Scope and Predictive Genetic/Phenotypic Signatures of Bicarbonate (NaHCO ₃ ) Responsiveness and β-Lactam Sensitization in Methicillin-Resistant Staphylococcus aureus

Abstract

Addition of sodium bicarbonate (NaHCO3) to standard antimicrobial susceptibility testing medium reveals certain methicillin-resistant Staphylococcus aureus (MRSA) strains to be highly susceptible to β-lactams. We investigated the prevalence of this phenotype (NaHCO3 responsiveness) to two β-lactams among 58 clinical MRSA bloodstream isolates. Of note, ∼75% and ∼36% of isolates displayed the NaHCO3 responsiveness phenotype to cefazolin (CFZ) and oxacillin (OXA), respectively. Neither intrinsic β-lactam MICs in standard Mueller-Hinton broth (MHB) nor population analysis profiles were predictive of this phenotype. Several genotypic markers (clonal complex 8 [CC8]; agr I and spa t008) were associated with NaHCO3 responsiveness for OXA.

A Combined Phenotypic-Genotypic Predictive Algorithm for In Vitro Detection of Bicarbonate: β-Lactam Sensitization among Methicillin-Resistant Staphylococcus aureus (MRSA)

“…However, current clinical guidelines discourage the use of β-lactams in MRSA infections because of their uniform “in vitro resistance” to such agents by standard MIC testing [ 15 , 32 ]. As noted above, we have previously identified that a significant proportion of clinical MRSA isolates exhibit susceptibility in vitro to standard β-lactams if tested in the presence of NaHCO 3 [ 26 ]. However, routine AST of all MRSA isolates in both standard and NaHCO 3 -supplemented media by BMD would involve additional reagent and materials costs, technical effort, as well as turnaround time implications for clinical microbiology laboratories.…”

“…Recent studies have highlighted potential limitations of relying on standard methods of AST to determine relevant antibiotic resistance in specific infection settings [ 8 , 9 , 23 , 24 , 25 , 26 ]. These studies utilized media that may be more reflective of the host environment, such as tissue culture media (e.g., RPMI) or MHB supplemented with NaHCO 3 , the body’s primary biological buffer, to perform in vitro AST.…”

“…Results of such studies suggest that it could be possible that “antibiotic resistance levels by MICs” determined in host-like media are better predictors of host infection outcomes in both murine and rabbit systemic infection models [ 8 , 9 , 23 , 24 , 25 ]. Our laboratory has recently published a series of investigations documenting the following in terms of MIC testing of MRSA in NaHCO 3 -supplemented MHB: (i) ~33% and ~75% frequency of in vitro susceptibility of MRSA to OXA and CFZ, respectively; (ii) better clearance of in vitro NaHCO 3 -responsive MRSA from experimental cardiac vegetations in both ex vivo and in vivo models; and (iii) definition of important mechanisms involved in this NaHCO 3 -responsive phenotype (e.g., penicillin-binding protein 2a (PBP2a) production, translocation and maturation) [ 23 , 26 , 27 , 28 ].…”

“…The aim of this exploratory and hypothesis-generating study was to combine current AST methods with targeted genetic sequencing to identify MRSA that may potentially respond to standard β-lactam therapy in vivo [ 23 , 26 ].…”

Reliability of mecA in Predicting Phenotypic Susceptibilities of Coagulase-Negative Staphylococci and Staphylococcus aureus

“…Additionally, we no longer have the isolates and therefore are unable to test isolates for the presence of mecA in co-pathogens or inducible resistance/empty cassettes in CoNS. If it is possible to investigate this discrepancy in the future with existing isolates, efforts will be made to explore the influence of bicarbonate on resistance expression [ 17 , 18 ], as well as performing inducibility studies. Also, our center uses Biofire Film Array for detection of the mecA gene; other platforms may perform differently, and BCID2 (recently Food and Drug Administration–approved version of the Biofire test) may not report mecA for CONS.…”