In Vitro Activity of Cefaclor, a New Orally Administered Cephalosporin Antibiotic

The in vitro antibacterial activity of cefaclor, cephalothin, and cephalexin against 261 clinical isolates of Staphylococcus aureus and Enterobacteriaceae was compared. Cefaclor and cephalexin were about equally active against S. aureus. Cefaclor was the most active cephalosporin against Escherichia coli, Proteus mirabilis, and Klebsiella pneumoniae. The effect on the antimicrobial activity using a relatively high and low inoculum was pronounced for cefaclor when compared with that of cephalothin.


MATERIALS AND METHODS
The following strains of bacteria used in this study were isolated from patients at The Medical College of Pennsylvania between July and September 1976: 59 strains of Staphylococcus aureus, 53 strains of Escherichia coli, 45 strains ofProteus mirabilis, 5 strains of indole-positive Proteus, 51 strains of Klebsiella pneumoniae, and 48 strains of Enterobacter species (22 strains of E. cloacae, 22 strains ofE. aerogenes, 2 strains ofE. hafniae, and 2 strains of E. agglomerans).
The susceptibility of the organisms to cefaclor, cephalothin, and cephalexin was determined by an agar-dilution method in Mueller-Hinton agar (MHA) (Difco) and antibiotic medium number 1 (AB1) (Difco). The antibiotics were diluted in twofold steps in phosphate buffer (pH 4.5). A 1-ml portion of each dilution of antibiotic was added to 9 ml of the respective molten agars to obtain final antibiotic concentrations of 0.098 to 50 ,ug/ml. The pH of MHA was 7.1 and that of AB1 was 6.6. Bacteria were inoculated onto the surface of the plates by the replicating device of Steers et al.
(2). The device delivered approximately 105 bacteria to the agar surface (0.001 ml of a 10-1 dilution of asn overnight culture in heart infusion broth) of each strain to be tested. The minimal inhibitory concentration (MIC) was taken to be the concentration of antibiotic that prevented the visible growth of more than one colonly after 24 h of incubation at 37°C. All three antibiotics were tested against a particular bacterial species on the same day.
To study the inoculum effect, 29 strains of S. aureus, 27 strains of E. coli, and 30 strains of P. mirabilis were randomly selected from among the total number of strains originally tested. A 10-2 dilution and an undiluted portion of an overnight culture were inoculated with the replicator onto the surface of plates containing MHA (approximately 104 and 106 bacteria, respectively). The lower inoculum size approximates that used in the WHO-ICS method and in the study of Bill and Washington (1). The MIC was determined as described above.
The activity of the antibiotics against S. aureus in both MHA and AB1 is given in Table 1. Cephalothin was the most active antibiotic against S. aureus. Cephalothin was more active in AB1 than in MHA. Cefaclor and cephalexin were about equally active against S. aureus in MHA. Both cefaclor and cephalexin were more active in AB1 than in MHA; the activity of cefaclor was enhanced to a much greater extent in AB1 and had activity similar to cephalothin.
Cefaclor was the most active cephalosporin againstE. coli (Table 1). The activity of cephalothin and cephalexin was similar in MHA and AB1 (Table  1). However, cefaclor was considerably more active in ABI. Cefaclor was as active as cephalothin against P. mirabilis and considerably more active than cephalexin (Table 1). Cefaclor was considerably more active in AB1 againstP. mirabilis than in MHA. Four of five strains of indole-positive Proteus were resistant to 50 ,ug of the three antibiotics per ml. All three antibiotics were inactive against Enterobacter species; 43 of 48 strains were resistant to 50 ,ug of cefaclor or cephalothin per ml, and 38 of 48 strains were resistant to 50 ,g of cephalexin per ml. Cefaclor was the most active antibiotic tested against K. pneumoniae; cephalothin activity was similar to that of cephalexin (Table 1).
The inoculum effect was considerable for S. aureus, E. coli, and P. mirabilis (Table 2). A total of 34% of S. aureus strains, 50% of the P. mirabilis strains, and 100% of E. coli strains showed greater than or equal to eightfold differences in MIC between high and low inocula with cefaclor. A total of 10% of S. aureus, 7% of P. mirabilis, and 67% ofE. coli showed similar differences in MIC between high and low inocula with cephalothin. Percentages for cephalexin were 38, 13, and 59%, respectively. Cefaclor was more active than the other two cephalospo-  rins even against a high inoculum size ofE. coli and P. mirabilis.
In both MHA and AB1, cefaclor was the most active of the three cephalosporins tested against Enterobacteriaceae, inhibiting 45% ofE. coli, 71% of P. mirabilis, and 83% of K. pneumoniae at 6.3 ,ugl ml. Compared with cephalexin, cefaclor was especially active against P. mirabilis. Our study confirms the increased activity of cefaclor compared with cephalexin against Enterobacteriaceae recently reported by Bill and Washington (1). However, our study differs from the latter in that cefaclor appeared to be much more active against P. mirabilis. Preliminary unpublished data indicate that peak serum levels after a 500-mg dose of cefaclor should readily exceed 6.3 ,ug/ml, and urine levels should be much higher. Cefaclor's greater activity in AB1 than in MHA may be due to the more acid pH of AB1 and greater stability of cefaclor at lower pH (a po-tential advantage in the treatment of many urinary tract infections) but may be also due to other differences between the two media. The inoculum effect was pronounced for cefaclor when compared with that of cephalothin. This study was supported by Eli Lilly and Co., Indianapolis, Ind.
We gratefully acknowledge Virginia Simpson for her technical assistance and Abby Kleiman and Vivian G. Sharp for typing the manuscript. An inocula replicating apparatus for routine testing of bacterial susceptibility to antibiotics. Antibiot. Chemother. 9:307-311.