Characterization of resistance phenotype and cephalosporin activity in oxacillin-resistant Staphylococcus aureus

Abstract

Forty isolates of methicillin-resistant Staphylococcus aureus were tested versus oxacillin at 30 and 35°C with and without 2% NaCl supplementation of Mueller-Hinton broth and classified as having resistance that was low (MIC,.16 ,ug/ml) or high (MIC, .32 gug/ml) and temperature or NaCl dependent. Only three isolates had low-grade resistance at both 30 and 35°C; for two isolates the MICs at 35°C were 24x the MICs at 30°C. NaCl usually increased the MICs two-to fourfold. Efficiency of plating studies were performed on strains selected for their level of oxacillin resistance and according to temperature-related difference in MICs. Most strains appeared to represent the heterogeneous resistance phenotype. Cefamandole MICs were little affected by temperature but increased with NaCl. With three exceptions, cefamandole MBCs were c4x MICs. For only six isolates were cefuroxime MICs <16 ,ug/ml. Four strains that were susceptible to both cefuroxime and cefamandole were selected for time-killing curve studies at inocula of 107 CFU/ml. At 8x MIC, cefuroxime failed to reduce the concentration of any strain by .3 x loglo CFU/ml. Killing of .3 x loglo CFU/ml was achieved by cefamandole at 4x and 8x MIC in one strain, at 8x MIC only in two strains, and by neither 4x nor 8x MIC in one strain. Within therapeutically attainable blood levels, cefuroxime is essentially inactive and cefamandole is variably bactericidal against oxacillin-resistant S. aureus.Methicillin-resistant Staphylococcus aureus should be considered resistant to P-lactam antibiotics, including all members of the cephalosporin group, according to the guidelines for antimicrobial susceptibility testing from the National Committee for Clinical Laboratory Standards (17). Some studies, however, have shown that cefamandole has good activity in vitro against both methicillin-resistant S. aureus and Staphylococcus epidermidis (7,16,20,29), although there seems to be inconsistent activity when cefamandole has been used for experimentally induced infections with methicillin-resistant S. aureus or when it has been used in clinical trials (4,6,8,11 , 1987). Discrepancies in results of treatment of staphylococcal infections with cefamandole could be due to the well-known heteroresistance found in methicillin-resistant S. aureus as well as to differences in susceptibility testing methodology, inoculum size, temperature of incubation, supplementation of culture medium with NaCl, and use of broth micro-versus macrodilution techniques (2,3,5,12,15,27).Hartman and Tomasz recently identified three different phenotypes of methicillin-resistant S. aureus: homogeneous resistance, heterogeneous resistance, and thermosensitive heterogeneous resistance (10). In the homogeneous resis-* Corresponding author. t Present address: Division of Infectious Diseases, Jackson Memorial Hospital, Miami, FL 33136. tance phenotype, methicillin resistance was expressed in approximately 100% of the cells present. In a second phenotype, the expression of resistance was heterogeneous and was not alter...

In Vivo Effect of Flucloxacillin in Experimental Endocarditis Caused by mecC -positive Staphylococcus aureus Showing Temperature-Dependent Susceptibility In Vitro

“…These results are in agreement with those of previous studies with the NCTC 13552 (formerly LGA251) strain (16). Temperature is known to have an impact in vitro on the susceptibility of heterogeneous mecA-MRSA strains to oxacillin (28)(29)(30), possibly due to PBP2a downregulation (31) or enzymatic inactivation at Ն37°C (2). Our results indicate that thermosensitive heterogeneous expression of oxacillin resistance is also present in mecC-MRSA isolates.…”

Inoculum effect on growth-delay time of oxacillin-resistant strains of Staphylococcus aureus and Staphylococcus epidermidis exposed to cefamandole, cefazolin, and cefuroxime

“…We do not know whether the bacteria were killed, and it remains a possibility that there was later regrowth. Nevertheless, better in vitro bactericidal activity for cefamandole than for cefazolin and cefuroxime has been demonstrated by other investigators (12,16,17).…”

“…The basic medium used was Mueller-Hinton agar supplemented with 4% NaCl and containing oxacillin at a concentration of 6 ,ug/ml. The EOP performed with Mueller-Hinton medium containing oxacillin at a concentration of 16 ,ugIml, as recommended by Mateos-Mora et al (12) in their study on the phenotypic characterization of oxacillin-resistant S. aureus strains, was greater than 1%. The phenotype of the oxacillin-resistant S. aureus strains included in our study appeared to be homogeneous, according to the classification of Hartman and Tomasz (7).…”

“…Oxacillin resistance was quantified by determining the efficiency of plating (EOP; in percent) (7,12). Growth obtained on MHS agar was inoculated into MHS broth and placed in a screw-cap tube on a rotator at 35°C.…”

Enhanced Detection of Community-Acquired Pneumonia Pathogens With the BioFire® Pneumonia FilmArray® Panel

“…The difference may result from differences in the PCR primers used for the nasal swab PCR in the MTB as compared with BPFA primers. In addition, replication of S. aureus is enhanced at 35°C (anterior nares) vs 37°C in the lower airway source of sputum sent for BPFA ( Matoos-Mora et al, 1988 , Milne et al, 1987 ).…”