Temperature Effect on the Susceptibility of Methicillin-Resistant Staphylococcus aureus to Four Different Cephalosporins

The activity of cefamandole and cefuroxime against oxacillin-resistant Staphylococcus epidermidis was studied in vitro to determine whether there was any relationship between oxacillin resistance phenotypes and cephalosporin activity. Oxacillin resistance phenotypes were determined by efficiency-of-plating studies on Mueller-Hinton agar containing oxacillin, with and without NaCl, and incubated at 30 and 35°C. On the basis of MIC and MBC determinations, cefamandole was more active than cefuroxime against oxacillin-resistant S. epidermidis. Although temperature had minimal effect on the activity of either cefamandole or cefuroxime, NaCl significantly decreased the activity of cefuroxime but not of cefamandole. Neither cephalosporin consistently produced -99.9% bactericidal activity within 24 h in timed killing-curve studies. No consistent relationship was observed between cefamandole or cefuroxime activity and oxacillin resistance phenotype.According to current standards for antimicrobial susceptibility tests of the National Committee for Clinical Laboratory Standards, all methicillin-resistant staphylococci should be considered resistant to cephalosporins (8). In recent studies, however, cefamandole has been shown to be active in vitro against methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis (3, 13). Furthermore, when compared with cefazolin for prophylaxis in cardiac surgery patients, both cefamandole (M. R. Petracek, A. B. Kaiser, J. W. Lea, D. S. Kernodle, and A. C. Roach, Clin. Res. 35:21a, 1987) and cefuroxime have been associated with significantly fewer postoperative staphylococcal infections (11). In contrast, cefamandole has not been consistently effective against methicillin-resistant staphylococci either in experimental models of infection (6) or in clinical studies (2, 4). These discrepant results may be due to differences in degrees of methicillin resistance among strains of staphylococci (7). Recently Hartman and Tomasz identified three phenotypes of methicillin-resistant S. aureus: homogeneous, heterogeneous, and thermosensitive heterogeneous (5). Studies identifying phenotypes of methicillin-resistant S. epidermidis, however, have not been done. The objectives of our study were first to determine whether methicillin (oxacillin) resistance phenotypes similar to those identified for methicillin-resistant S. aureus exist among oxacillinresistant S. epidermidis and second to determine whether the resistance phenotype relates to the activity of cefamandole and cefuroxime against oxacillin-resistant S. epidermidis. MATERIALS AND METHODSOrganisms. Blood culture isolates, identified specifically as S. epidermidis, were taken from 40 patients. Each isolate was initially characterized as being oxacillin resistant by disk diffusion susceptibility testing (16). * Corresponding author. Antimicrobial agents and susceptibility testing. Oxacillin was obtained from Sigma Chemical Co., St. Louis, Mo.; cefamandole was obtained from Eli Lilly & Co., Indianapolis, Ind.; and cefuroxime was ...

Section: Resultscontrasting

confidence: 51%

“…The turbidity of each culture was adjusted to equal that of a no. 1 McFarland standard and diluted 1:10 to yield a final inoculum of between 106 and 107 CFU/ml. A 1:10 dilution of this inoculum was made in cation-supplemented MHB containing cefamandole or cefuroxime at 1x, 2x, and 4x the MIC for the strain tested and incubated at 30 and 35°C.…”

Section: Methodsmentioning

confidence: 99%

Relationship between cefamandole and cefuroxime activity against oxacillin-resistant Staphylococcus epidermidis and oxacillin resistance phenotype

Woods

Knapp

Washington

1987

“…The MICs of ,-lactam antibiotics for MRSA are influenced frequently by growth conditions such as temperature (3), pH (14), and salt concentration (4). Thus, routine susceptibility tests may fail to detect MRSA (2).…”

mentioning

confidence: 99%

Comparison of a polymerase chain reaction assay and a conventional microbiologic method for detection of methicillin-resistant Staphylococcus aureus

Tokue

Shoji

Satoh

et al. 1992

117

The presence or absence of a methicillin resistance gene in 58 clinical isolates of Staphylococcus aureus was examined by the polymerase chain reaction (PCR) and Southern blot analyses. The results were analyzed in relation to those of the MIC assay of methicillin and oxacillin. PCR assay results were identical to those of Southern blot analysis of genomic DNA digested with HindIII (positive, 28 strains; negative, 30 strains). Among the 28 PCR-positive strains, 6 strains showed methicillin susceptibility by the conventional susceptibility test (MICs, less than or equal to 8 micrograms/ml). Culturing of the six strains with ceftizoxime led to an increase in the phenotypic level of resistance to methicillin and oxacillin, indicating that these strains should be classified as methicillin-resistant S. aureus (MRSA). The PCR assay was found to be a sensitive and reliable procedure for the rapid diagnosis of MRSA infection, even in cases in which the conventional MIC assay failed to detect MRSA.

“…Methicillin-resistant (MR) Staphylococcus aureus strains frequently demonstrate resistance to the older and newer cephalosporins (1,6,7). Currently, vancomycin remains the only antibiotic to which the microorganisms are consistently susceptible (5).…”

mentioning

confidence: 99%

In vitro susceptibility of methicillin-resistant and methicillin-susceptible Staphylococcus aureus strains to N-formimidoyl thienamycin

Witte¹,

Sapico²,

Canawati³

1982

A total of 82 clinical isolates of methicillin-resistant Staphylococcus aureus and 21 isolates of methicillin-susceptible S. aureus were studied for in vitro susceptibility to N-formimidoyl thienamycin at incubation temperatures of 30 and 35°C. The disk diffusion test results were correlated with the macrobroth dilution test by means of the error rate-bounded method of analysis. Both methicillin-susceptible and (to a lesser degree) methicillin-resistant strains were generally susceptible to the antibiotic as judged from their minimum inhibitory concentrations. The discrepancy between in vitro results obtained at 30 and at 35°C was not very remarkable. However, tolerance of N-formimidoyl thienamycin was observed in 37% of methicillin-resistant strains and 24% of methicillin-susceptible strains at an incubation temperature of 30°C; at 35°C, the values were 54% (methicillinresistant strains) and 14% (methicillin-susceptible strains).