What's New in beta-lactamases?

Structural Variabilities in β-Lactamase (blaA) of Different Biovars of Yersinia enterocolitica: Implications for β-Lactam Antibiotic and β-Lactamase Inhibitor Susceptibilities

Yersiniosis caused by Yersinia enterocolitica has been reported from all continents. The bacterial species is divided into more than fifty serovars and six biovars viz. 1A, 1B, 2, 3, 4 and 5 which differ in geographical distribution, ecological niches and pathogenicity. Most Y.enterocolitica strains harbor chromosomal genes for two β-lactamases, blaA an Ambler class A penicillinase and blaB an Ambler class C inducible cephalosporinase. In the present study, susceptibility to b-lactam antibiotics and β-lactamase inhibitor was studied for Y. enterocolitica strains of biovars 1A, 1B, 2 and 4. We observed that β-lactamases were expressed differentially among strains of different biovars. To understand the molecular mechanisms underlying such differential expression, the sequences of genes and promoters of blaA were compared. Also, the variants of blaA present in different biovars were modeled and docked with amoxicillin and clavulanic acid. The mRNA secondary structures of blaA variants were also predicted in-silico. Our findings indicated that neither variations in the promoter regions, nor the secondary structures of mRNA contributed to higher/lower expression of blaA in different biovars. Analysis of H-bonding residues of blaA variants with amoxicillin and clavulanic acid revealed that if amino acid residues of a β-lactamase interacting with amoxicillin and the clavulanic acid were similar, clavulanic acid was effective in engaging the enzyme, accounting for a significant reduction in MIC of amoxicillin-clavulanate. This finding might aid in designing better β-lactamase inhibitors with improved efficiencies in future.

Detection of antibiotic resistance genes in samples from acute and chronic endodontic infections and after treatment

OBJECTIVE

The purpose of this study was twofold: survey samples from acute and chronic endodontic infections for the presence of genes encoding resistance to beta-lactams, tetracycline and erythromycin, and evaluate the ability of treatment to eliminate these genes from root canals.

DESIGN

DNA extracts from samples of abscess aspirates (n=25) and root canals of teeth with asymptomatic apical periodontitis (n=24) were used as template for direct detection of the genes blaTEM, cfxA, tetM, tetQ, tetW, and ermC using real-time polymerase chain reaction (PCR). Bacterial presence was determined using PCR with universal bacterial primers. Root canals of the asymptomatic cases were also sampled and evaluated after chemomechanical procedures using NiTi instruments with 2.5% NaOCl irrigation.

RESULTS

All abscess and initial root canal samples were positive for bacteria. At least one of the target resistance genes was found in 36% of the abscess samples and 67% of the asymptomatic cases. The most prevalent genes in abscesses were blaTEM (24%) and ermC (24%), while tetM (42%) and tetW (29%) prevailed in asymptomatic cases. The blaTEM gene was significantly associated with acute cases (p=0.02). Conversely, tetM was significantly more prevalent in asymptomatic cases (p=0.008). Treatment eliminated resistance genes from most cases.

CONCLUSIONS

Acute and chronic endodontic infections harboured resistance genes for 3 classes of widely used antibiotics. In most cases, treatment was effective in eliminating these genes, but there were a few cases in which they persisted. The implications of persistence are unknown. Direct detection of resistance genes in abscesses may be a potential method for rapid diagnosis and establishment of proactive antimicrobial therapy.

Prevalence of zoonotic agents in dogs visiting hospitalized people in Ontario: implications for infection control

Visitation of hospitalized people by dogs is becoming commonplace, but little is known about the potential health risks of introducing dogs to healthcare settings. This cross-sectional study evaluated the prevalence of zoonotic agents in a group of 102 visitation dogs from a variety of sources across Ontario. Between May and July 2004, owners were interviewed by a standardized questionnaire while dogs underwent a standardized physical examination. One specimen of faeces, hair-coat brushings and one rectal, aural, nasal, oral and pharyngeal swab were collected from each dog and tested for 18 specific pathogens. All dogs were judged to be in good health. Zoonotic agents were isolated from 80 out of 102 (80%) dogs. The primary pathogen was Clostridium difficile, which was isolated from 58 (58%) faecal specimens. Seventy-one percent (41/58) of these isolates were toxigenic. Extended-spectrum beta-lactamase Escherichia coli was isolated from one (1%) dog, extended-spectrum cephalosporinase E. coli was isolated from three (3%) dogs, and organisms of the genus Salmonella were isolated from three (3%) dogs. Pasteurella multocida or Pasteurella canis was isolated from 29 (29%) oral swabs, and Malassezia pachydermatis was isolated from eight (8%) aural swabs. Giardia antigen was present in the faeces of seven (7%) dogs, while Toxocara canis and Ancylostoma caninum were detected in two (2%) dogs and one (1%) dog, respectively. Methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, Campylobacter spp., Microsporum canis, group A streptococci, Pseudomonas aeruginosa and Cryptosporidium spp. were not detected. Further information is needed before the full implications of these findings for infection control can be assessed properly.

Antimicrobial Resistance in the Hospital Setting: Impact, Trends, and Infection Control Measures

The growing threat posed by antibiotic-resistant pathogens is a major challenge for infectious disease practitioners and public health officials. In recent years, the prevalence of resistance among key bacterial pathogens, including Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Streptococcus pneumoniae, and Enterococcus sp, has increased at an alarming rate. The impact of antimicrobial resistance is manifold and can ultimately lead to treatment failure and increased morbidity and mortality. To control the spread of resistance and subsequent impact, a multifaceted approach is warranted. Awareness and surveillance of antimicrobial resistance, prudent use of antibiotics, and compliance with infection control techniques may help contain the emergence and spread of resistant organisms.

Diagnosis of infection in sepsis: An evidence-based review

OBJECTIVE

In 2003, critical care and infectious disease experts representing 11 international organizations developed management guidelines for the diagnosis of infection in sepsis that would be of practical use for the bedside clinician, under the auspices of the Surviving Sepsis Campaign, an international effort to increase awareness and improve outcome in severe sepsis.

DESIGN

The process included a modified Delphi method, a consensus conference, several subsequent smaller meetings of subgroups and key individuals, teleconferences, and electronic-based discussion among subgroups and among the entire committee.

METHODS

The modified Delphi methodology used for grading recommendations built on a 2001 publication sponsored by the International Sepsis Forum. We undertook a systematic review of the literature graded along five levels to create recommendation grades from A to E, with A being the highest grade. Pediatric considerations to contrast adult and pediatric management are in the article by Parker et al. on p. S591.

CONCLUSIONS

Obtaining a precise bacteriological diagnosis before starting antibiotic therapy is, when possible, of paramount importance for the success of therapeutic strategy during sepsis. Two to three blood cultures should be performed, preferably from a peripheral vein, without interval between samples to avoid delaying therapy. A quantitative approach is preferred in most cases when possible, in particular for catheter-related infections and ventilator-associated pneumonia. Diagnosing community-acquired pneumonia is complex, and a diagnostic algorithm is proposed. Appropriate samples are indicated during soft tissue and intraabdominal infections, but cultures obtained through the drains are discouraged.

Role of a mutation at position 167 of CTX-M-19 in ceftazidime hydrolysis

CTX-M-19 is a recently identified ceftazidime-hydrolyzing extended-spectrum beta-lactamase, which differs from the majority of CTX-M-type beta-lactamases that preferentially hydrolyze cefotaxime but not ceftazidime. To elucidate the mechanism of ceftazidime hydrolysis by CTX-M-19, the beta-lactam MICs of a CTX-M-19 producer, and the kinetic parameters of the enzyme were confirmed. We reconfirmed here that CTX-M-19 is also stable at a high enzyme concentration in the presence of bovine serum albumin (20 micro g/ml). Under this condition, we obtained more accurate kinetic parameters and determined that cefotaxime (k(cat)/K(m), 1.47 x 10(6) s(-1) M(-1)), cefoxitin (k(cat)/K(m), 62.2 s(-1) M(-1)), and aztreonam (k(cat)/K(m), 1.34 x 10(3) s(-1) M(-1)) are good substrates and that imipenem (k(+2)/K, 1.20 x 10(2) s(-1) M(-1)) is a poor substrate. However, CTX-M-18 and CTX-M-19 exhibited too high a K(m) value (2.7 to 5.6 mM) against ceftazidime to obtain their catalytic activity (k(cat)). Comparison of the MICs with the catalytic efficiency (k(cat)/K(m)) of these enzymes showed that some beta-lactams, including cefotaxime, ceftazidime, and aztreonam showed a similar correlation. Using the previously reported crystal structure of the Toho-1 beta-lactamase, which belongs to the CTX-M-type beta-lactamase group, we have suggested characteristic interactions between the enzymes and the beta-lactams ceftazidime, cefotaxime, and aztreonam by molecular modeling. Aminothiazole-bearing beta-lactams require a displacement of the aminothiazole moiety due to a severe steric interaction with the hydroxyl group of Ser167 in CTX-M-19, and the displacement affects the interaction between Ser130 and the acidic group such as carboxylate and sulfonate of beta-lactams.

Penicillin-derived inhibitors that simultaneously target both metallo- and serine-β-lactamases

The synthesis and beta-lactamase inhibitory activity of four 6-(mercaptomethyl)penicillinates and the four corresponding 6-(hydroxymethyl)penicillinates are described. These penicillins include both C6 stereoisomers as well as the sulfide and sulfone oxidation states of the penam thiazolidine sulfur. All compounds were evaluated as inhibitors of representative metallo- and serine-beta-lactamases enzymes. Selected (mercaptomethyl)penicillinates are shown to inactivate both metallo- and serine-beta-lactamases and to display synergism with piperacillin against beta-lactamase producing strains.

A detailed kinetic study of Mox-1, a plasmid-encoded class C beta-lactamase

Surveys of beta-lactamases in different parts of the world show an important increase in class C beta-lactamases, thus the study of these enzymes is becoming an important issue. We created an overproduction system for Mox-1, a plasmid class C beta-lactamase, by cloning the gene encoding this enzyme, and placing it under the control of a T7 promoter, using vector pET 28a. The enzyme, purified by ion exchange chromatography, was used to obtain the molecular mass (38246), the N-terminal sequence (GEASPVDPLRPVV), and pI (8.9), and to perform a detailed kinetic study. Cephalotin was used as reporter substrate in the case of poor substrates. The kinetic study showed that benzylpenicillin, cephalotin, cefcapene and moxalactam were good substrates for Mox-1 (k(cat)/K(m) values >2.5 x 10(6) M(-1) s(-1)). On the other hand, ceftazidime and cefepime were poor substrates for this enzyme (K(m) values >200 microM). Clavulanic acid had no inhibitory effect on Mox-1 (K(m)=30.2 mM), however aztreonam behaved as an inhibitor of Mox-1 (K(i)=2.85 microM).

Occurrence of newer beta-lactamases in Klebsiella pneumoniae isolates from 24 U.S. hospitals

Despite the discovery of novel beta-lactamases such as extended-spectrum beta-lactamases (ESBLs), imported AmpC, and carbapenem-hydrolyzing beta-lactamases at least a decade ago, there remains a low level of awareness of their importance and how to detect them. There is a need to increase the levels of awareness of clinical laboratories about the detection of newer beta-lactamases. Therefore, a study was conducted in 2000 to investigate the occurrence of these beta-lactamases in Klebsiella pneumoniae isolates at 24 U.S. medical centers. To enhance the likelihood of detecting imported AmpC and carbapenem-hydrolyzing beta-lactamases, participating laboratories were permitted to include archived strains (1996 to 2000) that were intermediate or resistant to either cefoxitin or imipenem. The beta-lactamase production of 408 isolates positive by screening of 1,123 isolates was investigated by ESBL phenotypic confirmation tests; and for AmpC and carbapenem-hydrolyzing beta-lactamases, three-dimensional tests, isoelectric focusing, beta-lactamase inhibitor studies, spectrophotometric assays, induction assays, and molecular tests were used. ESBL-producing isolates were detected at 18 of the 24 sites (75%), imported AmpC-producing isolates were detected at 10 sites (42%), inducible imported AmpC-producing isolates were detected at 3 sites (12.5%), and a molecular class A carbapenem-hydrolyzing enzyme was detected at 1 site (4%). No class B or D carbapenem-hydrolyzing enzymes were detected. ESBLs and imported AmpC beta-lactamases were detected at a significant number of sites, indicating widespread penetration of these enzymes into U.S. medical institutions. Because these enzymes may significantly affect therapeutic outcomes, it is vital that clinical laboratories be aware of them and be able to detect their occurrence.

Investigation of extended-spectrum beta-lactamase in Klebsiellae pneumoniae and Escherichia coli from China

Extended-spectrum beta-lactamases (ESBLs) are an increasing cause of resistance to third-generation cephalosporins in Enterobacteriaceae. However, they have not been well studied in China. We investigated the prevalence, resistance, and probable gene type of ESBLs using MICs testing and polymerase chain reaction in 559 Klebsiellae pneumoniae and 427 Escherichia coli isolates collected from patients in Huashan Hospital from 1 January to 31 December 1999. The incidence of ESBL-producing strains was 51% among Klebsiellae pneumoniae (285/559) and 23.6% among Escherichia coli (101/427), most of which were collected from patients in intensive care unit and neurosurgical ward. PFGE showed that some epidemic ESBL-producing strains were present in the ICU, especially among ESBL-producing Klebsiellae pneumoniae. The major source of ESBL-producing Klebsiellae pneumoniae and Escherichia coli was sputum specimen (63.5%) and urine (64.3%), respectively. These strains were resistant to most beta-lactams (including the third-generation cephalosporins and monobactams) and non-beta-lactams (such as fluoroquinolones, aminoglycosides, tetracycline, and chloramphenicol), all or most ESBL producers were susceptible to imipenem, cefmetazole and beta-lactam/clavulanic acid. TEM was the main type of beta-lactamases and the CTX-M type of ESBLs was common in these isolates. Some ESBL-producing Escherichia coli and most ESBL-producing Klebsiellae pneumoniae produced more than one type of beta-lactamase. These data confirm that ESBL producers are common among hospital strains of Escherichia coli and Klebsiellae pneumoniae in China. It is important to monitor such strains closely and prevent their spread.