Investigation of beta-lactam antibiotic-induced vancomycin-resistant MRSA (BIVR)

Combination Antibiotic Exposure Selectively Alters the Development of Vancomycin Intermediate Resistance in Staphylococcus aureus

Invasive methicillin-resistant Staphylococcus aureus (MRSA) treated with vancomycin (VAN) is associated with reduced VAN susceptibility and treatment failure. VAN combination therapy is one strategy to improve response, but comprehensive assessments of combinations to prevent resistance are limited. This study identifies optimal combinations to prevent the emergence of VAN-intermediate Staphylococcus aureus (VISA). Two standard MRSA and two heterogeneous VISA (hVISA) strains were exposed for 28 days in vitro to VAN alone, VAN with cefazolin (CFZ), fosfomycin, gentamicin, meropenem, rifampin, piperacillin-tazobactam (TZP), or trimethoprim-sulfamethoxazole. In addition to VAN susceptibility testing, cell wall thickness (CWT), carotenoid content, and membrane fluidity were determined for Mu3. VAN plus any β-lactam limited the VAN MIC increase to 1 to 4 mg/liter throughout the 28-day exposure, with CFZ and TZP being the most effective agents (VAN MIC = 1 to 2 mg/liter). Similar MIC trends occurred with the lipo-/glycopeptide agents daptomycin and telavancin, where β-lactam combinations with VAN prevented MIC increases to these agents as well. Combinations with non-β-lactams were ineffective in preventing VAN MIC increases with VAN MICs of 4 to 16 mg/liter emerging during weeks 2 to 4 of treatment. VAN plus β-lactam decreased CWT significantly, whereas VAN plus other antibiotics significantly increased the CWT. No correlation was observed between carotenoid content or membrane fluidity and antibiotic exposure. Only the combination exposures of VAN plus β-lactam suppress the development of VISA. Rational selection of VAN plus β-lactam should be further explored as a long-term combination treatment of MRSA infections due to their ability to suppress VAN resistance.

Antibiotic susceptibility survey of blood-borne MRSA isolates in Japan from 2008 through 2011

We conducted an antibiotic susceptibility survey of 830 blood-borne methicillin resistant Staphylococcus aureus collected from nationwide hospitals in Japan over a three-year period from January 2008 through May 2011. Antibiotic susceptibility was judged according to the criteria recommended by the Clinical Laboratory Standard Institute. Over 99% of the MRSA showed to be susceptible to teicoplanin, linezolid, sulfamethoxazole/trimethoprim and vancomycin, and over 97% of them were susceptible to daptomycin, arbekacin and rifampin. The majority of the MRSA strains showed resistant to minocycline, meropenem, imipenem, clindamycin, ciprofloxacin, cefoxitin, and oxacillin in the rates of 56.6, 72.9, 73.7, 78.7, 89.0, 99.5, and 99.9%, respectively. Among the MRSA strains, 72 showed reduced susceptibility to vancomycin, including 8 strains (0.96%) of vancomycin-intermediate S. aureus (VISA), 54 (6.51%) of heterogeneous vancomycin-intermediate S. aureus (hVISA), and 55 (5.63%) of β-lactam antibiotics-induced vancomycin resistant S. aureus (BIVR). Unexpectedly, among the 54 hVISA and 55 BIVR, 45 isolates (83.3% and 81.8%, respectively) showed both hVISA and BIVR phenotypes. A new trend of vancomycin resistance found in this study was that VISA strains were still prevalent among the bacteremic specimens. The high rates of the hVISA/BIVR two-phenotypic vancomycin resistance, and the prevalence of VISA in the bloodborne MRSA call attention in the MRSA epidemiology in Japan.

Exposure of Staphylococcus aureus to subinhibitory concentrations of β-lactam antibiotics induces heterogeneous vancomycin-intermediate Staphylococcus aureus

Glycopeptides are known to select for heterogeneous vancomycin-intermediate Staphylococcus aureus (h-VISA) from susceptible strains. In certain clinical situations, h-VISA strains have been isolated from patients without previous exposure to glycopeptides, such as cystic fibrosis patients, who frequently receive repeated treatments with beta-lactam antibiotics. Our objective was to determine whether prolonged exposure to beta-lactam antibiotics can induce h-VISA. We exposed 3 clinical vancomycin-susceptible methicillin-resistant Staphylococcus aureus (MRSA) strains to ceftazidime, ceftriaxone, imipenem, and vancomycin (as a control) at subinhibitory concentrations for 18 days in vitro. Population analyses showed progressive increases in vancomycin resistance; seven of the 12 derived strains obtained after induction were classified as h-VISA according to the following criteria: area under the curve (AUC) on day 18/AUC of Mu3 of ≥90% and/or growth on brain heart infusion (BHI) agar with 4 mg/liter vancomycin. The derived isolates had thickened cell walls proportional to the level of glycopeptide resistance. Genes known to be associated with glycopeptide resistance (vraSR, yvqF, SA1703, graRS, walKR, and rpoB) were PCR sequenced; no de novo mutations were observed upon beta-lactam exposure. To determine whether trfA, a gene encoding a glycopeptide resistance factor, was essential in the selection of h-VISA upon beta-lactam pressure, a trfA-knockout strain was generated by allelic replacement. Indeed, beta-lactam exposure of this mutated strain showed no capacity to induce vancomycin resistance. In conclusion, these results showed that beta-lactam antibiotics at subinhibitory concentrations can induce intermediate vancomycin resistance in vitro. This induction required an intact trfA locus. Our results suggest that prior use of beta-lactam antibiotics can compromise vancomycin efficacy in the treatment of MRSA infections.

Presence of both heterogeneous vancomycin-intermediate resistance and β-lactam antibiotic-induced vancomycin resistance phenotypes is associated with the outcome in methicillin-resistant Staphylococcus aureus bloodstream infection

BACKGROUND

Although the individual expression of heterogeneous vancomycin-intermediate resistance (hVISA) and β-lactam antibiotic-induced vancomycin resistance (BIVR) phenotypes has been associated with treatment failure and recurrence in methicillin-resistant Staphylococcus aureus (MRSA) infections, the effect of the co-expression of these phenotypic profiles on clinical outcome has not been fully elucidated. The aim of this study was to determine the impact of the combination of hVISA and BIVR phenotypes on the clinical outcome in MRSA bacteremia.

METHODS

One hundred and sixty-two MRSA blood isolates from a 21-y period, 1987-2007, were randomly selected. Screening for hVISA was done by the macromethod Etest and confirmed by population analysis profiles. BIVR was identified using Mu3 agar containing 4 μg/ml of vancomycin.

RESULTS

Thirty (18.5%) and 39 (24.1%) of the 162 MRSA blood isolates were positive for the hVISA and BIVR phenotypes, respectively. Eighteen (11.1%) isolates possessed both hVISA and BIVR phenotypes (hVISA(+)/BIVR(+)). In a subset of patients who received initial treatment with glycopeptides, only the patients whose isolates were hVISA(+)/BIVR(+) displayed a significantly higher mortality rate in comparison to those with non-hVISA(+)/BIVR(+) (80.0% vs 31.3%, p = 0.004). The presence of both hVISA and BIVR phenotypes was a predictor of mortality using a logistic regression analysis (p = 0.025).

CONCLUSIONS

The combined phenotype of hVISA and BIVR was associated with a higher probability of mortality in patients with MRSA bacteremia. Further prospective studies are warranted to delineate the clinical significance of the combined phenotype of hVISA and BIVR.

Low level ß-lactamase production in methicillin-resistant staphylococcus aureus strains with ß-lactam antibiotics-induced vancomycin resistance

Background

A class of methicillin-resistant Staphylococcus aureus (MRSA) shows resistance to vancomycin only in the presence of ß-lactam antibiotics (BIVR). This type of vancomycin resistance is mainly attributable to the rapid depletion of free vancomycin in the presence of ß-lactam antibiotics. This means that ß-lactam antibiotics remain active or intact in BIVR culture, although most MRSA cells are assumed to produce ß-lactamase. We hypothesised that the BIVR cells either did not harbour the ß-lactamase gene, blaZ, or the gene was quiescent. We tested this hypothesis by determining ß-lactamase activity and conducting PCR amplification of blaZ.

Results

Five randomly selected laboratory stock BIVR strains showed an undetectable level of ß-lactamase activity and were blaZ-negative. Five non-BIVR stock strains showed an average ß-lactamase activity of 2.59 ± 0.35 U. To test freshly isolated MRSA, 353 clinical isolates were collected from 11 regionally distant hospitals. Among 25 BIVR strains, only 16% and 8% were blaZ positive and ß-lactamase-positive, respectively. In contrast, 95% and 61% of 328 non-BIVR strains had the blaZ gene and produced active ß-lactamase, respectively. To know the mechanism of low ß-lactamase activity in the BIVR cells, they were transformed with the plasmid carrying the blaZ gene. The transformants still showed a low level of ß-lactamase activity that was several orders of magnitude lower than that of blaZ-positive non-BIVR cells. Presence of the ß-lactamase gene in the transformants was tested by PCR amplification of blaZ using 11 pairs of primers covering the entire blaZ sequence. Yield of the PCR products was consistently low compared with that using blaZ-positive non-BIVR cells. Nucleotide sequencing of blaZ in one of the BIVR transformants revealed 10 amino acid substitutions. Thus, it is likely that the ß-lactamase gene was modified in the BIVR cells to downregulate active ß-lactamase production.

Conclusions

We concluded that BIVR cells gain vancomycin resistance by the elimination or inactivation of ß-lactamase production, thereby preserving ß-lactam antibiotics in milieu, stimulating peptidoglycan metabolism, and depleting free vancomycin to a level below the minimum inhibitory concentration of vancomycin.

Identification of the active component that induces vancomycin resistance in MRSA

A fraction of methicillin-resistant Staphylococcus aureus (MRSA) shows resistance to vancomycin (VCM) in the presence of β-lactam antibiotics (BIVR) at low concentrations. We hypothesized that the BIVR phenomenon might be exerted by a peptidoglycan derivative(s) generated as a consequence of β-lactam antibiotic action. To verify this hypothesis, we isolated the fraction that mimicked the effect of β-lactam antibiotics by the enzymatic treatment of the crude cell wall. The active components were purified by a combination of reverse phase chromatographies, mass spectrum and amino-acid analyses, and were identified to be a muropeptide with the following formula: N-acetyglucosamyl-N-acetylmuramyl--Ala-D-isoGln-L-Lys-(ɛ-NH-4Gly)-D-Ala-2Gly. This is the very first identification of the active component, which induces VCM resistance in MRSA. We found that the BIVR cells are highly sensitive to this compound rendering the cells resistant to VCM compared with non-BIVR MRSA.

Rapid depletion of free vancomycin in medium in the presence of beta-lactam antibiotics and growth restoration in Staphylococcus aureus strains with beta-lactam-induced vancomycin resistance

A class of methicillin-resistant Staphylococcus aureus strains shows vancomycin resistance in the presence of beta-lactam antibiotics (beta-lactam-induced VAN-resistant methicillin-resistant S. aureus [BIVR]). Two possible explanations may be offered: (i) vancomycin in culture medium is depleted, and (ii) the D-Ala-D-Ala terminal of the peptidoglycan network is replaced with D-Ala-D-lactate. We tested these hypotheses by quantifying free vancomycin in the medium through the course of cell growth and by PCR amplification of the van genes. Growth of the BIVR cells to an absorption level of approximately 0.3 at 578 nm required about 24 h in the presence of vancomycin alone at the MIC (4.0 microg/ml). However, growth was achieved in only about 10 h when 1/1,000 to 1/2,000 the MIC of beta-lactam antibiotic was added 2 h prior to the addition of vancomycin, suggesting that the beta-lactams shortened the time to recovery from vancomycin-mediated growth inhibition. Free vancomycin in the culture medium decreased to 2.3 microg/ml in the first 8 h in the culture containing vancomycin alone, yet cell growth was undetectable. When the vancomycin concentration dropped below approximately 1.5 microg/ml at 24 h, the cells began to grow. In the culture supplemented with the beta-lactam 2 h prior to the addition of vancomycin, the drug concentration continuously dropped from 4 to 0.5 microg/ml in the first 8 h, and the cells began to grow at a vancomycin concentration of approximately 1.7 microg/ml or at 4 h of incubation. The gene encoding the enzyme involved in D-Ala-D-lactate synthesis was undetectable. Based on these results, we concluded that BIVR is attributable mainly to a rapid depletion of vancomycin in the medium triggered or promoted by beta-lactam antibiotics.

Potent activity of the lichen antibiotic (+)-usnic acid against clinical isolates of vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus

Vancomycin-resistant enterococci (VRE) and methicillin-resistant staphylococci, most notably methicillin-resistant Staphylococcus aureus (MRSA), are serious clinical problems. The antibiotic arsenal available against them is limited, and new mutants worsen the situation. We studied the activity of (+)-usnic acid, an old lichen-derived drug, and its sodium salt against clinical isolates of VRE and MRSA using the agar diffusion and minimal inhibitory concentration (MIC) methods. The acid and, especially, the sodium salt had potent antimicrobial activity against all clinical isolates of VRE and MRSA studied. The MIC values of the sodium salt against VRE strains ranged between 4 and 16 microg/ml (1-day test) and between 4 and 31 microg/ml (2-day test), being below 8 microg/ml for most strains. The salt had potent activity even against those strains that were not inhibited by ampicillin (125 microg/ml), and it never lost its activity after 24 h, in contrast to ampicillin. Thus, in spite of the fact that usnic acid can in some cases cause serious toxicity, it and its salts may be worth considering in clinical practice in cases where other therapies have failed or the microbe is resistant toward other agents.

Nosocomial infection of beta-lactam antibiotic-induced vancomycin-resistant Staphylococcus aureus (BIVR)

We report here an outbreak of beta-lactam-induced vancomycin-resistant methicillin-resistant Staphylococcus aureus (MRSA; BIVR) at one of the Cancer-Institute-affiliated hospitals in Tokyo. We examined a total of 500 strains (100 per year) of clinically isolated MRSA from 1998 to 2002. The detection rates of BIVR in the years 1998, 1999, 2000, 2001, and 2002 were 10%, 9%, 49%, 15%, and 19%, respectively. To investigate the cause of the high incidence of BIVR detection in the year 2000, we carried out pulsed-field gel electrophoresis (PFGE) of the SmaI-digested chromosomal DNA of BIVR and MRSA. The results showed that 96% of the BIVR strains isolated in 2000 were classified as an identical DNA type "A", while only 47% of the MRSA strains were classified as this type. We concluded, based on these results, that this hospital had a nosocomial infection of BIVR in the year 2000. An important message given by this study would be that nosocomial BIVR infection could occur in any hospital where MRSA infection is treated with vancomycin and beta-lactam antibiotics.

Gram-positive cocci infections in intensive care: guide to antibacterial selection

The incidence of multiresistance in Gram-positive cocci causing infections in critically ill patients admitted to the intensive care unit (ICU) has increased notably in recent years. Thus, therapeutic proposals have been modified according to the emergence of multiresistant cocci and adapted to epidemiological markers of individual infectious processes, geographical variations of these markers, the availability of new antibacterials, and advances in the knowledge of pharmacokinetic and pharmacodynamic aspects of their use. The current management of critically ill patients should consider new therapeutic approaches, such as the "de-escalating strategy", which includes the administration of empirical antibacterials active against multiresistant pathogens followed by directed treatment based on unequivocal data from antibacterial-susceptibility testing. Optimisation of antibacterial treatment should be viewed in the context of the need to determine plasma drug concentrations, pharmacoeconomic considerations and control of drug-related adverse events. Therapeutic proposals should be developed within the framework of the antibacterial policy of each hospital. The present review is focused on the description of the therapeutic strategies for the main infectious processes caused by Gram-positive cocci in severely ill patients admitted to the ICU according to a review of the pertinent literature and the clinical experience of the authors.