WILD-TYPE VARIANTS OF EXOPENICILLINASE FROM STAPHYLOCOCCUS AUREUS

Cefazolin inoculum effect in methicillin-susceptible Staphylococcus aureus clinical isolates

We investigated the prevalence and characteristics of Cefazolin inoculum effect (CInE) among clinical MSSA isolates in Japan. Although 35.5 % (39 isolates) were positive for the blaZ gene, none met the phenotypic criteria for CInE. Our findings suggested a very low prevalence of CInE among MSSA isolates in our clinical setting.

Molecular Determinants of β-Lactam Resistance in Methicillin-Resistant Staphylococcus aureus (MRSA): An Updated Review

The development of antibiotic resistance in Staphylococcus aureus, particularly in methicillin-resistant S. aureus (MRSA), has become a significant health concern worldwide. The acquired mecA gene encodes penicillin-binding protein 2a (PBP2a), which takes over the activities of endogenous PBPs and, due to its low affinity for β-lactam antibiotics, is the main determinant of MRSA. In addition to PBP2a, other genetic factors that regulate cell wall synthesis, cell signaling pathways, and metabolism are required to develop high-level β-lactam resistance in MRSA. Although several genetic factors that modulate β-lactam resistance have been identified, it remains unclear how they alter PBP2a expression and affect antibiotic resistance. This review describes the molecular determinants of β-lactam resistance in MRSA, with a focus on recent developments in our understanding of the role of mecA-encoded PBP2a and on other genetic factors that modulate the level of β-lactam resistance. Understanding the molecular determinants of β-lactam resistance can aid in developing novel strategies to combat MRSA.

Disc Test for Detecting Staphylococcus aureus Strains Producing Type A and Type C β-Lactamases

Staphylococcus aureus can produce β-lactamases capable of hydrolyzing penicillins and first-generation cephalosporins. The propensity of type A and type C β-lactamase-producing S. aureus (TAPSA and TCPSA) to hydrolyze cefazolin at a high inoculum is termed the cefazolin inoculum effect (CIE). Strains with a CIE have a theoretical risk of causing treatment failure and are unable to be detected routinely by most laboratories. We developed a high-performing yet straightforward β-lactamase disc test that identifies and differentiates both TAPSA and TCPSA and is suitable for routine diagnostic laboratory workflows. Clinical isolates of S. aureus resistant to penicillin were identified, and their blaZ genes were sequenced. MICs were determined at low and high inocula (5 × 105 CFU/mL and 5 × 107 CFU/mL), and isolates demonstrating a CIE were characterized. A semimechanistic model was established to describe differential hydrolysis patterns, and candidate models were iteratively assessed using area-under-the-curve analysis from competitor receiver operating characteristic (ROC) curves. Biomarker thresholds were derived from Youdon index-derived optimal cutoff values. Genetic analysis of 99 isolates identified 26 TAPSA isolates and 45 TCPSA isolates. The model best differentiating TAPSA from non-TAPSA utilized cefazolin-to-cephalothin ratio analysis (sensitivity, 96.2%; specificity, 98.6%). The model best differentiating TCPSA from non-TCPSA incorporated cefazolin, cephalothin, and oxacillin (sensitivity, 88.6%; specificity, 96.6%). TAPSA and TCPSA can be differentiated using three antibiotic discs on a single agar plate. The test has potential value in typing the β-lactamase type from isolates from patients that are candidates for or have failed cefazolin therapy. IMPORTANCE The key significance of this article is that it details a straightforward method of performing a disc test that can differentiate Staphylococcus aureus isolates that are likely to be associated with a cefazolin inoculum effect and theoretical risk of cefazolin treatment failure from isolates that are less likely to be associated with a cefazolin inoculum effect.

Cefazolin Inoculum Effect and Methicillin-Susceptible Staphylococcus aureus Osteoarticular Infections in Children

Select methicillin-susceptible Staphylococcus aureus (MSSA) strains may produce β-lactamases with affinity for first-generation cephalosporins (1GCs). In the setting of a high inoculum, these β-lactamases may promote the cleavage of 1GCs, a phenomenon known as the cefazolin inoculum effect (CzIE). We evaluated the prevalence and impact of CzIE on clinical outcomes among MSSA acute hematogenous osteomyelitis (AHO) cases. MSSA AHO isolates obtained from two children's hospitals between January 2011 and December 2018 were procured through ongoing surveillance studies. Isolates were tested for CzIE via a broth macrodilution assay using an inoculum of 10⁷ CFU/ml; CzIE was defined as a cefazolin MIC of ≥16 μg/ml. Isolates were characterized by accessory gene regulator group (agr). The progression from acute to chronic osteomyelitis was considered an important outcome. A total of 250 cases with viable isolates were included. Notably, 14.4% of isolates exhibited CzIE with no observed temporal trend; and 4% and 76% of patients received a 1GC as an empirical and definitive therapy, respectively. CzIE isolates were more often resistant to clindamycin, belonged to agrIII, and associated with the development of chronic osteomyelitis. In multivariable analyses, agrIII, multiple surgical debridements, delayed source control, and CzIE were independently associated with progression to chronic osteomyelitis. A higher rate of chronic osteomyelitis was observed with CzIE isolates regardless of definitive antibiotic choice. CzIE is exhibited by 14.4% of MSSA AHO isolates in children. CzIE is independently associated with progression to chronic osteomyelitis in cases of AHO irrespective of final antibiotic choice. These data suggest that negative outcomes reported with CzIE may more accurately reflect strain-dependent virulence factors rather than true antibiotic failure.

Novel Insights into the Classification of Staphylococcal β-Lactamases in Relation to the Cefazolin Inoculum Effect

Cefazolin has become a prominent therapy for methicillin-susceptible Staphylococcus aureus (MSSA) infections. However, an important concern is the cefazolin inoculum effect (CzIE), a phenomenon mediated by staphylococcal β-lactamases. Four variants of staphylococcal β-lactamases have been described based on serological methodologies and limited sequence information. Here, we sought to reassess the classification of staphylococcal β-lactamases and their correlation with the CzIE. We included a large collection of 690 contemporary bloodstream MSSA isolates recovered from Latin America, a region with a high prevalence of the CzIE. We determined cefazolin MICs at standard and high inoculums by broth microdilution. Whole-genome sequencing was performed to classify the β-lactamase in each isolate based on the predicted full sequence of BlaZ. We used the classical schemes for β-lactamase classification and compared it to BlaZ allotypes found in unique sequences using the genomic information. Phylogenetic analyses were performed based on the BlaZ and core-genome sequences. The overall prevalence of the CzIE was 40%. Among 641 genomes, type C was the most predominant β-lactamase (37%), followed by type A (33%). We found 29 allotypes and 43 different substitutions in BlaZ. A single allotype, designated BlaZ-2, showed a robust and statistically significant association with the CzIE. Two other allotypes (BlaZ-3 and BlaZ-5) were associated with a lack of the CzIE. Three amino acid substitutions (A9V, E112A, and G145E) showed statistically significant association with the CzIE (P = <0.01). CC30 was the predominant clone among isolates displaying the CzIE. Thus, we provide a novel approach to the classification of the staphylococcal β-lactamases with the potential to more accurately identify MSSA strains exhibiting the CzIE.

Comparative Effectiveness of Exclusive Exposure to Nafcillin or Oxacillin, Cefazolin, Piperacillin/Tazobactam, and Fluoroquinolones Among a National Cohort of Veterans With Methicillin-Susceptible Staphylococcus aureus Bloodstream Infection

OBJECTIVE

Beta-lactam antibiotics are recommended as first-line for treatment of methicillin-sensitive Staphylococcus aureus (MSSA) bacteremia. The objective of this study was to compare effectiveness of anti-MSSA therapies among bacteremia patients exclusively exposed to 1 antimicrobial.

METHOD

This was a national retrospective cohort study of patients hospitalized in Veterans Affairs medical centers with MSSA bacteremia from January 1, 2002, to October 1, 2015. Patients were included if they were treated exclusively with nafcillin, oxacillin, cefazolin, piperacillin/tazobactam, or fluoroquinolones (moxifloxacin and levofloxacin). We assessed 30-day mortality, time to discharge, inpatient mortality, 30-day readmission, and 30-day S. aureus reinfection. Hazard ratios (HRs) and 95% confidence intervals (CI) were calculated using propensity-score (PS) matched Cox proportional hazards regression model.

RESULTS

When comparing nafcillin/oxacillin (n = 105) with cefazolin (n = 107), 30-day mortality was similar between groups (PS matched n = 44; HR, 0.67; 95% CI, 0.11-4.00), as were rates of the other outcomes assessed. As clinical outcomes did not vary between nafcillin/oxacillin and cefazolin, they were combined for comparison with piperacillin/tazobactam (n = 113) and fluoroquinolones (n = 103). Mortality in the 30 days after culture was significantly lower in the nafcillin/oxacillin/cefazolin group compared with piperacillin/tazobactam (PS matched n = 48; HR, 0.10; 95% CI, 0.01-0.78), and similar when compared with fluoroquinolones (PS matched n = 32; HR, 1.33; 95% CI, 0.30-5.96).

CONCLUSIONS

In hospitalized patients with MSSA bacteremia, no difference in mortality was observed between nafcillin/oxacillin and cefazolin or fluoroquinolones. However, higher mortality was observed with piperacillin/tazobactam as compared with nafcillin/oxacillin/cefazolin, suggesting it may not be as effective as a monotherapy in MSSA bacteremia.

The Cefazolin Inoculum Effect Is Associated With Increased Mortality in Methicillin-Susceptible Staphylococcus aureus Bacteremia

Background

Recent studies have favored the use of cefazolin over nafcillin for the treatment of methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia. The clinical influence of the cefazolin inoculum effect (CzIE) in the effectiveness of cephalosporins for severe MSSA infections has not been evaluated.

Methods

We prospectively included patients from 3 Argentinian hospitals with S. aureus bacteremia. Cefazolin minimum inhibitory concentrations (MICs) were determined at standard (10⁵ colony-forming units [CFU]/mL) and high (10⁷ CFU/mL) inoculum. The CzIE was defined as an increase of MIC to ≥16 µg/mL when tested at high inoculum. Whole-genome sequencing was performed in all isolates.

Results

A total of 77 patients, contributing 89 MSSA isolates, were included in the study; 42 patients (54.5%) had isolates with the CzIE. In univariate analysis, patients with MSSA exhibiting the CzIE had increased 30-day mortality (P = .034) and were more likely to have catheter-associated or unknown source of bacteremia (P = .033) compared with patients infected with MSSA isolates without the CzIE. No statistically significant difference between the groups was observed in age, clinical illness severity, place of acquisition (community vs hospital), or presence of endocarditis. The CzIE remained associated with increased 30-day mortality in multivariate analysis (risk ratio, 2.65; 95% confidence interval, 1.10–6.42; P = .03). MSSA genomes displayed a high degree of heterogeneity, and the CzIE was not associated with a specific lineage.

Conclusions

In patients with MSSA bacteremia where cephalosporins are used as firstline therapy, the CzIE was associated with increased 30-day mortality. Clinicians should be cautious when using cefazolin as firstline therapy for these infections.

Mass spectrometry methods for predicting antibiotic resistance

Developing elaborate techniques for clinical applications can be a complicated process. Whole-cell MALDI-TOF MS revolutionized reliable microorganism identification in clinical microbiology laboratories and is now replacing phenotypic microbial identification. This technique is a generic, accurate, rapid, and cost-effective growth-based method. Antibiotic resistance keeps emerging in environmental and clinical microorganisms, leading to clinical therapeutic challenges, especially for Gram-negative bacteria. Antimicrobial susceptibility testing is used to reliably predict antimicrobial success in treating infection, but it is inherently limited by the need to isolate and grow cultures, delaying the application of appropriate therapies. Antibiotic resistance prediction by growth-independent methods is expected to reduce the turnaround time. Recently, the potential of next-generation sequencing and microarrays in predicting microbial resistance has been demonstrated, and this review evaluates the potential of MS in this field. First, technological advances are described, and the possibility of predicting antibiotic resistance by MS is then illustrated for three prototypical human pathogens: Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. Clearly, MS methods can identify antimicrobial resistance mediated by horizontal gene transfers or by mutations that affect the quantity of a gene product, whereas antimicrobial resistance mediated by target mutations remains difficult to detect.

Real-time PCR assay for detection of blaZ genes in Staphylococcus aureus clinical isolates

The Clinical and Laboratory Standards Institute recommends consideration of blaZ gene testing for cases of serious Staphylococcus aureus infection. Conventional PCR methods have demonstrated superior sensitivity and specificity to phenotypic tests. To our knowledge, this is the first description of real-time PCR detection of the blaZ gene.

Beta-lactamase induction and cell wall metabolism in Gram-negative bacteria

Production of beta-lactamases, the enzymes that degrade beta-lactam antibiotics, is the most widespread and threatening mechanism of antibiotic resistance. In the past, extensive research has focused on the structure, function, and ecology of beta-lactamases while limited efforts were placed on the regulatory mechanisms of beta-lactamases. Recently, increasing evidence demonstrate a direct link between beta-lactamase induction and cell wall metabolism in Gram-negative bacteria. Specifically, expression of beta-lactamase could be induced by the liberated murein fragments, such as muropeptides. This article summarizes current knowledge on cell wall metabolism, beta-lactam antibiotics, and beta-lactamases. In particular, we comprehensively reviewed recent studies on the beta-lactamase induction by muropeptides via two major molecular mechanisms (the AmpG-AmpR-AmpC pathway and BlrAB-like two-component regulatory system) in Gram-negative bacteria. The signaling pathways for beta-lactamase induction offer a broad array of promising targets for the discovery of new antibacterial drugs used for combination therapies. Therefore, to develop effective mitigation strategies against the widespread beta-lactam resistance, examination of the molecular basis of beta-lactamase induction by cell wall fragment is highly warranted.

Evidence for a purifying selection acting on the β-lactamase locus in epidemic clones of methicillin-resistant Staphylococcus aureus

Background

The β-lactamase (bla) locus, which confers resistance to penicillins only, may control the transcription of mecA, the central element of methicillin resistance, which is embedded in a polymorphic heterelogous chromosomal cassette (the SCCmec element). In order to assess the eventual correlation between bla allotypes and genetic lineages, SCCmec types and/or β-lactam resistance phenotypes, the allelic variation on the bla locus was evaluated in a representative collection of 54 international epidemic methicillin-resistant Staphylococcus aureus (MRSA) clinical strains and, for comparative purposes, also in 24 diverse methicillin-susceptible S. aureus (MSSA) strains.

Results

Internal fragments of blaZ (the β-lactamase structural gene) were sequenced for all strains. A subset of strains, representative of blaZ allotypes, was further characterized by sequencing of internal fragments of the blaZ transcriptional regulators, blaI and blaR1. Thirteen allotypes for blaZ, nine for blaI and 12 for blaR1 were found. In a total of 121 unique single-nucleotide polymorphisms (SNP) detected, no frameshift mutations were identified and only one nonsense mutation within blaZ was found in a MRSA strain. On average, blaZ alleles were more polymorphic among MSSA than in MRSA (14.7 vs 11.4 SNP/allele). Overall, blaR1 was the most polymorphic gene with an average of 24.8 SNP/allele. No correlation could be established between bla allotypes and genetic lineages, SCCmec types and/or β-lactam resistance phenotypes. In order to estimate the selection pressure acting on the bla locus, the average dN/dS values were computed. In the three genes and in both collections dN/dS ratios were significantly below 1.

Conclusions

The data strongly suggests the existence of a purifying selection to maintain the bla locus fully functional even on MRSA strains. Although, this is in agreement with the notion that in most clinical MRSA strains mecA gene is under the control of the bla regulatory genes, these findings also suggest that the apparently redundant function of blaZ gene for the MRSA resistant phenotype is still important for these strains. In addition, the data shows that the sensor-inducer blaR1 is the primary target for the accumulation of mutations in the bla locus, presumably to modulate the response to the presence of β-lactam antibiotic.

Antibiotic resistance in staphylococci

When penicillin was introduced in 1944 over 94% of Staphylococcus aureus isolates were susceptible; by 1950 half were resistant. By 1960 many hospitals had outbreaks of virulent multi-resistant S. aureus. These were overcome with penicillinase-stable penicillins, but victory was brief; methicillin-resistant S. aureus (MRSA) were recorded in the year of the drug's launch. MRSA owe their behaviour to an additional, penicillin-resistant peptidoglycan transpeptidase, PBP-2', encoded by mecA. Their spread is clonal, with transfer of mecA being extremely rare. MRSA accumulated and then declined in the 1960s and 1970s, but became re-established in the early 1980s. Some early MRSA strains were colonists rather than invaders and the proportion of MRSA among S. aureus bacteraemias in England remained under 3% until 1992. However, this proportion rose to 34-37% by 1998-1999, reflecting the dissemination of two new epidemic strains, EMRSA 15 and 16. These may be more virulent than earlier MRSA, or their success may reflect changing hospital practice. Until 1996, glycopeptides were universally active against S. aureus; then glycopeptide-intermediate S. aureus (GISA) were found in Japan, France, and the USA. This resistance is associated with increased wall synthesis. Coagulase-negative staphylococci (CNS) are less pathogenic than S. aureus but are important in line-associated bacteraemias and prosthetic device infections. They are even more often resistant than S. aureus, notably to teicoplanin. Few anti-staphylococcal agents were launched from 1970 to 1995, but the situation is now improving. Dalfopristin/quinupristin inhibits virtually all S. aureus, although its bactericidal activity is impaired against strains with constitutive MLSB-type resistance; other new agents are in advanced development. New agents give a renewed opportunity for control, but S. aureus is a resilient foe, able to regain its importance if drugs are used profligately or if hygiene is slackened.

Characterization of a chromosomal gene encoding type B beta-lactamase in phage group II isolates of Staphylococcus aureus

In contrast to most Staphylococcus aureus isolates in which the gene for staphylococcal beta-lactamase (blaZ) is plasmid borne, isolates typeable by group II bacteriophages frequently carry blaZ on the chromosome. Furthermore, the chromosomal gene encodes the type B variant of staphylococcal beta-lactamase for which the nucleotide and deduced amino acid sequences have not yet been reported. To better understand beta-lactamase production among phage group II staphylococci and the nature of the type B beta-lactamase, we determined the type and amount of enzyme produced by 24 phage group II isolates. Of these isolates, 1 did not produce beta-lactamase, 8 produced the type B enzyme, and 15 produced the type C enzyme. In all eight type B beta-lactamase-producing isolates, blaZ was located on the chromosome. This was in contrast to the type C beta-lactamase-producing isolates, in which blaZ was located on a 21-kb plasmid. The nucleotide sequence corresponding to the leader peptide and the N-terminal 85% of the mature exoenzyme form of type B S. aureus was determined. The deduced amino acid sequence revealed 3 residues in the leader peptide and 12 residues in the exoenzyme portion of the beta-lactamase that differ from the prototypic type A beta-lactamase sequence. These include the serine-to-asparagine change at residue 216 found in the kinetically similar type C enzyme, a threonine-to-lysine change at residue 128 close to the SDN loop (residues 130 to 132), and several substitutions not found in any of the other staphylococcal beta-lactamases. In summary, modern isolates of S. aureus typeable by group II phages produce type B or type C staphylococcal beta-lactamase. The type B gene resides on the chromosome and has a sequence that, when compared to the sequences of the other staphylococcal beta-lactamases, corresponds well with its kinetic properties.

Association of borderline oxacillin-susceptible strains of Staphylococcus aureus with surgical wound infections

Staphylococcus aureus isolates which produce type A staphylococcal beta-lactamase have been associated with wound infections complicating the use of cefazolin prophylaxis in surgery. To further evaluate this finding, 215 wound isolates from 14 cities in the United States were characterized by antimicrobial susceptibility and beta-lactamase type and correlated with the preoperative prophylactic regimen. Borderline-susceptible S. aureus isolates of phage group 5 (BSSA-5), which produce large amounts of type A beta-lactamase and exhibit borderline susceptibility to oxacillin, comprised a greater percentage of the 120 wound isolates associated with cefazolin prophylaxis than they did of the 95 isolates associated with other prophylactic regimens (25% versus 12.6%, respectively; P < 0.05). In contrast, methicillin-resistant S. aureus isolates were distributed evenly between the two groups (8.3% versus 11.6%, respectively). In vitro assays demonstrated that cefazolin was hydrolyzed faster by BSSA-5 strains than by other beta-lactamase-producing, methicillin-susceptible strains (1.54 versus 0.50 microg/min/10(8) CFU, respectively; P < 0.0001). These data demonstrate that BSSA-5 strains are a distinct subpopulation of methicillin-susceptible S. aureus which frequently cause deep surgical wound infections. Cefazolin use in prophylaxis is a risk factor for BSSA-5 infection.

Antimicrobial resistance in staphylococci. Epidemiology, molecular mechanisms, and clinical relevance

Staphylococcal infections continue to pose important clinical problems in children and adults. Antibiotic resistance among the staphylococci has rendered therapy of these infections a therapeutic challenge. Despite early, uniform susceptibility to penicillin, staphylococci acquired a gene elaborating beta-lactamase that rendered penicillin inactive and that is borne by nearly all clinical isolates. "Penicillinase-resistant beta-lactams," such as methicillin, were introduced in the early 1960s, but resistance to them has become an increasing concern. The mechanism of the so-called "methicillin resistance" is complex. Moreover, once confined to the ecology of hospitals and other institutions, a recent increase in community-acquired methicillin-resistant S. aureus infections has been observed. Glycopeptides, until now the only uniformly reliable therapeutic modality, have been increasingly used for therapy of staphylococcal infections. The recent recognition of clinical isolates with reduced susceptibility to glycopeptides is of concern.

DOMINANCE OF THE INDUCIBLE STATE IN STRAINS OF STAPHYLOCOCCUS AUREUS CONTAINING TWO DISTINCT PENICILLINASE PLASMIDS

Richmond, Mark H. (National Institute for Medical Research, Mill Hill, London, England). Dominance of the inducible state in strains of Staphylococcus aureus containing two distinct penicillinase plasmids. J. Bacteriol. 90:370-374. 1965.-The phenotypic expression of penicillinase synthesis is studied in a strain of S. aureus containing two distinct plasmids, each carrying the penicillinase structural gene and its associated control (inducibility) gene. One plasmid in the heterodiploid carries the genes responsible for the normal, fully inducible synthesis of a structural mutant of type A penicillinase, and the second plasmid carries the genes required for constitutive synthesis of C-type enzyme. The heterodiploid is phenotypically fully inducible, indicating that the inducible state dominates the constitutive in penicillinase synthesis. The heterodiploid synthesizes approximately equal quantities of A- and C-type enzyme when fully induced. Parental types are obtained from the heterodiploid by spontaneous segregation at a frequency of about 1 per 1,000 divisions of each character. Recombination between the two plasmids in such a persistent heterodiploid is a rare event.

Comparison of the beta-lactamase gene cluster in clonally distinct strains of Enterococcus faecalis

Ten beta-lactamase-producing Enterococcus faecalis isolates were examined for the presence of the staphylococcal beta-lactamase repressor and antirepressor genes. Four isolates, previously shown to be unrelated to each other by pulsed-field gel electrophoresis analysis, were positive for both genes by PCR, although beta-lactamase production was not induced with methicillin. Six isolates, previously shown to be clonally related, were negative for both genes by PCR. The blaZ sequences of eight beta-lactamase-producing E. faecalis isolates were determined. Seven isolates from five distinct clones had sequences identical to that previously reported for E. faecalis HH22, regardless of whether the repressor or antirepressor was demonstrated by PCR. However, blaZ from one isolate differed from those of the other enterococci by 11 nucleotides; this isolate is part of the large clone, as defined by pulsed-field gel electrophoresis and multilocus enzyme analysis, that includes HH22. These findings suggest either that enterococci have acquired the bla gene cluster from more than one source or that the gene cluster has undergone considerable change since acquisition by this clone.

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.

beta-Lactamase production and genetic location in Staphylococcus aureus: introduction of a beta-lactamase plasmid in strains of phage group II

Staphylococcus aureus strains of phage group II have increased in frequency in hospital-acquired infections during recent years. A total of 184 penicillin resistant group II strains from bacteraemia cases in the years 1961-1990 were analysed for the amount of beta-lactamase produced and the location of the beta-lactamase gene. Until 1977 all strains investigated had a chromosomally located beta-lactamase gene, but since then a 21 kb beta-lactamase plasmid has increased in occurrence among group II strains, especially among those strains typable only at high phage concentrations [100 x Routine Test Dilution (RTD) and 1000 x RTD]. In 1990, 84% of the group II strains contained this plasmid. Plasmid-containing strains produced more beta-lactamase than strains without the plasmid. S. aureus strains of the 94,96 complex, which since 1984 have decreased in frequency from 18 to 9% in 1993, have remained high producers of beta-lactamase.

Antimicrobial resistance in staphylococci

Staphylococci have developed a variety of strategies for dealing with the presence of antibiotics encountered in clinical environments. Resistance to beta-lactams and other antimicrobial agents has been accomplished by a diverse array of molecular mechanisms. Options available to treat infections caused by staphylococci resistant to methicillin are limited, and the next generation of antibiotics to be introduced, should glycopeptide resistance become an important clinical problem, is not yet on the horizon.

Production of A and C variants of staphylococcal beta-lactamase by methicillin-resistant strains of Staphylococcus aureus

Most methicillin-resistant Staphylococcus aureus (MRSA) strains produce beta-lactamase. To determine whether this enzyme(s) is identical to one or more of the four beta-lactamases produced by methicillin-susceptible strains, the beta-lactamases of 50 MRSA isolates were typed by using substrate profile analysis. Forty type A, no type B, ten type C, and no type D beta-lactamase-producing strains were identified. The beta-lactamase inhibitor sulbactam reduced the MICs of beta-lactamase-labile antibiotics, including ampicillin, penicillin G, and cefazolin, for type A and type C MRSA strains.

Antibiotic susceptibility and phage typing of methicillin-resistant Staphylococcus aureus clinical isolates from blood cultures of 692 patients in 15 Israeli hospitals

Six hundred ninety-two clinical isolates of Staphylococcus aureus were collected from blood cultures of 692 patients in 15 Israeli hospitals over a two year period. Antibiotic sensitivity was tested by the standard disk diffusion technique. Of these isolates, 41.6% were methicillin-resistant (MRSA). All 288 MRSA isolates were sensitive to vancomycin and pristinomycin; 98.6% were sensitive to fucidine; 97.9% to imipenem; 79% to rifampicin; 63.6% to amikacin; 54.5% to augmentin; 36.4% to clindamycin; 12.6% to ciprofloxacin; 11.9% to cotrimoxazole and ofloxacin; 10.5% to gentamicin; 9.8% to erythromycin; and 8.4% to norfloxacin. Phage typing was determined by using the international set of phages. All the isolates that were sensitive to Group I phages, and 91.8% of those sensitive to Group II phages, were sensitive to methicillin. Of the isolates that were sensitive to Group III phages, 79.2% were methicillin-resistant and 72.4% of the latter were sensitive to phages 75/85. Of the isolates that were sensitive to Group III and miscellaneous phages, 50.7% were methicillin-resistant and 71% of the latter were sensitive to phages 75/85 as well.

Characterization of four beta-lactamases produced by Staphylococcus aureus

Staphylococcus aureus produces four types of beta-lactamase (A, B, C, and D). To investigate the effect of specific beta-lactamase type upon staphylococcal resistance, each beta-lactamase was purified to homogeneity, and the Michaelis constants (Km values) and turnover numbers (kcat values) for various penicillin and cephalosporin substrates were determined. Whereas Km values of the four beta-lactamases were comparable for penicillin G, cephalothin, and cefamandole, the type A and D enzymes exhibited greater affinity than the type B and C beta-lactamases for nitrocefin, cefazolin, and cephapirin. Conversely, the type B and C beta-lactamases exhibited greater kcat values than the type A and D enzymes against most of the cephalosporin agents, excluding nitrocefin. In contrast to earlier reports suggesting that the type B beta-lactamase is relatively inefficient in hydrolyzing penicillin G, we found only minor differences in the specific activities and kcat values of the type A, B, and C beta-lactamases. The type D beta-lactamase was distinctly less active against penicillin G, however, exhibiting only 15 to 25% of the kcat values of the other beta-lactamases. More than a 2,000-fold difference between the relative efficiencies of hydrolysis (kcat/Km) of cefazolin and cefuroxime by the type A beta-lactamase exists. This greatly exceeds the 60-fold difference in the stability of penicillin G and cefazolin with the same enzyme. Whereas the isoelectric points of the type A, B, and C beta-lactamases were similar, the value for the type D beta-lactamase was distinguishably lower (10.1 for types A, B, and C and 9.7 for type D). We conclude that marked differences in the stability of commonly used beta-lactams to hydrolysis by the staphylococcal beta-lactamases are present. This heterogeneity and the clinical implication thereof need to be considered in the antibiotic management of staphylococcal infection.

Cephalosporins--cefotaxime 10 years later, a major drug with continued use

Cefotaxime has in the past decade proved to be a most useful agent. It has established the efficacy and safety suggested in the early in vitro, pharmacological and clinical papers. It remains an excellent agent to treat many community and hospital-acquired respiratory infections, urinary tract infections, meningitis, particularly in pediatrics, spontaneous bacterial peritonitis and selected abdominal and gynecological infections.

Hyperexpression in Escherichia coli, purification, and characterization of the metallo-beta-lactamase of Bacillus cereus 5/B/6

We used site-directed mutagenesis to introduce both a NdeI restriction endonuclease site and an initiator codon at the junction of the leader and structural gene sequences of the metallo-beta-lactamase of Bacillus cereus 5/B/6. This construct allowed us to clone just the beta-lactamase structural gene sequence into an Escherichia coli expression vector. E. coli cells were transformed with the recombinant plasmid, the B. cereus beta-lactamase was expressed, and these E. coli cells were disrupted by sonic oscillation. When the resultant suspensions were clarified by ultracentrifugation, the B. cereus beta-lactamase represented 15% of the total protein in the supernatant. Subsequent gel filtration and ion-exchange chromatography allowed the first reported purification to homogeneity of the B. cereus beta-lactamase from E. coli with an 87% recovery and an overall yield of 17 mg of enzyme per liter of cell culture. The electrophoretic mobilities of the enzyme expressed in and purified from E. coli and the enzyme purified directly from B. cereus were identical in both native and sodium dodecyl sulfate gel electrophoreses. As with the B. cereus enzyme, Km and Vmax (using cephalosporin C as substrate) for the enzyme purified from E. coli were 0.39 mM and 1333 units/mg protein, respectively. Likewise, the Co(II)-reconstituted enzyme purified from E. coli, which retained 29% of the activity of the Zn(II) enzyme, had electronic absorption spectra with maxima at 347, 551, 617, and 646 nm with extinction coefficients of 900, 250, 173, and 150 M-1 cm-1, respectively.

Purification of Staphylococcus aureus beta-lactamases by using sequential cation-exchange and affinity chromatography

Boronic acids are active-site inhibitors of serine beta-lactamases, and a phenylboronic acid-agarose affinity column has been used to purify beta-lactamase from crude cell extracts of several bacterial species. We applied phenylboronic acid-agarose chromatography to the purification of Staphylococcus aureus beta-lactamase. Two factors interfered with the success of the previously described single-step chromatographic protocol. First, staphylococcal beta-lactamase exhibited non-active-site-mediated adsorption to the agarose used as a support for the meta-aminophenylborate ligand, preventing the recovery of beta-lactamase from the column. Second, the staphylococcal beta-lactamases exhibited low affinity for meta-aminophenylborate with inhibition constants (Kis) ranging from 8.0 x 10(-3) to 20.0 x 10(-3) M. These problems were resolved by modifying the buffers utilized during chromatography and increasing the dimensions of the affinity column, and a two-stage procedure consisting of cation-exchange chromatography followed by affinity chromatography was used to purify each of the four variants of staphylococcal beta-lactamase. The mean specific activities of the purified type A, B, C, and D beta-lactamases were 44.6, 12.2, 10.6, and 30.8 mumol of nitrocefin hydrolyzed per min/mg of protein, respectively. Dimer formation, presumably from intramolecular cysteine-cysteine cross-linking, was observed with the type D beta-lactamase but not with the type A, B, or C enzyme.

Detection of borderline oxacillin-resistant Staphylococcus aureus and differentiation from methicillin-resistant strains

Eighty-eight Staphylococcus aureus clinical isolates meeting criteria for borderline oxacillin resistance (intermediate susceptibility or resistance to oxacillin but susceptibility to amoxicillin/clavulanic acid upon disk diffusion testing) were studied to determine optimal test techniques and conditions for differentiating borderline oxacillin-resistant Staphylococcus aureus (BORSA) from methicillin-resistant Staphylococcus aureus (MRSA). Further testing revealed three distinct resistance patterns: 61 strains (69%) consistently met BORSA criteria and had average beta-lactamase levels five- to six-fold higher than oxacillin-susceptible controls; 11 strains (13%) were markedly heteroresistant MRSA with delayed appearance of resistant colonies leading to spurious susceptibility to amoxicillin/clavulanic acid; 16 strains (18%) appeared to be oxacillin-susceptible on repetitive testing. Under conditions used to elicit intrinsic methicillin resistance in Staphylococcus aureus, a large percentage of BORSA appeared resistant to amoxicillin/clavulanic acid. This clearly shows that BORSA may be misidentified as MRSA while heteroresistant MRSA may appear to be BORSA. It is concluded that amoxicillin/clavulanic acid zone sizes should be measured after a full 24 hours of incubation, that susceptibility testing of Staphylococcus aureus under certain environmental conditions should be interpreted with caution, and that MIC testing is the most reliable technique for differentiating these two resistance patterns in Staphylococcus aureus.

Pathophysiologic basis for the use of third-generation cephalosporins

After 10 years of use, the third-generation cephalosporins remain excellent antibiotics. They have superior activity against selected streptococcal species compared with other cephalosporins, and superior activity against Haemophilus, Neisseria, Branhamella, and other less common oral gram-negative aerobic species. Despite a very broad spectrum of activity, the third-generation cephalosporins, like all other cephalosporins, have only poor activity against enterococci, Listeria, Corynebacterium jekeium, and methicillin-resistant staphylococci. Over the past 10 years, the activity of the third-generation cephalosporins against Escherichia coli, Klebsiella, Proteus, Providencia, Serratia, Haemophilus, and Neisseria has remained excellent. Equally as important, though perhaps less well recognized, is the activity of some of these agents against mouth anaerobic species and the anaerobic Bacteroides and Clostridium spp. of the pelvic area. At present, there are two main threats to the continued use of the third-generation cephalosporins. These are the increasing number of infections due to Enterobacter spp., which constitutively produce large amounts of a beta-lactamase that hydrolyzes cephalosporins, and the recent appearance of Klebsiella spp. in many parts of the world that possess new plasmid-mediated beta-lactamases that destroy cefotaxime, ceftazidime, and related third-generation parenteral cephalosporins. Correlation of pharmacologic properties with in vitro activity provides information as to reasonable dosage regimens for the third-generation cephalosporins. For most serious infections cefotaxime, ceftizoxime, and ceftazidime should be given three times a day provided that the patient has relatively normal renal function. Ceftriaxone can be administered once daily in less severe infections. The use of lower doses or less frequent dosing with cefotaxime, ceftizoxime, or ceftazidime is recommended in aged patients whose renal function is impaired. The unique interaction of cefotaxime with its active metabolite, desacetylcefotaxime, allows cefotaxime to be administered less frequently than three times a day in selected anaerobic infections. Correlation of the antibacterial activity and pharmacology of cephalosporins will help us to tailor their use more appropriately, so that the third-generation cephalosporins will remain useful antimicrobial agents for a further decade.

Use of extracts versus whole-cell bacterial suspensions in the identification of Staphylococcus aureus beta-lactamase variants

We previously have shown that extracts of S. aureus isolates which produce the recognized serotypes of staphylococcal beta-lactamase (A, B, C, D) differ in the rates at which they hydrolyze selected cephalosporins, exhibiting substrate profiles which are distinctive for each serotype. In an effort to simplify the methods employed in identifying the different staphylococcal beta-lactamases, we evaluated whether distinctive substrate profiles could be obtained by using whole-cell suspensions of 115 beta-lactamase-producing isolates of S. aureus. Compared with extracts from the same strains, the whole-cell bacterial suspensions not only were simpler to prepare but enabled beta-lactamase typing of a higher proportion of the evaluated strains (86 versus 97%, respectively). Furthermore, the use of whole-cell bacterial suspensions enabled the simultaneous quantitation of the beta-lactamase activity exhibited by each strain. Additionally, by comparing the quantitative activity of beta-lactamase-induced and -uninduced preparations of the same strain, induction ratios (i.e., induced/uninduced activity) could be derived, yielding information regarding the regulation of beta-lactamase production by each strain. We believe that the utilization of whole-cell methods, such as those employed in this study, will facilitate the investigation of qualitative and quantitative differences in beta-lactamase production among clinical and reference isolates of S. aureus.

Overview of mechanisms of bacterial resistance

Many antimicrobial agents have been either found in nature or synthesized in the past 45 years. Antibacterial agents inhibit cell-wall formation, disrupt cytoplasmic membrane function, prevent DNA synthesis, interfere with protein synthesis, and halt folate synthesis. Resistance to antibiotics is a result of three major mechanisms: prevention of the antibacterial agent from reaching its receptor site, production of altered targets, and destruction or modification of the agents. Bacterial resistance has occurred due to chromosomal changes or the presence of plasmids and transposons. Resistance to beta-lactams is the result of beta-lactamases and the production of altered penicillin-binding proteins as well as altered cell-wall permeability. Important examples of these resistance forms occur in staphylococci and pneumococci which have altered penicillin-binding proteins. A new form of target change has been the production of proteins in enterococci that inhibit the activity of glycopeptides. Beta-lactamases are present in both Gram-positive and Gram-negative species; recently, new plasmid beta-lactamases have been isolated that destroy iminomethoxy and iminocarboxy cephalosporins. Resistance to aminoglycosides is due to enzymes that acetylate, adenylate, or phosphorylate aminoglycosides that inhibit binding to ribosomes and thus cause the poor uptake of drug. Tetracycline resistance is due to plasmids which cause efflux of the agent from the cytoplasm. Macrolide and lincinoid resistance is the result of an altered 23S ribosomal component of the 50S ribosomes. Sulfonamide and trimethoprim resistance is due to production of altered synthetase and reductase enzymes essential in the synthesis of folate.(ABSTRACT TRUNCATED AT 250 WORDS)

Production, purification and spectral properties of metal-dependent beta-lactamases of Bacillus cereus

New methods for the production of consistently high levels of metal-dependent beta-lactamases (beta-lactamhydrolase, EC 3.5.2.6) from strains 569/H/9 and 5/B/6 of Bacillus cereus are described which have significant advantages over those reported previously. For example, these techniques do not require a fermentor with pH-stat capabilities. We also describe rapid very-high-yield purification schemes for the metal-dependent beta-lactamases from these strains, employing high-performance ultrafiltration (HPUF) and mass ion exchange techniques. Furthermore, we have developed improved methods for the removal of the active site Zn(II) and reconstitution of the beta-lactamase enzymatic activity with Co(II), which result in higher recovery of the original activity than previously reported. In order to characterize the purified beta-lactamases II of B. cereus 569/H/9 and 5/B/6 we have examined the molecular weights, and steady state kinetic parameters of Zn(II) enzymes, and the electronic and EPR spectra of the Co(II)-reconstituted enzymes. EPR spectra of CO(II)-reconstituted beta-lactamase from B. cereus 5/B/6 have not been previously reported.

Evaluation of cefotaxime alone and in combination with desacetylcefotaxime against strains of Staphylococcus aureus that produce variants of staphylococcal beta-lactamase

We evaluated cefotaxime (CTX) alone and in combination with its metabolite, desacetylcefotaxime (dCTX) against strains of Staphylococcus aureus that produce the four recognized variants of staphylococcal beta-lactamase and a beta-lactamase-producing isolate characterized by the expression of borderline resistance to methicillin. Although macrodilution MICs revealed that dCTX was less active than CTX against these strains (geometric means of 16 micrograms/ml and 4 micrograms/ml, respectively), the addition of clinically achievable concentrations of dCTX to CTX resulted in a reduction in the observed CTX MICs. This effect was similar to although less pronounced than that obtained by combining clavulanic acid with cefazolin. The increased antistaphylococcal activity noted by MIC determinations was confirmed with kill-kinetic studies. Determination of the relative rates of hydrolysis of selected cephalosporins showed that neither CTX nor dCTX were appreciably hydrolyzed by the variant staphylococcal enzymes. Evaluation of the effect of CTX and dCTX upon the staphylococcal beta-lactamases demonstrated that neither agent inhibited the destruction of a 100 microM solution of nitrocefin, although the reduction of CTX and cefazolin MICs by low concentrations of dCTX suggests that the dCTX metabolite may act as a competitive inhibitor of beta-lactamase. These observations may explain the previously demonstrated clinical efficacy of CTX used alone for the treatment of serious infections caused by S. aureus.

Bacterial resistance to beta-lactam antibiotics: crystal structure of beta-lactamase from Staphylococcus aureus PC1 at 2.5 A resolution

beta-lactamases are enzymes that protect bacteria from the lethal effects of beta-lactam antibiotics, and are therefore of considerable clinical importance. The crystal structure of beta-lactamase from the Gram-positive bacterium Staphylococcus aureus PC1 has been determined at 2.5 angstrom resolution. It reveals a molecule of novel topology, made up of two closely associated domains. The active site is located at the interface between the domains, with the key catalytic residue Ser70 at the amino terminus of a buried helix. Examination of the disposition of the functionally important residues within the active site depression leads to a model for the binding of a substrate and a functional analogy to the serine proteases. The unusual topology of the secondary structure units is relevant to questions concerning the evolutionary relation to the beta-lactam target enzymes of the bacterial cell wall.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Staphylococcus aureus: biology, mechanisms of virulence, epidemiology

The prominence of Staphylococcus aureus as a cause of serious human infection has prompted extensive studies of the microbiology, pathogenesis, and epidemiology of staphylococci and staphylococcal infections. Staphylococci are of the family Micrococcaceae, although there are diverse genetic and phenotypic differences between them and other members of this family. Of the more than 20 species of staphylococci, only three are clinically significant: S. aureus, S. epidermidis, and S. saprophyticus. These species can be distinguished by coagulase production and novobiocin resistance. Staphylococci produce a variety of structural, enzymatic, and toxic products, which are associated with adherence, invasion, toxicity, and avoidance of host defense mechanisms. In addition, a variety of host characteristics increase susceptibility to staphylococcal infection. Staphylococci are an important cause of infection in hospitals and the community. Following the introduction of antimicrobials, staphylococci rapidly developed resistance. A penicillin-resistant specific phage type, designated 80/81, caused severe outbreaks of nosocomial disease in the 1950s and 1960s. Staphylococci recently acquired resistance to methicillin and other antimicrobials, and persist as important nosocomial pathogens. Although S. aureus is one of the earliest recognized and most studied human pathogens, it is a perplexing, ever-changing, recurring public health problem.

In vitro studies on the antibacterial activities of YM-13115, a new broad-spectrum cephalosporin

The in vitro antibacterial activities of YM-13115, a new parenteral cephalosporin, were compared with those of ceftazidime, cefoperazone, and cefsulodin. The compound was highly active against the common members of the Enterobacteriaceae and 2 to 256 times more active than cefoperazone. YM-13115 was as active as ceftazidime against Citrobacter freundii, Proteus vulgaris, and Morganella morganii and two to four times more active than ceftazidime against Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Enterobacter aerogenes, Serratia marcescens, Proteus mirabilis, Providencia rettgeri, and Providencia stuartii. The activity of YM-13115 against Pseudomonas aeruginosa (with MICs of 0.78 and 3.13 micrograms/ml for 50 and 90% of the isolates, respectively) was ca. 2 times that of ceftazidime, 4 times that of cefsulodin, and 16 times that of cefoperazone. Against Haemophilus influenzae YM-13115 was more active than ceftazidime. YM-13115 was less active than ceftazidime, cefoperazone, and cefsulodin against Staphylococcus aureus and Staphylococcus epidermidis. The concentrations of YM-13115 required to inhibit the growth of 90% of the isolates of Streptococcus pyogenes and Streptococcus pneumoniae were 0.78 and 1.56 microgram/ml, respectively, but concentrations above 100 micrograms/ml were required to inhibit Streptococcus faecalis. YM-13115 was not hydrolyzed by the common plasmid and chromosomal beta-lactamases. YM-13115 is extremely active against P. aeruginosa and members of the Enterobacteriaceae.

Classification of beta-lactamase-producing bacteria

The resistance of bacteria to beta-lactam antibiotics, penicillins or cephalosporins, is usually associated with the production of beta-lactamase enzymes which are capable of inactivating these compounds. A number of recent surveys have reported a significant increase in the frequency of isolation of beta-lactamase-producing strains of bacteria from infections in general practice in the United Kingdom. A wide variety of bacterial beta-lactamases with different properties have been described and it is now recognised that most bacteria produce a chromosomally-mediated beta-lactamase which is characteristic of each species. In addition, Gram-negative bacteria may acquire beta-lactamases which are mediated by plasmids which are readily transferable by cell to cell contact. Schemes for the classification of bacterial beta-lactamases are outlined.

Characterization of penicillinase inhibitor in bovine serum

Bovine serum containing penicillinase inhibitor activity was fractioned by gel filtration chromatography and ion exchange chromatography. The penicillinase inhibitor was principally located in the IgG-fraction and therefore appears to be an antibody.

When serum antipenicillinase activity was screened in material consisting of 54 animals, mastitis cows showed higher incidence of penicillinase inhibitor than healthy animals.

Serum penicillinase inhibitor had a slight effect on the MIC-value of penicillin against penicillinase producing staphylococci which indicates that this inhibitor could have some clinical influence in increasing bacterial sensitivity to penicillin.

Transduction of methicillin resistance in Staphylococcus aureus: recipient effectiveness and beta-lactamase production

The effectiveness of Staphylococcus aureus strain 8325-4 as a recipient for the transduction of methicillin resistance requires the presence of a penicillinase plasmid but was found to be independent of the lysogenic state of the recipient. Effectiveness is conferred by the plasmid in either the autonomous or integrated states, although the transduction rate is higher in the former. Once established, the maintenance and expression of methicillin resistance were independent of continued carriage of the plasmid deoxyribonucleic acid. Analysis of penicillinase plasmid mutants indicated that beta-lactamase production was the plasmid function responsible for recipient effectiveness. Supportive evidence included the abrogation of recipient effectiveness by the beta-lactamase inhibitor clavulanic acid and the elimination of a plasmid requirement with recipient strains carrying a chromosomal beta-lactamase determinant. A possible role for beta-lactamase production in the transduction of methicillin resistance is discussed.

Site-specific recombination between plasmids of Staphylococcus aureus

Anomalous recombination between two similar but nonidentical, naturally occurring penicillinase plasmids, pI258 and pI524, leading to duplication and deletion of the beta-lactamase locus, is described. Physical mapping of these plasmids by heteroduplex and restriction analysis revealed that the beta-lactamase loci were homologous and in inverted orientation with respect to one another and that their respective locations were separated by a short region of homology. This intervening region of homology included one copy of a segment that was repeated on pI524 in inverted orientation at a distance of 2.2 kilobase pairs and contained a recognition sequence for a site-specific, rec-independent recombination function that caused reversible inversion of this segment on pI524. It is proposed that site-specific, intermolecular recombination involving this repeated sequence was responsible for the observed results.

Chromosomal penicillin resistance in Staphylococcus aureus strains of phage group II

The markers coding for serotype B penicillinase (PcB) in wild type strains of group II of S. aureus behaved as chromosomal characters in 6 out of 7 cases. The restriction deficient mutants also to some strains of group I. No extrachromosomal DNA could be detected in the transductants. The low frequency of transduction of chromosomal markers as well as the high restriction barrier seem to limit the spread of these markers outside group II.