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

Abstract

Cephalosporins have been recommended as prophylactic antibiotics in patients undergoing cardiovascular surgery. The major function of these antibiotics is to protect patients against Staphylococcus aureus and Staphylococcus epidermidis infections. The lowest inoculum amount responsible for infection during surgery is unknown but is probably low. To determine the comparative activities of cefazolin, cefuroxime, and cefamandole against S. aureus and S. epidermidis for prophylactic purposes, we selected five strains of S. aureus and S. epidermidis that presented homogeneous resistances to oxacillin. A continuously monitored turbidimetric method was used to evaluate cultures with variable inoculum sizes ranging from 106 to 1 CFU/ml and exposed to cefazolin, cefuroxime, and cefamandole at concentrations of 0.5, 1, 2, 4, 8, 16, and 32 ,ug/ml. Growth was defined as an increase of 0.1 optical density unit. The relationship between the time required for growth, the antibiotic concentration, and the initial bacterial density showed that cefamandole was more active than cefazolin, which, in turn, was revealed to be more active than cefuroxime against S. aureus and S. epidermidis.Although the benefits of antistaphylococcal prophylaxis in patients undergoing cardiac and orthopedic surgery remain uncertain, its use has become standard practice. The value of antimicrobial prophylaxis in patients undergoing cardiac surgery has never been assessed in a placebo-controlled trial, while information concerning infection rates in patients who do not receive antibiotics is scarce (1).Cephalosporins have been recommended as prophylactic antibiotics in patients undergoing cardiovascular surgery (9). The major purpose of these antibiotics is to protect patients against Staphylococcus aureus and Staphylococcus epidermidis infections. These two bacterial species are recognized as being the most important pathogens likely to infect different types of prostheses.Oxacillin-resistant strains of S. aureus and S. epidermidis may occur. Even though cephalosporins are not recommended for the treatment of infections caused by oxacillinresistant strains, it appears that these antibiotics provide effective protection early in infections caused by these bacteria (8,15).It is difficult to assess the amount of inoculum responsible for infections acquired during surgery (14). The amount is probably very low and is not comparable to the value of the inoculum used in experimental endocarditis (3).The strains included in the present study were isolated from patients with multiple positive blood cultures. They were recovered over a 3-year period, and these strains possibly may have differed in their phenotypic features.We submitted these oxacillin-resistant S. aureus and S. epidermidis strains to turbidimetric monitoring using inoculum sizes of 106 CFU/ml that were subsequently diluted 10-fold. The relative inoculum sizes (RISs) of oxacillinresistant S. aureus and oxacillin-resistant S. epidermidis were exposed to different concentrations of cefamandole, cef...

Prophylaxis with Teicoplanin and Cefuroxime Reduces the Rate of Prosthetic Joint Infection after Primary Arthroplasty

“…Recent guidelines for antimicrobial prophylaxis in surgery (3) concluded that recommendations for weight-based dosing for antimicrobial prophylaxis in obese patients cannot be made because data demonstrating clinically relevant decreases in the rates of surgical site infections from the use of such dosing strategies instead of standard doses in obese patients are not available in the published literature. Our result supports the concept that cefuroxime, even when it is used at 3 g (1.5 g before surgery and 1.5 g 2 h later), does not achieve concen- On the other hand, it is evident that narrow-or expanded-spectrum cephalosporins are not adequate for preventing infections caused by CoNS strains with a high rate of methicillin resistance (24). The incidence of these infections was also reduced in our study when teicoplanin was used, although the difference was not statistically significant.…”