New method for laboratory detection of AmpC beta-lactamases in Escherichia coli and Klebsiella pneumoniae

Evaluating the incidence of ampC-β-lactamase genes, biofilm formation, and antibiotic resistance among hypervirulent and classical Klebsiella pneumoniae strains

AIM

Both immunocompetent and healthy individuals can become life-threateningly ill when exposed to the hypervirulent (hvKp) strains of Klebsiella pneumoniae (Kp). The main objectives of this study were to evaluate the presence of ampC-lactamase genes, biofilm formation, and antibiotic resistance in clinical strains of hvKp and cKp (classical K. pneumoniae).

MATERIALS AND METHODS

Kp strains were collected from patients referred to Shahidzadeh Hospital in Behbahan City, Khuzestan Province, Iran. Several techniques were used to identify hvKp. The hypermucoviscosity phenotype was determined using the string test. Isolates that developed dark colonies on tellurite agar were assumed to be hvKp strains. If any of the iucA, iutA, or peg-344 genes were detected, the isolates were classified as hvKp. Phenotypic and genotypic detection of AmpC β-lactamases of hvKp strains was performed by the combined disk method and polymerase chain reaction, respectively. In addition, crystal violet staining was used to determine the biofilm formation of these isolates.

RESULTS

For this study, 76 non-duplicative isolates of Kp were collected. Overall, 22 (28.94%) strains had positive string test results, and 31 (40.78%) isolates were grown in tellurite-containing medium. The genes iucA and iutA or peg-344 were found in 23.68% of all Kp strains and in 50% of tellurite-resistant isolates, respectively. The most effective antibiotics against hvKp isolates were tetracycline (85.52%) and chloramphenicol (63.15%). Using the cefoxitin disc diffusion method, we observed that 56.57% (43/76) of the strains were AmpC producer. A total of 30.26% (n = 23/76) of the isolates tested positive for at least one ampC gene, including blaDHA (52.63%, n = 40), blaCIT (40.78%, n = 31), blaACC (19.76%, n = 15), blaMOX (25%, n = 19), and blaFOX (43.42%, n = 33). Biofilm formation analysis revealed that most hvKp isolates were weak (n = 6, 40%) and moderate (n = 5, 33.33%) biofilm producers.

CONCLUSION

Healthcare practitioners should consider the possibility of the existence and acquisition of hvKp everywhere. The exact mechanisms of bacterial acquisition are also unknown, and it is unclear whether the occurrence of infections is related to healthcare or not. Thus, there are still many questions about hvKp that need to be investigated.

Synthesis and Antibacterial Activity of Metal-Containing Ultraviolet-Cured Wood Floor Coatings

In our previous report, the antibacterial agents with different metals, mono(hydroxyethoxyethyl)phthalate [M(HEEP)₂, M = Zn, Mn, and Ca], were synthesized. For increasing their yields, modified synthesis and purified processes were further investigated. The result of energy-dispersive X-ray spectroscopy showed the M(HEEP)₂ could be stable and successfully synthesized, and their yields were raised to 73–85% from our previous report of 43–55%. For ultraviolet-cured wood floor coating application, the Zn(HEEP)₂ was selected as an antibacterial agent and mixed with commercial UV wood floor coating. The effects on the antibacterial activity of UV films with different Zn(HEEP)₂ additions of 0, 4, 8, and 12 phr as well as the commercial nano-Ag of 12 phr against Escherichia coli were evaluated. In the static antibacterial test, the UV films with Zn(HEEP)₂ additions had similar antibacterial activity of 57–59%. In another dynamic shaking antibacterial test, the film containing 12 phr Zn(HEEP)₂ had the best antibacterial activity among all the UV films. On the film properties, the Zn(HEEP)₂-containing UV films had lower gloss and abrasion resistance, and slightly increased the hardness than those of UV film without Zn(HEEP)₂ addition. However, there were no noticeable differences in mass retention, lightfastness, and thermal stability between UV films with and without the Zn(HEEP)₂ addition. In this study, the 12 phr Zn(HEEP)₂-containing UV film provided the best antibacterial activity against E. coli and had the balanced film properties for application on the UV wood floor coating.

Molecular Exploring of Plasmid-mediated Ampc beta Lactamase Gene in Clinical Isolates of Proteus mirabilis

Between September to December 2020, thirteen isolates of Proteus mirabilis were recovered among one hundred fifty; MacConkey agar was utilized to purify Gram-negative bacteria isolated from infections of the urinary tract. The primary identification of Proteus mirabilis isolates was relied on “colonial morphology, microscopic examination, and biochemical “tests; however, the confirmation of identification of antimicrobial susceptibility of isolates was conducted utilizing an automated VITEK-2 compact system. The result showed that Proteus mirabilis isolates were highly resistant to most antibiotics, making them multi-drug resistant (MDR). Phenotype methods were used to detect AmpC beta-lactamase. Initial and confirmatory methods showed that eight isolates were AmpC producers. Polymerase Chain Reaction (PCR) was employed to detect the blaampC gene.

Genomic Identity of Fluoroquinolone-Resistant blaCTX-M-15-Type ESBL and pMAmpC β-Lactamase Producing Klebsiella pneumoniae from Buffalo Milk, India

We investigated the occurrence of extended-spectrum β-lactamase (ESBL) and AmpC-type β-lactamase (ACBL) producing quinolone-resistant Klebsiella pneumoniae (KP) in milk samples of apparently healthy buffaloes (n = 348) and buffaloes (n = 19) with evidence of subclinical mastitis from seven districts of West Bengal, India. In total, 12 ESBL producing KP were isolated with blaCTX-M-15 gene and 7 of them were ACBL producers, as well. The blaCTX-M-15 genes were carried by transposable element ISEcp1. The plasmid-mediated quinolone resistance genes-qnrS, qnrA, qnrB, qepA, and aac(6')-Ib-cr were detected in five, one, three, four, and one isolate (s), respectively. In addition, eight isolates carried mutation in gyrase (gyrA) and six in topoisomerase IV (parC). Resistance markers/genes for sulfonamide (sul1), tetracycline [tet(A) and tet(B)], and aminoglycoside (aacC2) were also detected in eight, four, and one isolate(s), respectively. The class I integrons identified in five isolates carried aad2/aad5 and dfrA12/dfrA17 gene cassettes. The enterobacterial repetitive intergenic consensus-PCR revealed that all the isolates were genetically diverse and comprised a heterogeneous population. Isolation of multidrug-resistant KP, a typical nosocomial pathogen from buffalo milk, reiterates the need to monitor farm animals for ESBL producing Enterobacteriaceae and emphasizes on judicious use of antibiotics in animal husbandry sector.

Phenotypic detection of AmpC β-lactamases, extended-spectrum β-lactamases and metallo-β-lactamases in Enterobacteriaceae using a resazurin microtitre assay with inhibitor-based methods

Dissemination of antibiotic resistance in Enterobacteriaceae mediated by AmpC β-lactamase, extended-spectrum β-lactamase (ESBL) and metallo-β-lactamase (MBL) is clinically significant. A simple and relatively quick method for the detection of these resistance phenotypes would greatly improve chemotherapeutic recommendation. This technology would provide valuable input in our surveillance of resistance on a global stage, particularly if the methodology could be applicable to resource-poor settings. A resazurin microtitre plate (RMP) assay incorporating cloxacillin, clavulanic acid and EDTA for the rapid phenotypic identification of AmpC, ESBL and MBL and the co-existence of β-lactamases has been developed. A total of 47 molecularly characterized Enterobacteriaceae clinical isolates producing AmpCs, ESBLs, co-producers of ESBL and AmpC, MBLs and co-producers of ESBL and MBL were phenotypically examined using the RMP assay. The ceftazidime- and cefotaxime-based RMP assays successfully detected all 16 AmpC, 14 ESBL and 9 MBL producers, 6 ESBL-AmpC co-producers and 2 ESBL-MBL co-producers without false-positive results. The ceftazidime-based assay was more reliable in detecting AmpC alone, while the cefotaxime-based assay performed better in identifying co-producers of ESBL and AmpC. There was no difference in the detection of ESBL and MBL producers. The findings of the present study suggest that use of the RMP assay with particular β-lactamase inhibitors explicitly detects three different β-lactamases, as well as co-existence of β-lactamases, within 6 h of initial isolation of the pathogen. This assay is applicable to carry out in any laboratory, is cost-effective and is easy to interpret. It could be implemented in screening patients and controlling infection and for surveillance purposes.

Detection of expanded-spectrum β-lactamases in Gram-negative bacteria in the 21st century

Emerging β-lactamase-producing-bacteria (ESBL, AmpC and carbapenemases) have become a serious problem in our community due to their startling spread worldwide and their ability to cause infections which are difficult to treat. Diagnosis of these β-lactamases is of clinical and epidemiological interest. Over the past 10 years, several methods have been developed aiming to rapidly detect these emerging enzymes, thus preventing their rapid spread. In this review, we describe the range of screening and detection methods (phenotypic, molecular and other) for detecting these β-lactamases but also whole genome sequencing as a tool for detecting the genes encoding these enzymes.

Characterization of Plasmid-Mediated AmpC and Carbapenemases among Iranain Nosocomial Isolates of Klebsiella pneumoniae Using Phenotyping and Genotyping Methods

OBJECTIVES

Plasmid-mediated AmpC β-lactamases (PMABLs) and carbapenemases are emerging groups of antimicrobial-resistance determinants. The aims of the study were to evaluate the occurrence of PMABLs and carbapenemases in clinical isolates of Klebsiella pneumoniae and compare the test performance of various phenotypic methods for detection of these enzymes in Iran.

METHODS

A total of 100 K. pneumoniae isolates were collected from clinical specimens obtained in Valiasr Hospital. AmpC production in all isolates was determined using the AmpC disk test, the cephamycin Hodge test, the AmpC Etest, and the boronic acid combined-disk test. In addition, carbapenemase production was determined using the modified Hodge test, the EDTA disk synergy test, and the boronic acid combined-disk test. The performances of various phenotypic methods were evaluated by the comparison of their results with polymerase chain reaction (PCR) method as the gold standard.

RESULTS

Of the 100 isolates, 19 (19%) were demonstrated to harbor the PMABL-resistance gene by the multiplex PCR method. The PCR result indicated the presence of carbapenemase genes in 12 isolates. The performance of various phenotypic tests carried out for detection of carbapenemase-producing isolates varied widely, ranging in sensitivity from 30% to 100% and in specificity from 90.8% to 100%.

CONCLUSION

This is the first report of MOX-type AmpC β-lactamase and bla GES in K. pneumoniae in Iran. A comparison of the phenotypic methods showed that a combination of cefoxitin plus boronic acid is optimal for detecting plasmid-mediated AmpC enzymes in K. pneumoniae, whereas the implementation of molecular methods is often complex, requires specially trained personnel, and is associated with higher costs.

Detection of ESBL among AmpC producing enterobacteriaceae using inhibitor-based method

Introduction

The occurrence of multiple β-lactamases among bacteria only limits the therapeutic options but also poses a challenge. A study using boronic acid (BA), an AmpC enzyme inhibitor, was designed to detect the combined expression of AmpC β-lactamases and extended-spectrum β-lactamases (ESBLs) in bacterial isolates further different phenotypic methods are compared to detect ESBL and AmpC.

Methods

A total of 259 clinical isolates of Enterobacteriaceae were isolated and screened for ESBL production by (i) CLSI double-disk diffusion method (ii) cefepime- clavulanic acid method (iii) boronic disk potentiation method. AmpC production was detected using cefoxitin alone and in combination with boronic acid and confirmation was done by three dimensional disk methods. Isolates were also subjected to detailed antibiotic susceptibility test.

Results

Among 259 isolates, 20.46% were coproducers of ESBL and AmpC, 26.45% were ESBL and 5.40% were AmpC. All of the 53 AmpC and ESBL coproducers were accurately detected by boronic acid disk potentiation method.

Conclusion

The BA disk test using Clinical and Laboratory Standards Institute methodology is simple and very efficient method that accurately detects the isolates that harbor both AmpCs and ESBLs.

Detection of AmpC beta-lactamases using sodium salicylate

AmpC β-lactamases are enzymes that hydrolyze all β-lactam antibiotics except cefipime and imipenem. Currently, there is no standard phenotypic method for detection of such enzymes. This study aims to report the use of sodium salicylate for AmpC β-lactamases detection and to compare its sensitivity and specificity to other commonly known inhibitors. A total of 135 clinical isolates were used to test the effectiveness of sodium salicylate in detection of plasmid- as well as chromosomally encoded AmpC β-lactamases. All isolates were tested by multiplex PCR testing as well as inhibitor-based methods using cloxacillin, phenylboronic acid and sodium salicylate for the detection of AmpC enzymes. Four isolates were confirmed as producers of plasmid-encoded AmpC β-lactamase and a single isolate was confirmed to have both plasmid and chromosomal genes. Cloxacillin and phenylyboronic acid failed to detect most of the plasmid-encoded enzymes. Sodium salicylate was able to detect the Escherichia coli isolates with plasmid-encoded enzymes in addition to few other isolates that were chromosomally mediated. The sensitivity and specificity of sodium salicylate was 50% and 93%, respectively, higher than those of other known inhibitors. We thus conclude that sodium salicylate can be reliably used as an inhibitor in the detection of plasmid-encoded AmpC enzymes in E. coli.

AmpC β-lactamases in nosocomial isolates of Klebsiella pneumoniae from India

BACKGROUND & OBJECTIVES

AmpC β-lactamases are clinically significant since these confer resistance to cephalosporins in the oxyimino group, 7-α methoxycephalosporins and are not affected by available β-lactamase inhibitors. In this study we looked for both extended spectrum β-lactamases (ESBL) and AmpC β-lactamases in Klebsiella pneumoniae clinical isolates.

METHODS

One hundred consecutive, non-duplicate clinical isolates of K. pneumoniae collected over a period of one year (June 2008 - June 2009) were included in the study. An antibiotic susceptibility method was used with 10 antibiotics for Gram-negative infections which helped in screening for ESBL and AmpC β-lactamases and also in confirmation of ESBL production. The detection of AmpC β-lactamases was done based on screening and confirmatory tests. For screening, disc diffusion zones of cefoxitin <18 mm was taken as cefoxitin resistant. All cefoxitin resistant isolates were tested further by AmpC disk test and modified three dimensional test. Multiplex-PCR was performed for screening the presence of plasmid-mediated AmpC genes.

RESULTS

Of the 100 isolates of K. pneumoniae studied, 48 were resistant to cefoxitin on screening. AmpC disk test was positive in 32 (32%) isolates. This was also confirmed with modified three dimensional test. Indentation indicating strong AmpC producer was observed in 25 isolates whereas little distortion (weak AmpC) was observed in 7 isolates. ESBL detection was confirmed by a modification of double disk synergy test in 56 isolates. Cefepime was the best cephalosporin in synergy with tazobactam for detecting ESBL production in isolates co-producing AmpC β-lactamases. The subsets of isolates phenotypically AmpC β-lactamase positive were subjected to amplification of six different families of AmpC gene using multiplex PCR. The sequence analysis revealed 12 CMY-2 and eight DHA-1 types.

INTERPRETATION & CONCLUSIONS

Tazobactam was the best β-lactamase inhibitor for detecting ESBL in presence of AmpC β-lactamase as this is a very poor inducer of AmpC gene. Amongst cephalosporins, cefepime was the best cephalosporin in detecting ESBL in presence of AmpC β-lactamase as it is least hydrolyzed by AmpC enzymes. Cefepime-tazobactam combination disk test would be a simple and best method in detection of ESBLs in Enterobacteriaceae co-producing AmpC β-lactamase in the routine diagnostic microbiology laboratories.

Molecular description of plasmid-mediated AmpC β-lactamases among nosocomial isolates of Escherichia coli & Klebsiella pneumoniae from six different hospitals in India

BACKGROUND & OBJECTIVES

Plasmid mediated AmpC β-lactamase (PMABL) resistance in Escherichia coli and Klebsiella spp. is an emerging problem worldwide. Phenotypic methods are commonly used for detection of PMABL production in Gram-negative isolates, but molecular data about the prevalence of plasmid-mediated AmpC-type resistance at the national level are needed. Hence, a prospective study was undertaken to determine the occurrence of PMABL gene and its types among clinical isolates of E. coli and K. pneumoniae obtained from six different hospitals in India.

METHODS

A total of 241 nosocomial isolates of K. pneumoniae (n=109) and E.coli (n=132) from six geographically distant hospitals in India were included. These were screened for cefoxitin resistance. AmpC disk test and modified three dimensional extraction test were used for phenotypic detection of PMABL production. Molecular types were determined by a multiplex PCR.

RESULTS

Among the 241 isolates, 187 (77.5%) were found to be cefoxitin resistant (K. pneumoniae n=83, E. coli n=104). AmpC activity was detectable in 153 (63.4%) isolates, (K. pneumoniae n=69, E. coli n=84). By PCR, the plasmid encoded AmpC genes were found in 92 (38.1%) isolates and the molecular types of the genes detected predominantly were DHA, CIT followed by MOX and ACC types.

INTERPRETATION & CONCLUSIONS

A high percentage of plasmid-encoded AmpC enzymes was noted in E. coli and K. pneumonia isolates obtained from different parts of the country. Phenotypic methods alone may not reflect the true number of PMABL producers. Genotypic methods need to be employed in national surveillance studies.

Total alkaloids from Sophora alopecuroides L. increase susceptibility of extended-spectrum β-lactamases producing Escherichia coli isolates to cefotaxime and ceftazidime

OBJECTIVE

To evaluate the antimicrobial activity of total alkaloids extracted from Sophorea alopecuroides L. (TASA) against clinical isolated extended-spectrum beta-lactamases (ESBLs) producing Escherichia coli (E. coli) strains.

METHODS

The antibacterial activity of TASA either itself or in combination with cefotaxime (CTX) or ceftazidime (CAZ) was investigated by using the microbroth dilution method and phenotypic confirmatory disk diffusion test against three clinical isolated ESBLs-producing E. coli strains; the interactions of TASA and CTX or CAZ were ascertained by evaluating the fractional inhibitory concentration index (FICI).

RESULTS

The antibacterial activity of either TASA itself or in combination with CTX or CAZ was found. The minimum inhibitory concentration (MICs) of TASA against the ESBLs producing isolates was 12.5 mg/mL. In the combinations with a sub-inhibitory concentration of TASA, a synergistic effect on CTX and CAZ against the ESBLs producing isolates was observed. Similarly, the isolates exposed to lower dose of TASA yielded an increased susceptibility to CTX and CAZ by 8-16 folds determined by microdilution assay. Moreover, enzymatic detection of ESBLs demonstrated that TASA induced reversal resistance to CTX and CAZ partially by a mechanism of inhibition of ESBLs activity in these isolates. Additionally, in the tested isolates following the exposure of TASA, molecular analysis verified the SHV-type beta-lactamase encoding ESBL gene in these isolates, and no mutation was introduced into the ESBL gene.

CONCLUSIONS

These results suggest that TASA could be used as a source of natural compound with pharmacological activity of reversal resistance to antimicrobial agent. These findings also indicated that the application of the TASA in combination with antibiotics might prove useful in the control and treatment of infectious diseases caused by the ESBLs producing enterobacteriaceae.

Current concepts in laboratory testing to guide antimicrobial therapy

Antimicrobial susceptibility testing (AST) is indicated for pathogens contributing to an infectious process that warrants antimicrobial therapy if susceptibility to antimicrobials cannot be predicted reliably based on knowledge of their identity. Such tests are most frequently used when the etiologic agents are members of species capable of demonstrating resistance to commonly prescribed antibiotics. Some organisms have predictable susceptibility to antimicrobial agents (ie, Streptococcus pyogenes to penicillin), and empirical therapy for these organisms is typically used. Therefore, AST for such pathogens is seldom required or performed. In addition, AST is valuable in evaluating the activity of new and experimental compounds and investigating the epidemiology of antimicrobial resistant pathogens. Several laboratory methods are available to characterize the in vitro susceptibility of bacteria to antimicrobial agents. When the nature of the infection is unclear and the culture yields mixed growth or usual microbiota (wherein the isolates usually bear little relationship to the actual infectious process), AST is usually unnecessary and results may, in fact, be dangerously misleading. Phenotypic methods for detection of specific antimicrobial resistance mechanisms are increasingly being used to complement AST (ie, inducible clindamycin resistance among several gram-positive bacteria) and to provide clinicians with preliminary direction for antibiotic selection pending results generated from standardized AST (ie, β-lactamase tests). In addition, molecular methods are being developed and incorporated by microbiology laboratories into resistance detection algorithms for rapid, sensitive assessment of carriage states of epidemiologically and clinically important pathogens, often directly from clinical specimens (ie, presence of vancomycin-resistant enterococci in fecal specimens).

Comparison of the boronic acid disk potentiation test and cefepime-clavulanic acid method for the detection of ESBL among AmpC-producing Enterobacteriaceae

PURPOSE

Extended spectrum β-lactamase (ESBL) and AmpC β-lactamase are important mechanisms of betalactam resistance among Enterobacteriaceae . The ESBL confirmation test described by Clinical Laboratory Standards Institute (CLSI) is in routine use. This method fails to detect ESBL in the presence of AmpC. Therefore, we compared two different ESBL detection methods against the CLSI confirmatory test.

MATERIALS AND METHODS

A total 200 consecutive clinical isolates of Enterobacteriaceae from various clinical samples were tested for ESBL production using (i) CLSI described phenotypic confirmatory test (PCT), (ii) boronic acid disk potentiation test and (iii) cefepime-CA disk potentiation method. AmpC confirmation was done by a modified three-dimensional test.

RESULTS

Among total 200 Enterobacteriaceae isolates, 82 were only ESBL producers, 12 were only AmpC producers, 55 were combined ESBL and AmpC producers, 14 were inducible AmpC producers and 37 isolates did not harboured any enzymes. The CLSI described PCT detected ESBL-producing organisms correctly but failed to detect 36.3% of ESBLs among combined enzyme producers. The boronic acid disk potentiation test reliably detected all ESBL, AmpC, and combined enzyme producers correctly. The cefepime-CA method detected all ESBLs correctly but another method of AmpC detection has to be adopted.

CONCLUSION

The use of boronic acid in disk diffusion testing along with the CLSI described PCT enhances ESBL detection in the presence of AmpC betalactamases.

AmpC beta-lactamases

SUMMARY

AmpC beta-lactamases are clinically important cephalosporinases encoded on the chromosomes of many of the Enterobacteriaceae and a few other organisms, where they mediate resistance to cephalothin, cefazolin, cefoxitin, most penicillins, and beta-lactamase inhibitor-beta-lactam combinations. In many bacteria, AmpC enzymes are inducible and can be expressed at high levels by mutation. Overexpression confers resistance to broad-spectrum cephalosporins including cefotaxime, ceftazidime, and ceftriaxone and is a problem especially in infections due to Enterobacter aerogenes and Enterobacter cloacae, where an isolate initially susceptible to these agents may become resistant upon therapy. Transmissible plasmids have acquired genes for AmpC enzymes, which consequently can now appear in bacteria lacking or poorly expressing a chromosomal bla(AmpC) gene, such as Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. Resistance due to plasmid-mediated AmpC enzymes is less common than extended-spectrum beta-lactamase production in most parts of the world but may be both harder to detect and broader in spectrum. AmpC enzymes encoded by both chromosomal and plasmid genes are also evolving to hydrolyze broad-spectrum cephalosporins more efficiently. Techniques to identify AmpC beta-lactamase-producing isolates are available but are still evolving and are not yet optimized for the clinical laboratory, which probably now underestimates this resistance mechanism. Carbapenems can usually be used to treat infections due to AmpC-producing bacteria, but carbapenem resistance can arise in some organisms by mutations that reduce influx (outer membrane porin loss) or enhance efflux (efflux pump activation).

Evaluation of screening methods to detect plasmid-mediated AmpC in Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis

There are currently no standardized phenotypic methods for the screening and detection of AmpC enzymes. This study aimed to evaluate different methods to detect AmpC enzymes in Escherichia coli, Klebsiella spp., and Proteus spp., comparing the results from two disk-based methods and an agar dilution method. AmpC activity was determined for 255 clinical isolates by use of a three-dimensional enzyme assay combined with a multiplex PCR assay for plasmid-borne ampC genes. These results were compared against a disk-based inhibitor assay using various combinations of cefpodoxime and cefoxitin as antibiotic substrates and boronic acid or cloxacillin as an AmpC inhibitor. The presence of enzyme induction by disk approximation was evaluated using imipenem, cefoxitin, and amoxicillin-clavulanate as inducing agents against ceftazidime. Finally, an agar dilution assay was performed, using cefoxitin with and without added cloxacillin. AmpC activity was present in 49.8% of test isolates, 93.7% of which were positive for plasmid-borne ampC genes. CIT-like enzymes were predominant in E. coli, and DHA-like enzymes were predominant in Klebsiella spp. The disk-based inhibitor tests performed better than the agar dilution assay, while detection of AmpC by disk induction had a poor sensitivity. The cefoxitin-cloxacillin disk combination provided the best overall performance, with a sensitivity and specificity of 95%. This study confirmed the accuracy of disk-based inhibitor screening for AmpC enzymes, which proved reliable at detecting CIT- and DHA-like plasmid-borne ampC genes. The methods are simple enough for introduction into clinical microbiology laboratories.

Evaluation of detection of extended-spectrum beta-lactamases among Escherichia coli and Klebsiella spp. isolates by VITEK 2 AST-N029 compared to the agar dilution and disk diffusion methods

A total of 123 clinical Escherichia coli and Klebsiella spp. isolates were included in the study in order to evaluate VITEK 2 AST-NO29 (Nordic) card for detection of extended-spectrum beta-lactamases (ESBL) and to compare the results with genotypic ESBL verification. The results were also compared to alternative phenotypic methods, i.e. agar dilution and disk diffusion. The strains that were ESBL-positive according to AST-N029 were further analysed with the ESBL test card, VITEK 2 AST-N041. Using genotype as reference, Vitek 2 AES had the highest accuracy of the tested methods in classifying the strains as ESBL-positive or -negative (91.1%). When VITEK 2 gave ESBL as the only option for E. coli or K. pneumoniae, 44 of 45 (97.8%) strains had an ESBL gene. VITEK 2 achieved an accuracy of 94.9% and disk diffusion 95.9% compared to the agar dilution method as the phenotypic reference method for the E. coli and K. pneumoniae strains. For the K. oxytoca strains VITEK 2 achieved the highest accuracy (84.0%) of the methods used in this work.

An epidemic of plasmids? Dissemination of extended-spectrum cephalosporinases among Salmonella and other Enterobacteriaceae

CTX-M- and AmpC-type beta-lactamases comprise the two most rapidly growing populations among the extended-spectrum cephalosporinases. The evolution and dissemination of resistance genes encoding these enzymes occur mostly through the transmission of plasmids. The high prevalence of clinical isolates of Enterobacteriaceae producing the plasmid-mediated extended-spectrum cephalosporinases resembles an epidemic of plasmids, and has generated serious therapeutic problems. This review describes the emergence and worldwide spread of various classes of plasmid-mediated extended-spectrum cephalosporinases in Salmonella and other Enterobacteriaceae, the transfer mechanism of the plasmids, detection methods, and therapeutic choices.

Detection of plasmid-mediated class C β-lactamases

Plasmid-mediated class C beta-lactamases are reported from Enterobacteriaceae with increasing frequency. They likely originate from chromosomal AmpC of certain Gram-negative bacterial species and subsequently are mobilized onto transmissible plasmids. There are reports of unfavorable clinical outcomes in patients infected with these organisms and treated with broad-spectrum cephalosporins. However, unlike class A extended-spectrum beta-lactamases (ESBLs), no screening and confirmatory tests have been uniformly established for strains that produce class C beta-lactamases. Reduced susceptibility to cefoxitin is a sensitive but not specific indicator of class C beta-lactamase production. Simple confirmatory tests including tests using boronic acid compounds as specific class C beta-lactamase inhibitors have recently been developed. Their utilization will enable clinical microbiology laboratories to report those strains producing plasmid-mediated class C beta-lactamases as being resistant to all broad-spectrum cephalosporins, thus allowing physicians to prescribe appropriate antimicrobial therapy.

Evaluation of phenotypic screening methods for detecting plasmid-mediated AmpC beta-lactamases-producing isolates of Escherichia coli and Klebsiella pneumoniae

Detection of plasmid-mediated (P-M) AmpC beta-lactamase-producing isolates is considered critical for epidemiologic studies and hospital infection control, but the documents of the Clinical and Laboratory Standards Institute do not contain any recommendation for the phenotypic detection. In this study, phenotypic detection methods, cefoxitin-Hodge test and induction test, were evaluated using cefoxitin-resistant Escherichia coli and Klebsiella pneumoniae isolates. The cefoxitin-Hodge test detected all bla(CMY-10), and 97.4% of bla(CMY-2) allele-positive isolates, but only 57.3% of bla(DHA-1) allele-positive isolates. Induction test with an aztreonam and an amoxicillin-clavulanic acid disk was more sensitive than with cefoxitin disk, which detected 86.6% of bla(DHA-1) allele-positive isolates. These phenotypic tests should be useful to screen P-M AmpC beta-lactamase-producing E. coli and K. pneumoniae isolates.

Evaluation of beta-lactamase inhibitors in disk tests for detection of plasmid-mediated AmpC beta-lactamases in well-characterized clinical strains of Klebsiella spp

The diagnostic utility of the AmpC beta-lactamase inhibitors LN-2-128, 48-1220, and Syn 2190 in combination with cefotetan (CTT) or cefoxitin in a disk test for the detection of clinical isolates of Klebsiella spp. producing plasmid-mediated AmpC beta-lactamases (pAmpCs) was evaluated. The combination of Syn 2190 and CTT had a sensitivity of 91%, a specificity of 100%, and a reproducibility of 100% and showed the best potential of using an inhibitor for detection of Klebsiella spp. producing pAmpCs.

AmpC disk test for detection of plasmid-mediated AmpC beta-lactamases in Enterobacteriaceae lacking chromosomal AmpC beta-lactamases

Although plasmid-mediated AmpC beta-lactamases were first reported in the late 1980s, many infectious disease personnel remain unaware of their clinical importance. These enzymes are typically produced by isolates of Escherichia coli, Klebsiella spp., Proteus mirabilis, and Salmonella spp. and are associated with multiple antibiotic resistance that leaves few therapeutic options. Plasmid-mediated AmpC beta-lactamases have been associated with false in vitro susceptibility to cephalosporins. Many laboratories do not test for this resistance mechanism because current tests are inconvenient, subjective, lack sensitivity and/or specificity, or require reagents that are not readily available. In this study a new test, the AmpC disk test, based on filter paper disks impregnated with EDTA, was found to be a highly sensitive, specific, and convenient means of detection of plasmid-mediated AmpC beta-lactamases in organisms lacking a chromosomally mediated AmpC beta-lactamase. Using cefoxitin insusceptibility as a screen, the test accurately distinguished AmpC and extended-spectrum beta-lactamase production and differentiated AmpCs from non-beta-lactamase mechanisms of cefoxitin insusceptibility, such as reduced outer membrane permeability. The test is a potentially useful diagnostic tool. It can provide important infection control information and help to ensure that infected patients receive appropriate antibiotic therapy.

Practical methods using boronic acid compounds for identification of class C beta-lactamase-producing Klebsiella pneumoniae and Escherichia coli

Detection of the resistance mediated by class C beta-lactamases remains a challenging issue, considering that transferable plasmid-mediated class C beta-lactamases are of worldwide concern. Methods for the identification of strains that produce extended-spectrum beta-lactamases (ESBLs) or metallo-beta-lactamases (MBLs) have been developed and applied for routine use in clinical microbiology laboratories, but no practical methods for identification of plasmid-mediated class C producers have been established to date. We therefore developed three simple methods for clinical microbiology laboratories that allow identification of plasmid-mediated class C beta-lactamase-producing bacteria using a boronic acid derivative, 3-aminophenylboronic acid (APB), one of the specific inhibitors of class C beta-lactamases. Detection by the disk potentiation test was based on the enlargement of the growth-inhibitory zone diameter (by greater than or equal to 5 mm) around a Kirby-Bauer disk containing a ceftazidime (CAZ) or a cefotaxime (CTX) disk in combination with APB. In a double-disk synergy test, the discernible expansion of the growth-inhibitory zone around the CAZ or the CTX disk toward a disk containing APB was indicative of class C beta-lactamase production. A greater than or equal to eightfold decrease in the MIC of CAZ or CTX in the presence of APB was the criterion for detection in the microdilution test. By using these methods, Escherichia coli and Klebsiella pneumoniae isolates producing plasmid-mediated class C beta-lactamases, ACT-1, CMY-2, CMY-9, FOX-5, LAT-1, and MOX-1, were successfully distinguished from those producing other classes of beta-lactamases, such as ESBLs and MBLs. These methods will provide useful information needed for targeted antimicrobial therapy and better infection control.