to conventional susceptibility testing methods.

Synergy between penicillin and gentamicin against enterococci

The role of active uptake in aminoglycoside activity against penicillin-treated enterococci was studied by viable counts and ATP determinations. Penicillin and gentamicin gave synergistic bactericidal and post-antibiotic effects (PAEs) which were partially reduced by sodium azide, an electron transport inhibitor, and totally blocked in the presence of both sodium azide and EDTA, which chelates divalent cations. EDTA and gentamicin showed marked synergy in both 'killing curve' and PAE experiments. This synergy was completely inhibited by sodium azide. The data indicate that the activity of gentamicin against enterococci that have been damaged by penicillin or EDTA is energy-dependent. This is consistent with present theories of gentamicin uptake via transportation drive by a protonmotive force.

Penicillin-aminoglycoside synergy and post-antibiotic effect for enterococci

The post-antibiotic effect (PAE) on seven strains of enterococci was studied using a novel ATP method and conventional viable counting. There was a good correlation between the results of the two techniques. In strains exhibiting low-level resistance to penicillin and streptomycin, PAEs of 1 to 2.4 h (mean 1.8 h) resulted from exposure to penicillin; shorter PAEs were induced by streptomycin (0.2-0.3 h; mean 0.2 h). Addition of streptomycin to pencillin increased the duration of penicillin-induced PAEs by two-fold to three-fold. Enterococci exhibiting high-level streptomycin resistance but low-level penicillin resistance did not exhibit a PAE with streptomycin but exhibited a short PAE with gentamicin (0.3-0.6 h). For these strains, the addition of gentamicin, but not streptomycin, to penicillin increased the penicillin-induced PAEs. Penicillin alone or in combination with streptomycin or gentamicin did not induce PAEs for a single strain of Enterococcus faecium which exhibited high-level resistance to both penicillin and streptomycin. During penicillin-induced PAEs, extracellular ATP was detectable and only during this time period were enterococci susceptible to the action of gentamicin. The addition of aminoglycosides to penicillin not only extended the PAE for enterococci but also the periods when organisms leaked ATP and were susceptible to growth inhibition by gentamicin.

Antibiotic dependence in a strain of Neisseria pharyngis

A strain of Neisseria pharyngis was isolated in pure culture from a specimen of sputum. Growth was markedly enhanced by sub-lethal concentrations of rifampicin, spectinomycin, erythromycin and clindamycin. The effect was temporary with tetracycline. The possible mechanisms of this phenomenon are discussed.

Effect of temperature on antimicrobial susceptibilities of Pseudomonas maltophilia

After a case of peritonitis caused by Pseudomonas maltophilia had occurred 20 strains of the organism were investigated and the minimum inhibitory concentrations of a variety of antibiotics determined at 30 degrees C and 37 degrees C. There was a significant difference in susceptibility between 30 degrees C (most resistant) and 37 degrees C (most susceptible) for aminoglycosides and polymyxin B. No difference was seen with the other agents or in strains of Ps aeruginosa and Enterobacteriaceae tested under similar conditions. The possible mechanisms of this phenomenon are discussed below.