Unusual form of oxacillin resistance in methicillin-resistant Staphylococcus aureus clinical strains

Ceftaroline Is Active against Heteroresistant Methicillin-Resistant Staphylococcus aureus Clinical Strains despite Associated Mutational Mechanisms and Intermediate Levels of Resistance

“…Population analysis profiles (PAPs) were performed as described previously (16). Overnight cultures of SA13011-HeR and SA13011-(CPT)-HoR (SA13011-HeR selected with subinhibitory concentrations of CPT [0.125 g/ml]) were plated at various dilutions on Trypticase soy agar (TSA) plates containing a series of concentrations of CPT (0 to 128 g/ml); bacterial colonies were counted after incubation of the plates at 37°C for 48 h.…”

“…The activity of CPT was also tested against homoresistant (HoR)-MRSA derivatives obtained from HeR-MRSA strains SA13011, USA300, and SA352 exposed to subinhibitory concentrations of OXA (13,16). Importantly, as shown in Table 3, CPT was very active (MIC, 0.5 g/ml) against the highly resistant SA13011-HoR derivative, while still displaying high levels of resistance against most of the ␤-lactams tested (e.g., CTX, OXA, NAF, and FOX) (MICs of Ն16 g/ml).…”

“…For this study, isogenic clinical MRSA heteroresistant S. aureus 13011 (HeR) (OXA MIC, 2 g/ml) and its highly homoresistant derivative (HoR) (OXA MIC, 256 g/ml) were used. The SA13011 strain is representative of a heteroresistant MRSA collection previously described (13,16), which were determined to be OXA susceptible and mecA positive (13,16). SA13011-HeR and USA100 (OXA MIC, 256 g/ml) were identified as ST5, SCCmec type II, spa type 2, or TJMBMDMGMK.…”

β-Lactams Increase the Antibacterial Activity of Daptomycin against Clinical Methicillin-Resistant Staphylococcus aureus Strains and Prevent Selection of Daptomycin-Resistant Derivatives

“…Most strains of S. aureus also produce a ␤-lactamase encoded by blaZ, which can hydrolyze ␤-lactam antibiotics and render them inactive (18,49). Heterogeneous expression of ␤-lactam resistance in MRSA strains is a characteristic of both HA-MRSA and CA-MRSA strains, only a small part of the population of which expresses resistance to Ն10 g of oxacillin per ml (heterotypic resistance [He r ]), while among other isolates, most of the population expresses resistance to a high level (homotypic resistance [Ho r ]) (7,13,14,43). The cyclic anionic lipopeptide antibiotic daptomycin (DAP) is a produced by Streptomyces roseosporus (4) and is recommended for treatment of skin and skin structure infections, bacteremia, and right-sided endocarditis caused by MRSA, as well as patients with prolonged MRSA bacteremia (Ͼ7 days), which are at high risk for metastatic complications and death (2).…”

GeneXpert Captures Unstable Methicillin-Resistant Staphylococcus aureus Prone to Rapidly Losing the mecA Gene

“…It is also known that the genetic background of a strain can determine the stability of SCCmec in S. aureus (11). Certain MRSA strains seem to be more prone to losing SCCmec or the mecA gene upon storage (6,7,9,19). Isolates without the mecA gene but positive by Xpert MRSA have previously been described (18), but in this isolate, the GeneXpert test also became negative after storage, as did the mecA-specific PCR.…”