Rifampin combination therapy for nonmycobacterial infections

Characterization of a rifampin-inactivating glycosyltransferase from a screen of environmental actinomycetes

Identifying and understanding the collection of all antibiotic resistance determinants presented in the global microbiota, the antibiotic resistome, provides insight into the evolution of antibiotic resistance and critical information for the development of future antimicrobials. The rifamycins are broad-spectrum antibiotics that target bacterial transcription by inhibition of RNA polymerase. Although mutational alteration of the drug target is the predominant mechanism of resistance to this family of antibiotics in the clinic, a number of diverse inactivation mechanisms have also been reported. In this report, we investigate a subset of environmental rifampin-resistant actinomycete isolates and identify a diverse collection of rifampin inactivation mechanisms. We describe a single isolate, WAC1438, capable of inactivating rifampin by glycosylation. A draft genome sequence of WAC1438 (most closely related to Streptomyces speibonae, according to a 16S rRNA gene comparison) was assembled, and the associated rifampin glycosyltransferase open reading frame, rgt1438, was identified. The role of rgt1438 in rifampin resistance was confirmed by its disruption in the bacterial chromosome, resulting in a loss of antibiotic inactivation and a 4-fold decrease in MIC. Interestingly, examination of the RNA polymerase β-subunit sequence of WAC1438 suggests that it harbors a resistant target and thus possesses dual mechanisms of rifamycin resistance. Using an in vitro assay with purified enzyme, Rgt1438 could inactivate a variety of rifamycin antibiotics with comparable steady-state kinetics constants. Our results identify rgt1438 as a rifampin resistance determinant from WAC1438 capable of inactivating an assortment of rifamycins, adding a new element to the rifampin resistome.

Molecular epidemiology and mechanisms of rifampicin resistance in Acinetobacter baumannii isolates from Italy

Use of rifampicin (RIF) in combination with colistin (COL) has been proposed for the treatment of multidrug-resistant Acinetobacter baumannii infections owing to in vitro synergism. The aim of the present study was to evaluate the molecular epidemiology and mechanisms of RIF resistance in 57 clinical isolates of A. baumannii in two tertiary care hospitals in Naples (Italy) from 2006 to 2010. Amongst the collection, 36 isolates showed high RIF minimum inhibitory concentrations (MICs) (256 mg/L to ≥512 mg/L), 16 showed intermediate MICs (8-16 mg/L) and 5 had low MICs (4 mg/L). Of the 36 isolates with elevated RIF MICs, 35 were assigned to sequence type ST2 and 1 to ST78. Amongst the 57 isolates, 35 carried at least one mutation in rpoB, including H535L in 9 isolates and double mutations D525N and P544L in 7 isolates, whilst 22 showed no rpoB mutations. Treatment with the efflux pump inhibitor phenyl-arginine-β-naphthylamide (PAβN) of resistant isolates with no mutations in rpoB and different RIF MICs reduced the MIC by >10-fold and restored the synergism between RIF and COL in time-kill studies, whilst it had no effect on strains carrying rpoB mutations. In conclusion, the emergence of elevated RIF MICs in A. baumannii isolates from our geographical area was mostly caused by mutations in rpoB; low to intermediate RIF MICs were also caused by altered membrane permeability to the drug. The phenomenon was contributed by the selection of two prevalent clones both assigned to ST2 genotype. These data may have implications for the correct identification of cases with A. baumannii infection that would not benefit from addition of RIF to COL.

The management of community-acquired pneumonia in infants and children older than 3 months of age: clinical practice guidelines by the Pediatric Infectious Diseases Society and the Infectious Diseases Society of America

Evidenced-based guidelines for management of infants and children with community-acquired pneumonia (CAP) were prepared by an expert panel comprising clinicians and investigators representing community pediatrics, public health, and the pediatric specialties of critical care, emergency medicine, hospital medicine, infectious diseases, pulmonology, and surgery. These guidelines are intended for use by primary care and subspecialty providers responsible for the management of otherwise healthy infants and children with CAP in both outpatient and inpatient settings. Site-of-care management, diagnosis, antimicrobial and adjunctive surgical therapy, and prevention are discussed. Areas that warrant future investigations are also highlighted.

Combination therapy with daptomycin, linezolid, and rifampin as treatment option for MRSA meningitis and bacteremia

Methicillin-resistant Staphylococcus aureus (MRSA) meningitis is associated with a high mortality rate. Treatment is challenging in patients with allergy to vancomycin. Herein, we describe a case of MRSA bacteremia secondary to medical device infection with MRSA that was complicated by MRSA meningitis. This case provides evidence for a possible role of combination therapy of daptomycin, linezolid, and rifampin in cases of MRSA meningitis and bacteremia.

Impact of a low-oxygen environment on the efficacy of antimicrobials against intracellular Chlamydia trachomatis

Emergence of chronic inflammation in the urogenital tract induced by Chlamydia trachomatis infection in females is a long-standing concern. To avoid the severe sequelae of C. trachomatis infection, such as pelvic inflammatory diseases (PID), ectopic pregnancies, and tubal infertility, antibiotic strategies aim to eradicate the pathogen even in asymptomatic and uncomplicated infections. Although first-line antimicrobials have proven successful for the treatment of C. trachomatis infection, treatment failures have been observed in a notable number of cases. Due to the obligate intracellular growth of C. trachomatis, reliable antimicrobial susceptibility assays have to consider environmental conditions and host cell-specific factors. Oxygen concentrations in the female urogenital tract are physiologically low and decrease further during an inflammatory process. We compared MIC testing and time-kill curves (TKC) for doxycycline, azithromycin, rifampin, and moxifloxacin under hypoxia (2% O2) and normoxia (20% O2). While low oxygen availability only moderately decreased the antichlamydial activity of azithromycin in conventional MIC testing (0.08 μg/ml versus 0.04 μg/ml; P<0.05), TKC analyses revealed profound divergences for antibiotic efficacies between the two conditions. Thus, C. trachomatis was significantly less rapidly killed by doxycycline and azithromycin under hypoxia, whereas the efficacies of moxifloxacin and rifampin remained unaffected using concentrations at therapeutic serum levels. Chemical inhibition of multidrug resistance protein 1 (MDR-1), but not multidrug resistance-associated protein 1 (MRP-1), restored doxycycline activity against intracellular C. trachomatis under hypoxia. We suggest careful consideration of tissue-specific characteristics, including oxygen availability, when testing antimicrobial activities of antibiotics against intracellular bacteria.

Toxicity of five protein synthesis inhibiting antibiotics and their mixture to limnic bacterial communities

Antibiotics are common contaminants of aquatic environments, the protein synthesis inhibitors being one frequently detected class. Even though antibiotics target bacteria, little is known about the sensitivity of naturally occurring freshwater bacteria to these compounds. Limnic bacterial communities were therefore systematically exposed to five common protein synthesis inhibiting antibiotics, each representing a particular subgroup: streptomycin, chloramphenicol, fusidic acid, rifampicin and chlortetracycline. Full concentration-response curves and No Observed Effect Concentrations (NOECs) were determined for all antibiotics, using the (3H)leucine incorporation method. All test compounds were toxic to planktonic communities of limnic bacteria, with EC50s ranging from 0.138 micromol/L for chlortetracycline to 79.1 micromol/L for streptomycin. The order of toxicity was chlortetracycline>rifampicin>fusidic acid>chloramphenicol>streptomycin, based on the individual EC50 values. A comparison to reported chemical monitoring data shows that environmental concentrations of chlortetracycline are in a range that clearly inhibits the protein biosynthesis activity of planktonic bacterial communities. All compounds show exceptionally flat concentration-response relationship, for fusidic acid the ratio of EC50 to EC05 exceeds four orders of magnitude. This challenges the standard assessment factors of 10-100 for the extrapolation from high to low doses. Environmental exposure situations are often characterized by the presence of mixtures of antibiotics, e.g. in sewage effluents or river systems. Hence, also combined effects of the five antibiotics were determined, and compared to mixture toxicity predictions based on Concentration Addition and Independent Action. Concentration Addition slightly underestimated the observed EC50 by a factor of 1.5, independent on whether the prediction was based on single substance data that were recorded in parallel or whether historical data were used. Independent Action predicted higher mixture toxicity than Concentration Addition due to the flatness of the individual concentration-response curves. Implications of these findings for the environmental risk assessment of antibiotics and their mixtures are discussed.

Unpredictable effects of rifampin as an adjunctive agent in elimination of rifampin-susceptible and -resistant Staphylococcus aureus strains grown in biofilms

The use of rifampin as an adjunct in biofilm-associated infections is based on the ability to penetrate into biofilms and a presumed activity against dormant bacteria. Yet, its efficacy remains contradictory, and rifampin-resistant strains frequently emerge during therapy. Therefore, the efficacy against rifampin-susceptible and isogenic rifampin-resistant methicillin-susceptible Staphylococcus aureus (MSSA) strains was evaluated. Biofilms were generated under static conditions using MSSA with various genetic backgrounds. Oxacillin alone or with rifampin at various concentrations was subsequently added, and after 24 h biomass and viable cell counts were determined. Upon rifampin addition, interstrain variations in viable count change, ranging from a tendency toward antagonism to synergy, were observed among all strains tested, irrespective of the genetic background of the strain. Similar variations were observed in changes in biomass. The decrease in viable count upon rifampin addition was negatively correlated to formation of large amounts of biomass, since strains embedded by more biomass showed a diminished reduction in viable count. Rifampin (1 microg/ml) as adjunct to oxacillin achieved greater reductions in biomass produced by most rifampin-susceptible isolates, ranging from 17 to 54%, compared to 4% for oxacillin alone. In contrast, rifampin had no additional value in reduction of biomass of isogenic rifampin-resistant mutants. At subinhibitory concentrations of rifampin (0.008 microg/ml), none of the strains tested yielded an extra reduction in biomass that was > or = 40%. In conclusion, the effects of rifampin as adjunct on biomass and viable count were unpredictable, and the use of rifampin against biofilm containing rifampin-resistant strains seems unwarranted.