Increased Susceptibility to Vancomycin-Resistant Enterococcus Intestinal Colonization Persists After Completion of Anti-Anaerobic Antibiotic Treatment in Mice

Background:

Antibiotic-associated disruption of the indigenous intestinal microflora may persist beyond the treatment period. Although piperacillin/tazobactam inhibits the establishment of vancomycin-resistant Enterococcus (VRE) stool colonization in mice during treatment, we hypothesized that this agent and other anti-anaerobic antibiotics would increase susceptibility to colonization during the period of recovery of the intestinal microflora.

Design:

Mice received 104 colony-forming units of vancomycin-resistant E. faecium by orogastric inoculation 2, 5, or 10 days after completing 5 days of subcutaneous antibiotic treatment, or both during and 2 days after the completion of treatment. Denaturing gradient gel electrophoresis (DGGE) was performed to assess changes in the intestinal microflora.

Results:

Anti-anaerobic antibiotics (ie, piperacillin/tazobactam, cefoxitin, and clindamycin) caused significant disruption of the indigenous microflora (mean DGGE similarity indices ≤ 27% in comparison with saline controls) and promoted the establishment of high-density colonization when VRE was inoculated 2 or 5, but not 10, days following treatment (P < .001). Piperacillin/tazobactam exhibited a biphasic effect on the establishment of colonization (ie, inhibition when exposed to VRE during treatment and promotion when exposed to VRE after discontinuation of treatment), resulting in greater overall promotion of colonization than did agents with minimal anti-anaerobic activity (ie, levofloxacin, cefepime, and aztreonam) when VRE was inoculated both during and 2 days after treatment (P< .001).

Conclusion:

Patients receiving anti-anaerobic antibiotics, including piperacillin/tazobactam, may be susceptible to the establishment of high-density VRE colonization during the period of recovery of the anaerobic microflora.

Rational design of a beta-lactamase inhibitor achieved via stabilization of the trans-enamine intermediate: 1.28 A crystal structure of wt SHV-1 complex with a penam sulfone

beta-Lactamases are one of the major causes of antibiotic resistance in Gram negative bacteria. The continuing evolution of beta-lactamases that are capable of hydrolyzing our most potent beta-lactams presents a vexing clinical problem, in particular since a number of them are resistant to inhibitors. The efficient inhibition of these enzymes is therefore of great clinical importance. Building upon our previous structural studies that examined tazobactam trapped as a trans-enamine intermediate in a deacylation deficient SHV variant, we designed a novel penam sulfone derivative that forms a more stable trans-enamine intermediate. We report here the 1.28 A resolution crystal structure of wt SHV-1 in complex with a rationally designed penam sulfone, SA2-13. The compound is covalently bound to the active site of wt SHV-1 similar to tazobactam yet forms an additional salt-bridge with K234 and hydrogen bonds with S130 and T235 to stabilize the trans-enamine intermediate. Kinetic measurements show that SA2-13, once reacted with SHV-1 beta-lactamase, is about 10-fold slower at being released from the enzyme compared to tazobactam. Stabilizing the trans-enamine intermediate represents a novel strategy for the rational design of mechanism-based class A beta-lactamase inhibitors.

Sulbactam forms only minimal amounts of irreversible acrylate-enzyme with SHV-1 beta-lactamase

Sulbactam is a mechanism-based inhibitor of beta-lactamase enzymes used in clinical practice. It undergoes a complex series of chemical reactions in the active site that have been studied extensively in the past three decades. However, the actual species that gives rise to inhibition in a clinical setting has not been established. Recent studies by our group, using Raman microscopy and X-ray crystallography, have found that large quantities of enamine-based acyl-enzyme species are present within minutes in single crystals of SHV-1 beta-lactamases which can lead to significant inhibition. The enamines are formed by breakdown of the cyclic beta-lactam structures with further transformations leading to imine formation and subsequent isomerization to cis and/or trans enamines. Another favored form of inhibition arises from attack on the imine by a second nucleophilic amino acid side chain, e.g., from serine 130, to form a cross-linked species in the active site that can degrade to an acrylate-like species irreversibly bound to the enzyme. Thus, the imine is at a branch point on the reaction pathway. Using sulbactam and 6,6-dideuterated sulbactam we follow these alternate paths in WT and E166A SHV-1 beta-lactamase by means of Raman microscopic studies on single enzyme crystals. For the unlabeled sulbactam, the Raman data show the presence of an acrylate-like species, probably 3-serine acrylate, several hours after the reaction is started in the crystal. However, for the 6,6-dideutero analogue the acrylate signature appears on the time scale of minutes. The Raman signatures, principally an intense feature near 1530 cm-1, are assigned based on quantum mechanical calculations on model compounds that mimic acrylate species in the active site. The different time scales observed for acrylate-like product formation are ascribed to different rates of reaction involving the imine intermediate. It is proposed that for the unsubstituted sulbactam the conversion from imine to enamine, which involves breaking a C-H bond, is aided by quantum mechanical tunneling. For the 6,6-dideutero-sulbactam the same step involves breaking a C-D bond, which has little or no assistance from tunneling. Consequently the conversion to enamines is slower, and a higher population of imine results, presenting the opportunity for the competing reaction with the second nucleophile, serine 130 being the prime candidate. The hydrolysis of the resulting cross-linked intermediate leads to the observed rapid buildup of the acrylate product in the Raman spectra from the dideutero analogue. The protocol used here, essentially running the reactions with the two forms of sulbactam in parallel, provides an element of control and enables us to conclude that, for the unsubstituted sulbactam, the formation of the cross-linked intermediate and the final irreversible acrylate product is not a significant route to inhibition of SHV-1.

Raman crystallographic studies of the intermediates formed by Ser130Gly SHV, a beta-lactamase that confers resistance to clinical inhibitors

Antibiotic resistance to beta-lactam compounds in Gram-negative bacteria such as Escherichia coli and Klebsiella pneumoniae is often mediated by beta-lactamase enzymes like TEM and SHV. Previously, a limited number of inhibitors have shown efficacy in combating such bacterial drug resistance. However, many Gram-negative pathogens have evolved inhibitor resistant forms of these hydrolytic enzymes. A single point mutation of the active site residue Ser130 to a Gly in either TEM or SHV results in resistance to amoxicillin and clavulanic acid, an important clinical beta-lactam-beta-lactamase inhibitor combination antibiotic. Previous structural and modeling studies of the S130G mutants of TEM and SHV have shown differences in how these two distinct but closely related enzymes compensate for the loss of the Ser130 residue. In the case of S130G SHV, a structure of tazobactam in the active site has suggested that the inhibitor preferentially assumes a cis-enamine intermediate form when the Ser130 hydroxyl is absent. Raman crystallographic studies of S130G SHV inhibited with tazobactam, sulbactam, clavulanic acid, and 2'-glutaroxy penem sulfone (SA2-13) were performed with the aim of identifying the type and amount of intermediate formed with each drug to understand the role of the S130G mutation in formation of the important enamine intermediates. It is demonstrated that with the exception of sulbactam, each compound forms observable trans-enamine intermediates. For S130G reacted with tazobactam, identical steady state levels of enamine are achieved when compared to those of wild-type (WT) or even deacylation deficient forms of the enzyme. With clavulanic acid, slightly smaller amounts of enamine are observed within the first 30 min of the reaction but are not significantly different than those for tazobactam. Thus, the resistance mutation does not substantially affect the amount of trans-enamine formed with clavulanic acid during the critical early time period of inhibition. This finding has important implications in the design of beta-lactamase inhibitors for drug resistant variants like S130G SHV.

Structure and Raman Spectrum of Clavulanic Acid in Aqueous Solution

The calculation of the vibrational Raman spectrum of enzyme-bound beta-lactamase inhibitors may be of help to understand the mechanisms responsible for bacterial drug resistance. Here, we present a study of the solvation structure and the vibrational properties of clavulanate, an important beta-lactamase inhibitor, in aqueous solution as obtained from full quantum and hybrid empirical/quantum molecular dynamics simulations at ambient conditions. The analysis of the vibrational density of states indicates that hybrid empirical/quantum mechanical simulations are able to properly describe the vibrational levels of clavulanate in solution. In addition, we propose a computationally efficient protocol to calculate the vibrational Raman effect for large solute molecules in water, which is able to faithfully reproduce the experimentally recorded clavulanate Raman spectrum and discloses the possibility to employ hybrid simulations to assign the experimental Raman spectra of inhibitors bound to beta-lactamases.

Effect of the inhibitor-resistant M69V substitution on the structures and populations of trans-enamine beta-lactamase intermediates

The objective of this study was to determine the molecular factors that lead to beta-lactamase inhibitor resistance for the M69V variant in SHV-1 beta-lactamase. With mechanism-based inhibitors, the beta-lactamase forms an acyl-enzyme intermediate that consists of a trans-enamine derivative in the active site. This study focuses on these intermediates by introducing the E166A mutation that greatly retards deacylation. Thus, by comparing the properties of the E166A and M69V/E166A forms, we can explore the consequences of the resistance mutation at the level of the enamine acyl-enzyme forms. The reactions between the beta-lactamase and the inhibitors tazobactam, sulbactam, and clavulanic acid are followed in single crystals of the enzymes by using a Raman microscope. The resulting Raman difference spectroscopic data provide detailed information about conformational events involving the enamine species as well as an estimate of their populations. The Raman difference spectra for each of the inhibitors in the E166A and M69V/E166A variants are very similar. In particular, detailed analysis of the main enamine Raman vibration near 1595 cm(-1) reveals that the structure and flexibility of the enamine fragments are essentially identical for each of the three inhibitors in E166A and in the M69V/E166A double mutant. This finding is in accord with the X-ray-derived structures, presented herein at 1.6-1.75 A resolution, of the trans-enamine intermediates formed by the three inhibitors in M69V/E166A. However, a comparison of Raman results for M69V/E166A and E166A shows that the M69V mutation results in a 40%, 25%, and negligible reductions in the enamine population when the beta-lactamase crystals are soaked in 5 mM tazobactam, clavulanic acid, and sulbactam solutions, respectively. The levels of enamine from tazobactam and clavulanic acid can be increased by increasing the concentrations of inhibitor in the mother liquor. Thus, the sensitivity of population levels to the inhibitor concentration in the mother liquor focuses attention on the properties of the encounter complex preceding acylation. It is proposed that for small ligands, such as tazobactam, sulbactam, and clavulanic acid, the positioning of the lactam ring in the active site in the correct orientation for acylation is only one of a number of poorly defined conformations. For tazobactam and clavulanic acid, the correctly oriented encounter complex is even less likely in the M69V variant, leading to a reduction in the level of inhibition of the enzyme via formation of the acyl-enzyme intermediate and the onset of resistance. Analysis of the X-ray structures of the three intermediates in M69V/E166A demonstrates that, compared to the structures for the E166A form, the oxyanion hole becomes smaller, providing one explanation for why acylation may be less efficient following the M69V substitution.

Role of Asp104 in the SHV beta-lactamase

Among the TEM-type extended-spectrum beta-lactamases (ESBLs), an amino acid change at Ambler position 104 (Glu to Lys) results in increased resistance to ceftazidime and cefotaxime when found with other substitutions (e.g., Gly238Ser and Arg164Ser). To examine the role of Asp104 in SHV beta-lactamases, site saturation mutagenesis was performed. Our goal was to investigate the properties of amino acid residues at this position that affect resistance to penicillins and oxyimino-cephalosporins. Unexpectedly, 58% of amino acid variants at position 104 in SHV expressed in Escherichia coli DH10B resulted in beta-lactamases with lowered resistance to ampicillin. In contrast, increased resistance to cefotaxime was demonstrated only for the Asp104Arg and Asp104Lys beta-lactamases. When all 19 substitutions were introduced into the SHV-2 (Gly238Ser) ESBL, the most significant increases in cefotaxime and ceftazidime resistance were noted for both the doubly substituted Asp104Lys Gly238Ser and the doubly substituted Asp104Arg Gly238Ser beta-lactamases. Correspondingly, the overall catalytic efficiency (kcat/Km) of hydrolysis for cefotaxime was increased from 0.60 +/- 0.07 microM(-1) s(-1) (mean +/- standard deviation) for Gly238Ser to 1.70 +/- 0.01 microM(-1) s(-1) for the Asp104Lys and Gly238Ser beta-lactamase (threefold increase). We also showed that (i) k3 was the rate-limiting step for the hydrolysis of cefotaxime by Asp104Lys, (ii) the Km for cefotaxime of the doubly substituted Asp104Lys Gly238Ser variant approached that of the Gly238Ser beta-lactamase as pH increased, and (iii) Lys at position 104 functions in an energetically additive manner with the Gly238Ser substitution to enhance catalysis of cephalothin. Based on this analysis, we propose that the amino acid at Ambler position 104 in SHV-1 beta-lactamase plays a major role in substrate binding and recognition of oxyimino-cephalosporins and influences the interactions of Tyr105 with penicillins.

Current challenges in antimicrobial chemotherapy: the impact of extended-spectrum beta-lactamases and metallo-beta-lactamases on the treatment of resistant Gram-negative pathogens

Gram-negative bacteria that produce extended-spectrum- and metallo-beta-lactamases are being discovered at an alarming rate, while the development of new antimicrobial agents has almost ground to a standstill. A body of experience exists with the detection and treatment of extended-spectrum beta-lactamase-producing organisms (Klebsiella pneumoniae and Escherichia coli), suggesting that knowledge of their existence and dissemination might have an impact on therapeutic choices and patient outcomes via targeted empirical antimicrobial selection and infection control practices. It is unclear whether the same mandate exists for the detection of metallo-beta-lactamases. As dictated by local susceptibility patterns, in many settings worldwide empirical therapy in serious nosocomial infections now requires the use of carbapenems alone or in combination with a second antibiotic that is also effective against Gram-negative pathogens; colistin is advocated as the empirical drug of choice in the setting of multidrug-resistant Pseudomonas aeruginosa infections, although high doses of beta-lactams might prove to be effective.

Identification of a new allelic variant of the Acinetobacter baumannii cephalosporinase, ADC-7 beta-lactamase: defining a unique family of class C enzymes

Acinetobacter spp. are emerging as opportunistic hospital pathogens that demonstrate resistance to many classes of antibiotics. In a metropolitan hospital in Cleveland, a clinical isolate of Acinetobacter baumannii that tested resistant to cefepime and ceftazidime (MIC = 32 microg/ml) was identified. Herein, we sought to determine the molecular basis for the extended-spectrum-cephalosporin resistance. Using analytical isoelectric focusing, a beta-lactamase with a pI of > or = 9.2 was detected. PCR amplification with specific A. baumannii cephalosporinase primers yielded a 1,152-bp product which, when sequenced, identified a novel 383-amino-acid class C enzyme. Expressed in Escherichia coli DH10B, this beta-lactamase demonstrated greater resistance against ceftazidime and cefotaxime than cefepime (4.0 microg/ml versus 0.06 microg/ml). The kinetic characteristics of this beta-lactamase were similar to other cephalosporinases found in Acinetobacter spp. In addition, this cephalosporinase was inhibited by meropenem, imipenem, ertapenem, and sulopenem (K(i) < 40 microM). The amino acid compositions of this novel enzyme and other class C beta-lactamases thus far described for A. baumannii, Acinetobacter genomic species 3, and Oligella urethralis in Europe and South Africa suggest that this cephalosporinase defines a unique family of class C enzymes. We propose a uniform designation for this family of cephalosporinases (Acinetobacter-derived cephalosporinases [ADC]) found in Acinetobacter spp. and identify this enzyme as ADC-7 beta-lactamase. The coalescence of Acinetobacter ampC beta-lactamases into a single common ancestor and the substantial phylogenetic distance separating them from other ampC genes support the logical value of developing a system of nomenclature for these Acinetobacter cephalosporinase genes.

High resolution crystal structures of the trans-enamine intermediates formed by sulbactam and clavulanic acid and E166A SHV-1 {beta}-lactamase

Antibiotic resistance mediated by constantly evolving beta-lactamases is a serious threat to human health. The mechanism of inhibition of these enzymes by therapeutic beta-lactamase inhibitors is probed using a novel approach involving Raman microscopy and x-ray crystallography. We have presented here the high resolution crystal structures of the beta-lactamase inhibitors sulbactam and clavulanic acid bound to the deacylation-deficient E166A variant of SHV-1 beta-lactamase. Our previous Raman measurements have identified the trans-enamine species for both inhibitors and were used to guide the soaking time and concentration to achieve full occupancy of the active sites. The two inhibitor-bound x-ray structures revealed a linear trans-enamine intermediate covalently attached to the active site Ser-70 residue. This intermediate was thought to play a key role in the transient inhibition of class A beta-lactamases. Both the Raman and x-ray data indicated that the clavulanic acid intermediate is decarboxylated. When compared with our previously determined tazobactam-bound inhibitor structure, our new inhibitor-bound structures revealed an increased disorder in the tail region of the inhibitors as well as in the enamine skeleton. The x-ray crystallographic observations correlated with the broadening of the O-C=C-N (enamine) symmetric stretch Raman band near 1595 cm(-1). Band broadening in the sulbactam and clavulanic acid inter-mediates reflected a heterogeneous conformational population that results from variations of torsional angles in the O-(C=O)-C=C=NH-C skeleton. These observations led us to conclude that the conformational stability of the trans-enamine form is critical for their transient inhibitory efficacy.

Clavulanic acid inactivation of SHV-1 and the inhibitor-resistant S130G SHV-1 beta-lactamase. Insights into the mechanism of inhibition

Clavulanic acid is a potent mechanism-based inhibitor of TEM-1 and SHV-1beta-lactamases, enzymes that confer resistance to beta-lactams in many gram-negative pathogens. This compound has enjoyed widespread clinical use as part of beta-lactam beta-lactamase inhibitor therapy directed against penicillin-resistant pathogens. Unfortunately, the emergence of clavulanic acid-resistant variants of TEM-1 and SHV-1 beta-lactamase significantly compromise the efficacy of this combination. A single amino acid change at Ambler position Ser130 (Ser --> Gly) results in resistance to inactivation by clavulanate in the SHV-1 and TEM-1beta-lactamases. Herein, we investigated the inactivation of SHV-1 and the inhibitor-resistant S130G variant beta-lactamases by clavulanate. Using liquid chromatography electrospray ionization mass spectrometry, we detected multiple modified proteins when SHV-1 beta-lactamase is inactivated by clavulanate. Matrix-assisted laser desorption ionization-time of flight mass spectrometry was used to study tryptic digests of SHV-1 and S130Gbeta-lactamases (+/- inactivation with clavulanate) and identified peptides modified at the active site Ser70. Ultraviolet (UV) difference spectral studies comparing SHV-1 and S130Gbeta-lactamases inactivated by clavulanate showed that the formation of reaction intermediates with absorption maxima at 227 and 280 nm are diminished and delayed when S130Gbeta-lactamase is inactivated. We conclude that the clavulanic acid inhibition of the S130G beta-lactamase must follow a branch of the normal inactivation pathway. These findings highlight the importance of understanding the intermediates formed in the inactivation process of inhibitor-resistant beta-lactamases and suggest how strategic chemical design can lead to novel ways to inhibit beta-lactamases.

Effect of parenteral antibiotic administration on establishment of intestinal colonization by Candida glabrata in adult mice

We examined the effect of antibiotic treatment on establishment of intestinal colonization by Candida glabrata in adult mice. Subcutaneous ceftriaxone, piperacillin-tazobactam, clindamycin, and metronidazole promoted increased density of stool colonization, whereas cefepime, levofloxacin, and aztreonam did not. These findings suggest that antibiotics that inhibit intestinal anaerobes promote C. glabrata colonization.

Mechanisms by which anaerobic microbiota inhibit the establishment in mice of intestinal colonization by vancomycin-resistant Enterococcus

We used a mouse model to test the hypothesis that anaerobic microbiota in the colon inhibit the establishment of vancomycin-resistant enterococci (VRE) colonization by depleting nutrients within cecal contents and limiting the association of VRE with the mucus layer. Anaerobic growth of VRE was assessed in cecal contents and cecal mucus of mice that had received treatment with subcutaneous clindamycin or saline. VRE grew to high concentrations in cecal contents of clindamycin-treated mice and in cecal mucus of both groups but not in cecal contents of saline-treated mice, unless the cecal contents were autoclaved or converted into sterile filtrates. After orogastric inoculation of VRE, clindamycin-treated mice acquired high concentrations of VRE within the mucus layer, whereas saline-treated mice did not. These results suggest that colonic microbiota inhibit VRE by producing inhibitory substances or conditions rather than by depleting nutrients. The colonic mucus layer provides a potential niche for growth of VRE.

In Vitro Antienterococcal Activity Explains Associations between Exposures to Antimicrobial Agents and Risk of Colonization by Multiresistant Enterococci

We compared ceftriaxone and piperacillin-tazobactam at doses ranging from 0.1 to 2 times the human equivalent daily dose (HEDD), to determine their impact on gastrointestinal colonization by ampicillin- and vancomycin-resistant Enterococcus faecium C68 in a mouse model. Ceftriaxone failed to promote colonization at doses up to 0.25 times the HEDD, whereas piperacillin-tazobactam promoted colonization at doses up to 0.5 times the HEDD. Ceftriaxone promoted colonization at doses at least 0.5 times the HEDD, whereas piperacillin-tazobactam inhibited colonization at doses at least 0.75 times the HEDD. Both piperacillin-tazobactam and ceftriaxone inhibited colonization by an enterococcal strain devoid of low-affinity penicillin-binding protein-5 (significantly increasing its susceptibility to these agents), at doses that promoted colonization by E. faecium C68. These results support a model in which the impact that different beta -lactam agents have on colonization by VRE is related to the level of the beta -lactam agent's intrinsic antienterococcal activity against the colonizing strain.

Exploring the effectiveness of tazobactam against ceftazidime resistant Escherichia coli: insights from the comparison between susceptibility testing and beta-lactamase inhibition

Thirteen clinical isolates of Escherichia coli resistant to ceftazidime that possessed an AmpC and other (beta-lactamases were identified. The effectiveness of different formulations of piperacillin/tazobactam to other beta-lactams was compared. Antibiotic susceptibility testing, polymerase chain reaction, amplification of blaTEM, blaSHV and blaAmpC, and enzyme-linked immunosorbent assays to identify AmpC beta-lactamases were performed. Hydrolysis rates were obtained and residual enzymatic activity was determined. Cefepime and ertapenem were more active than piperacillin/tazobactam. In contrast, increasing the relative proportion of tazobactam improved susceptibility testing. Twenty micromolar tazobactam inhibited total beta-lactamase activity (as measured by nitrocefin hydrolysis rates) by greater than 75% against all isolates tested: in 11 of 13 E. coli isolates, total beta-lactamase activity was inhibited by 90%. The observed differences between MIC determinations and susceptibility to enzymatic inactivation by tazobactam against E. coli containing AmpC and other -lactamases may be due to the final tazobactam concentration achieved in the periplasmic space. Factors determining this are critical considerations in assessing beta-lactamase inhibitor potency.

Efficacy of oral ramoplanin for inhibition of intestinal colonization by vancomycin-resistant enterococci in mice

Ramoplanin is a glycolipodepsipeptide antibiotic with activity against gram-positive bacteria that is in clinical trials for prevention of vancomycin-resistant Enterococcus (VRE) bloodstream infections and treatment of Clostridium difficile diarrhea. Orally administered ramoplanin suppresses VRE intestinal colonization, but recurrences after discontinuation of treatment have frequently been observed. We used a mouse model to examine the efficacy of ramoplanin for inhibition of VRE colonization and evaluated the etiology of recurrences of colonization. Eight days of treatment with ramoplanin (100 microg/ml) in drinking water suppressed VRE to undetectable levels, but 100% of mice developed recurrent colonization; a higher dose of 500 microg/ml in water was associated with recurrent colonization in 50% of mice. Two of eight (25%) mice treated with the 100-microg/ml dose of ramoplanin had low levels of VRE in their cecal tissues on day 8 despite undetectable levels in stool and cecal contents. Mice that received prior ramoplanin treatment did not develop VRE overgrowth when challenged with 10(7) CFU of oral VRE 1, 2, or 4 days later. In communal cages, rapid cross-transmission and overgrowth of VRE was observed among clindamycin-treated mice; ramoplanin treatment effectively suppressed VRE overgrowth in such communal cages. Ramoplanin treatment promoted increased density of indigenous Enterobacteriaceae and overgrowth of an exogenously administered Klebsiella pneumoniae isolate. These results demonstrate the efficacy of ramoplanin for inhibition of VRE colonization and suggest that some recurrences occur due to reexpansion of organisms that persist within the lining of the colon. Ramoplanin treatment may be associated with overgrowth of gram-negative bacilli.

Tazobactam Inactivation of SHV-1 and the Inhibitor-resistant Ser130 → Gly SHV-1 β-Lactamase

The increasing number of bacteria resistant to combinations of beta-lactam and beta-lactamase inhibitors is creating great difficulties in the treatment of serious hospital-acquired infections. Understanding the mechanisms and structural basis for the inactivation of these inhibitor-resistant beta-lactamases provides a rationale for the design of novel compounds. In the present work, SHV-1 and the Ser(130) --> Gly inhibitor-resistant variant of SHV-1 beta-lactamase were inactivated with tazobactam, a potent class A beta-lactamase inhibitor. Apoenzymes and inhibited beta-lactamases were analyzed by liquid chromatography-electrospray ionization mass spectrometry (LC-ESI/MS), digested with trypsin, and the products resolved using LC-ESI/MS and matrix-assisted laser desorption ionization-time of flight mass spectrometry. The mass increases observed for SHV-1 and Ser(130) --> Gly (+ Delta 88 Da and + Delta 70 Da, respectively) suggest that fragmentation of tazobactam readily occurs in the inhibitor-resistant variant to yield an inactive beta-lactamase. These two mass increments are consistent with the formation of an aldehyde (+ Delta 70 Da) and a hydrated aldehyde (+ Delta 88 Da) as stable products of inhibition. Our results reveal that the Ser --> Gly substitution at amino acid position 130 is not essential for enzyme inactivation. By examining the inhibitor-resistant Ser(130) --> Gly beta-lactamase, our data are the first to show that tazobactam undergoes fragmentation while still attached to the active site Ser(70) in this enzyme. After acylation of tazobactam by Ser(130) --> Gly, inactivation proceeds independent of any additional covalent interactions.

Beta-lactam antibiotics and gastrointestinal colonization with vancomycin-resistant enterococci

We studied the effect of different subcutaneously administered beta-lactam antibiotics on the establishment of gastrointestinal colonization by vancomycin-resistant Enterococcus faecium C68 in a mouse model. Aztreonam, cefazolin, cefepime, and, to a lesser extent, ceftazidime, which neither have significant antienterococcal activity nor are secreted into human bile at high concentrations, did not promote significant vancomycin-resistant enterococci (VRE) colonization. Piperacillin-tazobactam, which has antienterococcal activity and is secreted in human bile at high concentrations, inhibited colonization after limited exposure to the inoculum but was associated with progressively increased VRE colony counts in stool samples after repeated exposure to the VRE inoculum. Ceftriaxone and cefotetan, which lack antienterococcal activity but are secreted into human bile at high concentrations, were associated with rapid and high-level colonization. These data suggest that the risk of VRE colonization varies during exposure to different beta-lactam antimicrobial agents and that the risk is related to biliary concentration and antienterococcal activity of the specific beta-lactam.

Effect of parenteral fluoroquinolone administration on persistence of vancomycin-resistant Enterococcus faecium in the mouse gastrointestinal tract

We examined the effect of subcutaneous fluoroquinolone antibiotic administration on persistence and density of vancomycin-resistant Enterococcus faecium stool colonization in mice. Levofloxacin and ciprofloxacin did not promote colonization in comparison to saline controls, whereas moxifloxacin and gatifloxacin promoted persistent overgrowth in a dose-dependent fashion.

Tazobactam Forms a Stoichiometric trans-Enamine Intermediate in the E166A Variant of SHV-1 β-Lactamase: 1.63 Å Crystal Structure,

Many pathogenic bacteria develop antibiotic resistance by utilizing beta-lactamases to degrade penicillin-like antibiotics. A commonly prescribed mechanism-based inhibitor of beta-lactamases is tazobactam, which can function either irreversibly or in a transient manner. We have demonstrated previously that the reaction between tazobactam and a deacylation deficient variant of SHV-1 beta-lactamase, E166A, could be followed in single crystals using Raman microscopy [Helfand, M. S., et al. (2003) Biochemistry 42, 13386-13392]. The Raman data show that maximal populations of an enamine-like intermediate occur 20-30 min after "soaking in" has commenced. By flash-freezing crystals in this time frame, we were able to trap the enamine species. The resulting 1.63 A resolution crystal structure revealed tazobactam covalently bound in the trans-enamine intermediate state with close to 100% occupancy in the active site. The Raman data also indicated that tazobactam forms a larger population of enamine than sulbactam or clavulanic acid does and that tazobactam's intermediate is also the most long-lived. The crystal structure provides a rationale for this finding since only tazobactam is able to form favorable intra- and intermolecular interactions in the active site that stabilize this trans-enamine intermediate. These interactions involve both the sulfone and triazolyl groups that distinguish tazobactam from clavulanic acid and sulbactam, respectively. The observed stabilization of the transient intermediate of tazobactam is thought to contribute to tazobactam's superior in vitro and in vivo clinical efficacy. Understanding the structural details of differing inhibitor effectiveness can aid the design of improved mechanism-based beta-lactamase inhibitors.

Oral Administration of β‐Lactamase Preserves Colonization Resistance of Piperacillin‐Treated Mice

We hypothesized that orally administered, recombinant class A beta-lactamase would inactivate the portion of parenteral piperacillin excreted into the intestinal tract, preserving colonization resistance of mice against nosocomial pathogens. Subcutaneous piperacillin or piperacillin plus oral beta-lactamase were administered 24 and 12 h before orogastric inoculation of piperacillin-resistant pathogens. Oral administration of beta-lactamase reduced piperacillin-associated alteration of the indigenous microflora and prevented overgrowth of pathogens.

Following the Reactions of Mechanism-Based Inhibitors with β-Lactamase by Raman Crystallography

The reactions between three clinically relevant inhibitors, tazobactam, sulbactam, and clavulanic acid, and SHV beta-lactamase (EC 3.5.2.6) have been followed in single crystals using a Raman microscope. The data are far superior to those obtained for the enzyme in aqueous solution and allow us to identify species on the reaction pathway and to measure the rates of the accumulation and decay of these species. A key intermediate on the reaction pathway is an acyl enzyme formed between Ser70 and the lactam ring's C=O group. By using the E166A deacylation deficient variant of the enzyme, we were able to focus on the process of acyl enzyme formation. The Raman data show that all three inhibitors form an enamine-type acyl enzyme reaching maximal populations at 10, 22, and 29 min for sulbactam, clavulanic acid, and tazobactam, respectively. The enamine intermediate exhibits a characteristic and relatively intense band near 1595 cm(-1) due to a stretching motion of the O=C-C=C-NH moiety that shifts to lower frequency upon NH <--> ND exchange. This feature was used to follow the kinetics of enamine buildup and decay in the crystal. Quantum mechanical calculations support the assignment of the 1595 cm(-1) band, as well as several other bands, to a trans-enamine species. The Raman data also demonstrate that the lactam ring opens prior to enamine formation since the lactam ring carbonyl (C=O) peak disappears prior to the appearance of the enamine 1595 cm(-1) band. Tazobactam appears to form approximately twice as much enamine intermediate as sulbactam and clavulanic acid, which correlates with its superior performance in the clinic, a finding that may bear on future drug design.

Understanding resistance to beta-lactams and beta-lactamase inhibitors in the SHV beta-lactamase: lessons from the mutagenesis of SER-130

Bacterial resistance to beta-lactam/beta-lactamase inhibitor combinations by single amino acid mutations in class A beta-lactamases threatens our most potent clinical antibiotics. In TEM-1 and SHV-1, the common class A beta-lactamases, alterations at Ser-130 confer resistance to inactivation by the beta-lactamase inhibitors, clavulanic acid, and tazobactam. By using site-saturation mutagenesis, we sought to determine the amino acid substitutions at Ser-130 in SHV-1 beta-lactamase that result in resistance to these inhibitors. Antibiotic susceptibility testing revealed that ampicillin and ampicillin/clavulanic acid resistance was observed only for the S130G beta-lactamase expressed in Escherichia coli. Kinetic analysis of the S130G beta-lactamase demonstrated a significant elevation in apparent Km and a reduction in kcat/Km for ampicillin. Marked increases in the dissociation constant for the preacylation complex, KI, of clavulanic acid (SHV-1, 0.14 microm; S130G, 46.5 microm) and tazobactam (SHV-1, 0.07 microm; S130G, 4.2 microm) were observed. In contrast, the k(inact)s of S130G and SHV-1 differed by only 17% for clavulanic acid and 40% for tazobactam. Progressive inactivation studies showed that the inhibitor to enzyme ratios required to inactivate SHV-1 and S130G were similar. Our observations demonstrate that enzymatic activity is preserved despite amino acid substitutions that significantly alter the apparent affinity of the active site for beta-lactams and beta-lactamase inhibitors. These results underscore the mechanistic versatility of class A beta-lactamases and have implications for the design of novel beta-lactamase inhibitors.

Role of fecal incontinence in contamination of the environment with vancomycin-resistant enterococci

BACKGROUND

We tested the hypothesis that patients with vancomycin-resistant Enterococcus (VRE) stool colonization who are continent of feces contaminate the environment less frequently than patients who are colonized and incontinent.

METHODS

We prospectively examined the frequency of environmental VRE contamination in the rooms of 15 patients who were continent and 15 who were incontinent and VRE-colonized. Broth-enrichment cultures of bed rails, bedside table, and call buttons were performed at baseline, and 2 and 5 days after environmental disinfection. The numbers of VRE colonies isolated after directly plating environmental swabs onto agar were compared for the continent and incontinent groups.

RESULTS

The percentages of patients with 1 or more positive environmental cultures for VRE were not significantly different for the groups of patients who were continent and incontinent at baseline (60% vs 73%, P =.45) or 2 days after disinfection (60% vs 80%, P =.24). The numbers of VRE colonies isolated by direct plating were not significantly different for the continent and incontinent groups (P =.42).

CONCLUSIONS

Environmental contamination occurs frequently in the rooms of patients who are continent, and those who are incontinent and VRE-colonized. Our findings suggest that similar infection control measures should be implemented for patients who are continent and incontinent.

Beta-lactamases: a survey of protein diversity

Bacterial resistance to beta-lactam antibiotics and beta-lactamase inhibitors is an ever increasing problem that threatens the clinical utility of drugs that form the cornerstone of the antibiotic armamentarium. Especially among Gram-negative pathogens, elaboration of structurally and mechanistically novel beta-lactamase enzymes is the most important means by which resistance occurs. An appreciation of the tremendous diversity of these drug-modifying enzymes will assist in understanding why so few generally effective inhibitory agents exist for these unique drug targets. This review will give a general background on the reaction mechanisms and classification schemes of the more than 340 beta-lactamase enzymes described to date. A discussion will follow highlighting the emerging Class A SHV and TEM-derived extended-spectrum (ESBLs), and inhibitor-resistant enzymes, non-TEM, non-SHV Class A ESBLs, and carbapenemases, Class B metallo-beta-lactamases and some of their novel inhibitors, plasmid and chromosomally encoded Class C enzymes, and finally, the OXA-type oxacillinases, ESBLs, and carbapenemases of Class D. The clinical importance of multiple resistance mechanisms in conjunction with the production of beta-lactamase enzymes is emphasized.

Unexpected advanced generation cephalosporinase activity of the M69F variant of SHV beta-lactamase

Infections with bacteria that contain hydrolytic beta-lactamase enzymes are becoming a serious problem in the United States. Mutations at Met-69, an amino acid proximal to the active site Ser-70 in the TEM-1 and SHV-1 beta-lactamases, have emerged as a puzzling cause of bacterial resistance to inhibitors of beta-lactamases. Site-saturation mutagenesis of the 69 position in SHV beta-lactamase was performed to determine how mutations of this non-catalytic residue play a role in increasing 50% inhibitory concentrations (IC(50) concentrations) for clinically important beta-lactamase enzyme inhibitors. Two distinct phenotypes are evident in the variant beta-lactamases studied: significantly increased minimum inhibitory concentrations (microg/ml) and IC(50) concentrations to clavulanic acid for the Met69Ile, Leu, and Val substitutions, and unanticipated increased minimum inhibitory concentrations and hydrolytic activity toward ceftazidime, an advanced generation cephalosporin antibiotic, for the Met69Lys, Tyr- and Phe-substituted enzymes. Molecular modeling studies emphasize the conserved structure of these substitutions despite great variation in substrate specificity. This study demonstrates the key role of Met-69 in defining substrate specificity of SHV beta-lactamases and alerts us to new phenotypes that may emerge clinically.

Amino acid substitutions at Ambler position Gly238 in the SHV-1 beta-lactamase: exploring sequence requirements for resistance to penicillins and cephalosporins

Site saturation mutagenesis of the 238 position in the SHV beta-lactamase was performed to identify the complete sequence requirements needed for the extended spectrum beta-lactamase (ESBL) phenotype. MICs (in micrograms per milliliter) in an isogenic background, Escherichia coli DH10B, demonstrated that the Gly238Ala mutation conferred the most resistance to penicillins and cephalosporins. The absolute increase in resistance was greatest against cefotaxime for the Gly238Ala mutant (0.06 to 8 micro g/ml). Except for the strain possessing the Gly238Pro beta-lactamase, ceftazidime MICs were also elevated. None of the mutant SHV beta-lactamases were expressed in as great an amount as the wild-type beta-lactamase. Kinetic analysis of the Gly238Ala mutant revealed that penicillin and cephalosporin substrates have a lower K(m) for the enzyme because of this mutation. Ampicillin and piperacillin MICs were inversely proportional to the side chain volume of the amino acid in cases larger than Ser, suggesting that steric considerations may be a primary requirement for penicillin resistance. Secondary structural effects explain increased resistance to oxyiminocephalosporins. Based upon this study, we anticipate that additional mutations of Gly238 in the SHV beta-lactamase will continue to be discovered with an ESBL (ceftazidime or cefotaxime resistant) phenotype.

Recurrence of vancomycin-resistant Enterococcus stool colonization during antibiotic therapy

OBJECTIVE

To test the hypothesis that antibiotic therapy may promote recurrence of vancomycin-resistant Enterococcus (VRE) stool colonization in patients who have previously had three consecutive negative stool cultures obtained at least 1 week apart.

DESIGN

One-year prospective cohort study examining the effect of antibiotic therapy on recurrence and density of VRE stool colonization in patients who have cleared colonization. Pulsed-field gel electrophoresis (PFGE) was performed to determine whether recurrent VRE strains were the same clone as the previous colonizing strain.

SETTING

A Department of Veterans Affairs medical center including an acute care hospital and nursing home.

PATIENTS

All patients with at least one stool culture positive for VRE who subsequently had three consecutive negative stool cultures obtained at least 1 week apart.

RESULTS

Of the 16 patients who cleared VRE colonization, 13 received antibiotic therapy during the study period. Eight (62%) of the 13 patients who received antibiotics developed recurrent high-density VRE stool colonization (range, 4.9 to 9.1 log10 colony-forming units per gram) during a course of therapy. Five patients had VRE strains available for PFGE analysis; recurrent strains were unrelated to the prior strain in 3 patients, closely related in 1 patient, and indistinguishable in 1 patient.

CONCLUSIONS

Antibiotic therapy may be associated with recurrent high-density VRE stool colonization in many patients who have previously had three consecutive negative stool cultures. These patients should be screened for recurrent stool colonization when antibiotic therapy is administered.

Development of a sensitive and specific enzyme-linked immunosorbent assay for detecting and quantifying CMY-2 and SHV beta-lactamases

Polyclonal rabbit antibodies against SHV-1 and CMY-2 beta-lactamases were produced and characterized, and enzyme-linked immunosorbent assays (ELISAs) were developed. Immunoblots revealed that the anti-SHV-1 antibody recognized SHV-1 but did not recognize TEM-1, K-1, OXA-1, or any AmpC beta-lactamase tested. The anti-CMY-2 antibody detected Escherichia coli CMY-2, Enterobacter cloacae P99, Klebsiella pneumoniae ACT-1, and the AmpC beta-lactamases of Enterobacter aerogenes, Morganella morganii, and Citrobacter freundii. No cross-reactivity of the anti-CMY-2 antibody was seen against laboratory strains of E. coli possessing TEM-1, SHV-1, K-1, or OXA-1 beta-lactamases. Operating conditions for performing ELISAs were optimized. Both anti-CMY-2 and anti-SHV-1 antibodies detected picogram quantities of purified protein in ELISAs. The reactivity of the anti-CMY-2 antibody was tested against a number of AmpC beta-lactamases by assaying known quantities of purified enzymes in ELISAs (AmpC beta-lactamases of M. morganii, C. freundii, E. coli, and E. cloacae). As the homology to CMY-2 beta-lactamase decreased, the minimum level needed for detection increased (e.g., 94% homology recognized at 1 ng/ml and 71% homology recognized at 10 ng/ml). The ELISAs were used to assay unknown clinical isolates for AmpC and SHV beta-lactamases, and the results were confirmed with PCR amplification of bla(AmpC) and bla(SHV) genes. Overall, we found that our ELISAs were at least 95% sensitive and specific for detecting SHV and AmpC beta-lactamases. The ELISA format can facilitate the identification of AmpC and SHV beta-lactamases and can be used to quantify relative amounts of beta-lactamase enzymes in clinical and laboratory isolates.

Immune activation, allergic drug toxicity and mortality in HIV-positive tuberculosis

SETTING

Tuberculosis Treatment Center, Kampala, Uganda.

OBJECTIVE

HIV-1 affects outcome in pulmonary tuberculosis (TB). Immune mechanisms triggered by Mycobacterium tuberculosis may lead to increased HIV expression and accelerated disease progression. This study was conducted to correlate serum levels of markers of immune activation with mortality and drug toxicity in HIV + TB.

DESIGN

Substudy of a randomized clinical trial of streptomycin-thiacetazone-isoniazid (STH) vs. rifampin-isoniazid-pyrazinamide (RHZ) in HIV + TB.

RESULTS

Neopterin > or = 14 ng/ml, TNF-alpha receptors > or = 6.5 ng/ml, and negative skin test were independently associated with increased mortality (P < 0.01). Among STH-treated subjects, dermatologic toxicity and mortality were respectively 13- and 6.3-fold more likely to occur in subjects with elevated neopterin (P < 0.05), although these two adverse events occurred independently. Activation markers increased from baseline after 2 months of therapy with the less rapidly bactericidal STH regimen, whereas they declined in those treated with RHZ, suggesting a relationship with continued mycobacterial replication.

CONCLUSIONS

Immune activation in HIV + TB is associated with shortened survival and increased risk of drug toxicity. HIV + TB patients with elevated serum neopterin should be treated with a rapidly-bactericidal drug regimen which does not include thiacetazone.