Cefoxitin Resistance to Beta-Lactamase: a Major Factor for Susceptibility of Bacteroides fragilis to the Antibiotic

Abstract

Toluene-treated cell suspensions of Bacteroides fragilis were used to screen clinical isolates for the production of f3lactamase. Approximately one-third of the isolates possessed considerable cephalosporinase activity. A significant correlation was found between f3lactamase production and resistance to cephalosporin antibiotics. Several isolates were resistant to cefuroxime and cefamandole and produced enzymes capable of hydrolyzing these antibiotics. However, none of the 79 strains tested could hydrolyze the cephamycin derivative, cefoxitin. A large percentage (>90%) of the strains were susceptible to cefoxitin. Therefore, resistance to lactamase hydrolysis is a major factor for the effectiveness of cefoxitin against B. fragilis. Detailed studies of four isolates suggest that two different enzymes may be produced. Both are cephalosporinases but differ with regard to cellular distribution and substrate specificity. Cefoxitin is not a substrate for either enzyme, but it is an excellent competitive inhibitor (Ki -0.1 FM).Cephalosporins are broad-spectrum antibiotics with activity against most gram-positive and gram-negative bacteria. Among the exceptions to this general rule are members of the gram-negative species Bacteroides fragilis. Martin et al. (11) in a study of 195 clinical isolates, found that only 9% were susceptible to 25 ,g or less of cephalothin per ml. Although cephaloridine has generally proven to be more effective, Kislak (9) found only 50% of 40 isolates to be sensitive to 25 ug or less per ml. These observations have since been confirmed by others (21,22).Recently, cefoxitin (a semisynthetic cephamycin analogue) was demonstrated to be considerably more effective against B. fragilis than the cephalosporins now in use (21,22). Over 95% of the strains tested by the two groups were susceptible to 32 ,ug or less of cefoxitin per ml. Of the several hypotheses available to explain the effectiveness of cefoxitin against this group of bacteria, perhaps the most attrac-

Properties of cephalosporinase from Proteus morganii.

“…On the other hand, new N-lactamase inhibitors such as clavulanic acid and CP-45899 had no affinity to the enzyme from P. morganii GN5407. These substrate specificities of P. morganii GN5407 cephalosporinase prove to be close to those reported for cephalosporinases from E. coli GN5482,2p) E. cloacae GN7471,21) C. freundii GN7391,22> S. marcescens GN10857, P. aeruginosa GN10362 and P. rettgeri GN4430.23) DARLAND et al 32) have shown that cefoxitin was an effective inhibitor against B. fragilis ,3-lactamase, but cefuroxime was hydrolyzed by the R-lactamase from B. fragilis. Studies in our laboratory also proved that cefuroxime-type cephalosporins were well hydrolyzed by the ~-lactamase from B. fragilis GN-1147725) and moreover, those from P. vulgaris GN791924> and P. cepacia GN11164.18) In addition the hydrolysis of cephalothin by (3-lactamases from B. fragilis, P. vulgaris and P. cepacia was competitively inhibited by both clavulanic acid and CP-45899.…”

Cefoxitin inactivation by Bacteroides fragilis

“…Of the currently available beta-lactam antibiotics, cefoxitin has the best in vitro activity (4,19). The enhanced activity of cefoxitin against these pathogens has been attributed to the resistance of the drug to the beta-lactamases of B. fragilis (3,5,7,11,22). Although there have been rare reports documenting the inactivation of the drug, the most common reported mechanism of cefoxitin resistance in this group of organisms has been the failure of the drug to penetrate through the outer membrane (6).…”

Beta-Lactamase Stability of HR 756, a Novel Cephalosporin, Compared to That of Cefuroxime and Cefoxitin

“…Changes in the chemical structure of the cephalosporins have resulted in an increased antibacterial spectrum, more useful pharmacokinetic properties, and, in some cases, a high level of resistance to fi-lactamase hydrolysis. The presence of fi-lactamases in gram-negative organisms has been associated with resistance to cephalosporins, although these enzymes are not the only factor contributing to the resistance of gram-negative bacilli to f3-lactam compounds (3,9,10,15). There is great interest in antibiotics that are resistant to fl-lactamase hydrolysis and in compounds that are inhibitors of 8B-lactamases (2,5,11 (11).…”

“…Recently, naturally occurring inhibitors of ,B-lactamases produced by streptomyces have been discovered (1,14). Cefoxitin and cefuroxime, two new fi-lactam compounds, have been reported to be resistant to B8-lactamases of gram-negative bacilli (3,12).…”