Plasmid-determined beta-lactamase indistinguishable from the chromosomal beta-lactamase of Escherichia coli

Genetic Characterization of AmpC and Extended-Spectrum Beta-Lactamase Phenotypes in Escherichia coli and Salmonella From Alberta Broiler Chickens

Horizontal gene transfer is an important mechanism which facilitates bacterial populations in overcoming antimicrobial treatment. In this study, a total of 120 Escherichia coli and 62 Salmonella enterica subsp. enterica isolates were isolated from broiler chicken farms in Alberta. Fourteen serovars were identified among Salmonella isolates. Thirty one percent of E. coli isolates (37/120) were multiclass drug resistant (resistant to ≥ 3 drug classes), while only about 16% of Salmonella isolates (10/62) were multiclass drug resistant. Among those, eight E. coli isolates had an AmpC-type phenotype, and one Salmonella isolate had an extended-spectrum beta-lactamase (ESBL)-type beta-lactamase phenotype. We identified both AmpC-type (bla_CMY-2) and ESBL-type (bla_TEM) genes in both E. coli and Salmonella isolates. Plasmids from eight of nine E. coli and Salmonella isolates were transferred to recipient strain E. coli J53 through conjugation. Transferable plasmids in the eight E. coli and Salmonella isolates were also transferred into a lab-made sodium azide-resistant Salmonella recipient through conjugation. The class 1 integrase gene, int1, was detected on plasmids from two E. coli isolates. Further investigation of class 1 integron cassette regions revealed the presence of an aadA gene encoding streptomycin 3’’-adenylyltransferase, an aadA1a/aadA2 gene encoding aminoglycoside 3’’-O-adenyltransferase, and a putative adenylyltransferase gene. This study provides some insight into potential horizontal gene transfer events of antimicrobial resistance genes between E. coli and Salmonella in broiler chicken production.

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.

Prevalence of AmpC and other beta-lactamases in enterobacteria at a large urban university hospital in Brazil

Production of extended-spectrum beta-lactamases (ESBLs) has been reported in virtually all species of Enterobacteriaceae, which greatly complicates the therapy for infections caused by these organisms. However, the frequency of isolates producing AmpC beta-lactamases, especially plasmid-mediated AmpC (pAmpC), is largely unknown. These beta-lactamases confer resistance to extended-spectrum cephalosporins and aztreonam, a multidrug-resistant (MDR) profile. The aim of the present study was to determine the occurrence of ESBL and pAmpC beta-lactamases in a hospital where MDR enterobacterial isolates recently emerged. A total of 123 consecutive enterobacterial isolates obtained from 112 patients at a university hospital in Rio de Janeiro, Brazil, during March to June 2001 were included in the study. ESBL was detected by the addition of clavulanate to cephalosporin containing disks and by double diffusion. AmpC production was evaluated by a modified tridimensional test and a modified Hodge test. The presence of plasmid-mediated ampC beta-lactamase genes was evaluated by multiplex polymerase chain reaction. Sixty-five (53%) of 123 enterobacterial isolates were MDR obtained from 56 patients. ESBL production was detected in 35 isolates; 5 clonal Escherichia coli isolates exhibited high levels of chromosomal AmpC and ESBL production. However, no isolates contained pAmpC genes. Infection or colonization by MDR enterobacteria was not associated with any predominant resistant clones. A large proportion of hospital infections caused by ESBL-producing enterobacteria identified during the study period were due to sporadic infections rather than undetected outbreaks. This observation emphasizes the need to improve our detection methods for ESBL- and AmpC-producing organisms in hospitals where extended-spectrum cephalosporins are in wide use.

Amp C beta-lactamase-producing Escherichia coli in neonatal meningitis: diagnostic and therapeutic challenge

Antibiotic resistance is a global health priority. Major defenses for Gram-negative bacteria are beta-lactamase enzymes, which have co-evolved with the development and increasing utilization of new antibiotics. Bacteria harboring the plasmid-mediated AmpC enzymes are increasingly prevalent among adult patients, but have not previously been reported in neonates. Early-onset neonatal meningitis caused by an AmpC beta-lactamase-producing Escherichia coli is described for the first time; the plasmid was identified as a transferable CMY-2 family beta-lactamase. Limited experience with newer antibiotics and pharmacokinetics in neonates presents a therapeutic challenge. Currently, there are no Clinical Laboratory Standards Institute (CLSI) recommendations for detecting AmpC nor is the optimal treatment for AmpC-producing organisms known. Thus, it is imperative that clinicians have a high index of suspicion when antimicrobial susceptibility patterns are inconsistent. Development of better microbiology screening tests to rapidly detect resistance is essential. Additionally, pharmacokinetic studies with newer antibiotics in neonates are warranted.

Characterization of β-Lactamases Responsible for Resistance to Extended-Spectrum Cephalosporins inEscherichia coliandSalmonella entericaStrains from Food-Producing Animals in the United Kingdom

Nine epidemiologically unrelated isolates [1 Salmonella Bredeney from turkeys, and 8 Escherichia coli [3 environmental isolates (2 from chickens, 1 from pigs), and 5 isolates from cattle with neonatal diarrhea]] were examined both pheno- and genotypically for extended-spectrum beta-lactam (ESBL) resistance. Resistance phenotypes (ampicillin, aztreonam, cefotaxime, cefpodoxime, ceftazidime, and ceftriaxone) suggested the presence of an ESBL enzyme, but cefoxitin MICs (>/= 32 mg/L) suggested the presence of an AmpC-like enzyme. Synergism experiments with benzo(b)thiophene-2-boronic acid (BZBTH2B) and isoelectric focusing (IEF) revealed the presence of an AmpC beta-lactamase with a pI >/= 9. amp C multiplex PCR, sequence, and Southern analyses indicated that only the Salmonella isolate had a plasmid-encoded AmpC beta-lactamase CMY-2 on a nonconjugative 60-MDa plasmid. PCR and sequence analysis of the E. coli ampC promoter identified mutations at positions -88(T), -82(G), -42(T), -18(A), -1(T) and +58(T) in all the isolates. In addition one strain had two extra-mutations at positions +23(A) and +49(G), and another strain had one extra-mutation at position +32(A). DNA fingerprinting revealed that all the E. coli isolates were different clones. It also showed that the U.K. Salmonella isolate was indistinguisable from a Canadian Salmonella isolate from turkeys; both had identical resistance phenotypes and produced CMY-2. This is the first report of a CMY-2 Salmonella isolate in the United Kingdom. These data imply that beta-lactam resistance in animal isolates can be generated de novo as evidenced by the E. coli strains, or in the case of the Salmonella strains be the result of intercontinental transmission due to an acquired resistance mechanism.

Plasmid-borne AmpC beta-lactamases

Historically, it was thought that ampC genes encoding class C beta-lactamases were located solely on the chromosome but, within the last 12 years, an increasing number of ampC genes have been found on plasmids. These have mostly been acquired by ampC-deficient pathogenic bacteria, which consequently are supplied with new and additional resistance phenotypes. This review discusses the phylogenetic origin of the plasmid-encoded AmpC beta-lactamases, their occurrence, and mode of spread, as well as their hydrolytic properties.

Prevalence of beta-lactamases among 1,072 clinical strains of Proteus mirabilis: a 2-year survey in a French hospital

beta-Lactam resistance was studied in 1,072 consecutive P. mirabilis clinical strains isolated at the Clermont-Ferrand teaching hospital between April 1996 and March 1998. The frequency of amoxicillin resistance was 48.5%. Among the 520 amoxicillin-resistant isolates, three resistance phenotypes were detected: penicillinase (407 strains [78.3%]), extended-spectrum beta-lactamase (74 strains [14. 2%]), and inhibitor resistance (39 strains [7.5%]). The penicillinase phenotype isolates were divided into three groups according to the level of resistance to beta-lactams, which was shown to be related to the strength of the promoter. The characterization of the different beta-lactamases showed that amoxicillin resistance in P. mirabilis was almost always (97%) associated with TEM or TEM-derived beta-lactamases, most of which evolved via TEM-2.

Occurrence and detection of AmpC beta-lactamases among Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis isolates at a veterans medical center

AmpC beta-lactamases are cephalosporinases that confer resistance to a wide variety of beta-lactam drugs and that may thereby create serious therapeutic problems. Although reported with increasing frequency, the true rate of occurrence of AmpC beta-lactamases in Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis remains unknown. We tested a total of 1,286 consecutive, nonrepeat isolates of these three species and found that, overall, 45 (3.5%) yielded a cefoxitin zone diameter less than 18 mm (screen positive) and that 16 (1.2%) demonstrated AmpC bands by isoelectric focusing. Based on the species, of 683 E. coli, 371 K. pneumoniae, and 232 P. mirabilis isolates tested, 13 (1.9%), 28 (7.6%), and 4 (1.7%), respectively, demonstrated decreased zone diameters and 11 (1.6%), 4 (1.1%), and 1 (0.4%), respectively, demonstrated AmpC bands. Cefoxitin resistance was transferred for all but 8 (E. coli) of the 16 AmpC producers. We also describe a three-dimensional extract test, which was used to detect phenotypically isolates that harbor AmpC beta-lactamase. Of the 45 cefoxitin-resistant isolates, the three-dimensional extract test accurately identified all 16 AmpC producers and 28 of 29 (97%) isolates as non-AmpC producers. Interestingly, most (86%) isolates in the latter group were K. pneumoniae isolates. These data confirm that, at our institution, E. coli, K. pneumoniae, and P. mirabilis harbor plasmid-mediated AmpC enzymes.

Diversity of TEM mutants in Proteus mirabilis

In a survey of resistance to amoxicillin among clinical isolates of Proteus mirabilis, 10 TEM-type beta-lactamases were characterized: (i) the well-known penicillinases TEM-1 and TEM-2, the extended-spectrum beta-lactamases (ESBLs) TEM-3 and TEM-24, and the inhibitor-resistant TEM (IRT) TEM-44 and (ii) five novel enzymes, a penicillinase TEM-57 similar to TEM-1, an ESBL TEM-66 similar to TEM-3, and three IRTs, TEM-65, TEM-73, and TEM-74. The penicillinase TEM-57 and the ESBL TEM-66 differed from TEM-1 and TEM-3, respectively, by the amino acid substitution Gly-92-->Asp (nucleotide mutation G-477-->A). This substitution could have accounted for the decrease in pIs (5.2 for TEM-57 and 6.0 for TEM-66) but did not necessarily affect the intrinsic activities of these enzymes. The IRT TEM-65 was an IRT-1-like IRT (Cys-244) related to TEM-2 (Lys-39). The two other IRTs, TEM-73 and TEM-74, were related to IRT-1 (Cys-244) and IRT-2 (Ser-244), respectively, and harbored the amino acid substitutions Leu-21-->Phe and Thr-265-->Met. In this study, the ESBLs TEM-66, TEM-24, and TEM-3 were encoded by large (170- to 180-kb) conjugative plasmids that exhibited similar patterns after digestion and hybridization with the TEM and AAC(6')I probes. The three IRTs TEM-65, TEM-73, and TEM-74 were encoded by plasmids that ranged in size from 42 to 70 kb but for which no transfer was obtained. The characterization of five new plasmid-mediated TEM-type beta-lactamases and the first report of TEM-24 in P. mirabilis are evidence of the wide diversity of beta-lactamases produced in this species and of its possible role as a beta-lactamase-encoding plasmid reservoir.

Chromosomally encoded ampC-type beta-lactamase in a clinical isolate of Proteus mirabilis

A clinical strain of Proteus mirabilis (CF09) isolated from urine specimens of a patient displayed resistance to amoxicillin (MIC >4,096 microg/ml), ticarcillin (4,096 microg/ml), cefoxitin (64 microg/ml), cefotaxime (256 microg/ml), and ceftazidime (128 microg/ml) and required an elevated MIC of aztreonam (4 microg/ml). Clavulanic acid did not act synergistically with cephalosporins. Two beta-lactamases with apparent pIs of 5.6 and 9.0 were identified by isoelectric focusing on a gel. Substrate and inhibition profiles were characteristic of an AmpC-type beta-lactamase with a pI of 9.0. Amplification by PCR with primers for ampC genes (Escherichia coli, Enterobacter cloacae, and Citrobacter freundii) of a 756-bp DNA fragment from strain CF09 was obtained only with C. freundii-specific primers. Hybridization results showed that the ampC gene is only chromosomally located while the TEM gene is plasmid located. After cloning of the gene, analysis of the complete nucleotide sequence (1,146 bp) showed that this ampC gene is close to blaCMY-2, from which it differs by three point mutations leading to amino acid substitutions Glu --> Gly at position 22, Trp --> Arg at position 201, and Ser --> Asn at position 343. AmpC beta-lactamases derived from that of C. freundii (LAT-1, LAT-2, BIL-1, and CMY-2) have been found in Klebsiella pneumoniae, E. coli, and Enterobacter aerogenes and have been reported to be plasmid borne. This is the first example of a chromosomally encoded AmpC-type beta-lactamase observed in P. mirabilis. We suggest that it be designated CMY-3.

beta-Lactamases in laboratory and clinical resistance

beta-Lactamases are the commonest single cause of bacterial resistance to beta-lactam antibiotics. Numerous chromosomal and plasmid-mediated types are known and may be classified by their sequences or phenotypic properties. The ability of a beta-lactamase to cause resistance varies with its activity, quantity, and cellular location and, for gram-negative organisms, the permeability of the producer strain. beta-Lactamases sometimes cause obvious resistance to substrate drugs in routine tests; often, however, these enzymes reduce susceptibility without causing resistance at current, pharmacologically chosen breakpoints. This review considers the ability of the prevalent beta-lactamases to cause resistance to widely used beta-lactams, whether resistance is accurately reflected in routine tests, and the extent to which the antibiogram for an organism can be used to predict the type of beta-lactamase that it produces.

Screening method for beta-lactamase substrate profiles

A disc diffusion method, based on the idea of Klundert, for screening of substrate profiles of beta-lactamases was developed in order to perform epidemiological studies. The method was tested against 30 different reference beta-lactamases and 59 clinical isolates known to produce TEM-1, SHV-1 and BRO-1. The reproducibility and discriminating power of the disc diffusion method made it possible to differentiate between eight types of substrate profiles for the 30 reference beta-lactamases and to differentiate between TEM-1, SHV-1 and BRO-1 from clinical isolates. In combination with analytical isoelectric focusing the disc diffusion method gives a reliable identification of beta-lactamases.

In vitro and in vivo transferrable beta-lactam resistance due to a new plasmid-mediated oxyiminocephalosporinase from a clinical isolate of Proteus mirabilis

A new plasmid-mediated beta-lactamase (FPM-1) with an isoelectric point of 7.2 and a molecular weight of 26,000 was found in a cefuroxime-resistant clinical isolate of Proteus mirabilis strain 6003. FPM-1 can be classified as a type I oxyimino-cephalosporinase on the basis of its substrate specificity and inhibition pattern by clavulanic acid etc., and its conferred resistance on both the strain and transconjugants against most oxyme-type cephalosporins as well as the older ones but not against cefamycins and a few exceptional oxyme-type cephalosporins such as ceftizoxime, ceftazidime and cefixime. In a murine systemic infection model, only these FPM-1-stable drugs exhibited protective activity against the FPM-1-producing P. mirabilis 6003 similar to that against a nonproducing derivative strain. The FPM-1-mediated cefuroxime resistance in P. mirabilis 6003 was transferred to co-infected Escherichia coli 7004 at frequencies between 3.8 x 10(-3) and 4.0 x 10(-2) in a murine ascending urinary tract infection model. In the same infection model due to the FPM-1-producing E. coli transconjugant, FPM-1-stable cefixime was significantly more effective than FPM-1-labile cefteram pivoxil, although both drugs had similar therapeutic effect against its FPM-1-nonproducing counterpart strain.

Novel plasmid-mediated beta-lactamase (MIR-1) conferring resistance to oxyimino- and alpha-methoxy beta-lactams in clinical isolates of Klebsiella pneumoniae

Klebsiella pneumoniae isolates from 11 patients at the Miriam Hospital were identified as resistant to cefoxitin and ceftibuten as well as to aztreonam, cefotaxime, and ceftazidime. Resistance could be transferred by conjugation or transformation with plasmid DNA into Escherichia coli and was due to the production of a beta-lactamase with an isoelectric point of 8.4 named MIR-1. In E. coli, MIR-1 conferred resistance to aztreonam, cefotaxime, ceftazidime, ceftibuten, ceftriaxone, and such alpha-methoxy beta-lactams as cefmetazole, cefotetan, cefoxitin, and moxalactam. In vitro, MIR-1 hydrolyzed cephalothin and cephaloridine much more rapidly than it did penicillin G, ampicillin, or carbenicillin. Cefotaxime was hydrolyzed at 10% the rate of cephaloridine. Cefoxitin inactivation could only be detected by a microbiological test. The inhibition profile of MIR-1 was similar to that of chromosomally mediated class I beta-lactamases. Potassium clavulanate had little effect on cefoxitin or cefibuten resistance and was a poor inhibitor of MIR-1 activity. Cefoxitin or imipenem did not induce MIR-1. The gene determining MIR-1 was cloned on a 1.4-kb AccI-PstI fragment. Under stringent conditions, probes for TEM-1 and SHV-1 genes and the E. coli ampC gene failed to hybridize with the MIR-1 gene. However, a provisional sequence of 150 bp of the MIR-1 gene proved to be 90% identical to the sequence of ampC from Enterobacter cloacae but only 71% identical to that of E. coli, thus explaining the lack of hybridization to the E. coli ampC probe. Plasmid profiles of the 11 K. pneumoniae clinical isolates were not identical, but each contained a plasmid from 40 to 60 kb that hybridized with the cloned MIR-1 gene. Both transfer-proficient and transfer-deficient MIR-1 plasmids belonged to the N incompatibility group. Thus, the resistance of these K. pneumoniae strains was the result of plasmid acquisition of a class I beta-lactamase, a new resistance determinant that expands the kinds of beta-lactam resistance capable of spread by plasmid dissemination among clinical isolates.

OHIO-1 beta-lactamase is part of the SHV-1 family

The OHIO-1 beta-lactamase gene was subcloned in a 1.16-kilobase TaqI fragment in the 2.4-kilobase chimeric plasmid pSK04. After directional subcloning into M13, the DNA sequence of this fragment was determined. The results showed an open reading frame of 858 base pairs (bp) encoding a protein of 286 amino acids. The structural gene showed 95, 87, and 60% DNA sequence identity with SHV-1, LEN-1, and TEM-1, respectively, and 93, 85, and 62% predicted amino acid sequence identity, respectively. The 87 bp upstream of the OHIO-1 structural gene had 96% identity with the upstream flanking sequence of SHV-1, including the -35 and -10 consensus sequences and the putative ribosomal binding site. A 223-bp DNA probe derived from a PstI-HaeII fragment in the C-terminal sequence of OHIO-1 had predicted 96, 88, and 61% sequence identity with SHV-1, LEN-1, and TEM-1, respectively. This probe hybridized to SHV-1 and poorly to LEN-1, but not to TEM-1 or a variety of other plasmid-mediated beta-lactamase genes, under stringent conditions. Screening of plasmid DNA derived from 40 ampicillin-resistant clinical isolates by Southern hybridization with the 223-bp probe uncovered no strains encoding OHIO-1. Isoelectric focusing of the same collection did identify two strains producing enzymes resembling SHV-1, however. We have also performed a kinetic comparison of OHIO-1, SHV-1, and TEM-1. OHIO-1 and SHV-1 were indistinguishable from each other but could be distinguished from TEM-1. These data clearly place OHIO-1 within the SHV-1 family of beta-lactamases.

SAR-2: identification of a novel plasmid-encoded beta-lactamase from India

A novel beta-lactamase has been identified in an Escherichia coli strain isolated in South India. The beta-lactamase gene was carried on a plasmid (pUK734) along with resistance determinants to sulphonamides and tetracycline. The novel enzyme has a pI of 8.3 and an Mr of 36,000. The enzyme has a broad-spectrum of activity against both penicillins and cephalosporins. It is also active against oxacillin and methicillin.

Novel plasmid-mediated beta-lactamase from Escherichia coli that inactivates oxyimino-cephalosporins

A highly cephem-resistant Escherichia coli strain, FP1546, isolated from the fecal flora of laboratory dogs previously administered beta-lactam antibiotics was found to produce a beta-lactamase, FEC-1, of 48-kilodalton size and pI 8.2. FEC-1 hydrolyzed cefuroxime, cefotaxime, cefmenoxime, and ceftriaxone, as well as the enzymatically less-stable antibiotics cephaloridine, cefotiam, and cefpiramide. Of the oxyimino-cephalosporins, ceftizoxime was fairly stable to FEC-1. FEC-1 differed notably from chromosomal E. coli cephalosporinase, especially in its broad-spectrum substrate profile and its high inhibition by clavulanic acid, sulbactam, and imipenem. A conjugation study revealed that FEC-1 was encoded by a 74-megadalton plasmid, pFCX1. This may be the first instance of a plasmid-mediated oxyimino-cephalosporinase from E. coli.

Monoclonal antibodies to TEM-1 plasmid-mediated beta-lactamase

At least 28 plasmid-mediated beta-lactamases have been described in gram-negative bacteria. To assess the relationship among these enzymes, we produced and characterized 28 murine monoclonal antibodies to the TEM-1 plasmid-mediated beta-lactamase. Radial immunodiffusion identified 3 monoclonal antibodies as immunoglobulin M (IgM), 18 as subclass IgG1, 2 as IgG2a, and 5 as IgG2b. Using a newly described enzyme immunoassay, cross-reactivity of 16 of these monoclonal antibodies was tested against 24 plasmid-determined beta-lactamases. The 16 monoclonal antibodies cross-reacted with TEM-2 and TLE-1 and, to a certain extent, SHV-1. Different levels of cross-reactivity were also observed with OXA-3 (11 of 16), OXA-7 (8 of 16), OXA-1 (2 of 16), OXA-6 (2 of 16), and AER-1 (2 of 16). Six monoclonal antibodies demonstrated partial neutralization of beta-lactamase activity. This study suggests that common epitopes are shared by nine biochemically distinct plasmid-mediated beta-lactamases. On the basis of cross-reactivities with these monoclonal antibodies, we identified four epitopes on TEM-1, TEM-2, TLE-1, and SHV-1 beta-lactamases.

Mechanisms of resistance to cephalosporin antibiotics

Cephalosporins, like other beta-lactams, bind to the bacterial penicillin-binding proteins (PBPs). These correspond to the D-ala-D-ala trans-, carboxy- and endo-peptidases responsible for catalysing the cross-linking of newly formed peptidoglycan. Resistance arises when the PBPs-and particularly the transpeptidases-are modified, or when they are protected by beta-lactamases or 'permeability barriers'. Target-mediated cephalosporin resistance can involve either reduced affinity of an existing PBP component, or the acquisition of a supplementary beta-lactam-insensitive PBP. beta-lactamases are produced widely by bacteria and may be determined by chromosomal or plasmid DNA. The chromosomal beta-lactamases are species-specific, but can be classified into a few broad groups. The plasmid-mediated enzymes cross interspecific and intergeneric boundaries. The level of beta-lactamase-mediated resistance relates to the amount of enzyme produced with or without induction; to the location of the enzyme (extracellular for Gram-positive organisms and periplasmic in Gram-negative ones); and to the kinetics of the enzyme's activity. In Gram-positive organisms the PBPs are located on the outer aspect of the cytoplasmic membrane and so shielding by permeability barriers is minimal. In Gram-negative cells, however, the PBPs are protected by the outer membrane, which most beta-lactams cross by diffusion through aqueous pores composed of 'porin' proteins. In enterobacteria, a clear correlation exists between porin quantity and cephalosporin resistance, suggesting that the outer membrane is the sole barrier to drug entry. Such relationships are less clear for Pseudomonas aeruginosa, where the cell may contain additional barriers between the outer membrane and the PBPs. Although elevated cephalosporin resistance often is attributed to a single factor (PBP-modification, beta-lactamase action or impermeability) an organism's response to a drug often reflects the interplay of several factors. Mathematical models can be proposed to describe this interplay.

Novel plasmid-mediated beta-lactamase in members of the family Enterobacteriaceae from Ohio

Epidemiologic studies of plasmid-mediated resistance at the Cleveland Veterans Administration Medical Center revealed that related plasmids had disseminated among members of the family Enterobacteriaceae. We studied the beta-lactamases encoded by these plasmids in Escherichia coli C600 transformants or transconjugants. Substrate and inhibition profiles of the enzymes determined by two of these plasmids suggested an activity resembling TEM-1; however, isoelectric focusing revealed a pI of 7.0. These two plasmids were originally found in a Serratia marcescens (pDS076) and an Enterobacter cloacae (pDS075) strain isolated from the same sink in the medical intensive care unit and later, in an Enterobacter cloacae (pDS142 identical to pDS076) isolate colonizing a patient in the same unit. The plasmids also carried the aminoglycoside resistance determinant, 2"-aminoglycoside nucleotidyl transferase. A 2-kilobase AvaI restriction endonuclease digestion fragment of pSD075 known to carry the beta-lactamase determinant was used as a molecular probe. This probe did not recognize sequences of any plasmid-mediated beta-lactamase tested including the recently described determinants ROB-1, TLE-1, and OXA-4-7. A TEM-1 probe derived from the 0.7-kilobase PstI-EcoRI fragment of pBR322 failed to recognize the new beta-lactamase gene. Four additional Enterobacter cloacae and two Enterobacter aerogenes strains isolated in Columbus, Ohio, have been shown to produce a pI 7.0 beta-lactamase and to carry plasmids recognized by the 2-kilobase probe. These data suggest dissemination of a novel plasmid-mediated beta-lactamase among members of the family Enterobacteriaceae in Ohio and demonstrate the development and utility of a molecular probe for the new determinant. We suggest that the novel beta-lactamase be named OHIO-1.

Insertion of IS2 creates a novel ampC promoter in Escherichia coli

A class of ampC beta-lactamase-hyperproducing mutants of Escherichia coli were shown to have the insertion element IS2 inserted into the ampC promoter. The insertion of IS2 in orientation II created a novel promoter in which the -35 region and the 17 bp long spacing sequence between the two consensus sequences are present in IS2 DNA, whereas the -10 region from the original ampC promoter is retained. In vitro transcription revealed that the transcription initiation site in the ampC::IS2 mutants was identical with that of ampC wild-type promoter. The novel promoter exhibited a 20-fold increase in promoter strength relative to the original ampC promoter, presumably due to the increase in the spacing sequence from 16 to 17 bp. The evolution of transposable elements and of control elements such as promoters are discussed on the basis of the findings described herein.

Mapping of the plasmid (pLQ3) from Achromobacter and cloning of its cephalosporinase gene in Escherichia coli

We have constructed a physical map of the plasmid pLQ3 which was originally isolated from Achromobacter and which codes for a beta-lactamase. The enzyme specified by pLQ3 is expressed in Escherichia coli and is unusual in that it is a cephalosporinase, an enzyme usually coded for by chromosome. Plasmid pLQ3 is 12.4 kb in length and has a unique Bam HI site and two BglII sites. From a BamHI + BglII double digest of pLQ3, we have constructed a "shortened" plasmid, pLQ10, in which a 2.96-kb fragment is deleted. We have constructed a clone, pLQ22, in which a 3.27-kb fragment of pLQ3, carrying the beta-lactamase gene, is inserted into the BamHI site of pACYC184. By "comparative mapping" of single and multiple digests of each of these plasmids, we have been able to locate the cleavage sites for PstI, which makes seven cuts in pLQ3.

Resistance types in Escherichia coli. II. Transfer of ampicillin resistance

The ability to transfer resistance traits was investigated in 49 E. coli strains isolated from clinical specimens. The strains were divided into the three groups according to sensitivity to penicillin derivatives. Group 1 contained 8 ampicillin-carbenicillin sensitive (A-s/Ca-s) strains, group 2 contained 16 ampicillin resistent-carbenicillin sensitive (A-r/Ca-s) strains and group 3 contained 25 ampicillin-carbenicillin resistant strains. In group 3, 17 strains could transfer A-resistance (range of transfer frequency was 10(0.0) to 10(-7.5). The 16 strains in group 2 that did not transfer A-resistance more often than mutants arose from the recipient (10(-8.8). The mutants of the recipient selected on A-plates were A-r/Ca-s/cephalothin-resistant, exactly as the strains in group 2. The A-resistance in group 3 was probably based on plasmids, and that of group 2 was based on mobilizable plasmids and/or chromosomal resistance. The frequencies of transfer of sulphonamide-, tetracycline- and streptomycin resistance of the strains in groups 1 and 2 did transfer, were in the same range as the frequencies of transfer in group 3.

Types of beta-lactamase determined by plasmids in gram-negative bacteria

Two species of beta-lactamase determined by plasmids in enteric bacteria that show some resemblance to TEM enzymes are described. Both are distinct from all other plasmid-mediated beta-lactamases and differ from the TEM beta-lactamases in ability to hydrolyze some substrates, in isoelectric point, in immunological specificity, and in susceptibility to inhibition. One of the enzyme species, mediated by plasmid p453, has been briefly described previously. We have discovered that this beta-lactamase, designated SHV-1, is unique in its response to inhibition by the sulfhydryl group reagent p-chloromercuribenzoate, because the hydrolysis of cephaloridine but not that of benzylpenicillin is affected. This enzyme is found in a variety of plasmid types which were transferred from several bacterial species collected from a wide geographic range. The other enzyme species is novel; only a single plasmid determining this kind of beta-lactamase (designated HMS-1) has been detected.

Transconjugant analysis: limitations on the use of sequence-specific endonucleases for plasmid identification

We used the sequence-specific endonucleases EcoRI, SmaI, BamHI, HsuI, and HaeIII as identification tools in following the conjugal transfer of the well-studied R plasmids Sa, R388, RP4, and R6K. Transfers were both intergeneric and intrageneric. Plasmid fingerprints were generated from both single- and combination-enzyme digests. The Sa transconjugants yielded plasmids showing consistent fingerprints for each of the respective endonucleases used, whereas the three other R-plasmid transconjugants showed fingerprint changes.

Genetic map of the bacteriocinogenic plasmid CLO DF13 derived by insertion of the transposon Tn901

An ampicillin transposon Tn901 was used as a "mutagen" to isolate insertion mutants of the bacteriocinogenic plasmid Clo DF13. By combining the obtained heteroduplex and restriction maps of the Clo DF13::Tn901 plasmids (van Emboden et al., 1977b) with their polypeptide pattern in minicells, we were able to map five genes on the Clo DF13 genome. These five genes designated A (cloacin gene), B, C, D, and G cover 55% of the coding capacity of Clo DF13 DNA. Since integration of Tn901 within these five genes did not result in a loss of the Clo DF13::Tn901 plasmids involved, it is suggested that these genes do not play an essential role in the maintenance of these plasmid insertion mutants. In addition, the described methods allowed us to indicate the initiation site of cloacin synthesis and to propose the counter-clockwise direction of transcription of the cloacin gene. The Tn901 DNA directed the synthesis of at least three polypeptides one of which is shown to be a TEM-1 beta-lactamase.

Escherichia coli K-12 mutants hyperproducing chromosomal beta-lactamase by gene repetitions

Escherichia coli K-12 ampicillin-resistant mutants hyperproducing chromosomal beta-lactamase arose spontaneously from strains carrying ampA1 ampC(+). Such mutants were found even in a recA background. Two Amp(r)-100 strains were analyzed genetically. The Amp(r)-100 resistance level of both strains could be transduced by direct selection for ampicillin resistance. Several classes of ampicillin-resistant transductants were found that differed from one another in the beta-lactamase activity and the ampicillin resistance mediated by an ampA1 ampC(+)-carrying strain. The data suggested that beta-lactamase hyperproduction was due to repetitions of the chromosomal amp genes. The size of the repeated region was calculated from cotransduction estimates, using the formula of Wu (Genetics 54:405-410, 1966), and was found to be about 1 min in one strain and 1.5 min in the other. Second-step Amp(r)-400 mutants were isolated from an Amp(r)-100 strain. The resistance of these mutants was apparently also due to repetitions, each mediating a resistance to about 10 mug/ml. Mutants of wild-type strains that were moderately resistant to ampicillin also gave rise to intermediate-resistance classes, suggesting repetitions of the wild-type amp alleles. F' factors hyperproducing chromosomal beta-lactamase by gene repetitions were constructed. They mediated levels of ampicillin resistance comparable to that of naturally occurring resistance plasmids. The expression of beta-lactamase hyperproduction was not affected by the presence of ampA and ampC alleles in trans and did not act in trans on the other alleles.

[Kinetics of a new cephalosporinase from Pseudomonas aeruginosa]

Pseudomonas aeruginosa strain RL 39 produces an inducible cephalosporinase possessing an isoelectric point of 8,66. The kinetic constants have been measured by computerized microacidimetry. The results allow to differenciate this enzyme from the one produced by Ps. aeruginosa GN 918 (Yaginuma et al. (1973) Jap. J. Microbiol., 17, 141-149) showing a similar isoelectric point.