Detection of plasmid-mediated AmpC in Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis

Detection of β-Lactamase Resistance and Biofilm Genes in Pseudomonas Species Isolated from Chickens

Bacteria of the genus Pseudomonas are pathogens in both humans and animals. The most prevalent nosocomial pathogen is P. aeruginosa, particularly strains with elevated antibiotic resistance. In this study, a total of eighteen previously identified Pseudomonas species strains, were isolated from chicken. These strains were screened for biofilm formation and antibiotic resistance. In addition, we evaluated clove oil’s effectiveness against Pseudomonas isolates as an antibiofilm agent. The results showed that Pseudomonas species isolates were resistant to most antibiotics tested, particularly those from the β-lactamase family. A significant correlation (p < 0.05) between the development of multidrug-resistant isolates and biofilms is too informal. After amplifying the AmpC-plasmid-mediated genes (blaCMY, blaMIR, DHA, and FOX) and biofilm-related genes (psld, rhlA, and pelA) in most of our isolates, PCR confirmed this relationship. Clove oil has a potent antibiofilm effect against Pseudomonas isolates, and may provide a treatment for bacteria that form biofilms and are resistant to antimicrobials.

Systematic Review of Plasmid AmpC Type Resistances in Escherichia coli and Klebsiella pneumoniae and Preliminary Proposal of a Simplified Screening Method for ampC

Beta-lactamase (BL) production is a major public health problem. Although not the most frequent AmpC type, AmpC-BL is increasingly isolated, especially plasmid AmpC-BL (pAmpC-BL). The objective of this study was to review information published to date on pAmpC-BL in Escherichia coli and Klebsiella pneumoniae, and on the epidemiology and detection methods used by clinical microbiology laboratories, by performing a systematic review using the MEDLINE PubMed database. The predictive capacity of a screening method to detect AmpC-BL using disks with cloxacillin (CLX) was also evaluated by studying 102 Enterobacteriaceae clinical isolates grown in CHROMID ESBL medium with the addition of cefepime (FEP), cefoxitin (FOX), ertapenem (ETP), CLX, and oxacillin with CLX. The review, which included 149 publications, suggests that certain risk factors (prolonged hospitalization and previous use of cephalosporins) are associated with infections by pAmpC-BL-producing microorganisms. The worldwide prevalence has increased over the past 10 years, with a positivity rate ranging between 0.1 and 40%, although AmpC was only detected when sought in a targeted manner. CMY-2 type has been the most prevalent pAmpC-BL-producing microorganism. The most frequently used phenotypic method has been the double-disk synergy test (using CLX disks or phenyl-boronic acid and cefotaxime [CTX] and ceftazidime) and the disk method combined with these inhibitors. In regard to screening methods, a 1-µg oxacillin disk with CLX showed 88.9% sensitivity, 100% specificity, 100% positive predictive value (PPV), 98.9% negative predictive value (NPV), and 98.9% validity index (VI). This predictive capacity is reduced with the addition of extended-spectrum beta-lactamases, showing 62.5% sensitivity, 100% specificity, 100% PPV, 93.5% NPV, and 94.1% VI. In conclusion, there has been a worldwide increase in the number of isolates with pAmpC-BL, especially in Asia, with CMY-2 being the most frequently detected pAmpC-BL-producing type of microorganism. Reduction in its spread requires routine screening with a combination of phenotypic methods (with AmpC inhibitors) and genotypic methods (multiplex PCR). In conclusion, the proposed screening technique is an easy-to-apply and inexpensive test for the detection of AmpC-producing isolates in the routine screening of multidrug-resistant microorganisms.

Plasmid-Mediated AmpC β-Lactamase CITM and DHAM Genes Among Gram-Negative Clinical Isolates

Background

Antibiotic resistance mediated by the production of extended-spectrum β-lactamases (ESBLs) and AmpC β-lactamases is posing a serious threat in the management of the infections caused by Gram-negative pathogens. The aim of this study was to determine the prevalence of two AmpC β-lactamases genes, bla_CITM and bla_DHAM, in Gram-negative bacterial isolates.

Materials and Methods

A total of 1151 clinical samples were obtained and processed at the microbiology laboratory of Annapurna Neurological Institute and Allied Science, Kathmandu between June 2017 and January 2018. Gram-negative isolates thus obtained were tested for antimicrobial susceptibility testing (AST) using Kirby–Bauer disk diffusion method. AmpC β-lactamase production was detected by disk approximation method using phenylboronic acid (PBA). Confirmed AmpC β-lactamase producers were further screened for bla_CITM and bla_DHAM genes by conventional polymerase chain reaction (PCR).

Results

Out of 1151 clinical specimens, 22% (253/1152) had bacterial growth. Of the total isolates, 89.3% (226/253) were Gram-negatives, with E. coli as the most predominant species (n=72) followed by Pseudomonas aeruginosa (n=41). In the AST, 46.9% (106/226) of the Gram-negative isolates were multidrug resistant (MDR). In disk diffusion test, 113 (50%) isolates showed resistance against cefoxitin, among which 91 isolates (83 by disk test and Boronic acid test, 8 by Boronic test only) were confirmed as AmpC β-lactamase-producers. In PCR assay, 90.1% (82/91) and 87.9% (80/91) of the isolates tested positive for production of bla_CITM and bla_DHAM genes, respectively.

Conclusions

High prevalence of AmpC β-lactamase-producers in our study is an alarming sign. This study recommends the use of modern diagnostic facilities in the clinical settings for early detection and management which can optimize the treatment therapies, curb the growth and spread of the drug-resistant pathogens.

Klebsiella pneumoniae in Singapore: Hypervirulent Infections and the Carbapenemase Threat

Klebsiella pneumoniae remains a major pathogen responsible for localized infections such as cystitis and pneumonia, and disseminated infections that may result in severe sepsis and death. Invasive disease such as liver abscesses and endogenous endophthalmitis are associated with capsular serotypes K1 and K2. These infections require a prolonged course of antimicrobial treatment which has evolved over the years from inpatient treatment to outpatient parenteral antibiotic therapy. The emergence of plasmid-mediated resistance began with extended-spectrum β-lactamases (ESBLs) and AmpC β-lactamases. This was followed by carbapenemase genes and now plasmid transmissible colistin resistance (mcr), thus limiting viable treatment options. Plasmid-mediated carbapenemase production in Singapore was first reported in 1996. Carbapenemase production has since become the predominant mechanism of carbapenem resistance and incidence rates continue to increase over time. Although carbapenemases can occur in all Enterobacteriaceae, K. pneumoniae are the most common carrier of carbapenemase genes. Alternative treatment options are urgently required before the simplest infections, let alone invasive infections are left potentially untreatable. Clinical management requires guidance from robust laboratory testing methods to optimize patient outcomes. We explore past and present trends in treatment of K. pneumoniae infections, and discuss future treatment options and gaps in knowledge for further study.

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.

Escherichia coli β-Lactamases: What Really Matters

Escherichia coli strains belonging to diverse pathotypes have increasingly been recognized as a major public health concern. The β-lactam antibiotics have been used successfully to treat infections caused by pathogenic E. coli. However, currently, the utility of β-lactams is being challenged severely by a large number of hydrolytic enzymes – the β-lactamases expressed by bacteria. The menace is further compounded by the highly flexible genome of E. coli, and propensity of resistance dissemination through horizontal gene transfer and clonal spread. Successful management of infections caused by such resistant strains requires an understanding of the diversity of β-lactamases, their unambiguous detection, and molecular mechanisms underlying their expression and spread with regard to the most relevant information about individual bacterial species. Thus, this review comprises first such effort in this direction for E. coli, a bacterial species known to be associated with production of diverse classes of β-lactamases. The review also highlights the role of commensal E. coli as a potential but under-estimated reservoir of β-lactamases-encoding genes.

Nosocomial Isolates and Their Drug Resistant Pattern in ICU Patients at National Institute of Neurological and Allied Sciences, Nepal

Multidrug resistant organisms are increasing day by day and the cause is poorly known. This study was carried out from June 2011 to May 2012 at National Institute of Neurological and Allied Sciences Kathmandu, Nepal, with a view to determining drug resistant pathogens along with detection of extended spectrum β-lactamase (ESBL), AmpC β-lactamase (ABL), and metallo-β-lactamase (MBL) producing bacteria causing infection to ICU patients. A standard methodology was used to achieve these objectives as per recommendation of American Society for Microbiology. ESBL was detected by combined disc assay using cefotaxime and cefotaxime clavulanic acid, ABL by inhibitor based method using cefoxitin and phenylboronic acid, and MBL by imipenem-EDTA combined disk method. Two hundred and ninety-four different clinical samples such as tracheal aspirates, urine, pus, swabs, catheter tips, and blood were processed during the study. Most common bacteria were Acinetobacter spp. Of the total 58 Acinetobacter spp., 46 (79%) were MDR, and 27% were positive for ABL and 12% were for MBL. Of the 32 cases of Staphylococcus aureus, 18 (56%) were MDR. Findings of this study warrant routine β-lactamase testing in clinical isolates.

Detection and occurrence of plasmid-mediated AmpC in highly resistant gram-negative rods

OBJECTIVES

The aim of this study was to compare the current screening methods and to evaluate confirmation tests for phenotypic plasmidal AmpC (pAmpC) detection.

METHODS

For this evaluation we used 503 Enterobacteriaceae from 18 Dutch hospitals and 21 isolates previously confirmed to be pAmpC positive. All isolates were divided into three groups: isolates with 1) reduced susceptibility to ceftazidime and/or cefotaxime; 2) reduced susceptibility to cefoxitin; 3) reduced susceptibility to ceftazidime and/or cefotaxime combined with reduced susceptibility to cefoxitin. Two disk-based tests, with cloxacillin or boronic acid as inhibitor, and Etest with cefotetan-cefotetan/cloxacillin were used for phenotypic AmpC confirmation. Finally, presence of pAmpC genes was tested by multiplex and singleplex PCR.

RESULTS

We identified 13 pAmpC producing Enterobacteriaceae isolates among the 503 isolates (2.6%): 9 CMY-2, 3 DHA-1 and 1 ACC-1 type in E. coli isolates. The sensitivity and specificity of reduced susceptibility to ceftazidime and/or cefotaxime in combination with cefoxitin was 97% (33/34) and 90% (289/322) respectively. The disk-based test with cloxacillin showed the best performance as phenotypic confirmation method for AmpC production.

CONCLUSIONS

For routine phenotypic detection of pAmpC the screening for reduced susceptibility to third generation cephalosporins combined with reduced susceptibility to cefoxitin is recommended. Confirmation via a combination disk diffusion test using cloxacillin is the best phenotypic option. The prevalence found is worrisome, since, due to their plasmidal location, pAmpC genes may spread further and increase in prevalence.

Detection of plasmid-mediated AmpC β-lactamase in Escherichia coli and Klebsiella pneumoniae

BACKGROUND

Detecting plasmid-mediated AmpC (pAmpC) β-lactamase-producing organism is important for optimal infection control and providing accurate and effective treatment option for physicians.

OBJECTIVES

The aim of this study was to investigate the prevalence of pAmpC β-lactamase and compare the results of boronic acid (BA) disk test with other phenotypic tests detecting AmpC positive isolates.

MATERIALS AND METHODS

A total of 273 clinical isolates of Klebsiella pneumoniae (n: 82) and Escherichia coli (n: 191) were analysed. The presence of pAmpC β-lactamase was determined by BA disk test, cefoxitin (FOX) screening test, modified three dimensional test (M3DT), and multiplex polymerase chain reaction (PCR). Pulsed-field gel electrophoresis was performed to evaluate the genetic similarities between isolates. To detect extended spectrum β-lactamases (ESBL) in the presence of AmpC β-lactamase, ESBL confirmation test was carried out with and without BA solution.

RESULTS

Of the 273 strains tested, 127 strains were found FOX resistant, 114 were positive by M3DT, 108 were positive in BA disk test, and the multiplex PCR detected 24 pAmpC β-lactamase-positive isolate. The prevalence of AmpC-producing strains was 10.9% in E. coli and 3.6% in K. pneumoniae in the tested population by PCR. CIT and MOX group genes were predominant type in these strains.

CONCLUSION

These results emphasize that clinical laboratories should consider testing the presence of pAmpC enzymes particularly in FOX-resistant isolates, and BA disk test will improve detection of this emerging resistance phenotype.

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.

The antibiotic resistance characteristics of non-typhoidal Salmonella enterica isolated from food-producing animals, retail meat and humans in South East Asia

Antimicrobial resistance is a global problem. It is most prevalent in developing countries where infectious diseases remain common, the use of antibiotics in humans and animals is widespread, and the replacement of older antibiotics with new generation antibiotics is not easy due to the high cost. Information on antibiotic resistance phenotypes and genotypes of Salmonella spp. in food animals and humans in different countries and geographic regions is necessary to combat the spread of resistance. This will improve the understanding of antibiotic resistance epidemiology, tracing of new emerging pathogens, assisting in disease treatment, and enhancing prudent use of antibiotics. However, the extent of antibiotic resistance in food-borne pathogens and humans in many developing countries remains unknown. The goal of this review is to discuss the current state of antibiotic resistance of non-typhoid Salmonella spp. in food-producing animals, retail meat and humans from South East Asia. It is focused on resistance characteristics of traditional and "critically important" antibiotics in this region, and the emergence of multidrug resistant strains and genetic elements that contribute to the development of multidrug resistance, including integrons and the Salmonella Genomic Island (SGI).

Occurrence of Plasmid-Mediated AmpC β-Lactamases Among Escherichia coli and Klebsiella pneumoniae Clinical Isolates in a Tertiary Care Hospital in Bangalore

Therapeutic options for infections caused by gram-negative organisms expressing plasmid-mediated AmpC β-lactamases are limited because these organisms are usually resistant to all the β-lactam antibiotics, except for cefepime, cefpirome and the carbapenems. These organisms are a major concern in nosocomial infections and should therefore be monitored in surveillance studies. Hence, this study was aimed out to determine the prevalence of plasmid-mediated AmpC β-lactamases in E. coli and K. pneumoniae from a tertiary care in Bangalore. A total of 63 E. coli and 27 K. pneumoniae were collected from a tertiary care hospital in Bangalore from February 2008 to July 2008. The isolates with decreased susceptibility to cefoxitin were subjected to confirmation test with three dimensional extract tests. Minimum inhibitory concentrations (MICs) were determined by agar dilution method. Conjugation experiments, plasmid profiling and susceptibility testing were carried out to investigate the underlying mechanism of resistance. In our study, 52 (57.7%) isolates showed resistance to cefoxitin, the occurrence of AmpC was found to be 7.7% of the total isolates. Plasmid analysis of the selected isolates showed the presence of a single plasmid of 26 kb in E. coli and 2 Kb in K. pneumoniae. Plasmid-mediated AmpC β-lactamases were found in 11.1% of K. pneumoniae and in 6.3% of E. coli. Curing and conjugation experiments showed that resistance to cephamycins and cephalosporins was plasmid-mediated. Our study has demonstrated the occurrence of plasmid-mediated AmpC in E. coli and K. pneumoniae which illustrates the importance of molecular surveillance in tracking AmpC-producing strains at general hospitals and emphasizes the need for epidemiological monitoring.

Comparison of methods for AmpC β-lactamase detection in Enterobacteriaceae

AmpC β-lactamases (Bla(AmpC)) are an emerging group of antimicrobial resistance determinants. The lack of an agreed Bla(AmpC) detection method hinders investigation of their epidemiology and understanding of their clinical significance. This study compared the sensitivity and specificity of phenotypic methods of Bla(AmpC) detection in a collection of 246 Enterobacteriaceae with a diverse range of β-lactam resistance profiles. The Bla(AmpC) screening methods evaluated were based on cephamycin, ceftazidime and cefepime susceptibility. These were compared with Bla(AmpC) screening using conventional ESBL detection methods. The confirmatory methods evaluated were biologically based assays, inhibitor-based assays, an AmpC Etest and a rapid chromogenic assay. A multiplex nucleic acid amplification test and the three-dimensional enzyme extraction assay were used as reference methods. Bla(AmpC) activity was present in 74 isolates. The majority of the enzymes were plasmid-encoded and belonged to the CMY, DHA and EBC families. The screening methods had sensitivities between 47 and 99 % and specificities of 45-95 %. The performance of confirmatory tests varied widely, ranging in sensitivity from 19 % to 97 % and in specificity from 88 % to 100 %. Only the Tris-EDTA and MAST ID D68C disc tests had a sensitivity and a specificity above 90 %. Further investigation is needed to establish the most suitable enzyme substrates, inhibitor types, inhibitor concentrations and interpretative cut-offs in order to refine the inhibitor-based methods. A simple disc-based protocol using cefoxitin non-susceptibility as a screening tool, followed by the Tris-EDTA method for confirmation, detects Bla(AmpC) activity with 95 % sensitivity and 98 % specificity.

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 boronic acid disk tests for differentiating KPC-possessing Klebsiella pneumoniae isolates in the clinical laboratory

The worldwide increase in the occurrence and dissemination of KPC beta-lactamases among gram-negative pathogens makes critical the early detection of these enzymes. Boronic acid disk tests using different antibiotic substrates were evaluated for detection of KPC-possessing Klebsiella pneumoniae isolates. A total of 57 genotypically confirmed KPC-possessing K. pneumoniae isolates with varying carbapenem MICs were examined. To measure the specificity of the tests, 106 non-KPC-possessing isolates (89 K. pneumoniae and 17 Escherichia coli isolates) were randomly selected among those exhibiting reduced susceptibility to cefoxitin, expanded-spectrum cephalosporins, or carbapenems. As many as 56, 53, and 40 of the non-KPC-possessing isolates harbored extended-spectrum beta-lactamases, metallo-beta-lactamases, and plasmid-mediated AmpC beta-lactamases, respectively. By use of CLSI methodology and disks containing imipenem, meropenem, or cefepime, either alone or in combination with 400 microg of boronic acid, all 57 KPC producers gave positive results (sensitivity, 100%) whereas all 106 non-KPC producers were negative (specificity, 100%). The meropenem duplicate disk with or without boronic acid demonstrated the largest differences in inhibition zone diameters between KPC producers and non-KPC producers. By use of disks containing ertapenem, all isolates were correctly differentiated except for five AmpC producers that gave false-positive results (sensitivity, 100%; specificity, 95.3%). These practical and simple boronic acid disk tests promise to be very helpful for the accurate differentiation of KPC-possessing K. pneumoniae isolates, even in regions where different broad-spectrum beta-lactamases are widespread.

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.

Antibiotic Resistance in Gram-negative Bacilli: A Singapore Perspective

Introduction: Antibiotic resistance in gram-negative bacilli is an area of increasing importance. This prospective study was performed to survey antibiotic resistance in Escherichia coli (E. coli), Klebsiella spp., Pseudomonas aeruginosa and Acinetobacter spp. over a 1-year period. Materials and Methods: Non-duplicate isolates of E. coli, Klebsiella spp., P. aeruginosa and Acinetobacter spp. were collected from participating Singapore hospitals during defined collection periods in 2006 and 2007. Confirmatory identification and antibiotic susceptibility testing were performed at Changi General Hospital. Minimum inhibitory concentrations (MIC) to a defined panel of antibiotics were determined using microbroth dilution methods. The presence of extended-spectrum beta lactamases and AmpC beta-lactamases in Enterobacteriaceae was determined by phenotypic methods, and susceptibility results were defined using current breakpoints from the Clinical Laboratory Standards Institute (CLSI). Results: Seven hundred and forty-six gram-negative bacilli were received for testing. Resistance to extended-spectrum cephalosporins was present in a third of Enterobacteriaceae isolates, and extended-spectrum beta-lactamases (ESBL) carriage was present in 19.6% and 30.1% of E. coli and Klebsiella pneumoniae, respectively. AmpC enzymes were also detected in 8.5% and 5.6% of E. coli and K. pneumoniae isolates respectively. All Enterobacteriaceae were susceptible to imipenem and meropenem. The most active antibiotics against P. aeruginosa were amikacin, meropenem and piperacillin-tazobactam. A third of P. aeruginosa showed reduced susceptibility to polymyxin B. Carbapenem resistance was significantly higher in Acinetobacter baumannii (70.5%) than in other Acinetobacter species (25.0%). The most active antibiotic against A. baumannii was polymyxin B. Conclusion: Antibiotic resistance is prevalent in gram-negative bacilli isolated from Singapore hospitals. The MIC testing surveillance programme complemented susceptibility data from wider laboratory-based surveillance, and has revealed emerging mechanisms of antibiotic resistance. Keywords: Enterobacteriaceae, Acinetobacter baumannii, Pseudomonas aeruginosa, Microbial sensitivity tests

The prevalence of plasmid-mediated AmpC beta-lactamases among clinical isolates of Escherichia coli and Klebsiella pneumoniae from five children's hospitals in China

The purpose of this study was to investigate the prevalence of plasmid-mediated AmpC beta-lactamases in Escherichia coli and Klebsiella pneumoniae from five children's hospitals in China. A total of 494 E. coli and 637 K. pneumoniae isolates were collected from five children's hospitals in China from 2005 to 2006. The isolates with decreased susceptibility to cefoxitin were subjected to confirmation test with 3-aminophenyl boronic acid. Polymerase chain reaction (PCR) amplification of the blaAmpC, blaTEM, blaCTXM, and blaSHV genes and their gene sequencing were performed. Transconjugants were achieved by conjugation experiments. Plasmid-mediated AmpC beta-lactamases were found in 10.1% of K. pneumoniae (64/637) and in 2.0% of E. coli (10/494) strains. The proportion of plasmid-mediated AmpC-producing strains significantly increased from 2005 (2.6%) to 2006 (9.3%) (p<0.001). The DHA-1-producing isolates were the most prevalent type (93.2%, 69/74). The sequences of blaDHA-1 genes were all identical to those from the GenBank. Strains of blaCMY-2 were isolated from five isolates (6.8%), which were all from E. coli. One sequence of blaCMY-2 differs from blaCMY-2 in the GenBank. Eighteen of the 74 (24.3%) AmpC-producing K. pneumoniae and E. coli isolates coproduced an extended-spectrum beta-lactamase (ESBL). Cefoxitin resistance was transferred to 15 of the 74 positive strains (20.3%). Our study has demonstrated the occurrence of plasmid-mediated AmpC beta-lactamases in E. coli and K. pneumoniae in Chinese pediatric patients and DHA-1 type AmpC enzymes had the highest prevalent rate. The CMY-2 AmpC beta-lactamases from the children's hospitals in China in this study are the first reported. Hence, continuous surveillance of the prevalence and evolution of AmpC beta-lactamase is important.