Bacterial tolerance to antibiotics reduces the ability to prevent resistance

Objectives: Daptomycin non-susceptibility in Staphylococcus aureus can emerge via the accumulation of single or multiple mutations, each resulting in a slight increase in the daptomycin MIC. The daptomycin-non-susceptible phenotype may include other features such as daptomycin tolerance. This study identifies S. aureus genomic regions that frequently develop mutations following prolonged daptomycin exposure but have not been previously associated with daptomycin non-susceptibility. Methods: Sequence variations in the same eight loci independently observed following 28 day parallel serial passages of S. aureus J01 in daptomycin were introduced in isolation into S. aureus J01. MICs were determined by microbroth dilution. Daptomycin killing and tolerance were determined by kill curve analysis. Results: Single mutations in snoF, hmp1, sspA, rimP, hepT, rsh, map1 and amaP had only a modest impact on the daptomycin MIC (<= 2-fold). In contrast, individual mutation in several of these regions resulted in pronounced changes to daptomycin tolerance. Conclusions: This study demonstrates that less characterized mutations in S. aureus following daptomycin exposure do not result in significant daptomycin susceptibility changes, but rather allow for enhanced survival characteristics during treatment. This sheds new light on genetic adaptations that may play a role in persistent infection. Further studies are needed to elucidate the prevalence of these mutations in clinical isolates.

Understating how antibiotic tolerance impacts subsequent resistance development in the clinical setting is important to identifying effective therapeutic interventions and prevention measures. This study describes a patient case of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia which rapidly developed resistance to three primary MRSA therapies and identifies genetic and metabolic changes selected in vivo that are associated with rapid resistance evolution. Index blood cultures displayed susceptibility to all (non-beta-lactam) antibiotics with the exception of trimethoprim/ sulfamethoxazole. One month after initial presentation, during the same encounter, blood cultures were again positive for MRSA, now displaying intermediate resistance to vancomycin and ceftaroline and resistance to daptomycin. Two weeks later, blood cultures were positive for a third time, still intermediate resistant to vancomycin and ceftaroline and resistant to daptomycin. Mutations in mprF and vraT were common to all multidrug resistant isolates whereas mutations in tagH, agrB and saeR and secondary mprF mutation emerged sequentially and transiently resulting in distinct in vitro phenotypes. The baseline mutation rate of the patient isolates was unremarkable ruling out the hypermutator phenotype as a contributor to the rapid emergence of resistance. However, the index isolate demonstrated pronounced tolerance to the antibiotic daptomycin, a phenotype that facilitates the subsequent development of resistance during antibiotic exposure. This study exemplifies the capacity of antibiotic-tolerant pathogens to rapidly develop both stable and transient genetic and phenotypic changes, over the course of a single patient encounter.

We present a case of endocarditis wherein organisms cultured from different valve leaflets yielded different daptomycin susceptibilities from each other and from organisms obtained from peripheral blood culture. Genomic analyses showed mutations in mprF, purR , and agrA . Pharmacokinetic simulations showed consistent activity of daptomycin plus beta-lactam against all subpopulations.

Daptomycin (DAP) is a cyclic lipopeptide antibiotic that targets the cell membrane and is commonly used for the treatment of multidrug-resistant Staphylococcus aureus infections, especially methicillin-resistant strains (MRSA). Emergence of DAP resistance during DAP therapy is increasingly reported; however, the mechanisms by which resistance occurs are diverse. Using phages in combination with antibiotics is of high interest to circumvent antibiotic resistance, due to their minimal side effects, accessibility, and synergistic effects with antibiotics. Here, we have investigated the relationship between DAP resistance and sensitivity to phage Sb-1 in a cohort of 14 clinical MRSA strains. This cohort consists of four isogenic pairs (a clinical DAP-susceptible parent and its isogenic DAP-resistant variant isolated during DAP treatment), and three DAP-susceptible/DAP-resistant strain pairs are created by traditional serial passaging in the presence of increasing DAP concentrations. Efficiency of plating (EOP) and bacteriophage plaque sizes were recorded, and synergy between the antibacterial agents was tested using the phage-antibiotic combination (PAC) checkerboard method. Plaque sizes and EOP significantly increased ( P = 0.019 and P = 0.008, respectively) as strains developed resistance to DAP. The PAC checkerboards conveyed less antagonistic patterns in DAP-resistant mutant strains compared with their DAP-susceptible counterparts. Antagonism was displayed in 71.4% (5/7) of the DAP-susceptible strains, while it was only present in 14.3% (1/7) of the DAP-resistant mutants. Transmission electron microscopy images showed significantly thicker cell walls in DAP-susceptible strains exposed to DAP monotherapy compared with either growth control or DAP-phage ( P = 0.0002 and 0.0007, respectively). These data indicate increased sensitivity to phage Sb-1 concurrent with the emergence of DAP resistance. IMPORTANCE: Multidrug-resistant Staphylococcus aureus is a threat to the health care system, especially cross-resistance between daptomycin (DAP) and glycopeptides through various mutations such as mprF (which is involved in the modification of membrane phospholipids in some bacteria) and yycG (part of a two-component regulatory system in bacteria that is important for regulating cell wall biosynthesis and other cellular processes) has been reported previously. Our current study shows adjunctive treatment with phage in DAP-resistant strains will lead to synergistic activity and larger phage plaque sizes, translating to elevated lytic performance. The addition of bacteriophage to standard-of-care antibiotic therapies for multidrug-resistant S. aureus infections has the potential to hinder, and possibly revert, resistance to antibiotics. Applying this strategy can potentially lead to the preservation of the current antibiotics. Verification of this salutary outcome in relevant ex vivo and in vivo models of endovascular infections is required to validate translatability.