Resistance to extended-spectrum cephalosporins, caused by PER-1 beta-lactamase, in Salmonella typhimurium from Istanbul, Turkey

On-Farm Anaerobic Digestion of Dairy Manure Reduces the Abundance of Antibiotic Resistance-Associated Gene Targets and the Potential for Plasmid Transfer

The present study investigated the impact of on-farm anaerobic digestion on the abundance of enteric bacteria, antibiotic resistance-associated gene targets, and the horizontal transfer potential of extended-spectrum β-lactamase (ESBL) genes. Samples of raw and digested manure were obtained from six commercial dairy farms in Ontario, Canada. Digestion significantly abated populations of viable coliforms in all six farms. Conjugative transfer of plasmids carrying β-lactamase genes from manure bacteria enriched overnight with buffered peptone containing 4 mg/liter cefotaxime into a β-lactam-sensitive green fluorescent protein (GFP)-labeled Escherichia coli recipient strain was evaluated in patch matings. Digestion significantly decreased the frequency of the horizontal transfer of ESBL genes. Twenty-five transconjugants were sequenced, revealing six distinct plasmids, ranging in size from 40 to 180 kb. A variety of ESBL genes were identified: bla_CTX-M-1, bla_CTX-M-14, bla_CTX-M-15, bla_CTX-M-27, bla_CTX-M-55, and bla_PER-1. bla_CTX-M-15 was the most prevalent ESBL gene detected on plasmids harbored by transconjugants. Various mobile genetic elements were found located proximal to resistance genes. Ten gene targets, including sul1, str(A), str(B), erm(B), erm(F), intI1, aadA, incW, bla_PSE, and bla_OXA-20, were quantified by quantitative PCR on a subset of 18 raw and 18 digested samples. Most targets were significantly more abundant in raw manure; however, erm(B) and erm(F) targets were more abundant in digested samples. Overall, on-farm digestion of dairy manure abated coliform bacteria, a number of antibiotic resistance-associated gene targets, and the potential for in vitro conjugation of plasmids conferring resistance to extended-spectrum β-lactams and other classes of antibiotics into E. coli CV601. IMPORTANCE Using livestock manure for fertilization can entrain antibiotic-resistant bacteria into soil. Manure on some dairy farms is anaerobically digested before being land applied. Recommending the widespread implementation of the practice should be founded on understanding the impact of this treatment on various endpoints of human health concern. Although lab-scale anaerobic treatments have shown potential for reducing the abundance of antibiotic resistance genes, there are very few data from commercial farms. Anaerobic digestion of manure on six dairy farms efficiently abated coliform bacteria, E. coli, and a majority of antibiotic resistance-associated gene targets. In addition, the conjugation potential of plasmids carrying ESBL genes into introduced E. coli strain CV601 was reduced. Overall, anaerobic digestion abated coliform bacteria, the genes that they carry, and the potential for ESBL-carrying plasmid transfer.

Distribution and Classification of Serine β-Lactamases in Brazilian Hospital Sewage and Other Environmental Metagenomes Deposited in Public Databases

β-lactam is the most used antibiotic class in the clinical area and it acts on blocking the bacteria cell wall synthesis, causing cell death. However, some bacteria have evolved resistance to these antibiotics mainly due the production of enzymes known as β-lactamases. Hospital sewage is an important source of dispersion of multidrug-resistant bacteria in rivers and oceans. In this work, we used next-generation DNA sequencing to explore the diversity and dissemination of serine β-lactamases in two hospital sewage from Rio de Janeiro, Brazil (South Zone, SZ and North Zone, NZ), presenting different profiles, and to compare them with public environmental data available. Also, we propose a Hidden-Markov-Model approach to screen potential serine β-lactamases genes (in public environments samples and generated hospital sewage data), exploring its evolutionary relationships. Due to the high variability in β-lactamases, we used a position-specific scoring matrix search method (RPS-BLAST) against conserved domain database profiles (CDD, Pfam, and COG) followed by visual inspection to detect conserved motifs, to increase the reliability of the results and remove possible false positives. We were able to identify novel β-lactamases from Brazilian hospital sewage and to estimate relative abundance of its types. The highest relative abundance found in SZ was the Class A (50%), while Class D is predominant in NZ (55%). CfxA (65%) and ACC (47%) types were the most abundant genes detected in SZ, while in NZ the most frequent were OXA-10 (32%), CfxA (28%), ACC (21%), CEPA (20%), and FOX (19%). Phylogenetic analysis revealed β-lactamases from Brazilian hospital sewage grouped in the same clade and close to sequences belonging to Firmicutes and Bacteroidetes groups, but distant from potential β-lactamases screened from public environmental data, that grouped closer to β-lactamases of Proteobacteria. Our results demonstrated that HMM-based approach identified homologs of serine β-lactamases, indicating the specificity and high sensitivity of this approach in large datasets, contributing for the identification and classification of a large number of homologous genes, comprising possible new ones. Phylogenetic analysis revealed the potential reservoir of β-lactam resistance genes in the environment, contributing to understanding the evolution and dissemination of these genes.

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.

Molecular Characterization of β-lactam Resistance ofSalmonellaIsolates from Pediatric Patients in Romania

The molecular characterization of 16 clinical isolates of Salmonella enterica (14 serotype Typhimurium and 2 serotype Kingston) obtained between January and June 1999 from feces of children hospitalized in Iasi, Romania were genotypically compared by pulse field gel electrophoresis of XbaI restricted bacterial DNA. The majority of the clinical isolates (12/16) belonged to cluster A and (4/16) to unrelated strains, correlating to the OMP profile. Two major different patterns of beta-lactamases were identified: the first with pI of 5.4, 8.2 in 6/16 strains and the second with pI of 5.4 in 5/16. The blaTEM beta-lactamase was identified in 14/16 of the clinical isolates and the blaSHV-5 gene in one strain. We concluded that extended spectrum beta-lactamase (ESBL) with pIs of 8.2 was the most frequent enzyme produced by serotype Typhimurium isolates which were related.

Rate of isolation and trends of antimicrobial resistance of multidrug resistant pseudomonas aeruginosa from otorrhea in chronic suppurative otitis media

Objectives

To assess the rate of isolation of Pseudomonas aeruginosa (PA) and multidrug-resistant PA (MDR-PA) from patients with chronic suppurative otitis media (CSOM) otorrhea and the annual trend of antibiotic-resistance.

Methods

Otorrhea samples were collected aseptically from 1,598 CSOM patients. The rate of bacterial isolation and the results of antibiotic susceptibility testing were evaluated retrospectively.

Results

The PA isolation rate from CSOM otorrhea was 24.4%. Of the 398 isolated strains tested for their susceptibilities to 10 antibiotics, 395 strains showed definitive results. Of these, 183 (46.3%) were susceptible to whole antibiotics and 212 (53.7%) was resistant to more than 1 antibiotics, with the frequency of antibiotics-resistance increasing significantly over time. Although strains susceptible to all antibiotics decreased over time, the rate of isolation of MDR-PA did not change significantly. Resistance to aminoglycosides and quinolones was higher than to other antibiotics and significantly increased over time, whereas resistance to other antibiotics showed no trend.

Conclusion

MDR-PA, assessed using five individual antibiotics and six antibiotic-classes, showed no tendency to increase or decrease over time. This may have been due to increased concern about antibiotic-resistant bacterial strains, leading to improved infection control within hospitals and healthcare centers.

Interspecies dissemination of the bla gene encoding PER-1 extended-spectrum β-lactamase

PER-1 extended-spectrum β-lactamase-producing Gram-negative bacilli are resistant to oxyimino-cephalosporins. However, the bla(PER-1) gene has never been reported in Klebsiella pneumoniae. Here, we studied interspecies dissemination of the bla(PER-1) gene by horizontal transfer of Tn1213 among Acinetobacter baumannii, Pseudomonas aeruginosa, and K. pneumoniae. In a K. pneumoniae clinical isolate, the bla(PER-1) gene was located on a 150-kbp incompatibility group A/C plasmid.

Infective endocarditis caused by Salmonella enteritidis in a dialysis patient: a case report and literature review

Background

Infective endocarditis is significantly more common in haemodialysis patients as compared with the general population, the causative pathogen is generally Staphylococcus aureus; there have been no previously reported cases of infective endocarditis caused by a Salmonella species in haemodialysis patients.

Case Presentation

We report the case of a 68 year-old woman on haemodialysis who developed infective endocarditis as a result of Salmonella enteritidis. Although we treated the patient with ceftriaxone combined with ciprofloxacin, infective endocarditis was not detected early enough and unfortunately developed into cerebral septic emboli, which ultimately resulted in death.

Conclusion

Although there are several reports that Salmonella endocarditis without cardiac failure can be successfully treated with antibiotics alone, early surgical intervention is essential for some cases to prevent life-threatening complications. Transesophageal echocardiography should be performed in any patient with high clinical suspicion of infective endocarditis. To the best of our knowledge, this is the first case-report of Salmonella endocarditis in a haemodialysis patient.

Prosthetic valve endocarditis caused by salmonella enteritidis

Until now, only 12 cases of prosthetic valve endocarditis caused by Salmonella spp. have been reported in the English literature. High complication and mortality rates, a fulminant course and the requirement for early surgical intervention deserve special attention in this kind of infective endocarditis. A new case of Salmonella prosthetic valve endocarditis complicated by sepsis-induced cholestasis and a literature review are presented in this report.

Prevalence, antimicrobial resistance, and extended-spectrum beta-lactamases characterization of Salmonella isolates in Apulia, southern Italy (2001-2005)

The prevalence, antimicrobial susceptibility, and production of extended-spectrum beta-lactamases (ESBLs) of Salmonella collected from several hospitals in Apulia (southern Italy) were evaluated. The most common Salmonella isolates were Salmonella enterica serovar Typhimurium (44.6%), S. enterica serovar Enteritidis (33.4 %), S. enterica serovar Infantis (3.2 %), S. enterica serovar Typhi (1.5%), and S. enterica serovar Bovismorbificans (1.5%). The other serovars accounted for less than 1% each. Our data show a high resistance to ampicillin, tetracycline, and chloramphenicol. The isolates were pansensitive (53.5%), resistant to one antimicrobial agent (10.5%), resistant to two antimicrobial agents (22.1%), resistant to three antimicrobial agents (10.8%), and to four antimicrobial agents (2.7%). Resistance to more than four antibiotics was observed in 0.5% of strains. The presence of ESBL was found in only one strain of S. enterica serovar Bovismorbificans. The CTX-M-1 type-producing strain was identified by isoelectric focusing and molecular analysis. Results were consistent with the presence of a pI 8.6 ESBL active on cefotaxime, ceftazidime, cefepime, and aztreonam. Mating experiments showed that the CTX-M-1 determinant was transferable. To the best of our knowledge, this is the first report of CTX-M-1 type ESBL in Salmonella serovar Bovismorbificans.

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.

Outbreak of Pseudomonas aeruginosa infections with PER-1 extended-spectrum beta-lactamase in Warsaw, Poland: further evidence for an international clonal complex

Forty-one Pseudomonas aeruginosa isolates with extended-spectrum beta-lactamases (ESBLs) from a hospital in Warsaw, Poland, were analyzed. Thirty-seven isolates from several wards were collected over 9 months in 2003 and 2004. The isolates were recovered from patients with multiple types of infections, mostly respiratory tract and postoperative wound infections. All 41 isolates produced the PER-1 ESBL, originally observed in Turkey but recently also identified in several countries in Europe and the Far East. The bla(PER-1) gene resided within the Tn1213 composite transposon, which was chromosomally located. Pulsed-field gel electrophoresis and multilocus sequence typing (MLST) revealed the presence of three separate clones among the isolates. Two of these, corresponding to sequence types (STs) ST244 and ST235, were responsible for parallel outbreaks. Apart from PER-1, all the isolates produced OXA-2 oxacillinase. ST235 isolates additionally expressed a novel enzyme, OXA-74, differing by one amino acid from the OXA-17 ESBL identified originally in PER-1- and OXA-2-positive P. aeruginosa isolates from Ankara, Turkey, in 1992. These earlier Ankara isolates with PER-1, OXA-2, and OXA-17 were also classified into ST235, which is a single-locus variant of two other STs, ST227 and ST230. ST227, ST230, and ST235 all correspond to the recently described clonal complex BG11, which seems to be internationally distributed, having spread in Turkey, Greece, Italy, Hungary, Poland, Sweden, and much of Russia. It is associated with various beta-lactamases, including PER-1 and VIM metalloenzymes. This work further demonstrates the value of MLST of P. aeruginosa.

Biochemical characterization of PER-2 and genetic environment of blaPER-2

PER-2 was the first detected and the second most prevalent extended-spectrum beta-lactamase in clinical pathogens isolated in Argentina and was also reported only in other South American countries. Citrobacter freundii 33587 was isolated in 1999 in Buenos Aires and was resistant to all tested beta-lactams except cephamycins and carbapenems. The strain produced both plasmid-borne TEM-1 and PER-2 (pI 5.4), which could be transferred by conjugation. By PCR screening, thermal asymmetric interlaced PCR, and DNA sequencing, we detected an ISPa12/IS1387a insertion sequence upstream of bla(PER-2), previously reported as also being associated with bla(PER-1). The presence of similar structures upstream of bla(PER-1) and bla(PER-2) suggests a common origin and mobilization. Compared to bla(PER-1) genes, an additional putative promoter for bla(PER-2) was found. PER-2 kinetic analysis showed its high hydrolysis efficiencies toward both CTX and CAZ (k(cat)/K(m), 0.76 and 0.43 microM(-1).s(-1), respectively).

Salmonella resistant to extended-spectrum cephalosporins: prevalence and epidemiology

Salmonella resistant to extended-spectrum cephalosporins (ESCs) have emerged worldwide since 1988. By 2004, 43 countries had reported this public health problem. Resistance was mediated by classical extended-spectrum beta-lactamases, plasmid-mediated cephalosporinases, and recently a class A carbapenemase. Of these, CMY-2 is the most widely disseminated enzyme. Salmonella enterica serotype Typhimurium and S. enterica serotype Enteritidis are the most common serovars associated with ESC resistance in human infections. Many outbreaks in humans have been reported, most often among children and neonates. ESC-resistant Salmonella is frequently recovered from animals and food, with poultry as primary food source, suggesting that humans are often infected by these routes.

Pseudomonas aeruginosa bloodstream infections: risk factors and treatment outcome related to expression of the PER-1 extended-spectrum beta-lactamase

BACKGROUND

Bloodstream infection (BSI) due to Pseudomonas aeruginosa (Pa) has relevant clinical impact especially in relation to drug resistance determinants. The PER-1 extended-spectrum beta-lactamase (ESBL) is a common enzyme conferring high-level resistance to anti-pseudomonal cephalosporins. Risk factors and treatment outcome of BSI episodes caused by PER-1-positive Pa (PER-1-Pa) strains were compared to those caused by ESBL-negative Pa isolates (ESBL-N-Pa).

METHODS

Twenty-six BSI cases due to ceftazidime-resistant Pa strains have been investigated. MIC values of anti-pseudomonal drugs were determined by the Etest method (AB Biodisk, Solna, Sweden). The double-disk synergy test was used to detect ESBL production. PCR amplification and DNA sequencing were used to characterize ESBL types. Clinical records of BSI-patients were examined retrospectively. Demographic data, underlying diseases (McCabe-Jackson classification and Charlson weighted index), risk factors, antimicrobial therapy, and treatment outcome were evaluated in cases due to ESBL-positive and cases due to ESBL-N-Pa isolates. Unpaired Student's t-test, Mann-Whitney U-test, Fisher's exact test and the chi2 test were used for statistical analysis.

RESULTS

Nine Pa isolates expressed the PER-1 ESBL; the remaining 17 isolates did not produce ESBLs. Severe sepsis (P = 0.03), bladder and intravascular catheters (both P = 0.01), immunosuppressive therapy (P = 0.04), and mechanical ventilation (P = 0.03) were significantly associated with BSI due to PER-1-Pa. Empirical treatment (P = 0.02) and treatment after ID/AST (P < 0.01) were rarely adequate in PER-1-Pa cases. With regard to treatment outcome, 77.8% BSI cases due to PER-1-Pa vs. 28.6% cases due to ESBL-N-Pa isolates failed to respond (P < 0.03). All cases due to PER-1-Pa that were treated with carbapenems (alone or in combination with amikacin) failed to respond. In contrast, 7/8 cases due to ESBL-N-Pa given carbapenems were responders.

CONCLUSION

Therapeutic failure and increased hospital costs are associated with BSI episodes caused by PER-1-Pa strains. Thus, recognition and prompt reporting of ESBL-production appears a critical factor for the management of patients with serious P. aeruginosa infections.

Extended-spectrum beta-lactamases: a clinical update

Extended-spectrum beta-lactamases (ESBLs) are a rapidly evolving group of beta-lactamases which share the ability to hydrolyze third-generation cephalosporins and aztreonam yet are inhibited by clavulanic acid. Typically, they derive from genes for TEM-1, TEM-2, or SHV-1 by mutations that alter the amino acid configuration around the active site of these beta-lactamases. This extends the spectrum of beta-lactam antibiotics susceptible to hydrolysis by these enzymes. An increasing number of ESBLs not of TEM or SHV lineage have recently been described. The presence of ESBLs carries tremendous clinical significance. The ESBLs are frequently plasmid encoded. Plasmids responsible for ESBL production frequently carry genes encoding resistance to other drug classes (for example, aminoglycosides). Therefore, antibiotic options in the treatment of ESBL-producing organisms are extremely limited. Carbapenems are the treatment of choice for serious infections due to ESBL-producing organisms, yet carbapenem-resistant isolates have recently been reported. ESBL-producing organisms may appear susceptible to some extended-spectrum cephalosporins. However, treatment with such antibiotics has been associated with high failure rates. There is substantial debate as to the optimal method to prevent this occurrence. It has been proposed that cephalosporin breakpoints for the Enterobacteriaceae should be altered so that the need for ESBL detection would be obviated. At present, however, organizations such as the Clinical and Laboratory Standards Institute (formerly the National Committee for Clinical Laboratory Standards) provide guidelines for the detection of ESBLs in klebsiellae and Escherichia coli. In common to all ESBL detection methods is the general principle that the activity of extended-spectrum cephalosporins against ESBL-producing organisms will be enhanced by the presence of clavulanic acid. ESBLs represent an impressive example of the ability of gram-negative bacteria to develop new antibiotic resistance mechanisms in the face of the introduction of new antimicrobial agents.

PER-1 is still widespread in Turkish hospitals among Pseudomonas aeruginosa and Acinetobacter spp

PER-1 type beta-lactamases were screened among ceftazidime-resistant clinical isolates of Acinetobacter spp. and Pseudomonas aeruginosa. A total of 176 non-repetitive isolates (84 Acinetobacter spp. and 92 P. aeruginosa) were collected during a three month surveillance period. Isolates were obtained from seven intensive care units of seven university hospitals. All strains were screened for bla(PER-1) alleles by PCR. Of the strains, 31% and 55.4% of Acinetobacter spp. and P. aeruginosa were positive for bla(PER-1) type genes, respectively.

Multidrug resistance in Gram-negative bacteria that produce extended-spectrum beta-lactamases (ESBLs)

In 1983, just two years after the introduction of the oxymino-beta-lactams to the market , the first extended-spectrum beta-lactamases were isolated in Germany from Klebsiella pneumoniae strains. Since then several outbreaks have been reported in many European countries and the USA, and nowadays in several places worldwide the problem seems to reach endemic dimensions, with rates exceeding 50% in some countries, such as Portugal and Turkey. On the other hand not only K. pneumoniae but also Escherichia coli strains, with Enterobacter aerogenes predominating among the other enterobacteriaceal species, are increasingly reported as ESBL producers. In this review types, molecular characteristics, detection methods, epidemiology as well as interventions for therapy and antibiotic strategies to prevent and control infections caused by ESBL-producing microorganisms, are presented and discussed.

Tn5393d, a complex Tn5393 derivative carrying the PER-1 extended-spectrum beta-lactamase gene and other resistance determinants

In Alcaligenes faecalis FL-424/98, a clinical isolate that produces the PER-1 extended-spectrum beta-lactamase, the bla(PER-1) gene was found to be carried on a 44-kb nonconjugative plasmid, named pFL424, that was transferred to Escherichia coli by electroporation. Investigation of the genetic context of the bla(PER-1) gene in pFL424 by means of a combined cloning and PCR mapping approach revealed that the gene is associated with a transposonlike element of the Tn3 family. This 14-kb element is a Tn5393 derivative of original structure, named Tn5393d, which contains the transposition module and the strAB genes typical of other members of the Tn5393 lineage plus additional resistance determinants, including the bla(PER-1) gene and a new allelic variant of the aphA6 aminoglycoside phosphotransferase gene, named aphA6b, whose product is active against kanamycin, streptomycin, and amikacin. Tn5393d apparently originated from the consecutive insertion of two composite transposons into a Tn5393 backbone carrying the aphA6b and the bla(PER-1) genes, respectively. The putative composite transposon carrying bla(PER-1), named Tn4176, is made of two original and nonidentical insertion sequences of the IS4 family, named IS1387a and IS1387b, of which one is interrupted by the insertion of an original insertion sequence of the IS30 family, named IS1066. In pFL424, Tn5393d is inserted into a Tn501-like mercury resistance transposon. Transposition of Tn5393d or modules thereof containing the bla(PER-1) gene from pFL424 to small multicopy plasmids or to a bacterial artificial chromosome was not detected in an E. coli host harboring both replicons.

Ceftriaxone resistance of nontyphoidal Salmonella enterica isolates in Northern Taiwan attributable to production of CTX-M-14 and CMY-2 beta-lactamases

Among 3,027 nontyphoidal Salmonella enterica isolates identified between January 1999 and December 2002 in a medical center in northern Taiwan, 31 were resistant to the extended-spectrum cephalosporin ceftriaxone (1.02% [31/3,027]), including 2 in 1999 (0.36% [2/549]), 13 in 2000 (1.49% [13/870]), 7 in 2001 (0.78% [7/893]), and 9 in 2002 (1.26% [9/715]). Sixteen of these isolates belonged to Salmonella serogroup B, nine belonged to serogroup C, four belonged to serogroup D, and two belonged to serogroup E. The majority were from stool cultures. The mechanism of resistance was investigated for eight isolates, including three S. enterica serovar Typhimurium, one S. enterica serovar Wagenia, one S. enterica serovar Senftenberg, one S. enterica serovar Derby, one S. enterica serovar Panama, and one S. enterica serovar Duesseldorf isolate. All eight patients from whom these isolates were recovered had community-acquired infections. All eight isolates were resistant to ampicillin, ceftriaxone, and cefotaxime but susceptible to imipenem and ciprofloxacin. Ceftriaxone resistance was due to the production of the CMY-2 AmpC beta-lactamase by seven isolates and the CTX-M-14 beta-lactamase by the remaining isolate. Both beta-lactamase genes were carried on conjugative plasmids. In a 2.5-kb region encompassing the bla(CMY-2) gene, at nucleotide 49 upstream of the start codon of bla(CMY-2), three of the seven bla(CMY-2)-positive isolates had an A nucleotide and four had a G nucleotide. In conclusion, the ceftriaxone resistance of nontyphoidal Salmonella isolates in our hospital was attributed to the CTX-M-14 and CMY-2 beta-lactamases.

Spread of novel expanded-spectrum β-lactamases in Enterobacteriaceae in a university hospital in the Paris area, France

In 2002, 28 non-duplicate enterobacterial isolates producing extended-spectrum beta-lactamases (ESBLs) were collected from infected patients at the Bicêtre Hospital in Paris, France. Escherichia coli was the predominant ESBL-positive enterobacterial species, comprising ten (36%) of the isolates. CTX-M enzymes (CTX-M-3, CTX-M-10, CTX-M-14 and CTX-M-15) were produced by 11 (39%) of the isolates (six E. coli, two Enterobacter cloacae, one Enterobacter aerogenes, one Proteus mirabilis and one Citrobacter freundii). Other ESBLs, such as VEB-1 and PER-1, were also detected, but less frequently.

PER-1- and OXA-10-like beta-lactamases in ceftazidime-resistant Pseudomonas aeruginosa isolates from intensive care unit patients in Istanbul, Turkey

The presence of PER-1- and OXA-10-like beta-lactamases was investigated by PCR in 49 ceftazidime-resistant Pseudomonas aeruginosa isolates from patients hospitalised in the 24-bed general intensive care unit of the Istanbul Faculty of Medicine during a 12-month period between February 1999 and February 2000. The clonal relatedness of the isolates was investigated by random amplified polymorphic DNA (RAPD) analysis, and the sequences of the PER-1 and OXA genes from all isolates were determined. The rates of resistance of the isolates to imipenem, aztreonam and cefepime were 98%, 92% and 96%, respectively, and to piperacillin and piperacillin-tazobactam were 41% and 37%, respectively. Using the double-disk synergy test, 37% (18/49) of the isolates were identified as extended-spectrum beta-lactamase producers. The PER-1 gene was identified in 86% (42/49) and the OXA-10 gene in 55% (27/49) of the ceftazidime-resistant isolates. Of isolates carrying the PER-1 gene, 48% (20/42) also carried the OXA-10 gene. The respective nucleotide sequences were identical for each isolate. Sixteen RAPD patterns were detected among the PER-1-positive isolates, but 60% (25/42) of the PER-1-positive isolates belonged to two distinct patterns, while the remainder exhibited a wide clonal diversity. The results indicated that the prevalence of PER-1- and OXA-10-like beta-lactamases remains high among ceftazidime-resistant P. aeruginosa isolates in Turkey.

Multiple outbreaks of nosocomial salmonellosis in Russia and Belarus caused by a single clone of Salmonella enterica serovar Typhimurium producing an extended-spectrum beta-lactamase

Thirty-four cefotaxime-resistant Salmonella enterica serovar Typhimurium isolates representative of the isolates that caused outbreaks of gastroenteritis in 10 hospitals in seven regions of Russia and Belarus from 1994 to 2003 were analyzed. All isolates produced the CTX-M-5-like extended-spectrum beta-lactamase, which confers high-level resistance to cefotaxime and ceftriaxone and decreased susceptibility to ceftazidime. The bla(CTX-M) genes were located on small (7.4- to 12-kb) non-self-transferable plasmids approximately 20 bp downstream of the ISEcp1 insertion sequences. Some isolates carried additional conjugative plasmids mediating resistance to penicillin-inhibitor combinations and various non-beta-lactam agents, including tetracycline, chloramphenicol, gentamicin, tobramycin, and co-trimoxazole. Despite the minor differences in susceptibility patterns, all isolates were considered clonally related on the basis of arbitrarily primed PCR and pulsed-field gel electrophoresis analysis. The similarities of the restriction profiles of the CTX-M-coding plasmids further supported the clonal origin of these isolates.

Extended-spectrum beta-lactamases in Klebsiella pneumoniae bloodstream isolates from seven countries: dominance and widespread prevalence of SHV- and CTX-M-type beta-lactamases

A huge variety of extended-spectrum beta-lactamases (ESBLs) have been detected during the last 20 years. The majority of these have been of the TEM or SHV lineage. We have assessed ESBLs occurring among a collection of 455 bloodstream isolates of Klebsiella pneumoniae, collected from 12 hospitals in seven countries. Multiple beta-lactamases were produced by isolates with phenotypic evidence of ESBL production (mean of 2.7 beta-lactamases per isolate; range, 1 to 5). SHV-type ESBLs were the most common ESBL, occurring in 67.1% (49 of 73) of isolates with phenotypic evidence of ESBL production. In contrast, TEM-type ESBLs (TEM-10 type, -12 type, -26 type, and -63 type) were found in just 16.4% (12 of 73) of isolates. The finding of TEM-10 type and TEM-12 type represents the first detection of a TEM-type ESBL in South America. PER (for Pseudomonas extended resistance)-type beta-lactamases were detected in five of the nine isolates from Turkey and were found with SHV-2-type and SHV-5-type ESBLs in two of the isolates. CTX-M-type ESBLs (bla(CTX-M-2) type and bla(CTX-M-3) type) were found in 23.3% (17 of 73) of isolates and were found in all study countries except for the United States. We also detected CTX-M-type ESBLs in four countries where they have previously not been described-Australia, Belgium, Turkey, and South Africa. The widespread emergence and proliferation of CTX-M-type ESBLs is particularly noteworthy and may have important implications for clinical microbiology laboratories and for physicians treating patients with serious K. pneumoniae infections.

Expanded-spectrum cephalosporin resistance in non-typhoid Salmonella

Expanded-spectrum cephalosporins (ESCs) such as ceftriaxone, together with fluorinated quinolones, are the choice antibiotics in the treatment of invasive salmonella infections. Resistance to ESCs among non-typhoid salmonella has been recognised since the late 1980s. Currently, ESC-resistant salmonella strains are reported world-wide and in some areas their incidence is significant. Resistance is mainly due to acquisition of multi-resistant plasmids encoding a variety of extended-spectrum and AmpC-type beta-lactamases. The origins of ESC-resistant salmonellae are diverse. Exchange of resistance determinants between salmonellae and nosocomial enterobacteria seems to be frequent, at least in developing countries. Also, the use of newer beta-lactams in animal husbandry and veterinary medicine may have facilitated the spread of ESC-resistant salmonella strains in livestock.

Predictive value of nalidixic acid resistance for detecting salmonellae with decreased ciprofloxacin susceptibility

Seventy-three Salmonella isolates classified as ciprofloxacin susceptible when using the criteria of the National Committee for Clinical Laboratory Standards were studied for nalidixic acid (NA) resistance. The aim of the study was to determine the predictive value of nalidixic acid resistance in screening for decreased ciprofloxacin susceptibility. We observed that isolates with decreased ciprofloxacin susceptibility were all resistant to nalidixic acid. Identification of nalidixic acid resistance by the disk diffusion method provided 100% sensitivity and a specificity of 98.4% in strains with minimum inhibitory concentrations (MICs) >0.008 mg/l.

Molecular epidemiology of extended-spectrum beta-lactamases produced by clinical isolates in a university hospital in Greece: detection of SHV-5 in Pseudomonas aeruginosa and prevalence of SHV-12

To assess the nature and diversity of various types of SHV and TEM derivatives in our hospital a survey was conducted. Sixty-seven extended-spectrum beta-lactamases (ESBL)-producing nosocomial pathogens, isolated over a 12-month period, were analyzed by means of PCR and direct sequencing. SHV-5 was the predominant ESBL found in our region (38 strains). Other less frequent variants included SHV-2 and SHV-12 with two and three isolates, respectively. For the first time, an outbreak of 11 Pseudomonas aeruginosa producing SHV-5 was encountered. All blaTEM-positive strains carried the non-ESBL TEM-1. The incidence of non-SHV non-TEM ESBLs was remarkably high as almost one out of three isolates harbored such an ESBL. The epidemiological and clinical impact of these findings must be carefully investigated and interpreted.

High prevalence of PER-1 extended-spectrum beta-lactamase-producing Acinetobacter spp. in Korea

PER-1, an extended-spectrum beta-lactamase, has been reported only in Europe. We detected PER-1 in 53 of 97 acinetobacters in Korea, mainly in the sputum of intensive care unit patients. Pulsed-field gel electrophoresis analysis suggested that clonal spread had occurred. Only PCR reliably detected PER-1 producers. PER-1 producers may also exist in other Asian countries.

Imipenem resistance in a Salmonella clinical strain due to plasmid-mediated class A carbapenemase KPC-2

A Salmonella enterica serotype Cubana isolate exhibiting resistance to most beta-lactam antibiotics, including oxyimino-cephalosporins and imipenem, was isolated from a 4-year-old boy with gastroenteritis in Maryland. beta-Lactam resistance was mediated by a conjugative plasmid that encoded KPC-2, a class A carbapenemase previously found in a Klebsiella pneumoniae isolate from the Maryland area as well. Sequence analysis of the flanking regions indicated a potential association of bla(KPC-2) with mobile structures.

Treatment of Salmonella meningitis: two case reports and a review of the literature

BACKGROUND

Salmonella species now represent a leading cause of Gram-negative bacterial meningitis in the developing world. Various drugs have been used for the treatment of Salmonella meningitis over the past decades, but mortality, neurologic sequelae and relapse rates remain high. In this report we describe two children aged 8 and 9 months who presented within a week to our hospital with Salmonella meningitis. They were treated with penicillin and chloramphenicol but progressed rapidly to death within 48 h.

AIM

The aim of this article is to review all published English literature on the treatment of Salmonella meningitis and identify the best drug option for its treatment. This was done by comparing the outcomes such as cure, failure, relapse, and death rates.

METHOD

A Medline electronic search was carried out to find and retrieve articles that have been published since 1987, when the last review of Salmonella meningitis was done.

RESULTS

Salmonella typhimurium was the commonest organism reported, and 89.7% of infections occurred in children less than 1 year old. Fluoroquinolones had a cure rate of 88.9%, while the third-generation cephalosporins had a cure rate of 84.6%. Conventional antibiotics (chloramphenicol, ampicillin, and cotrimoxazole) had a cure rate of 41.2%, a relapse rate of 11.8%, and an associated mortality of 44.7%. Treatment with fluoroquinolone and imipenem resulted in no deaths. There were, however, only two cases that were treated with imipenem.

CONCLUSION

When Salmonella meningitis is suspected, third-generation cephalosporins, with or without a fluoroquinolone, may be the best option for treatment.

Emergence of ceftriaxone-resistant Salmonella isolates and rapid spread of plasmid-encoded CMY-2-like cephalosporinase, Taiwan

Of 384 Salmonella isolates collected from 1997 to 2000 in a university hospital in Taiwan, six ceftriaxone-resistant isolates of Salmonella enterica serovar Typhimurium were found in two patients in 2000. The resistance determinants were on conjugative plasmids that encoded a CMY-2-like cephalosporinase. During the study period, the proportion of CMY-2-like enzyme producers among Escherichia coli increased rapidly from 0.2% in early 1999 to >4.0% in late 2000. Klebsiella pneumoniae isolates producing a CMY-2-like beta-lactamase did not emerge until 2000. The presence of bla(CMY)-containing plasmids with an identical restriction pattern from Salmonella, E. coli, and K. pneumoniae isolates was found, which suggests interspecies spread and horizontal transfer of the resistance determinant. Various nosocomial and community-acquired infections were associated with the CMY-2-like enzyme producers. Our study suggests that the spread of plasmid-mediated CMY-2-like beta-lactamases is an emerging threat to hospitalized patients and the public in Taiwan.

Characterization of the first extended-spectrum beta-lactamase-producing Salmonella isolate identified in Canada

A single Salmonella enterica serovar Typhimurium isolate with an UT2 phage type producing an extended-spectrum beta-lactamase (ESBL) was identified in Canada in 2000. The isolate harbored two plasmids, one containing a bla(TEM-1) gene and the other containing a bla(SHV-2a) gene. The ESBL gene was located on a 70-kb transferable plasmid which also carried tetracycline and trimethoprim resistance elements.

Expanded-spectrum cephalosporin-resistant salmonella strains in Romania

Thirteen Salmonella enterica serotype Typhimurium and one Salmonella enterica serotype Heidelberg strain resistant to expanded-spectrum cephalosporins were isolated from October 2000 to February 2001 from infants with gastroenteritis in Iasi, Romania. In all but one serotype Typhimurium isolate, resistance was due to the production of a CMY-2 cephalosporinase encoded by a nonconjugative plasmid. The remaining isolate produced an SHV-5-type beta-lactamase. Typing by pulsed-field gel electrophoresis indicated that the CMY-2-producing serotype Typhimurium isolates were related.

Plasmidic extended-spectrum beta-lactamases in Vibrio cholerae O1 El Tor isolates in Argentina

Since 1992 there have been seven major outbreaks of cholera in Argentina. Susceptibility analysis of 1,947 isolates (40% of reported cases) of Vibrio cholerae O1 biotype El Tor suggested the presence of extended-spectrum beta-lactamases (ESBLs) in 28 isolates. Because of their different susceptibility profiles, V. cholerae isolates M1502, M1516, M1573, and M3030 (all of which are of the Ogawa serotype) were selected for the present study. By susceptibility analysis, isoelectric focusing, and PCR-based restriction fragment length polymorphism analysis, CTX-M-type enzymes were identified in three isolates, whereas a PER-2-type enzyme, in addition to a TEM-1-like enzyme, was identified in the other isolate. The presence of these ESBLs in V. cholerae isolates resulted in MICs well below those commonly observed for members of the family ENTEROBACTERIACEAE: Genes that encode both ESBLs were transferred to Escherichia coli by conjugation, together with all determinants of resistance to non-beta-lactam antibiotics (gentamicin, kanamycin, and sulfamethoxazole for all isolates; amikacin and streptomycin for three isolates; trimethoprim, tetracycline, and chloramphenicol for two isolates). Plasmid profile analysis and Southern blotting revealed the presence of single plasmids of about 150 kb in the four V. cholerae isolates and their respective transconjugants and revealed that the plasmids harbored genes encoding CTX-M-type or PER-2-type ESBLs. These results strongly suggest the broad spread of these ESBLs among genera belong to families other than the ENTEROBACTERIACEAE:

Surveillance of antimicrobial resistance in gram-negative isolates from intensive care units in Turkey: analysis of data from the last 5 years

A multicenter antimicrobial surveillance program was established in Turkey in 1995 to monitor the predominant Gram-negative pathogens from intensive care units (ICUs) and antimicrobial resistance patterns of these isolates. Sixteen hospitals participated in the study and a total of 1479 isolates from 1,100 patients were collected. The isolates were tested for their susceptibility against 13 antibiotics by E-test method. Minimum inhibitory concentrations (MICs) for each isolate were determined for imipenem, ceftazidime, ceftazidime-clavulanate, cefoperazone-sulbactam, ceftriaxone, cefepime, cefuroxime, piperacillin-tazobactam, ticarcillin-clavulanate, gentamicin, amikacin and ciprofloxacin. The most common isolates were Pseudomonas spp. (28.2%), Escherichia coli (19.2%) and Klebsiella spp. (19.1%). We found very high resistance rates to all major antibiotics that are used to treat serious infections. Although imipenem is the most active agent, it had an overall susceptibility rate of 68%. Half of the tested Klebsiella spp. strains were found to produce ESBL. This is a very high rate when compared with the literature. Cross-resistance among species was also investigated. 52% of ciprofloxacin-resistant strains were also resistant to imipenem, 80% to ceftazidime, 97% to ceftriaxone, 86% to amikacin and 19% of imipenem-resistant strains were susceptible to ceftazidime and 18% to amikacin. When susceptibilities of the years 1995 and 1999 were compared, the most interesting finding was the decrease in resistance to 3rd generation cephalosporins. In conclusion, this national clinical isolate database shows that resistance rates are high, the change over years is not predictable and continuous surveillance is necessary to monitor antimicrobial resistance and to guide antibacterial therapy.

Extended-spectrum beta-lactamases in the 21st century: characterization, epidemiology, and detection of this important resistance threat

Beta-lactamases continue to be the leading cause of resistance to beta-lactam antibiotics among gram-negative bacteria. In recent years there has been an increased incidence and prevalence of extended-spectrum beta-lactamases (ESBLs), enzymes that hydrolyze and cause resistance to oxyimino-cephalosporins and aztreonam. The majority of ESBLs are derived from the widespread broad-spectrum beta-lactamases TEM-1 and SHV-1. There are also new families of ESBLs, including the CTX-M and OXA-type enzymes as well as novel, unrelated beta-lactamases. Several different methods for the detection of ESBLs in clinical isolates have been suggested. While each of the tests has merit, none of the tests is able to detect all of the ESBLs encountered. ESBLs have become widespread throughout the world and are now found in a significant percentage of Escherichia coli and Klebsiella pneumoniae strains in certain countries. They have also been found in other Enterobacteriaceae strains and Pseudomonas aeruginosa. Strains expressing these beta-lactamases will present a host of therapeutic challenges as we head into the 21st century.

Dynamics of a nosocomial outbreak of multidrug-resistant Pseudomonas aeruginosa producing the PER-1 extended-spectrum beta-lactamase

From November 1998 to August 1999, a large outbreak occurred in the general intensive care unit of the Ospedale di Circolo in Varese (Italy), caused by Pseudomonas aeruginosa producing the PER-1 extended-spectrum beta-lactamase. A total of 108 clinical isolates of P. aeruginosa resistant to broad-spectrum cephalosporins were recovered from 18 patients. Epidemic isolates were characterized by synergy between clavulanic acid and ceftazidime, cefepime, and aztreonam. Isoelectric focusing of crude bacterial extracts detected two nitrocefin-positive bands with pI values of 8.0 and 5.3. PCR amplification and characterization of the amplicons by restriction analysis and direct sequencing indicated that the epidemic isolates carried a bla(PER-1) determinant. The outbreak was of clonal origin as shown by pulsed-field gel electrophoresis analysis. This technique also indicated that the epidemic strain was not related to three other PER-1-positive isolates obtained at the same hospital in 1997. Typing by enterobacterial repetitive intergenic consensus-PCR showed that minor genetic variations occurred during the outbreak. The epidemic strain was characterized by a multiple-drug-resistance phenotype that remained unchanged over the outbreak, including extended-spectrum cephalosporins, monobactams, aminoglycosides, and fluoroquinolones. Isolation of infected patients and appropriate carbapenem therapy were successful in ending the outbreak. Our report indicates that the bla(PER-1) resistance determinant may become an emerging therapeutic problem in Europe.

Characterization of extended-spectrum beta-lactamase (TEM-52)-producing strains of Salmonella enterica serovar typhimurium with diverse resistance phenotypes

Two Salmonella enterica serovar Typhimurium strains from different clonal origins, both producing an extended-spectrum beta-lactamase (TEM-52), were isolated from a patient. This enzyme was encoded on a single plasmid and was found at very low levels in one strain, while being encoded on multiple plasmids and in multiple different EcoRI fragments in the other strain.

What's New in beta-lactamases?

beta-lactamases continue to be the leading cause of resistance to beta-lactam antibiotics, among gram-negative bacteria. In recent years, both the incidence and the prevalence of extended-spectrum beta-lactamases, inhibitor-resistant beta-lactamases, AmpC-type enzymes, and both metallo- carbapenemases and nonmetallo-carbapenemases have increased. These beta-lactamases provide resistance to oximino-cephalosporins, beta-lactam/beta-lactamase inhibitor combinations, cephamycins, and carbapenems, respectively. Strains expressing these beta-lactamases will generate a host of therapeutic challenges as we begin the 21st century.

Phage types, antibiotic susceptibilities and plasmid profiles of Salmonella typhimurium and Salmonella enteritidis strains isolated in Istanbul, Turkey

OBJECTIVE

To examine 13 Salmonella typhimurium and 22 S. enteritidis strains isolated from individual cases of gastroenteritis for their phage types, antibiotic susceptibilities and plasmid profiles.

METHODS

The phage typing of S. typhimurium strains was done according to the method of Anderson et al, and the phage typing scheme of Ward et al was used for phage typing of S. enteritidis strains. Antibiotic susceptibility testing was performed by the Kirby-Bauer disk diffusion method. Extended-spectrum beta-lactamase production of the strains was determined by the three-dimensional method. Plasmid profiles of the strains were examined using the method described by Kado and Liu with some modification by Graeber et al.

RESULTS

Two S. typhimurium strains were DT 193 and one was DT 22, whereas 10 strains were untypable. PT 4 was the predominant phage type among S. enteritidis strains. Four S. enteritidis strains were DT 6a, three strains were PT 1 and one strain was PT 8, whereas only one strain was untypable. Eleven of 13 S. typhimurium and three of 22 S. enteritidis strains were found to be multiresistant. Ten different resistance patterns among S. typhimurium and four different resistance patterns among S. enteritidis strains were detected. Extended-spectrum beta-lactamase production was detected in 10 of 13 S. typhimurium and in three of 22 S. enteritidis strains. All S. typhimurium strains but one were found to contain at least one plasmid, with molecular masses varying between 4 and 107 MDa, and 11 different plasmid patterns were determined. Plasmid pattern analysis permitted further differentiation of the S. enteritidis strains into nine groups. A serovar-specific virulence plasmid of 36 MDa was detected in 13 of 22 S. enteritidis strains.

CONCLUSIONS

The results suggest that the majority of S. typhimurium strains were closely related.

Antimicrobial resistance of gram-negative isolates from intensive care units in Turkey: comparison to previous three years

Resistance rates to selected antibiotics of gram-negative bacteria isolated from intensive care units (ICU) of 16 Turkish hospitals during 1998 were evaluated and compared to data from the previous 3 years. Antibiotic susceptibilities to imipenem, ceftazidime, ceftazidime-clavulanate, cefoperazone-sulbactam, ceftriaxone, cefepime, cefodizime, cefuroxime, piperacillin-tazobactam, ticarcillin-clavulanate, gentamicin, amikacin and ciprofloxacin were determined by Etest. A total of 1,404 isolates from 1,060 patients were collected, mainly from urinary and respiratory tracts. As in the previous 3 years, Pseudomonas spp. was the most frequently isolated gram-negative species (29.7%), followed by Escherichia coli, Acinetobacter and Klebsiella spp. Imipenem was the most active in vitro agent (73.4% susceptible), followed by ciprofloxacin (60.6%), cefoperazone-sulbactam (58.7%), cefepime (56.7%), piperacillin-tazobactam (55.0%) and amikacin (54.7%). In 1996, a decline in susceptibility rates of all antibiotics was evident. With the exception of imipenem, resistance to which remained stable, rates somewhat increased in 1997. In 1998, susceptibility to imipenem and cefepime remained stable, amikacin resistance tended to increase and susceptibility rates to other antibacterials showed a favorable increase. These results may in part be due to the implementation of a surveillance program and increased understanding of the magnitude of the resistance problem.

Ceftriaxone-resistant salmonella infection acquired by a child from cattle

BACKGROUND

The emergence of resistance to antimicrobial agents within the salmonellae is a worldwide problem that has been associated with the use of antibiotics in livestock. Resistance to ceftriaxone and the fluoroquinolones, which are used to treat invasive salmonella infections, is rare in the United States. We analyzed the molecular characteristics of a ceftriaxone-resistant strain of Salmonella enterica serotype typhimurium isolated from a 12-year-old boy with fever, abdominal pain, and diarrhea.

METHODS

We used pulsed-field gel electrophoresis and analysis of plasmids and beta-lactamases to compare the ceftriaxone-resistant S. enterica serotype typhimurium from the child with four isolates of this strain obtained from cattle during a local outbreak of salmonellosis.

RESULTS

The ceftriaxone-resistant isolate from the child was indistinguishable from one of the isolates from cattle, which was also resistant to ceftriaxone. Both ceftriaxone-resistant isolates were resistant to 13 antimicrobial agents; all but one of the resistance determinants were on a conjugative plasmid of 160 kb that encoded the functional group 1 beta-lactamase CMY-2. Both ceftriaxone-resistant isolates were closely related to the three other salmonella isolates obtained from cattle, all of which were susceptible to ceftriaxone.

CONCLUSIONS

This study provides additional evidence that antibiotic-resistant strains of salmonella in the United States evolve primarily in livestock. Resistance to ceftriaxone, the drug of choice for invasive salmonella disease, is a public health concern, especially with respect to children, since fluoroquinolones, which can also be used to treat this disease, are not approved for use in children.

Propagation of TEM- and PSE-type beta-lactamases among amoxicillin-resistant Salmonella spp. isolated in France

A survey conducted between 1987 and 1994 at the University Hospital of Besançon, France, demonstrated a dramatic increase (from 0 to 42. 5%) in the prevalence of amoxicillin resistance among Salmonella spp. Of the 96 resistant isolates collected during this period (including 77 Typhimurium), 54 were found to produce TEM-1 beta-lactamase, 40 produced PSE-1 (equivalent to CARB-2), one produced PSE-1 plus TEM-2, and one produced OXA-1 in isoelectric focusing and DNA hybridization experiments. Plasmids coding for these beta-lactamases were further characterized by (i) profile analysis, (ii) restriction fragmentation pattern analysis, (iii) hybridization with an spvCD-orfE virulence probe, and (iv) replicon typing. In addition, isolates of S. typhimurium were genotypically compared by pulsed-field gel electrophoresis of XbaI-macrorestricted chromosomal DNA. Altogether, these methods showed that 40 of the 41 PSE-1 producers were actually the progeny of a single epidemic S. typhimurium strain lysotype DT104. Isolates of that strain were found to harbor RepFIC virulence plasmids with somewhat different restriction profiles, but which all carried the bla(PSE-1) gene. Of these virulence/resistance plasmids, 15 were transmissible to Escherichia coli. TEM-1-producing S. typhimurium displayed much greater genotypic and plasmidic diversities, suggesting the acquisition of the bla(TEM-1) gene from multiple bacterial sources by individual strains. In agreement with this, 32 of the 35 S. typhimurium plasmids encoding TEM-1 were found to be conjugative. These data show that development of amoxicillin resistance among Salmonella, especially in serovar Typhimurium, results from both gene transfers and strain dissemination.